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Waldron, M.C.B.; Carter, G.A., and Biber, P.D., 2021. Using aerial imagery to determine the effects of sea-level rise on fluvial marshes at the mouth of the Pascagoula River (Mississippi, USA). Journal of Coastal Research, 37(2), 389–407. Coconut Creek (Florida), ISSN 0749-0208.Coastal marshes provide valuable ecosystem services yet are increasingly vulnerable to sea level rise (SLR).To facilitate a better understanding of how fluvial marshes along the Gulf of Mexico coast are responding to regional SLR of around 3.7 mm per year, this study used aerial imagery to map land cover at the mouth of the Pascagoula River at 20-year intervals, beginning in 1955 and ending in 2014. High-resolution land cover maps were created for each image date based on a maximum likelihood classification scheme using spectral and textural image features. This marsh ecosystem, at the mouth of the largest free-flowing river by volume in the contiguous United States, should be more resilient to sea level rise than other Gulf Coast marshes, with little restriction to sediment supply and relatively low subsidence rates measured nearby. However, the results of this study show that marsh area declined by 1073 ha (17.5%) and rates of marsh conversion to open water increased over the studied time period. Although modeling studies indicate that coastal marshes worldwide may persist under accelerated SLR, these observations suggest that marsh extent in the sediment-rich Pascagoula River Estuary will continue to decline, signifying vulnerability among other marsh ecosystems along the northern Gulf of Mexico coast.

Feedbacks among inundation, sediment trapping, and vegetation productivity help maintain coastal wetlands facing sea‐level rise (SLR). However, when the SLR rate exceeds a threshold, coastal wetlands can collapse. Understanding the threshold helps address key challenges in ecology—nonlinear response of ecosystems to environmental change, promotes communication between ecologists and resource managers, and facilitates decision‐making in climate change policies. We studied the threshold of SLR rate and developed a new threshold of SLR acceleration rate on sustainability of coastal wetlands as SLR is likely to accelerate due to enhanced anthropogenic forces. Deriving these two thresholds depends on the temporal scale, the interaction of SLR with other environmental factors, and landscape metrics, which have not been fully accounted for before this study. We chose a representative marine‐dominated estuary in the northern Gulf of Mexico, Grand Bay in Mississippi, to test the concept of SLR thresholds. We developed a mechanistic model to simulate wetland change and then derived the SLR thresholds for Grand Bay. The model results show that the threshold of SLR rate in Grand Bay is 11.9 mm/year for 2050, and it drops to 8.4 mm/year for 2100 using total wetland area as a landscape metric. The corresponding SLR acceleration rate thresholds are 3.02 × 10−4 m/year2 and 9.62 × 10−5 m/year2 for 2050 and 2100, respectively. The newly developed SLR acceleration rate threshold can help quantify the temporal lag before the rapid decline in wetland area becomes evident after the SLR rate threshold is exceeded, and cumulative SLR a wetland can adapt to under the SLR acceleration scenarios. Based on the thresholds, SLR that will adversely impact the coastal wetlands in Grand Bay by 2100 will fall within the likely range of SLR under a high warming scenario (RCP8.5), highlighting the need to avoid RCP8.5 to preserve these marshes.

