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MODIS satellite imagery and in situ water measurements were used to quantify inorganic sediment concentration (ISC) and, subsequently, sediment plume motion and evolution during the 2016 diversion of the Mississippi River through the Bonnet Carré Spillway (BCS) within Lake Pontchartrain (LP) estuary in Louisiana, USA. Satellite images revealed that sediment plume areal distribution was closely linked to BCS discharge rate. The plume flowed eastward along the southern LP coastline at an average speed of ~ 0.3 m s−1 during the first week of the event and attained its maximum areal extent (~ 1117 km2; 68% of the total LP area) on 23 January. In an attempt to examine the plume kinematics and potential sediment resuspension impacts, wind information was also assessed. Following BCS closure, the prevailing wind direction, but not wind speed, corresponded to significant differences in LP ISC. LP was further subdivided into four separate quadrants to assess relationships between ISC within each quadrant to BCS discharge rate and winds. While the BCS was open, the plume's eastward motion was associated with a high correlation (r2 = 0.93) between BCS discharge and ISC in the southeast quadrant. However, the weak relationship (r2 = 0.42) between BCS discharge and ISC in the northwest quadrant was attributed to influences from other tributaries. These results will benefit future research regarding impacts of freshwater diversions on coastal and estuarine processes that are sensitive to sediment influx by providing a satellite-based sediment map time series that can be used to help validate numerical circulation and sediment transport models. [ABSTRACT FROM AUTHOR]

Wave‐supported fluid mud (WSFM) plays an important role in sediment downslope transport on the continental shelves. In this study, we incorporated WSFM processes in the wave boundary layer (WBL) into the Community Sediment Transport Modeling System (CSTMS) on the platform of the Coupled Ocean‐Atmosphere‐Wave‐and‐Sediment Transport modeling system (COAWST). The WSFM module was introduced between the bottommost water layer and top sediment layer, which accounted for the key sediment exchange processes (e.g., resuspension, vertical settling, diffusion, and horizontal advection) at the water‐WBL and WBL‐sediment bed boundaries. To test its robustness, we adapted the updated model (CSTMS + WBL) to the Atchafalaya shelf in the northern Gulf of Mexico and successfully reproduced the sediment dynamics in March 2008, when active WSFM processes were reported. Compared with original CSTMS results, including WSFM module weakened the overall intensity of sediment resuspension, and the CSTMS + WBL model simulated a lutocline between the WBL and overlying water due to the formation of WSFM. Downslope WSFM transport resulted in offshore deposition (>4 cm), which greatly changed the net erosion/deposition pattern on the inner shelf off the Chenier Plain. WSFM flux was comparable with suspended sediment flux (SSF) off the Atchafalaya Bay, and it peaked along the Chenier Plain coast where wave activities were strong and the bathymetric slope was steep. The influence of fluvial sediment supply on sediment dynamics was limited in the Atchafalaya Bay. Sensitivity tests of free settling, flocculation, and hindered settling effects suggested that sediments were transported further offshore due to reduced settling velocity in the WBL once fluid mud was formed. Although sediment concentration in the WBL was sensitive to surface sediment critical shear stress, cohesive bed behavior was less important in WSFM dynamics when compared with strong hydrodynamic during cold fronts. Plain Language Summary: Fluid mud is an intermediate stage between a consolidated seafloor and dilute fine sediment suspension. Its high density (>10 g/L) and downslope movement are important in sediment transport over the inner continental shelf (water depths < 15 m). In this study we adopted a numerical model to simulate fluid mud transport on the Atchafalaya continental shelf in coastal Louisiana. We found that fluid mud formed during the passages of cold fronts, and its transport contributed to offshore sediment deposition. This downslope sediment transport process was more affected by the settling velocity of sediment particles rather than fluvial supply from the Atchafalaya River or sediment consolidation on the seabed. Key Points: Model results well reproduce hydrodynamics and WSFM dynamics during cold frontsWSFM transport results in onshore erosion and offshore deposition over the Atchafalaya inner shelfBed consolidation might influence WSFM concentration after several rounds of erosion [ABSTRACT FROM AUTHOR]

A deep-sea time-lapse camera and several temperature probes were deployed on the Gulf of Mexico continental shelf at a biological community associated with a gas hydrate outcropping to study topographic and hydrologic changes over time. The deployment site, Bush Hill (GC-185), is located at 27°47.5′ N and 91°15.0′ W at depths of ∼540 m. The digital camera recorded one still image every 6 h for July-October in 2001, every 2 h for the month of June 2002, and every 6 h for the month of July 2002. Temperature probes were in place at the site for the entire experimental period. The data recovered provide a record of processes that occur at gas hydrate mounds. Sediment resuspension over the mound causes significant variation in luminosity of the time-lapse photographs. A marked diurnal pattern can be seen in the temperature and luminosity records. No major change in shape or size of the gas hydrate outcrop at this site was observed during this study. Stable topography of the gas hydrate mound, combined with high bacterial activity and sediment turnover, appears to focus biological activity in the mound area. Frequency and recurrence of sediment resuspension indicate that short-term change in the depth and distribution of surface sediments is a feature of the benthos at the site. Because the sediment interface is a critical environment for hydrocarbon oxidation and chemosynthesis, short-term variability and heterogeneity may be important characteristics of these settings. [ABSTRACT FROM AUTHOR]

The seabed and the water column are tightly coupled in shallow coastal environments. Numerical models of seabed‐water interaction provide an alternative to observational studies that require concurrent measurements in both compartments, which are hard to obtain and rarely available. Here, we present a coupled model that includes water column biogeochemistry, seabed diagenesis, sediment transport and hydrodynamics. Our model includes realistic representations of biogeochemical reactions in both seabed and water column, and fluxes at their interface. The model was built on algorithms for seabed‐water exchange in the Regional Ocean Modeling System and expanded to include carbonate chemistry in seabed. The updated model was tested for two sites where benthic flux and porewater concentration measurements were available in the northern Gulf of Mexico hypoxic zone. The calibrated model reproduced the porewater concentration‐depth profiles and benthic fluxes of O2, dissolved inorganic carbon (DIC), TAlk, NO3 and NH4. We used the calibrated model to explore the role of benthic fluxes in acidifying bottom water during fair weather and resuspension periods. Under fair weather conditions, model results indicated that bio‐diffusion in sediment, labile material input and sediment porosity have a large control on the importance of benthic flux to bottom water acidification. During resuspension, the model indicated that bottom water acidification would be enhanced due to the sharp increase of the DIC/TAlk ratio of benthic fluxes. To conclude, our model reproduced the seabed‐water column exchange of biologically important solutes and can be used for quantifying the role of benthic fluxes in driving bottom water acidification over continental shelves. Plain Language Summary: In coastal environments where water is shallow, significant interactions occur between the seabed and the overlying water column. In the Northern Gulf of Mexico (NGoM) hypoxic zone, the seabed is believed to play an important role in the acidification of bottom ocean water. In this study we use a numerical model to understand how the seabed can affect bottom water acidification over the NGoM hypoxic zone. We found that during fair weather periods, mixing in the sediment bed due to biological activity, organic matter supply from the water column to the sediment, and the porosity of sediment itself can largely affect the role of the seabed in acidifying the bottom water. When the ocean condition is highly dynamic and resuspension occurs, the contribution of the seabed to bottom water acidification will likely be enhanced. Key Points: A coupled water column biogeochemical‐seabed diagenetic model is updated to include dissolved inorganic carbon (DIC) and TAlk for ocean acidificationThe modeled impact of benthic fluxes on bottom water acidification during quiescent periods is impacted by the intensity of bio‐diffusionResuspension enhances bottom water acidification by increasing the benthic flux DIC/TAlk ratio [ABSTRACT FROM AUTHOR]

Objective: The Atlantic Tripletail Lobotes surinamensis is a globally distributed subtropical and tropical fish species that inhabits estuaries throughout the northern Gulf of Mexico (GOM), particularly during warm months. Little is known about distribution and residency patterns within estuaries, as the species is rarely caught in the recreational fishery, and virtually no commercial fishery exists for the species in the GOM. Methods: We used data from a long‐term fishery‐independent gill‐net survey to model estuarine distribution throughout Texas and to relate environmental variables to the Atlantic Tripletail catch. Result: Although there were no observable temporal trends in catch over the time series (1990–2022), the most recent 6 years included record catch in six of the 10 major Texas estuaries, possibly indicating a recent pulse in abundance. Catch throughout the time series was spatially aggregated in a small number of "hot spots" observed coastwide. Latitude was the best predictor of catch, although wind fetch and wind aspect (wind direction in relation to shoreline direction) were important predictors, and catch was highest near GOM inlets. The Texas Parks and Wildlife Department gill‐net sampling program caught a range of Atlantic Tripletail between 171 and 880 mm total length, indicating a potential gear bias against juveniles. Conclusion: Despite this gear bias, these data shed light on the factors that drive Atlantic Tripletail estuarine distribution and abundance in the northwestern GOM. Wind‐driven passive movements in the estuary, combined with active selection of polyhaline habitats near GOM inlets, might be primary drivers of Atlantic Tripletail catch, thus supporting findings from previous studies. Impact statementAtlantic Tripletail are a popular recreational fishing species throughout the Gulf of Mexico. In Texas estuaries, Atlantic Tripletail are clustered in hot spots, which are typified by windward shorelines, and present mainly during the high‐use fishing season. They are rarely observed at water temperatures below 20°C. [ABSTRACT FROM AUTHOR]

