Search

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]

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]

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]

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]