Seagrasses are submerged marine plants that are anchored to the substrate and are therefore limited to assimilating nutrients from the surrounding water column or sediment, or by translocating nutrients from adjacent shoots through the belowground rhizome. As a result, seagrasses have been used as reliable ecosystem indicators of surrounding nutrient conditions. The Chandeleur Islands are a chain of barrier islands in the northern Gulf of Mexico that support the only marine seagrass beds in Louisiana, USA, and are the sole location of the seagrass Thalassia testudinum across nearly 1000 km of the coastline from west Florida to central Texas. Over the past 150 years, the land area of the Chandeleur Islands has decreased by over half, resulting in a decline of seagrass cover. The goals of this study were to characterize the status of a climax seagrass species at the Chandeleur Islands, T. testudinum, in terms of leaf nutrient (nitrogen [N] and phosphorus [P]) changes over time, from 1998 to 2015, and to assess potential drivers of leaf nutrient content. Thalassia testudinum leaf nutrients displayed considerable interannual variability in N and P content and molar ratios, which broadly mimicked patterns in annual average dissolved nutrient concentrations in the lower Mississippi River. Hydrological modeling demonstrated the potential for multiple scenarios that would deliver Mississippi River water, and thus nutrients, to T. testudinum at the Chandeleur Islands. Although coastal eutrophication is generally accepted as the proximate cause for seagrass loss globally, there is little evidence that nutrient input from the Mississippi River has driven the dramatic declines observed in seagrasses at the Chandeleur Islands. Rather, seagrass cover along the Chandeleur Islands appears to be strongly influenced by island geomorphological processes. Although variable over time, the often elevated nutrient levels of the climax seagrass species, T. testudinum, which are potentially driven by river-derived nutrient inputs, raises an important consideration of the potential loss of the ecosystem functions and services associated with these declining seagrass meadows.

The U. S. Gulf of Mexico is experiencing a dramatic increase in tidal marsh restoration actions, which involves planting coastal areas with smooth cordgrass (Spartina alterniflora) and black needlerush (Juncus roemerianus) for erosion control and to provide habitat for fish and wildlife. It can take decades for sedimentary cycles in restored marshes to approach reference conditions, and the contribution of the sediment microbial communities to these processes is poorly elucidated. In this study, we addressed this gap by comparing rhizosphere microbiomes of S. alterniflora and J. roemerianus from two restored marshes and a natural reference marsh located at Deer Island, MS. Our results revealed that plants from the restored and reference areas supported similar microbial diversity indicating the rapid colonization of planted grasses with indigenous soil microbiota. Although close in composition, the microbial communities from the three studied sites differed significantly in the relative abundance of specific taxa. The observed differences are likely driven by the host plant identity and properties of sediment material used for the creation of restored marshes. Some of the differentially distributed groups of bacteria include taxa involved in the cycling of carbon, nitrogen, and sulfur, and may influence the succession of vegetation at the restored sites to climax condition. We also demonstrated that plants from the restored and reference sites vary in the frequency of culturable rhizobacteria that exhibit traits commonly associated with the promotion of plant growth and suppression of phytopathogenic fungi. Our findings will contribute to the establishment of benchmarks for the assessment of the outcome of coastal restoration projects in the Gulf of Mexico and better define factors that affect the long-term resiliency of tidal marshes and their vulnerability to climate change.

In response to hurricane and oil-spill environmental impacts along the northern Gulf of Mexico, coastal and marine habitat restoration has become a priority. In particular, restoration of submerged aquatic plants is vital for ecosystem health. To facilitate restoration, developing propagation protocols for Gulf coast plants is necessary, but challenging due to the lack of information on many species. Previous seed germination research of Vallisneria americana, a submerged aquatic species with declining abundance in coastal habitats, from northern latitudinal populations reported germination percentages between 80–90%. Germination experiments using Mississippi Gulf coast plants revealed unexpected outcomes. Less than 8% germination occurred when seeds were germinated in a 16 hr photoperiod or 24 hr dark period at 10, 20, 30 or 40 °C. To enhance germination, cold stratification and gibberellic acid soak treatments were conducted, but germination was below 10%. A subsequent seed scarification experi...