Bio-optical and physical measurements were collected in the Mississippi Sound (Northern Gulf of Mexico) during the spring of 2018 as part of the Integrated Coastal Bio-Optical Dynamics project. The goal was to examine the impact of atmospheric and tidal fronts on fine-scale physical and bio-optical property distributions in a shallow, dynamic, coastal environment. During a 25-day experiment, eight moorings were deployed in the vicinity of a frontal zone. For a one-week period in the middle of the mooring deployment, focused ship sampling was conducted with aircraft and unmanned aerial vehicle overflights, acquiring hyperspectral optical and thermal data. The personnel in the aircraft located visible color fronts indicating the convergence of two water masses and directed the ship to the front. Dye releases were performed on opposite sides of a front, and coincident aircraft and unmanned aerial vehicle overflights were collected to facilitate visualization of advection/mixing/dispersion processes. Radiometric calibration of the optical hyperspectral sensor was performed. Empirical Line Calibration was also performed to atmospherically correct the aircraft imagery using in situ remote sensing reflectance measurements as calibration sources. Bio-optical properties were subsequently derived from the atmospherically corrected aircraft and unmanned aerial vehicle imagery using the Naval Research Laboratory Automated Processing System. [ABSTRACT FROM AUTHOR]

Abstract This study utilizes suspended particles and seafloor sediments collected from the northern Gulf of Mexico (GOM) continental margin to study the fate, transport, residence times and accumulation rates of particle-bound polycyclic aromatic hydrocarbons (PAHs). Total particulate-PAHs and particulate organic carbon (POC) varied between 0.9 and 7.0 ng/L, and 4–131 μg/L, respectively. Particulate-PAHs were positively correlated with POC, while both particulate-PAHs and POC were negatively correlated with salinity (P-value < 0.05). These results show that the river-derived particle influx and associated POC are important vectors for transport and fate of particulate-PAHs in the river-dominated northern GOM continental ecosystems. The composition of underlying seafloor sediment-PAHs were not correlated to the water column particulate-PAHs, which is attributed to re-mineralization, sediment resuspension/redistribution and different timescales of comparison. The 210Pb-derived residence time of particles and associated particulate-PAHs in water column varied between 2 and 39 days. Residence times of particulate-PAHs were significantly correlated with seafloor sediment-PAHs accumulation rates, shorter water column residence times leading to higher PAHs accumulation rates. Graphical abstract Unlabelled Image Highlights • First estimates of 210Pb-based residence time of particulate-PAHs in the northern GOM. • Concentration of particulate-PAHs and POC varied between 0.9 –7.0 ng/L, and 4–131 µg/L, respectively. • Total and individual PAHs in suspended particles had strong positive correlation with POC. • The Mississippi River-derived particles play a major role in transport and fate of particulate-PAHs in the northern GOM. • 210Pb-derived residence time of particles in the northern GOM varies between 2-39 days. [ABSTRACT FROM AUTHOR]

Context: Seagrass ecosystems are lauded for storing organic carbon in underlying sediments, but storage is highly variable, even at relatively small spatial scales. While environmental setting and seagrass cover are known drivers of carbon storage capacity, it is unclear how other seagrass features such as species composition influence carbon storage, and whether historical vs. contemporary features are better predictors of storage. Objectives: We examined the influence of historical and contemporary seagrass variables on surface (0–10 cm) sediment organic carbon storage at the meadow-scale (~ 25 km2), in addition to the influence of environmental drivers. Our study area was located within a subtropical mixed-species seagrass meadow along a low-energy coastline in the northeastern Gulf of Mexico (Cedar Key, Florida, USA). Methods: We derived historical metrics of seagrass cover and composition from 14-year seagrass monitoring datasets and measured surface sediment carbon densities and grain size, contemporary seagrass biomass and species composition, as well as environmental characteristics related to hydrology and physical disturbance (i.e., relative exposure, elevation, and distance to navigation channels). We assessed bivariate relationships between predictor variables and surface carbon densities with linear regression analyses and used path analysis to assess hypothesized relationships between a subset of predictor variables and carbon densities. Results: While low relative to global values, surface carbon densities in Cedar Key seagrass meadows varied by an order of magnitude. Sediment grain size was strongly related to carbon densities, but environmental variables had only indirect effects on carbon densities. Historical seagrass cover, variability in cover, and species diversity were generally better predictors of storage than contemporary variables. Historical and contemporary species identity–specifically the presence of Thalassia testudinum–were also significant drivers of storage. Conclusions: In Cedar Key, historically diverse and persistent seagrass meadows dominated by late-successional species contained the largest surface carbon stores. Our results highlight the importance of site history in terms of meadow stability (inversely measured as variability in cover) as well as species identity and diversity in enhancing surface carbon storage. The environmental variables we examined had comparatively weak effects on carbon densities, however, relative exposure and elevation may not be the most relevant hydrological drivers of carbon storage at the meadow scale. Together, these findings suggest that drivers of seagrass meadow carbon storage are context and scale dependent. [ABSTRACT FROM AUTHOR]

Recent studies have shown that Lake Pontchartrain Estuary in Louisiana experiences frequent harmful cyanobacterial blooms (cyanoHABs). In 2019, the Bonnet Carré Spillway (BCS) that diverts Mississippi River water into the estuary opened twice in the same year for the first time in history to prevent flooding in New Orleans. Short-term water quality monitoring was conducted in shoreline areas with high public use for the presence of cyanoHABs and cyanotoxins to assess the public health risks. Field sampling methods and satellite imagery were used to determine water quality and quantify bloom intensity and toxicity across time and space. Long-term BCS opening created a fresh (salinity < 0.2) and nutrient-rich estuary that supported several cyanoHABs in warmer months during and after the second BCS closure. Cyanobacterial biomass ranged from 35 to 4972 µg PC L−1, while toxin microcystin ranged from undetected to 8.41 µg MC L−1. The highest biomass and toxin were detected on June 25 at the north shore, station LP8, Mandeville Beach, dominated by Microcystis and Dolichospermum species. CyanoHABs occurred mostly in the northern part of the estuary, where tributary discharge is also a strong influence. Some of these blooms exited the estuary and were transported to the Gulf of Mexico following passage through Lake Borgne and then Mississippi Sound. Modifications in the timing and duration of river diversion operations can create prolonged cyanobacterial blooms that can cause environmental and public health risks, especially in warmer months, and this may intensify due to a changing climate. [ABSTRACT FROM AUTHOR]

The southward propagation of cold-air frontal boundaries into the Gulf of Mexico region initiates a cascade of coupled air–sea processes that manifests along the coastlines as an apparent brightness anomaly in the ocean color signals. Our hypothesis is that the color anomaly is largely due to the turbulent resuspension of sedimentary particles. Initially, there is significant wind-driven ocean turbulence as the frontal boundary passes, followed by the potential for sustained convective instability due to significant heat losses from the ocean surface. These cold front events occur during boreal autumn, winter, and into early spring, and the latter episodes occur in the context of the seasonally recurring thermal stratification of shelf waters. Here, we show that as stratification reasserts thermal stability in the waning days of a cold front episode, daily to hourly ocean color patterns are temporally coherent with the air–sea heat flux changes and the resulting impact on water column stability. Concomitant results from a nested, data-assimilative, and two-way coupled ocean-atmosphere numerical modeling system provides both corroboration and insight into how surface air–sea fluxes and in-water turbulent mixing manifest as hourly changes in apparent surface water turbidity due to the potential excitation and settling of reflective particles. A simple model of particle mixing and settling driven by the simulated turbulence mimics patterns seen in the satellite image sequences. This study offers a preview of potential application areas that may emerge following the launch of a dedicated ocean color geostationary sensor. [ABSTRACT FROM AUTHOR]

The ecological impacts of multiple stressors are hard to predict but important to understand. When multiple stressors influence foundation species, the effects can cascade throughout the ecosystem. Gulf of Mexico seagrass ecosystems are currently experiencing a suite of novel stressors, including warmer water temperatures and increased herbivory due to tropicalization and conservation efforts. We investigated the impact of warming temperatures and grazing history on plant performance, morphology, and palatability by integrating a mesocosm study using the seagrass Thalassia testudinum with feeding trials using the sea urchin Lytechinus variegatus. Warming temperatures negatively impacted T. testudinum tolerance traits, reducing belowground biomass by 34%, productivity by 74%, shoot density by 10%, and number of leaves per plant by 24%, and negatively impacted resistance traits through 13% lower toughness of young leaves and a trend for reduced leaf carbon:nitrogen. Lytechinus variegatus individuals preferred to consume plants grown under heated conditions, which supports findings of enhanced palatability. Simulated turtle grazing impacted more plant traits than grazing by other herbivores, potentially diminishing plant resilience to future disturbances through reduced rhizome non‐structural carbohydrate concentrations and increasing palatability through reduced fiber content and 23% lower leaf carbon:phosphorus. Simulated turtle, simulated parrotfish, and urchin grazing reduced leaf carbon:nitrogen by 11%, also potentially increasing nutritive value. Interactions between warming temperatures and grazers on plant traits were additive for 16 out of 19 response variables. However, the stressors non‐additively impacted the number of leaves per plant, fiber content, and epiphyte load. We suggest that the impacts of grazers on leaf turnover rate and leaf age may vary based on water temperature, potentially driving these interactions. Overall, increased temperatures and grazing pressure will likely reduce seagrass resilience, structure, and biomass, potentially impacting feedback systems and producing negative consequences for seagrass cover, associated species, and ecosystem services. [ABSTRACT FROM AUTHOR]

