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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.

Along the mainland coastline of eastern Mississippi Sound, the seagrass species Ruppia maritima (Wigeongrass) and Halodule wrightii (Shoalgrass) co-occur in persistent and sometimes extensive beds within Grand Bay. This paper presents seasonal and annual changes in those Ruppia-Halodule beds that are part of the Grand Bay National Estuarine Research Reserve in western Grand Bay using 2005–2011 transect survey data of percent cover and 2009–2013 biomass sampling data. The seagrass beds exhibited significant interannual variation in areal coverage and seasonal variations in biomass, with peak total aboveground biomass (up to 140 g dry wt/m2) in July and August. Intact R. maritima and H. wrightii seeds were found in the sediment, with densities as high as 4160 seeds/m2. The Ruppia-Halodule beds did not show any short-term declines in their growth or abundance following 2005 Hurricanes Katrina and Rita nor the 2010 Deepwater Horizon oil spill. Resumen: A lo largo de la costa continental del este del Mississippi Sound, las especies de pastos marinos Ruppia maritima (Wigeongrass) y Halodule wrightii (Shoalgrass) coexisten en camas persistentes ya veces extensas dentro de Grand Bay. Este articulo presenta cambios estacionales y anuales en los lechos de Ruppia-Halodule que forman parte de la Reserva de Investigacion Estuarina Nacional de Grand Bay en el oeste de Grand Bay usando datos de la encuesta de transectos de 2005–2011 de cobertura porcentual y datos de muestreo de biomasa 2009–2013. Los lechos de pastos marinos mostraron una variacion interanual significativa en la cobertura aerea y las variaciones estacionales en la biomasa, con el pico de biomasa total sobre el suelo (hasta 140 g de peso seco/m2) en julio y agosto. Se encontraron semillas intactas de R. maritima y H. wrightii en el sedimento, con densidades tan altas como 4160 semillas/m2. Las camas Ruppia-Halodule no mostraron disminuciones a corto plazo en su crecimiento o abundancia despues de los huracanes Katrina y Rita en el 2005 ni en el derrame de petroleo Deepwater Horizon del 2010.

Vertical aerial image data were used with an edge-detection procedure and visual image interpretation to determine yearly to decadal changes in seagrass (predominantly Halodule wrightii Ascherson) coverage on the Mississippi barrier islands. On Horn Island, seagrass coverage declined from 77 ha in 1940 to 19 ha in 1971, but returned to its 1940 value by 2006. Coverage on Petit Bois declined from 54 ha in 1940 to 8–19 ha from 1952 through 2007. On East Ship, seagrass coverage varied at 2–19 ha from 1963 to 2007. On West Ship, coverage dropped to zero in 2003, but by 2007 it had increased to approximate its 1975 value of 1.8 ha. On Cat Island, coverage increased from 22 ha in 2003 to 71 ha in 2007. There was no apparent negative impact of Hurricane Camille or Hurricane Katrina on seagrass coverage, which could vary annually by a factor of two or more.

Seagrasses are considered important indicators of decline in water quality resulting in increased light attenuation that negatively influences their growth and survival. Chronic light-limitation interspersed with unpredictable acute attenuation events have had poorly understood effects on seagrass recovery dynamics. Zostera marina (eelgrass) and Halodule wrightii (shoalgrass) were subject to a matrix of light-deprivation events followed by recovery periods to mimic repeated acute shading events. Plant survival, morphology, biomass, chlorophyll content, and Fv/Fm were assessed over time to determine recovery. At the end of the experiment, all plants were harvested and species-specific treatment effects were determined. Significant differences due to treatments were noted in all parameters measured. In general, responses were similar for both life-stages and between species, suggesting similar physiological tolerance to repeated acute light-attenuation events. Only plants in treatments where light-deprivation was followed by a recovery interval of at least the same duration showed signs of long-term survival. Chlorophyll fluorescence (Fv/Fm) was an important metric for assessing recovery, but it failed to detect the onset of mortality in many plants. Other metrics of plant condition need to be assessed and coupled with chlorophyll fluorescence data to assess seagrass “health”. This is of particular importance in field studies, where the history of the plants is largely unknown.

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.

Seagrass fragments and seeds are important dispersal mechanisms by which individuals can be transported to new habitats. While dispersal distances of these free-floating stages have been recently investigated in some detail, almost nothing is known about how long fragments or seedlings may remain viable in the water-column. This study reports on the results of an experiment in which both mature and seedling life-history stages of the temperate seagrass, Zostera marina L. were successfully maintained hydroponically over a 1-month-period. It is suggested that a potential application of this hydroponic growth approach could be seedling culture for restoration activities.

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 (

Seagrasses provide habitat for a wide variety of aquatic species, and reduce coastal erosion via sediment stabilization. The areal coverage of seagrass beds on the Mississippi barrier islands appears to have declined substantially since the 1960s. However, difficulties in sampling benthic vegetation, differences among previous studies in survey methods, and a lack of data on year-to-year variation in coverage have impeded quantitative study of this decline. Thus, the goal of the present study was to utilize historical remote sensing data to quantitatively determine changes in seagrass (predominantly Halodule wrightii Ascherson) coverage on the Mississippi barrier islands. Analyses were limited to imagery acquired during late summer through autumn, when seagrass canopies remained fully developed and water turbidity was low. Given these conditions, seagrass beds were readily identifiable in older black-and-white to color aerial photographs as well as in more recent multi-spectral imagery.On Horn, the largest Mississippi island, seagrass coverage declined from 77 ha in 1940 to 19 ha in 1971. Coverage had returned to its 1940 value by 2006, but declined again in 2007. Seagrass coverage on Petit Bois, the most rapidly-migrating island, declined from 54 ha in 1940 to an approximately stable 8-19 ha from 1952 through 2007. Petit Bois was the only island for which decline in seagrass coverage corresponded with decline in island land area. On the smallest island, East Ship, seagrass coverage remained nearly constant at 16-19 ha from 1963-2007 despite dramatic reductions in land area during Hurricanes Camille (1969) and Katrina (2005). On West Ship, coverage dropped to zero in 2003 but increased by 2007 to slightly exceed its 1975 value of 18 ha. Data for Cat Island extended only from 2003-2007. Seagrass coverage more than tripled during this period, from 22 ha in 2003 to 71 ha in 2007. Protection from trawling may have been a significant factor in the post-1995 recovery of seagrass on Horn Island. The absence of a similar recovery on Petit Bois may be linked with its rapid westward migration in combination with a nearly 40% reduction in land area from 1940-2007. There was no discernable negative impact of Hurricane Camille or Hurricane Katrina on seagrass coverage. Methodological differences (e.g., mapping potential seagrass habitat rather than existing seagrass beds) are important in explaining the dramatic decline in seagrass coverage that is apparent when recent data are compared with results of earlier surveys.