Two small-scale Black needlerush (Juncus roemerianus) marsh restoration designs were examined for cost-effectiveness by analyzing a suite of morphological and physiological metrics, along with vegetated area over time. The restoration was conducted by harvesting marsh sods from an adjacent natural marsh and planting in the restoration site. Both restoration designs are on suitable scales for private property owners to conduct, but differed in initially planted coverage area. One design was fully planted (100% coverage of planted marsh sods; termed full density design) and the other design was planted at half the density of the fully planted design (50% coverage of planted marsh sods; termed half density design). We found no consistent differences in the measured metrics between the two restoration designs and few differences between restored sites and reference natural marsh stands. These findings suggest the potential similar functionality across all treatments. The only metrics with consistent differences among treatments were increased leaf nutrient and chlorophyll content in the restored plots when compared to natural stands. These differences are potentially attributable to nutrient-rich runoff from an adjacent parking lot to the restoration site. Total vegetated coverage area for half density plots was similar to full density plots at 2.1 years after planting. Cost-effectiveness analysis of both designs across eight differing restoration scenarios (based on hiring or donation of cost categories) resulted in half density plots having higher or equal cost-effectiveness in seven of the eight scenarios. Half density plots were approximately twice as cost-effective in scenarios with donated pre-planting site construction. Based on the similar vegetated area between the two designs and lower cost and restoration effort, we suggest the half density design as a more cost-effective restoration strategy than the full density design and should be considered for small-scale Black needlerush restoration projects.

Seagrasses are typically light limited in many turbid estuarine systems. Light attenuation is due to water and three optically active constituents (OACs): nonalgal particulates, phytoplankton, and colored dissolved organic matter (CDOM). Using radiative transfer modeling, the inherent optical properties (IOPs) of these three OACs were linked to the light attenuation coefficient, K PAR, which was measured in North River, North Carolina, by profiles of photosynthetically active radiation (PAR). Seagrasses in the southern portion of Albemarle-Pamlico Estuarine System (APES), the second largest estuary in the USA, were found to be light limited at depths ranging from 0.87 to 2 m. This corresponds to a range of K PAR from 0.54 to 2.76 m−1 measured during a 24-month monitoring program. Turbidity ranged from 2.20 to 35.55 NTU, chlorophyll a from 1.56 to 15.35 mg m−3, and CDOM absorption at 440 nm from 0.319 to 3.554 m−1. The IOP and water quality data were used to calibrate an existing bio-optical model, which predicted a maximum depth for seagrasses of 1.7 m using annual mean water quality values and a minimum light requirement of 22% surface PAR. The utility of this modeling approach for the management of seagrasses in the APES lies in the identification of which water quality component is most important in driving light attenuation and limiting seagrass depth distribution. The calibrated bio-optical model now enables researchers and managers alike to set water quality targets to achieve desired water column light requirement goals that can be used to set criteria for seagrass habitat protection in North Carolina.

Drifting macroalgae are unattached seaweeds that are commonly found in many South Florida and Gulf of Mexico shallow-water seagrass habitats. They are primarily comprised of species of red algae (Rhodophyta) and some brown algae (Phaeophyta). Because of the unattached nature of these species, drift algae have the ability to be moved around the landscape primarily by tidal, as well as wind-driven and alongshore currents. Numerous invertebrates and some fish species are typically found associated with drift algal clumps and aggregations. Transport of drift algae is an important dispersal mechanism for both the plants and their associated fauna. Dispersal distances have been studied in numerous locations over a range of spatial scales. However, little is known about quantities of algal material that are involved. In this study I report on composition and biomass of drifting algae that are transported through a tidal inlet in Biscayne Bay, Florida. Sargassum (a brown alga) and about 12 genera of red algae were found in three seasonal collections (Aug., Dec., May). Total biomass collected varied among seasons, with larger average amounts of drift algae collected in May than the other two months sampled. From this data, I calculate the approximate quantities of drift algae that are potentially moving in, or out of, Biscayne Bay, about a half to one ton of biomass per day.

Currents induced by tidal circulation and wind in shallow tropical seagrass habitats can influence the distribution of drifting macroalgae. In Florida, drift algae are mostly comprised of 5 to 10 genera of Rhodophyta (red algae), with 1 or 2 dominant species. Drift clump transport was investigated through manipulative experiments. Increasing flow velocities entrained and transported clumps of drift algae; transport speeds were 50 to 67% of flow velocity and did not vary statistically significantly between clump sizes tested. The roughness of the substratum influenced transport speeds, with moderate to dense monospecific turtlegrass Thalassia testudinum reducing the speed of transport compared to bare substratum. Mixed seagrass substrata (T. testudinum and Halodule wrightii) further inhibited transport of drift clumps by more frequent entangling compared to the bare and monospecific substrata. Persistence of drift algae was inversely related to flow conditions, with longer persistence at low flow velocities.