In April of 2010, the Macondo well blowout in the northern Gulf of Mexico resulted in an unprecedented release of oil into the water column at a depth of approximately 1500 m. A time series of surface and subsurface sediment samples were collected to the northwest of the well from 2010 to 2013 for molecular biomarker and bulk carbon isotopic analyses. While no clear trend was observed in subsurface sediments, surface sediments (0–3 cm) showed a clear pattern with total concentrations of n -alkanes, unresolved complex mixture (UCM), and petroleum biomarkers (terpanes, hopanes, steranes) increasing from May to September 2010, peaking in late November 2010, and strongly decreasing in the subsequent years. The peak in hydrocarbon concentrations were corroborated by higher organic carbon contents, more depleted Δ 14 C values and biomarker ratios similar to those of the initial MC252 crude oil reported in the literature. These results indicate that at least part of oil discharged from the accident sedimented to the seafloor in subsequent months, resulting in an apparent accumulation of hydrocarbons on the seabed by the end of 2010. Sediment resuspension and transport or biodegradation may account for the decrease in sedimented oil quantities in the years following the Macondo well spill. [ABSTRACT FROM AUTHOR]

Florida's coastal salt marshes are vulnerable to both direct and indirect human impacts, including climate change and consequent sea-level rise. For a salt marsh to survive in the face of ongoing sea-level rise, organic and/or mineral sediment must accumulate at a rate equal to or faster than that of sea-level increase. We explored the effects of Late Holocene sea-level variations in the Suwannee River Estuary within the Big Bend region of Florida (USA). We conducted a paleoenvironmental study of a sediment core collected from a salt marsh near Cedar Key, on Florida's Gulf of Mexico coast. The core spans the last ~ 320 years of sediment accumulation. Carbon isotope (δ13C) data and diatom assemblages indicate the salt marsh was relatively stable during that time frame and was dominated by C3 vegetation, likely Juncus roemerianus, but experienced moderate variations in salinity that likely reflect changes in sea-level, with an increase in salinity and marine incursions between ~ 1850 and 1930 CE. Whereas small vertical changes in sea-level have the potential to inundate large areas of the low-gradient salt marsh, as observed during the interval ~ 1850–1930 CE, the salt-marsh vegetation recovered quickly after 1930 CE, indicating that the rate of aggradation and vegetation growth kept pace with the rate of sea-level rise. Despite the apparent resiliency of Big Bend salt marshes and likelihood that they will persist through accretion and migration, we expect to see major changes in salt-marsh ecology if rates of sea-level rise continue to accelerate. [ABSTRACT FROM AUTHOR]

Multi‐sensor data collected with in situ and satellite instruments during August 2015 were used to understand how the three‐dimensional bio‐optical properties of a Loop Current Eddy (LCE) in the Gulf of Mexico (GoM) contrast those of the background waters, and how these properties are related to physical and chemical properties. With a surface radius of ∼150 km and vertical extension to 1,400–1,500 m, the LCE was found to have highly stratified waters in two layers, with one lying just below the mixed layer (16 m) and the other coinciding with the pycnocline (∼120–200 m within the eddy). Strong contrasts were found in the bio‐optical properties (chlorophyll‐a concentration, absorption of particulate and dissolved matters, particulate backscattering, and beam‐c attenuation) across the eddy core, eddy edge, and surrounding waters. Absorption coefficients (400 nm) of surface particulate and dissolved matters were ∼4 times higher in the surrounding waters than in the eddy core, while surface reflectance (400 nm) in the eddy core was ∼7 times higher than in the surrounding waters. The magnitude of deep chlorophyll maximum (DCM) was comparable (0.3–0.33 mg/m3) in all waters, but the depth of DCM in the eddy core (∼115 m) was much deeper than in the surrounding waters (60–75 m). These contrasts were found to correspond to different water masses with different physical (temperature, density, and buoyancy frequency) and chemical properties (salinity and dissolved oxygen concentration), where physical processes (river plume advection and eddy‐induced downwelling) appeared to drive the changes in bio‐optical properties. Plain Language Summary: Loop Current Eddies (LCEs) are dominant circulation features in the Gulf of Mexico (GoM), which sporadically shed from the Loop Current (LC) with westward propagations. LCEs can transport heat, salt, dissolved oxygen, and other particulate and dissolved matters both horizontally and vertically. LCEs have been studied extensively, including their formations, shedding variability and prediction, moving paths, and interactions with topography. However, little is known about their bio‐optical properties, especially in the context of their physical and chemical contrasts from the background waters. Here, using multiple observational data sets collected during August 2015, we characterize the three‐dimensional bio‐optical, physical, and chemical properties of a LCE. With a mean radius of ∼150 km at the sea surface, the influence of the LCE extended to 1,400–1,500 m depth. The LCE was also found to have different bio‐optical, physical, and chemical properties from the surrounding waters at both surface and depth, where physical processes appeared to drive the different water masses that led to the observed changes. Key Points: The 3‐D bio‐optical, physical, and chemical properties of a Loop Current Eddy (LCE) were systemically characterized with observational data setsDifferences in bio‐optical and chemical properties of the eddy core, eddy edge, and surrounding waters were controlled by physical processesThe influence of the LCE extended to depths of 1,400–1,500 m [ABSTRACT FROM AUTHOR]

The first measurements of bottom nepheloid layers in the central and southern deep waters of the Gulf of Mexico west of the Yucatan peninsula were made during the three summers of 2015–2017. Particulate matter concentrations (PM) were estimated from optical profiles of beam attenuation due to particles (c p). Near-bottom maps and vertical sections of c p and PM converted from c p show evidence of sediment resuspension, possibly linked with topographic Rossby waves, loop current eddies, or eddy-topography interactions. Additional c p profiles were made along cross-slope transects around the entire Gulf of Mexico, including across the Yucatan Channel and Straits of Florida in 2017. Near-bottom PM concentrations were barely elevated in the deep Yucatan Channel and Straits of Florida at that time, except in about the surface 200 m along the northern and western boundaries. Comparison was made between areas with benthic nepheloid layers and Eddy Kinetic Energy (EKE) patterns in the deep Gulf of Mexico. Regions of high EKE or strong bottom currents in the central and eastern Gulf were found over a large region of deeply eroded furrows in the seafloor previously imaged using 3-D seismic profiling and submersible observations. Few PM measurements were obtained in the high EKE areas during these expeditions, however, historical and recent sampling show very strong nepheloid layers at stations within and westward of the region of the actively eroding furrows. • Beam c p data reveal weak to very strong nepheloid layers in the deep Gulf of Mexico. • Loop Current eddies and other currents may create varying bottom nepheloid layers. • Bottom currents create mega-furrows and nepheloid layers at Sigsbee escarpment base. • Nepheloid layers were weak in the deep Yucatan Channel and Straits of Florida. [ABSTRACT FROM AUTHOR]

Trophic transfer and bioaccumulation of trace metals have a profound impact on the structure and function of coastal areas; however, the metal accumulation patterns in detritus-based food webs and the influence of climatic variability have not been thoroughly investigated. The Tampamachoco Lagoon (Gulf of Mexico) is a coastal system impacted by emissions from a thermoelectric plant. We evaluated the spatial–temporal distribution patterns of Al, Cd, Hg, Cr, Cu, and Pb in water, sediments, and in organisms categorized by trophic levels (TLs), trophic guilds, and habitat preferences. The sediments had the highest concentrations of metals with no significant differences between seasons. The indices of geo-accumulation and potential ecological risk classified sediments as "moderately contaminated", evidencing a threat to human health through consumption of detritivores and filter-feeders. The lowest TLs (filter-feeders and detritivorous) reached the maximum Metal Pollution Index in the rainy season. According to discriminant analyses of metals and species, omnivorous and zoobentivorous organisms were associated with Hg during the rainy and dry seasons; while Al, Cd, and Cu were related to low TLs, and seston was associated with Pb. Food web magnification factor analysis showed that: (a) Pb, Cu, and Cr were biodiluted as trophic levels increased; (b) Cd and Hg showed temporal biomagnification trends; (c) Al, Pb, Cu, and Cd showed significant biodilution from the lowest TL to intermediate TLs; and (d) Hg was transferred from the lowest to intermediate TLs with clear biomagnification effects. [ABSTRACT FROM AUTHOR]

Estuarine and coastal geomorphology, biogeochemistry, water quality, and coastal food webs in river-dominated shelves of the Gulf of Mexico (GoM) are modulated by transport processes associated with river inputs, winds, waves, tides, and deep-ocean/continental shelf interactions. For instance, transport processes control the fate of river-borne sediments, which in turn affect coastal land loss. Similarly, transport of freshwater, nutrients, and carbon control the dynamics of eutrophication, hypoxia, harmful algal blooms, and coastal acidification. Further, freshwater inflow transports pesticides, herbicides, heavy metals, and oil into receiving estuaries and coastal systems. Lastly, transport processes along the continuum from the rivers and estuaries to coastal and shelf areas and adjacent open ocean (abbreviated herein as "river-estuary-shelf-ocean") regulate the movements of organisms, including the spatial distributions of individuals and the exchange of genetic information between distinct subpopulations. The Gulf of Mexico Research Initiative (GoMRI) provided unprecedented opportunities to study transport processes along the river-estuary-shelf-ocean continuum in the GoM. The understanding of transport at multiple spatial and temporal scales in this topographically and dynamically complex marginal sea was improved, allowing for more accurate forecasting of the fate of oil and other constituents. For this review, we focus on five specific transport themes: (i) wetland, estuary, and shelf exchanges; (ii) river-estuary coupling; (iii) nearshore and inlet processes; (iv) open ocean transport processes; and (v) river-induced fronts and cross-basin transport. We then discuss the relevancy of GoMRI findings on the transport processes for ecological connectivity and oil transport and fate. We also examine the implications of new findings for informing the response to future oil spills, and the management of coastal resources and ecosystems. Lastly, we summarize the research gaps identified in the many studies and offer recommendations for continuing the momentum of the research provided by the GoMRI effort. A number of uncertainties were identified that occurred in multiple settings. These include the quantification of sediment, carbon, dissolved gasses and nutrient fluxes during storms, consistent specification of the various external forcings used in analyses, methods for smooth integration of multiscale advection mechanisms across different flow regimes, dynamic coupling of the atmosphere with sub-mesoscale and mesoscale phenomena, and methods for simulating finer-scale dynamics over long time periods. Addressing these uncertainties would allow the scientific community to be better prepared to predict the fate of hydrocarbons and their impacts to the coastal ocean, rivers, and marshes in the event of another spill in the GoM. [ABSTRACT FROM AUTHOR]

Biological oil weathering facilitated by specialized heterotrophic microbial communities plays a key role in the fate of petroleum hydrocarbon in the ocean. The most common methods of assessing oil biodegradation involve (i) measuring changes in the composition and concentration of oil over time and/or (ii) biological incubations with stable or radio-labelled substrates. Both methods provide robust and invaluable information on hydrocarbon biodegradation pathways; however, they also require extensive sample processing and are expensive in nature. More convenient ways to assess activities within microbial oil degradation networks involve measuring extracellular enzyme activity. This perspective article synthesizes previously published results from studies conducted in the aftermath of the 2010 Deepwater Horizon (DwH) oil spill in the northern Gulf of Mexico (nGoM), to test the hypothesis that fluorescence assays of esterases, including lipase activity, are sensitive indicators for microbial oil degradation in the ocean. In agreement with the rates and patterns of enzyme activity in oil-contaminated seawater and sediments in the nGoM, we found close correlations between esterase activity measured by means of methylumbeliferyl (MUF) oleate and MUF butyrate hydrolysis, and the concentration of petroleum hydrocarbons in two separate laboratory incubations using surface (<1 m) and deep nGoM waters (>1200 m). Correlations between esterase activities and oil were driven by the presence of chemical dispersants, suggesting a connection to the degree of oil dissolution in the medium. Our results clearly show that esterase activities measured with fluorogenic substrate proxies are a good indicator for oil biodegradation in the ocean; however, there are certain factors as discussed in this study that need to be taken into consideration while utilizing this approach. [ABSTRACT FROM AUTHOR]

North African dust is known to be deposited in the Gulf of Mexico, but its deposition rate and associated supply of lithogenic dissolved metals, such as the abiotic metal thorium or the micronutrient metal iron, have not been well‐quantified. 232Th is an isotope with similar sources as iron and its input can be quantified using radiogenic 230Th. By comparing dissolved 232Th fluxes at three sites in the northern Gulf of Mexico with upwind sites in the North Atlantic, we place an upper bound on North African dust contributions to 232Th and Fe in the Gulf of Mexico, which is about 30% of the total input. Precision on this bound is hindered by uncertainty in the relative rates of dust deposition in the North Atlantic and the northern Gulf of Mexico. Based on available radium data, shelf sources, including rivers, submarine groundwater discharge, and benthic sedimentary releases are likely as important if not more important than dust in the budget of lithogenic metals in the Gulf of Mexico. In other words, it is likely there is no one dominant source of Th and Fe in the Gulf of Mexico. Finally, our estimated Fe input in the northern Gulf of Mexico implies an Fe residence time of less than 6 months, similar to that in the North Atlantic despite significantly higher supply rates in the Gulf of Mexico. Plain Language Summary: The ocean's nutrients generally come from land. The specific route a nutrient takes depends on the nutrient. For the nutrient iron, the rain of airborne dust blown from deserts is thought to be a significant source, especially in the remote ocean. This is because iron is very insoluble and usually cannot travel far in the water. The Gulf of Mexico is a semi‐enclosed sea with significant contact with ocean margin sediments and rivers. It is uncertain how this margin source might compare to airborne dust in the Gulf of Mexico. In this study, we used another element, thorium, to trace these two sources and found that dust is likely a significant but not dominant source. Key Points: An upper bound on the contribution of North African dust toward dissolved thorium supply in the northern Gulf of Mexico may be around 30%Thorium and other tracers suggest significant additional sources from the Gulf of Mexico shelf as well as slope and abyssal sedimentsThorium‐based lithogenic supply of iron suggests dissolved iron residence time in the northern Gulf of Mexico is less than 6 months [ABSTRACT FROM AUTHOR]

Low bottom water dissolved oxygen conditions (hypoxia) occur almost every summer in the northern Gulf of Mexico due to a combination of nutrient loadings and water column stratification. Several statistical and mechanistic models have been used to forecast the midsummer hypoxic area, based on spring nitrogen loading from major rivers. However, sub-seasonal forecasts are needed to fully characterize the dynamics of hypoxia over the summer season, which is important for informing fisheries and ecosystem management. Here, we present an approach to forecasting hypoxic conditions at a daily resolution through Bayesian mechanistic modeling that allows for rigorous uncertainty quantification. Within this framework, we develop and test different representations and projections of hydrometeorological model inputs. We find that May precipitation over the Mississippi River basin is a key predictor of summer discharge and loading that substantially improves forecast performance. Accounting for spring wind conditions also improves forecast performance, though to a lesser extent. The proposed approach generates forecasts for two different sections of the Louisiana–Texas shelf (east and west), and it explains about 50 % of the variability in the total hypoxic area when tested against historical observations (1985–2016). Results also show how forecast uncertainties build over the summer season, with longer lead times from the nominal forecast release date of 1 June, due to increasing stochasticity in riverine and meteorological inputs. Consequently, the portion of overall forecast variance associated with uncertainties in data inputs increases from 26 % to 41 % from June–July to August–September, respectively. Overall, the study demonstrates a unique approach to assessing and reducing uncertainties in temporally resolved hypoxia forecasting. [ABSTRACT FROM AUTHOR]

Abstract: To investigate the effects of tropical cyclones on the water chemistry of Gulf of Mexico coastal rivers, time series samples from the lower Pearl River at Stennis Space Center, Mississippi, were collected on August and September, 2008, during Hurricanes Gustav and Ike. Hurricane Gustav, which landed near the sampling site, caused intensive storm surge and strong seawater intrusion, resulting in an elevated salinity of 7.5 in the lower Pearl River and subsequent flooding induced by heavy rainfall. Hurricane Ike, which passed further away from the sampling site, caused only a mild seawater intrusion with a salinity of 1.2 at the sampling site. The river showed distinct variations in water chemistry corresponding to different hydrographic disturbance of hurricanes. Abrupt increase of suspended particulate matter and associated organic carbon and nitrogen concentrations coincided with the intensive storm surge due to coastal sediment resuspension. A remarkable drop in the concentrations of phosphate and dissolved organic matter was also observed during the intense seawater intrusion, a result of both dilution by seawater and resultant flocculation of dissolved organic matter. During hurricane-induced flooding, the river showed a mild increase in the concentrations of organic matter, reflecting a dominant contribution of terrestrial inputs from the watershed by surface runoffs while the concentrations of inorganic nutrient species in the river water decreased. In contrast, water chemistry in the Pearl River underwent little change in most carbon and nutrient species under the mild seawater intrusion. Overall, tropical cyclones could induce unique variations in coastal river water chemistry and variable material export which would further alter the coastal water quality. [Copyright &y& Elsevier]

Abstract: Dissolved (DOM) and particulate organic matter (POM) samples were collected from the Bay of St. Louis (BSL) in the northern Gulf of Mexico during March–September, 2007 for chemical and isotopic characterization to examine the distribution and sources of organic matter species in the estuarine environment. Similar to the variations in hydrographic parameters and nutrients, concentrations of organic C, N, and P and stable isotopic composition show large spatial and seasonal variations during the sampling period. Concentrations of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) decreased with increasing salinity while δ13C-DOC increased with salinity from −30.23‰ to −19.04‰, suggesting a shift of DOC sources from terrestrial- to marine-dominated inputs during estuarine mixing. In contrast to both DOC and DON, the concentration of dissolved organic phosphorus (DOP) increased with increasing salinity, indicating additional DOP sources at higher salinity stations. Concentrations of particulate organic carbon (POC) and nitrogen (PN) decreased with increasing salinity, showing a negative correlation with the concentration of suspended particulate mater. Both POC/PN ratio (8.6–19.6) and δ13C-POC (−28.51‰ to −23.79‰) increased in general with increasing salinity, indicating the predominance of terrestrially derived organic matter in the upper bay and increasing diagenetically altered marine POM component in the lower bay. Fresh microalgae might account for about one third of POM in the BSL as estimated from POC/Chl a ratio. Overall, the terrestrial inputs, in situ primary production and diagenetically altered marine POM mostly from sediment resuspension are the major sources of POM in the BSL. [Copyright &y& Elsevier]