A model of three functional groups of macroalgae, drift algae, rhizophytic calcareous algae, and seagrass epiphytes, was developed to complement an existing seagrass production model for tropical habitats dominated by Thalassia testudinum(Turtle-grass). The current modeling effort simulates annual biomass dynamics for each of the three functional groups under a variety of stress regimes imposed by nutrient-laden freshwater discharges into a well-flushed coastal bay in South Florida. The model is parameterized based on multiple years of experimental data collected from Biscayne Bay, as well as literature values reported for other Florida and Caribbean seagrass habitats. Calibration of the model yields a good fit of predicted-to-observed biomass (r 2 > 0.85) for sheet-flow and oceanic-influenced sites, but a poor fit ( r 2 = 0.13) under canal-influenced conditions. This is hypothesized to be related to the lack of adequate observed data on seagrass epiphyte dynamics to parameterize the model. Sensitivity analysis showed that the model is easily perturbed by changes in intrinsic growth parameters of the algae (daily growth rate, mortality rate, carrying capacity), while it is much less sensitive to changes in the functional form of the response curves that are used to characterize the stress tolerances of the algae to light, temperature, salinity, and nutrient conditions. This model is considered suitable for use in seagrass habitats similar to those occurring in South Florida, as these are the conditions to which the model was calibrated. © 2003 Elsevier B.V. All rights reserved.

Determination of the percentage cover and residence time of drift algae over perma- nently marked 0.25 m 2 plots at 8 study sites indicated that drift algal cover was relatively low (

1. Hardbottom habitats of Biscayne Bay, a shallow lagoon adjacent to the city of Miami, Florida, USA, contain a limited number of coral species that represent a small subset of the species found at nearby offshore hardbottom and reef habitats of the Florida Reef Tract. Although the physical characteristics of this basin make it a marginal environment for coral growth, the presence of dense populations of Siderastrea radians and Porites furcata indicate that these, as well as other corals that are found at lower densities, are able to tolerate extreme and fluctuating conditions. Three factors, temperature, sedimentation, and salinity, appear to limit coral abundance, diversity, and distribution within Biscayne Bay. 2. Temperatures exhibit high frequencies of extreme high and low values known to cause coral stress and mortality elsewhere. Similarly, sedimentation rates are very high and sediment resuspension caused by currents, storms and boating activities commonly bury corals under sediment layers. Sediment burial was shown experimentally to influence growth and mortality of S. radians. 3. The salinity of Biscayne Bay is influenced by freshwater inputs from canal, sheetflow and groundwater sources that create a near-shore environment with low mean salinity and high salinity fluctuation. Coral communities along this western margin have the lowest coral density and species richness. Chronic exposure to low salinity was shown experimentally to cause a decrease in the growth of S. radians. 4. The location of Biscayne Bay, downstream of a large restoration effort planned for the Everglades watershed, highlights the need to understand the relationship between the physical environment and the health of benthic communities. The data presented here provide the type of scientific information needed so that management decisions can take into account the potential impacts of human activities on the health of coral populations that are already near their tolerance limits for temperature, salinity, and sedimentation. Copyright © 2003 John Wiley & Sons, Ltd.