Abstract: It is generally assumed that estuarine mixing is continuous for metals from terrestrial sources, gradually decreasing towards the open ocean endmember. Here we show that, chemical reactivity, determined by ion exchange method, and molecular weight distributions, obtained using cross-flow ultrafiltration, of dissolved Cd, Cu, and Ni in the surface waters of the Gulf of Mexico varied systematically across the estuarine mixing zone of the Mississippi River. Most size or chemical affinity fractions of dissolved metals (<0.4 μm) were linearly related to salinity (10.8–36.6), suggesting that the distribution of these elements was mainly controlled by continuous mixing processes. Dissolved concentrations across the salinity gradient ranged for Cd: 87–187 pM; Cu: 1.4–18.3 nM; and Ni: 2.6–18.8 nM, with highest values near the Mississippi river mouth, and lowest concentrations in a warm core ring in the Gulf of Mexico. Dissolved Cd was mostly present as a truly dissolved (<10 kDa, 97 ± 1%) and cationic fraction (Chelex-100 extractable, 94 ± 4%). A novel observation across the estuarine mixing zone was that colloidal metal concentrations were identical to either inert (for Cu, Ni) or AMPG-labile anionic (Cu, Cd) fractions. The difference in behavior between Cu and the other two metals might indicate differences in the biopolymeric nature of the metal–organic chelates. In particular, the anionic-organic Cd fractions accounted for just 3 ± 1%, on average. However, for Cu, it was 24 ± 4%, and for Ni, it was 9 ± 6%. The fractions of the total dissolved metal fractions that were “inert” averaged 31 ± 10% for Cu and 29 ± 12% for Ni. Small but noticeable amounts (6 ± 3%) of dissolved inert Cd fractions were also present. Apparent non-local transport processes, likely associated with cross-shelf sediment resuspension processes, could have been responsible for the relatively high concentrations of ‘inert’ and ‘anionic’ metal fractions in high salinity coastal waters, and accounting for the persistence of metals bound to humic substances in the Gulf of Mexico. [Copyright &y& Elsevier]

Oyster reef chains grow in response to local hydrodynamics and can redirect flows, particularly when reef chains grow perpendicular to freshwater flow paths. Singularly, oyster reef chains can act as porous dams that may facilitate nearshore accumulation of fresh or low-salinity water, in turn creating intermediate salinities that support oyster growth and estuarine conditions. However, oyster-driven freshwater detention has only been confirmed by limited, point-scale observational data, and simplified models. Oyster reef-driven freshwater detention in real ecosystems at the estuary scale remains largely unexplored. In this study, we analyzed the visible bands in 30-m resolution remote sensing (RS) images recorded by the Operational Land Imager aboard Landsat-8 to characterize the freshwater detention effect of oyster reef chains across a set of hydrologic conditions. Our results support prior findings indicating that 30-m resolution RS images recorded by the Operational Land Imager aboard Landsat-8 are useful for analyzing coastal dynamics after atmospheric correction, despite having been originally designed for terrestrial studies. Statistical models of water-leaving reflectance revealed that freshwater detention by oyster reefs was evident across the estuary, with the greatest effect occurring in the region closest to shore. Additionally, statistical modeling results and spatial patterns apparent in the satellite images suggested that reef-driven freshwater detention occurred under high riverine discharge conditions, but was less evident when flow was low. Beyond offering insight on the potential role of oyster reefs as mediators of estuarine hydrology, this study presents a transferable methodological framework for exploring estuarine biophysical feedbacks in blackwater river estuaries using satellite remote sensing. [ABSTRACT FROM AUTHOR]

Surface water samples from the Yucatan shelf presented Cd concentrations similar to those reported internationally for non-polluted coastal and marine waters. V concentrations, on the other hand, fall within the range of anthropogenically polluted waters (25% of the sampling sites). In the study area, the probable sources of V could be: (1) carbonate sediments leaching V into the water column and co-transported with fine sediments resuspending as a result of the complex hydrodynamics in the area or, (2) accidental spills from cargo ships transporting oil between the Atlantic and the Gulf of Mexico. Significant spatial and temporal differences were found for both metals; therefore, a regional interval concentration is suggested for V from 1.28 to 1.84 μg L−1 and Cd from 0.003 to 0.09 μg L−1. These differences could primarily be the result of the observed hydrology and hydrodynamics created by the Yucatan current, submarine groundwater discharges and upwelling. [ABSTRACT FROM AUTHOR]

Estuaries exist across a large climatic gradient in the northwestern Gulf of Mexico, capturing a range of hydrologic conditions and estuarine functioning. We examined freshwater inflow, salinity, and stable isotope compositions (δ13C, δ15N) of oysters, suspended particulate organic matter (SPOM), and surface sediment organic matter (SSOM) from five estuaries across the hydrologic gradient. All five estuaries experienced large decreases in freshwater inflow over the last 40 yr, with three estuaries being subject to freshwater inflow reductions of more than 85%. Generally, these freshwater inflow decreases were associated with estuarine salinity increases. Across the spatial gradient, average salinity generally increased from northeast to southwest estuaries. SPOM in the northeastern, lower salinity estuaries generally contained more continental organic matter and was of higher quality (i.e., lower C/chlorophyll a ratio), as compared to southwestern, higher salinity estuaries. Similarly, both SSOM and oyster δ13C values were positively correlated with salinity, further highlighting that food webs in lower salinity estuaries are more greatly influenced by continental organic matter than those in higher salinity estuaries. A decrease in the connectivity between continental and coastal habitats may have broad consequences for flows of organic matter, and estuarine function and health. Conducting studies across large‐scale hydrologic gradients can provide a useful approach to informing and predicting shifts in estuarine functioning. [ABSTRACT FROM AUTHOR]

Wave-seabed interaction is known to be a fundamental cause of sediment instability and entrainment. Observations of burial for surrogate munitions were made with a high frequency, sector scanning sonar acquiring acoustic images of the seabed every 12 min during a storm event in May 2013 in the northern Gulf of Mexico offshore of Panama City Beach, FL. Surrogate munitions burial depth was verified by divers. Driven by time series of pressure observed near the seabed, an existing poro-elastic wave-sediment interaction model in combination with a sediment failure criterion due to liquefaction (loss of vertical effective stress) was used to correctly estimate depth and timing of burial. We introduce the concept of liquefaction degree, which is defined as the portion of vertical effective stress at the sediment surface that is, counteracted by wave-induced pore pressures. The relationship between wave-induced pressure at the seabed and liquefaction degree is expressed by a complex transfer function, indicating the importance of the swell band with respect to infra-gravity and short-wave components. The liquefaction degree was used to construct and calibrate a dynamical relationship between wave action and acoustic backscatter observations near the seabed, taken as a surrogate for suspended sediment concentration. Predictions of backscatter based on liquefaction degree were accurate at time scales of 15 s-30 min during most of the time span analyzed and superior to predictions of backscatter based on shear stress. The observations and analysis provide evidence for the importance of wave-induced seepage forces in describing sediment liquefaction and entrainment processes. [ABSTRACT FROM AUTHOR]

Water management operations can drive long-term water quality patterns in nutrient-enriched lakes and reservoirs. This study identified multidecadal trends of rainfall, flow, water levels, total phosphorous (TP), total nitrogen (TN), and Chlorophyll-a in the water column and eight (dis)tributaries during four specific operation schedules (1974–1990, 1991–1999, 2000–2007, and 2008–2019) in Lake Okeechobee (Florida). We found an increasing trend in water discharges towards the Gulf of Mexico (via the Caloosahatchee River) during the dry season for the entire 46 years, and a decreasing trend in discharges towards the Atlantic Ocean (via the St. Lucie Canal) during 2008–2019. Higher flows, combined with increasing TP concentrations in the lake and the Caloosahatchee, led to significant increases in monthly TP mass loads to the Gulf of Mexico by up to 122%–158% during 2008–2019. Higher flows in the Caloosahatchee also resulted in a net TN load increase to the Gulf, despite system-wide decreases in TN concentrations. Overall, results from this study demonstrate that reservoir operations could have long-term effects on nutrient status and exports; thus, modifying operations should be considered as a potential nutrient management tool. [ABSTRACT FROM AUTHOR]

Gulf of Mexico ecosystems are interconnected by numerous physical and biological processes. As a result of the Deepwater Horizon (DWH) spill, some normal ecological processes including resource connectivity and trophic interactions and were damaged or broken. A considerable portion of post-DWH research has focused on higher levels of biological organization (i.e., populations, communities, and ecosystems) spanning at least four environments (onshore, coastal, open ocean, and deep benthos). Damage wrought by the oil spill and mitigation efforts varied considerably across ecosystems. Whereas all systems show prolonged impacts because of cascading effects that impacted functional connections within and between communities, deepsea and mesopelagic environments were particularly hard hit and have shown less resilience than shallow environments. In some environments, such as marshes or the deep-sea benthos, products from the spill are still biologically accessible. Some shallow ecosystems show signs of recovery, and populations of some species show resilience; however, a return to a "pre-spill" state is questionable. Importantly, habitats in which large amounts of energy flow through the ecosystem (marshes, coastal regions) recovered more quickly than low energy habitats (deep-sea benthos). Functional interactions between Gulf of Mexico systems are more complex and widespread than generally recognized. Moreover, ecosystems in the Gulf are subject to multiple stressors that can combine to impart greater, and less predictable, impacts. To help mitigate the effects of future insults, we identify four salient areas of research that should be addressed for each of the major environments within the Gulf of Mexico: establishing monitoring systems; quantifying coupling between Gulf ecosystems; developing criteria for assessing the "vulnerability" and "resilience" of species, communities, and ecosystems; and developing holistic predictive modeling. [ABSTRACT FROM AUTHOR]

International efforts to restore degraded ecosystems will continue to expand over the coming decades, yet the factors contributing to the effectiveness of long‐term restoration across large areas remain largely unexplored. At large scales, outcomes are more complex and synergistic than the additive impacts of individual restoration projects. Here, we propose a cumulative‐effects conceptual framework to inform restoration design and implementation and to comprehensively measure ecological outcomes. To evaluate and illustrate this approach, we reviewed long‐term restoration in several large coastal and riverine areas across the US: the greater Florida Everglades; Gulf of Mexico coast; lower Columbia River and estuary; Puget Sound; San Francisco Bay and Sacramento–San Joaquin Delta; Missouri River; and northeastern coastal states. Evidence supported eight modes of cumulative effects of interacting restoration projects, which improved outcomes for species and ecosystems at landscape and regional scales. We conclude that cumulative effects, usually measured for ecosystem degradation, are also measurable for ecosystem restoration. The consideration of evidence‐based cumulative effects will help managers of large‐scale restoration capitalize on positive feedback and reduce countervailing effects. [ABSTRACT FROM AUTHOR]

Floodplains are some of the most productive and diverse ecosystems on Earth. The Usumacinta River Basin, in the Southern Gulf of Mexico, hosts several floodplain lakes, whose integrity is threatened by water resource development. We aimed to investigate how food webs in these ecosystems vary in relation to their hydrological connectivity and seasonal flood cycles. To do this, we combined analysis of water quality and stable isotopes in two lakes with permanent connection to the Usumacinta River and two isolated lakes during dry, early rainy and late rainy season. Significant differences in chlorophyll-a, nutrients and suspended solids were reflected in distinct trophic dynamics among lakes. Stable isotope analysis revealed that fish from all lakes relied predominantly on autochthonous primary production, but while phytoplankton was the dominant resource for fish in the isolated lakes, aquatic macrophytes made the highest contribution to consumers' production in the connected lakes. Fish tended to have broader isotopic niches during the early rainy season, likely reflecting consumption of a wider range of resources following the first flow pulses of the year. Our study highlights the importance of hydrological connectivity and natural flow regimes to maintain the outstanding biodiversity and fishery yield of these floodplain ecosystems. [ABSTRACT FROM AUTHOR]

The cycling and fate of polycyclic aromatic hydrocarbons (PAHs) is not well understood in estuarine systems. It is critical now more than ever given the increased ecosystem pressures on these critical coastal habitats. A budget of PAHs and cycling has been created for Galveston Bay (Texas) in the northwestern Gulf of Mexico, an estuary surrounded by 30–50% of the US capacity of oil refineries and chemical industry. We estimate that approximately 3 to 4 mt per year of pyrogenic PAHs are introduced to Galveston Bay via gaseous exchange from the atmosphere (ca. 2 mt/year) in addition to numerous spills of petrogenic PAHs from oil and gas operations (ca. 1.0 to 1.9 mt/year). PAHs are cycled through and stored in the biota, and ca. 20 to 30% of the total (0.8 to 1.5 mt per year) are estimated to be buried in the sediments. Oysters concentrate PAHs to levels above their surroundings (water and sediments) and contain substantially greater concentrations than other fish catch (shrimp, blue crabs and fin fish). Smaller organisms (infaunal invertebrates, phytoplankton and zooplankton) might also retain a significant fraction of the total, but direct evidence for this is lacking. The amount of PAHs delivered to humans in seafood, based on reported landings, is trivially small compared to the total inputs, sediment accumulation and other possible fates (metabolic remineralization, export in tides, etc.), which remain poorly known. The generally higher concentrations in biota from Galveston Bay compared to other coastal habitats can be attributed to both intermittent spills of gas and oil and the bay's close proximity to high production of pyrogenic PAHs within the urban industrial complex of the city of Houston as well as periodic flood events that transport PAHs from land surfaces to the Bay. [ABSTRACT FROM AUTHOR]

Marine dynamics plays an important role in understanding the variations in offshore suspended sediment distribution and transport direction. In this study, variations in total suspended sediment concentration (TSSC) over Terrebonne Bay in the northern Gulf of Mexico were investigated by using remote sensing technology in relation to ocean-atmospheric dynamics. The terrestrial rainfall/runoff and river discharge in the lower Mississippi River estuarine area during the study period played an important role in the surface TSSC distribution. In most cases, the wave period and wind speed affected TSSC and its distribution. There was an inverse relationship between TSSC and wave period, but wind speed exhibited a positive relationship with TSSC distribution; however, there was a slight delay. The southeast (onshore) wind accounted for 57.0% of the wind direction over the study area and played a major role in the control of the surface sediment concentration in Terrebonne Bay in July 2010. The onshore wind lifted the underlying sediment, increasing the TSSC at the sea surface. The maximum TSSC was 1056.47 mg/l in the southeast direction (96.75°, onshore) on July 1. Other higher TSSCs were also observed in the southeast wind direction: 996.38, 820.55, and 605.27 mg/l with wind directions of 99.63, 127.33, and 120.17° on 2, 5, and 6 July 2010, respectively. The correlation coefficients between TSSC and wave period and wind speed were 0.45 and 0.47, respectively (significant level of 99%). The sediment concentration in the coastal area was related to coastal erosion and river discharge. The sediment transport not only was related to the physical properties of the sediment, but also depended on the flow velocity. The wind-driven flow field is one of the main factors that drive sediment transport. The sediments in the estuary and bay have many similarities in their dynamic environment and development evolution. The study of the dynamic response of offshore sediments is a supplement to the theory of sediment transport in the estuary and coastal zones. The study of the relationship between ocean dynamics and sediment transport can provide a reference for the site selection of offshore engineering structures. [ABSTRACT FROM AUTHOR]

We introduced a sediment-induced light attenuation algorithm into a biogeochemical model of the Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) modeling system. A fully coupled ocean–atmospheric–sediment–biogeochemical simulation was carried out to assess the impact of sediment-induced light attenuation on primary production in the northern Gulf of Mexico during the passage of Hurricane Gustav in 2008. When compared with model results without sediment-induced light attenuation, our new model showed a better agreement with satellite data on both the magnitude of nearshore chlorophyll concentration and the spatial distribution of offshore bloom. When Hurricane Gustav approached, resuspended sediment shifted the inner shelf ecosystem from a nutrient-limited one to a light-limited one. Only 1 week after Hurricane Gustav's landfall, accumulated nutrients and a favorable optical environment induced a posthurricane algal bloom in the top 20 m of the water column, while the productivity in the lower water column was still light-limited due to slow-settling sediment. Corresponding with the elevated offshore NO3 flux (38.71 mmol N m -1 s -1) and decreased chlorophyll flux (43.10 mg m -1 s -1), the outer shelf posthurricane bloom should have resulted from the cross-shelf nutrient supply instead of the lateral dispersed chlorophyll. Sensitivity tests indicated that sediment light attenuation efficiency affected primary production when sediment concentration was moderately high. Model uncertainties due to colored dissolved organic matter and parameterization of sediment-induced light attenuation are also discussed. [ABSTRACT FROM AUTHOR]

An adaptive selection of the near/shortwave infrared (NIR/SWIR) reflectance correction and the quasi-analytic algorithms (QAAs) is proposed for the Moderate Resolution Imaging Spectroradiometer (MODIS-Aqua) to utilize the strengths of different correction algorithms and QAAs in a single satellite scene with water types ranging from turbid coastal to clear open ocean waters. A blended satellite product is generated by merging three atmospheric-correction algorithms(AD-ATCOR): 1) iterative NIR correction; 2) management unit of the north sea mathematical models (MUMM); and 3) SWIR, using a spectral threshold-based selection for different water types. The validation analysis of a blended remote sensing reflectance product showed overall good agreement with AERONET-OC observations followed by NASA bio-optical marine algorithm data set (NOMAD) at the blue wavelengths and the estuarine data set at the green and red wavelengths. The results suggest that the adaptive method is a better alternative to address the challenging problem of selecting different correction algorithms for different water types in a single satellite scene. Likewise, an adaptive selection of a QAA (AD-QAA) used the QAA-v5 and the QAA-V to obtain merged inherent optical property (IOP) products in a single MODIS-Aqua scene with varying water types. As a case study, the two adaptive selection procedures were sequentially applied to the MODIS-Aqua imagery representing four environmental conditions in the northern Gulf of Mexico. Improved retrievals of the total absorption and backscattering coefficients along an estuarine to ocean continuum demonstrated the effectiveness of this method in an optically complex and dynamic river-dominated system. [ABSTRACT FROM AUTHOR]

Molluscs are an important component of the macrofauna at coastal lagoons and a relevant invertebrate group used as an ecological indicator of environmental impacts on these ecosystems. In this study, we used a multivariate approach to assess spatial distribution changes of molluscan assemblages along a salinity gradient in four habitats from a tropical coastal lagoon. Throughout 1 year, we collected samples at six sites in four identified habitats: i) muddy bottom near a fringing mangrove (MAN), ii) mangrove (Avicennia germinans) roots (RAI), iii) sandy-muddy bottom (ARE) and iv) submerged aquatic vegetation (VAS). We identified 29 molluscs species. The gastropods Neritina virginea and N. reclivata were the dominant species in the lagoon and bivalves Rangia flexuosa, Ischadium recurvum, and Crassostrea virginica occurred in all habitats and sites. Greater species richness was found during the rainy season and in the VAS habitat. Molluscan assemblages varied among seasons, habitats, and sites nested within habitats. Environmental variables that best explained seasonal molluscan assemblages were temperature and turbidity; along the lagoon, the best variables that explained molluscan assemblages were salinity and dissolved oxygen. This study supports the argument that preventing habitat loss or habitat conversion is an important issue to be considered for molluscan conservation at estuarine ecosystems. [ABSTRACT FROM AUTHOR]

Increasing green turtle abundance will lead to increased grazing within seagrass habitats—ecosystems that are important for carbon sequestration and storage. However, it is not well understood how carbon dynamics in these ecosystems respond to grazing and whether a response differs among meadows or locations.We measured seagrass ecosystem metabolism in grazed and ungrazed areas of Thalassia testudinum meadows with established green turtle foraging areas across the Greater Caribbean and Gulf of Mexico. We sampled meadows from five locations that differed in seagrass and environmental characteristics. Established meadows of the invasive seagrass Halophila stipulacea were also present at two of these locations, and we measured ecosystem metabolism in these meadows for comparison to grazed and ungrazed areas of the native T. testudinum.Across all individual sites, rates of net ecosystem production (NEP) ranged from 56% to 96% lower in grazed areas than ungrazed areas of T. testudinum meadows. Rates of NEP were also strongly, positively correlated with above‐ground seagrass biomass across sites. While metabolic carbon capture rates were lower in grazed areas, heterotrophic respiration was not stimulated, and grazing therefore did not result in significant metabolic remineralization of carbon in these meadows. NEP in H. stipulacea meadows was similar to rates in T. testudinum meadows at all three sites, suggesting that metabolic carbon capture may remain similar in Caribbean meadows where this invasive seagrass is replacing native species.Synthesis. Our results show that there is a consistent response in metabolic carbon dynamics to green turtle grazing in T. testudinum meadows across the Greater Caribbean region. An increase in grazing will not likely stimulate remineralization of carbon as these important habitats are returned to a natural grazed state. [ABSTRACT FROM AUTHOR]