Wu, W.; Huang, H.; Biber, P., and Bethel, M., 2017. Litter decomposition of Spartina alterniflora and Juncus roemerianus: Implications of climate change in salt marshes. Decomposition of plant litter in salt marshes plays an important role in coastal trophodynamics, nutrient cycling, sediment trapping, and short-term carbon storage, all of which are likely to be affected by climate change and accelerated sea-level rise. Warmer temperatures, altered precipitation patterns, longer and more frequent inundation, and saltwater intrusion will all interact to affect decomposition of plant litter in a complex way. A combination of field experiments and model selection techniques was applied to study how these environmental factors affected litter decomposition of two dominant salt marsh species, Spartina alterniflora and Juncus roemerianus, along the northern Gulf of Mexico. The results from summer field experiments conducted between June and August of 2013 showed that S. alterniflora in the low marsh ha...

Salvinia molesta Mitchell, a floating invasive aquatic plant, is one of the top 10 worst invasive aquatic weeds in the world. It was discovered in the lower Pascagoula River in 2005 and evidence suggests that this non-native species is spreading along the northern Gulf of Mexico. These plants exhibit rapid growth and nutrient uptake rates, allowing them to out compete other plants in similar habitats. Distributional observations suggest that non-native S. molesta is able to survive in salinities of up to 7 ppt in the lower Pascagoula River. The response of S. molesta to three salinity levels (0, 5, 10 ppt) was tested using chlorophyll fluorescence. The health of the plants was measured over a period of one month, using a log scale series of observation intensities (hourly, daily, weekly). Plant responses indicated an acute salinity effect after about 4-6 hrs and then a gradual chronic decline. Compared to initial measurements, the final actual quantum yield (∆F/Fm’) dropped by 5%, 6% and 29%, while the final potential quantum yield (Fv/Fm) dropped 6%, 27% and 39% in the 0, 5, and 10 ppt treatments, respectively. Only plants in the 0 ppt treatment showed significant new growth. Plants in 5 ppt appeared to maintain themselves, but plants at 10 ppt all exhibited signs of severe stress and loss of color, turgor, and tissue viability after 10 d. Tolerance to brackish salinities has been reported in the past, and has implications for the use of the biological control agent, the weevil Cyrtobagous salviniae, that can only tolerate freshwater conditions. IntroductIon Aquatic plants can be grouped into three types: emergent, floating, and submerged (Pieterse and Murphy 1993), with some of the most successful invasive aquatic plants being in the floating group (e.g., Eichhornia crassipes and Salvinia molesta). These plants exhibit rapid growth rates, rapid nutrient uptake rates, are aggressive, and are competitive species that can impact aquatic environments, local economies, and human health (Holm et al. 1977). The impact of these species on a freshwater body is dramatically illustrated by S. molesta, one of the top 10 worst non-native invasive aquatic weeds in the world (Room and Julien 1995, Carley and Brown 2006). Salvinia molesta has become a worldwide problem, with invasions into freshwater bodies in most tropical countries, and was introduced into the United States in 1995 (Julien and Tipping 2002, USGS 2005). Salvinia molesta has a doubling time of 4-6 d (Mitchell and Tur 1975) and was found in the lower Pascagoula River in 2005 (MS DMR 2005). Evidence reported in McFarland et al. (2004) suggests that this non-native species is spreading into the northern Gulf of Mexico (GOM). In Alabama and Mississippi, there are many suitable habitats for native and non-native invasive aquatic plants; there are four river drainage systems along the 121 km (75 mile) coastline of the state of Mississippi alone. The largest of these is the Pascagoula River, which holds the distinction of being the longest un-dammed, natural river remaining in the continental USA and provides habitat for numerous important and endangered salt marsh species (Schueler 2002). Much of this river system remains relatively unimpacted by development, except for the very lower reaches between the towns of Gautier and Pascagoula, MS. Distributional observations during an outbreak in 2005 by personnel with the Mississippi Department of Marine Resources (DMR) suggest that non-native S. molesta was able to survive in salinities of up to 7 parts per thousand (ppt) in the lower Pascagoula River; a similar tolerance has been reported earlier by Divakaran et al. (1980) from growth tests conducted on salinities of 0 to 11 ppt. This has implications for the use of the biological control agent Cyrtobagous salviniae on this infestation, as this weevil can only tolerate freshwater conditions (Thomas and Room 1986, Julien et al. 2002). This observation is distressing in two respects: (1) potential for a portion of the non-native S. molesta population in the Pascagoula River to escape biological control; and (2) a more salinity-tolerant variety of this species could easily spread into similar habitats that abound along the GOM and elsewhere. Pulse amplitude modulated (PAM) fluorescence is a tool to measure photophysiological processes in vivo. While it cannot be used to directly measure the mechanisms of osmoregulation, it has been used successfully to demonstrate the physiological stress resulting from salinity change in a number of aquatic plant species (Ralph 1998, Kamermans et al. 1999, Murphy et al. 2003, Biber 2006). PAM fluorescence has been used in submerged aquatic plants to measure acute stress, such as desiccation (Adams and Bates 1994, Bjork et al. 1999), temperature or salinity shifts (Ralph et al. 1998, Ralph 1999), and even changes in ambient light over short time durations (Beer and Bjork 2000