To fully assess the long‐term impacts of oil spills like the 2010 Deepwater Horizon incident in the northern Gulf of Mexico, the potential for organisms other than microbes to affect the fate and distribution of the oil may have to be considered. This influence could be substantial for abundant bioturbating benthic animals like the ghost shrimp Lepidophthalmus louisianensis. An assessment of the influence of these ghost shrimp on petroleum hydrocarbons was conducted in laboratory micro‐ and mesocosms containing coastal Gulf of Mexico sediment, seawater, and oil or the polynuclear aromatic hydrocarbon (PAH) pyrene. In an experiment with pyrene added to the water column, the ghost shrimp presence lowered water‐column pyrene concentrations. In an experiment with oil added to the sediment surface, the ghost shrimp presence decreased PAH concentrations in the sediment surface layer but increased these in the water column and subsurface sediment. A companion study and a mass‐balance analysis indicated a net loss of PAHs through an enhancement of microbial degradation. In an experiment in which oil was added as a narrow subsurface layer in the sediment, the ghost shrimp presence appeared to broaden the oil's depth distribution. Taken together, these results demonstrate that ghost shrimp can significantly influence the biodegradation and distribution of spilled oil in coastal ecosystems. Environ Toxicol Chem 2020;39:637–647. © 2019 SETAC [ABSTRACT FROM AUTHOR]

To better understand lateral dispersion of buoyant and nonbuoyant pollutants within the surface waters of large lakes, two lateral dispersion experiments were carried out in Lake Michigan during the stratified period: (1) a dye tracking experiment lasting 1 d; and (2) a drifter tracking experiment lasting 24 d. Both the dye patch and drifters were surface‐released at the center of Lake Michigan's southern basin. Near‐surface shear induced by near‐inertial Poincaré waves partially explains elevated dye dispersion rates (1.5–4.2 m2 s−1). During the largely windless first 5 d of the drifter release, the drifters exhibited nearly scale‐independent dispersion (K ∼ L0.2), with an average dispersion coefficient of 0.14 m2 s−1. Scale‐dependent drifter dispersion ensued after 5 d, with K ∼ L1.09 and corresponding dispersion coefficients of 0.3–2.0 m2 s−1 for length scales L = 1500–8000 m. The largest drifter dispersion rates were found to be associated with lateral shear‐induced spreading along a thermal front. Comparisons with other systems show a wide range of spreading rates for large lakes, and larger rates in both the ocean and the Gulf of Mexico, which may be caused by the relative absence of submesoscale processes in offshore Lake Michigan. [ABSTRACT FROM AUTHOR]

Prior to the Deepwater Horizon oil spill, we lacked a comprehensive baseline of oil contamination in the Gulf of Mexico's sediments, water column, and biota. Gaps in prespill knowledge limit our ability to determine the aftereffects of the Deepwater Horizon blowout or prepare to mitigate similar impacts during future oil spill disasters. We examined spatiotemporal differences in exposure to and metabolism of polycyclic aromatic hydrocarbons (PAHs) in 2 hake species (Urophycis spp.) to establish a current baseline for these ecologically important, abundant, and at‐risk demersal fishes. Gulf hake (Urophycis cirrata) and southern hake (Urophycis floridana) were collected throughout the Gulf of Mexico during extensive longline surveys from 2012 to 2015. Analyses of biliary PAH metabolites and liver PAH concentrations provided evidence of exposures to di‐ and tricyclic compounds, with the highest concentrations measured in the northern Gulf of Mexico. Species‐specific differences were not detected, but temporal trends observed in biliary PAHs suggest a decrease in acute exposures, whereas increasing liver PAHs suggest chronic exposures marked by greater assimilation than metabolism rates. To our knowledge, the present study provides the first multitissue contaminant analyses, as well as the most exhaustive biometric analyses, for both gulf and southern hakes. Though sources of exposure are complex because of multiple natural and anthropogenic PAH inputs, these results will facilitate the development of much needed health metrics for Gulf of Mexico benthos. Environ Toxicol Chem 2019;38:2740–2749. © 2019 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC. [ABSTRACT FROM AUTHOR]

A time series of polycyclic aromatic hydrocarbon (PAH) data was collected for Gulf of Mexico demersal fishes in the years following the Deepwater Horizon oil spill (2012–2017). Tilefish (Lopholatilus chamaeleonticeps) were sampled via demersal longline at repeat stations in the northern Gulf of Mexico in 2012 to 2015 and 2017. Bile samples (n = 256) were analyzed via high‐performance liquid chromatography with fluorescence detection for PAH metabolites as a biomarker of exposure to PAHs. Liver tissues (n = 230) were analyzed for accumulation of PAHs and alkylated homologs via quick, easy, cheap, effective, rugged, and safe (QuEChERS) extractions and gas chromatography–tandem mass spectrometry quantification. Over the 6‐yr time series, exposure to petrogenic PAHs increased by an average of 178%, correlating with an average 22% decline in Fulton's condition factor. The decline in Fulton's condition factor was positively correlated with a 53% decline in percentage of liver lipid. There was no accumulation of PAHs in liver tissue over time. Together, these results suggest that increasing and chronic PAH exposure and metabolism may be taxing the energy budgets of tilefish, particularly adult females, with potentially negative impacts on fitness. Environ Toxicol Chem 2019;38:2659–2671. © 2019 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC. [ABSTRACT FROM AUTHOR]

Acetate is a key intermediate of organic matter mineralization, but its metabolism remains largely unconstrained in the pelagic ocean. We conducted an integrated biogeochemical study to investigate microbial acetate cycling in the northern Gulf of Mexico with the goal of elucidating the importance of acetate as a carbon and energy source. Acetate was used primarily as an energy source, as evidenced by observed oxidation rates (rate constant k: 0.06–0.22 day−1) that varied between 42% and 96% of total biological acetate uptake (i.e., assimilation + oxidation; k: 0.06–0.34 day−1). The assimilation of acetate into biomass (k: 0.01–0.20 day−1) illustrated the potential significance of acetate as a biomass carbon source, particularly in nutrient‐rich coastal waters. No relationship between acetate assimilation or oxidation and environmental factors, such as chlorophyll and nutrients, was observed. However, elevated acetate uptake in reduced oxygen waters characterized by particulate organic carbon mineralization suggests that acetate metabolism may be a good proxy for particulate organic carbon breakdown. Molecular genetic analysis revealed that SAR11 Alphaproteobacteria were the most abundant heterotrophic bacteria and suggest that they may utilize acetate. At some sites, acetate carbon may have accounted for up to 50.4% of the bacterial carbon production. These results suggest that acetate may serve as an important carbon and energy source for heterotrophic bacteria thus revealing a potentially significant role of acetate for dissolved organic carbon cycling in the ocean. Key Points: Acetate was used primarily as an energy source but was also an important structural carbon source, especially in coastal watersAcetate metabolism was not correlated with chlorophyll or nutrients but tracked POC degradation and O2 consumption in the water columnThe carbon incorporated into biomass from acetate may support a substantial fraction of bacterial production [ABSTRACT FROM AUTHOR]

Agricultural non-point sources of nutrients and sediments have caused eutrophication and other water quality issues in aquatic and marine ecosystems, such as the annual occurrence of hypoxia in the Gulf of Mexico. Management practices have been implemented adjacent to and in agricultural drainage ditches to promote their wetland characteristics and functions, including reduction of nitrogen, phosphorus, and sediment losses downstream. This review: (1) summarized studies examining changes in nutrient and total suspended solid concentrations and loads associated with management practices in drainage ditches (i.e., riser and slotted pipes, two-stage ditches, vegetated ditches, low-grade weirs, and organic carbon amendments) with emphasis on the Lower Mississippi Alluvial Valley, (2) quantified management system effects on nutrient and total suspended solid concentrations and loads and, (3) identified information gaps regarding water quality associated with these management practices and research needs in this area. In general, management practices used in drainage ditches at times reduced losses of total suspended solids, N, and P. However, management practices were often ineffective during storm events that were uncommon and intense in duration and volume, although these types of events could increase in frequency and intensity with climate change. Studies on combined effects of management practices on drainage ditch water quality, along with research towards improved nutrient and sediment reduction efficiency during intense storm events are urgently needed. [ABSTRACT FROM AUTHOR]

Dissolved dimethylsulfoniopropionate (DMSPd) is an important substrate for marine microbes and a precursor of sulfur gases. We compared DMSPd turnover flux rates in coastal seawater measured with a 35S-DMSPd tracer to those obtained with the DMSP-uptake inhibitor glycine betaine (GBT). The 35S-DMSP tracer method yielded DMSPd turnover fluxes (35.7-215 nM day-1) that were 1.7 to 152 times higher than those obtained in parallel samples with the GBT inhibitor method (0.34-21.6 nM day-1). Tests confirmed that GBT functioned as planned by strongly inhibiting DMSPd degradation and that 35S-DMSPd gave accurate estimates of DMSPd loss rate constants. This left the initial DMSPd concentrations, determined by small volume drip filtration (SVDF) through Whatman GF/F filters (0.7-µm nominal retention) ([DMSPd]SVDF), as a potential cause of the discrepancy in rate estimates. Indeed, GF/F filtrate incubations showed that the initial [DMSPd]SVDF overestimated the bioavailable DMSPd concentrations for at least two reasons: (1) a significant fraction (10-37 %) of DMSP passing through GF/F filters was in particles >0.2 µm (likely bacteria) and therefore not dissolved, and (2) a significant pool (0.44-1.0 nM) of operationally dissolved, non-particle DMSP ([DMSPd]<0.2 µm), comprising 40-99% of [DMSPd][sub SVDF], was refractory to degradation on a time scale of days. The nature of this refractory DMSP is currently unknown. Accounting for DMSP-containing particles and the refractory DMSP pool in GF/F filtrates is necessary to obtain the true bioavailable DMSPd concentrations, which we estimate to be very low (0.006-1.0 nM; mean of 0.41 nM) in the coastal waters examined, and to avoid overestimation of DMSPd turnover fluxes when using the 35S-DMSP tracer technique. [ABSTRACT FROM AUTHOR]

This review covers selected 2019 articles on the biological effects of pollutants, including human physical disturbances, on marine and estuarine plants, animals, ecosystems, and habitats. The review, based largely on journal articles, covers field, and laboratory measurement activities (bioaccumulation of contaminants, field assessment surveys, toxicity testing, and biomarkers) as well as pollution issues of current interest including endocrine disrupters, emerging contaminants, wastewater discharges, marine debris, dredging, and disposal. Special emphasis is placed on effects of oil spills and marine debris due largely to the 2010 Deepwater Horizon oil blowout in the Gulf of Mexico and proliferation of data on the assimilation and effects of marine debris microparticulates. Several topical areas reviewed in the past (e.g., mass mortalities ocean acidification) were dropped this year. The focus of this review is on effects, not on pollutant sources, chemistry, fate, or transport. There is considerable overlap across subject areas (e.g., some bioaccumulation data may be appeared in other topical categories such as effects of wastewater discharges, or biomarker studies appearing in oil toxicity literature). Therefore, we strongly urge readers to use keyword searching of the text and references to locate related but distributed information. Although nearly 400 papers are cited, these now represent a fraction of the literature on these subjects. Use this review mainly as a starting point. And please consult the original papers before citing them., (© 2020 Water Environment Federation.)

Despite over seven decades of production and hundreds of oil spills per year, there were no comprehensive baselines for petroleum contamination in the Gulf of Mexico (GoM) prior to this study. Subsequent to the 2010 Deepwater Horizon (DWH) spill, we implemented Gulf-wide fish surveys extending over seven years (2011-2018). A total of 2,503 fishes, comprised of 91 species, were sampled from 359 locations and evaluated for biliary polycyclic aromatic hydrocarbon (PAH) concentrations. The northern GoM had significantly higher total biliary PAH concentrations than the West Florida Shelf, and coastal regions off Mexico and Cuba. The highest concentrations of biliary PAH metabolites occurred in Yellowfin Tuna (Thunnus albacares), Golden Tilefish (Lopholatilus chamaeleonticeps), and Red Drum (Sciaenops ocellatus). Conversely, biliary PAH concentrations were relatively low for most other species including economically important snappers and groupers. While oil contamination in most demersal species in the north central GoM declined in the first few years following DWH, more recent increases in exposure to PAHs in some species suggest a complex interaction between multiple input sources and possible re-suspension or bioturbation of oil-contaminated sediments. This study provides the most comprehensive baselines of PAH exposure in fishes ever conducted for a large marine ecosystem.

Nutrient loads from the Mississippi River watershed are associated with seasonal development of bottom-water hypoxia in the northern Gulf of Mexico. Microbial nitrogen (N) transformations at the sediment-water interface are important in determining system productivity and the development and maintenance of hypoxia. Intact sediment cores were incubated in a continuous-flow system with stable isotope tracers to identify and quantify important N sources (e.g., N fixation), sinks (e.g., denitrification and anammox), and links (e.g., dissimilatory nitrate reduction to ammonium, DNRA). Microbial N sinks on the Louisiana-Texas continental shelf remove up to 68 % of the total N load from the Mississippi River watershed, and up to 29 % of this N removal (mean = 11.8 ± 1.7 %) may be due to anammox. The highest N production rates and ammonium effluxes were observed at low bottom-water oxygen concentrations, and sediments were a significant source of ammonium to the water column at all times. DNRA and heterotrophic N fixation were not consistent pathways for total sediment N fluxes, but their potential importance to N balance and productivity in the system warrants further study and inclusion in ecosystem models. Physical disturbance from passage of two hurricanes in 2008 resulted in lower N cycling rates and sediment oxygen consumption, with sediment processes migrating into the water column. Denitrification is the dominant N sink in the northern Gulf of Mexico and provides a valuable ecosystem service by mitigating N loads from the Mississippi River watershed, particularly during seasonal bottom-water hypoxia events. [ABSTRACT FROM AUTHOR]

Seagrass meadows are important for their ecological functions and ecosystem services, including habitat and shelter for a high diversity of species. The present study aims to assess the spatial distribution patterns of macroinvertebrate assemblages associated with Thalassia testudinum beds in a tropical coastal lagoon of the southern Gulf of Mexico. Environmental variables and core samples were taken from three shallow beds with different degrees of exposure to wind-driven waves: protected, exposed and semi-exposed. A total of 40 benthic macroinvertebrate species were recorded, molluscs being the most important group in terms of diversity and abundance. The highest species richness was recorded at the semi-exposed site (31 species), while the highest abundance was at the exposed site (5207 ind. m−2). Despite inter-site variations in environmental variables and physical descriptors of seagrass (seagrass biomass, shoot density, leaf length), the species richness and abundance of macroinvertebrates correlated only with seagrass biomass. Multivariate analyses showed a clear separation of macroinvertebrate assemblages into two main groups according to the degree of exposure of the sites as exposed/semi-exposed or protected. It was also determined that seagrass biomass, hydrodynamism, sedimentation/resuspension rates and the proportion of coarse particles in the sediment appear to be the variables that best explain the differences between sites. These findings support the importance of including sites with different degrees of exposure in assessing the biodiversity of seagrass beds at the landscape scale, because species composition, species richness and the abundance of associated fauna may vary considerably between sites within the same coastal region according to a combination of biotic and abiotic factors. [ABSTRACT FROM AUTHOR]

Submarine groundwater discharge (SGD) is now recognized as an important source of nutrients and freshwater to some coastal environments. We studied a shallow coastal lagoon (Little Lagoon, AL, USA) in the northern Gulf of Mexico that lacks riverine inputs but has been suspected to receive significant SGD. We observed persistent salinity gradients between the east and west ends of the lagoon and the pass connecting it to the Gulf of Mexico. Covariance between salinity in the lagoon and the groundwater tracer Rn indicated that SGD was responsible for the salinity gradients and is the primary source of freshwater to the lagoon. Cluster analysis of 246 biweekly samples based on temperature, salinity, and two proxies of SGD revealed two hydrographic regimes with different drivers for nutrient inputs. In samples characterized by high discharge and low temperatures (generally December-April), total nitrogen (TN) was negatively correlated with salinity, while total phosphorus (TP) was positively correlated with temperature. Total nitrogen in the groundwater was very high (0.36-4.80 mM) while total phosphorus was relatively low (0.3-2.3 μM), consistent with SGD as the source of TN during the high-discharge periods. In periods with low discharge and higher temperatures (approx. May-November), TN and TP had strong positive correlations with temperature and are inferred to originate from benthic efflux. Seasonal changes in nutrient stoichiometry in the lagoon water column also indicate an alternation between low TN/TP sediments and high TN/TP groundwater as the primary sources of nitrogen in this system. [ABSTRACT FROM AUTHOR]

Profiling floats equipped with bio-optical sensors well complement ship-based and satellite ocean color measurements by providing highly-resolved time-series data on the vertical structure of biogeochemical processes in oceanic waters. This is the first study to employ an autonomous profiling (APEX) float in the Gulf of Mexico for measuring spatiotemporal variability in bio-optics and hydrography. During the 17-month deployment (July 2011 to December 2012), the float mission collected profiles of temperature, salinity, chlorophyll fluorescence, particulate backscattering (bbp), and colored dissolved organic matter (CDOM) fluorescence from the ocean surface to a depth of 1,500 m. Biogeochemical variability was characterized by distinct depth trends and local “hot spots”, including impacts from mesoscale processes associated with each of the water masses sampled, from ambient deep waters over the Florida Plain, into the Loop Current, up the Florida Canyon, and eventually into the Florida Straits. A deep chlorophyll maximum (DCM) occurred between 30 and 120 m, with the DCM depth significantly related to the unique density layer ρ = 1023.6 (R2 = 0.62). Particulate backscattering, bbp, demonstrated multiple peaks throughout the water column, including from phytoplankton, deep scattering layers, and resuspension. The bio-optical relationship developed between bbp and chlorophyll (R2 = 0.49) was compared to a global relationship and could significantly improve regional ocean-color algorithms. Photooxidation and autochthonous production contributed to CDOM distributions in the upper water column, whereas in deep water, CDOM behaved as a semi-conservative tracer of water masses, demonstrating a tight relationship with density (R2 = 0.87). In the wake of the Deepwater Horizon oil spill, this research lends support to the use of autonomous drifting profilers as a powerful tool for consideration in the design of an expanded and integrated observing network for the Gulf of Mexico. [ABSTRACT FROM AUTHOR]