Coral communities worldwide are undergoing intense degradation in response to natural and human disturbances, and many reef systems have already experienced significant declines in live coral cover associated with an increase in macroalgal abundance. Here, we document the seasonal dynamics of the macroalgal communities of the Northern Florida Reef Tract, providing a baseline for long-term studies of coral-algal competition in the area. Both macroalgal biomass and percent cover on reefs showed an increasing trend from January to July, when both light and temperature conditions were favorable for growth. Maximum percent cover (56.7 %) was found in July and minimum levels in December (25.8 %). During these peaks in algal cover, many corals were completely covered by dense mats of algae. Two genera, Halimeda and Dictyota, represented the largest proportion (77299 %) of the total algal biomass. In the summer, Dictyota spp. dominated the algal community, occupying up to 40 % of the reef bottom with a dry biomass of up to 20 g.m 22 . In addition, two species, Stypopodium zonale and Trichogloea requienii, showed a significant bloom in April 1998, covering a significant percentage of the bottom (up to 25 %) at an inshore reef. Species that exhibited rapid space monopolization on Florida reefs, such as Dictyota spp. and Stypopodium zonale, also showed rapid growth in microcosm and field growth studies. No correlations were found between fish grazer abundance and algal biomass or percent cover, indicating that present grazer population abundance and composition are not adequate to prevent space monopolization and coral overgrowth by algae such as Dictyota spp. and Halimeda spp. Only a continued monitoring effort will determine whether the seasonal dynamics of the algal community may result in the decline of coral populations in the Northern Florida Reef Tract.

To study the impact of the Deepwater Horizon oil spill on photosynthesis of coastal salt marsh plants in Mississippi, we developed a hierarchical Bayesian (HB) model based on field measurements collected from July 2010 to November 2011. We sampled three locations in Davis Bayou, Mississippi (30.375 N, 88.790 W) representative of a range of oil spill impacts. Measured photosynthesis was negative (respiration only) at the heavily oiled location in July 2010 only, and rates started to increase by August 2010. Photosynthesis at the medium oiling location was lower than at the control location in July 2010 and it continued to decrease in September 2010. During winter 2010‐2011, the contrast between the control and the two impacted locations was not as obvious as in the growing season of 2010. Photosynthesis increased through spring 2011 at the three locations and decreased starting with October at the control location and a month earlier (September) at the impacted locations. Using the field data, we developed an HB model. The model simulations agreed well with the measured photosynthesis, capturing most of the variability of the measured data. On the basis of the posteriors of the parameters, we found that air temperature and photosynthetic active radiation positively influenced photosynthesis whereas the leaf stress level negatively affected photosynthesis. The photosynthesis rates at the heavily impacted location had recovered to the status of the control location about 140 days after the initial impact, while the impact at the medium impact location was never severe enough to make photosynthesis significantly lower than that at the control location over the study period. The uncertainty in modeling photosynthesis rates mainly came from the individual and micro-site scales, and to a lesser extent from the leaf scale.