No abstract

Despite the importance of coastal ecosystems for the global carbon budgets, knowledge of their carbon storage capacity and the factors driving variability in storage capacity is still limited. Here we provide an estimate on the magnitude and variability of carbon stocks within a widely distributed marine foundation species throughout its distribution area in temperate Northern Hemisphere. We sampled 54 eelgrass (Zostera marina) meadows, spread across eight ocean margins and 36° of latitude, to determine abiotic and biotic factors influencing organic carbon (Corg) stocks in Zostera marina sediments. The Corg stocks (integrated over 25‐cm depth) showed a large variability and ranged from 318 to 26,523 g C/m2 with an average of 2,721 g C/m2. The projected Corg stocks obtained by extrapolating over the top 1 m of sediment ranged between 23.1 and 351.7 Mg C/ha, which is in line with estimates for other seagrasses and other blue carbon ecosystems. Most of the variation in Corg stocks was explained by five environmental variables (sediment mud content, dry density and degree of sorting, and salinity and water depth), while plant attributes such as biomass and shoot density were less important to Corg stocks. Carbon isotopic signatures indicated that at most sites <50% of the sediment carbon is derived from seagrass, which is lower than reported previously for seagrass meadows. The high spatial carbon storage variability urges caution in extrapolating carbon storage capacity between geographical areas as well as within and between seagrass species.

An unprecedented series of ecological disturbances have been recurring within Florida Bay since the summer of 1987. Persistent and widespread phytoplankton and cyanobacteria blooms have coincided with the large scale decimation of sponge communities. One hypothesis is that the large scale loss of suspension-feeding sponges has rendered the Florida Bay ecosystem susceptible to these recurring blooms. The primary objective of this study was to experimentally evaluate the potential for suspension-feeding sponges to control nuisance phytoplankton blooms within Florida Bay prior to a large sponge die-off event. To achieve this objective, we determined the extent and biomass of the surviving sponge community in the different basins of Florida Bay. Many areas within Florida Bay possessed sponge densities and biomasses of 1 to 3 ind. m -2 or 100 to 300 g m -2 respectively. The dominant species included Spheciospongia vesparia, Chondrilla nucula, Cinachyra alloclada, Tedania ignis and Ircinia sp., which accounted for 68% of individual sponges observed and 88% of sponge biomass. Laboratory grazing rates of these dominant sponges were experimentally determined on 4 different algal food treatments: a monoculture of cyanobacteria Synechococcus elongatus, a monoculture of the diatom Cyclotella choctawhatcheeana, a monoculture of the dinoflagellate Prorocentrum hoffmanianum, and an equal volume of the 3 monocultures combined. To estimate the impact of a mass sponge mortality event on the system-wide filtration rate of Florida Bay, we combined estimates of the current sponge biomass and laboratory sponge filtration rates with estimates of mean volumes of the sub-basins of Florida Bay. This study implies that the current blooms occurring within the central region of Florida Bay can be explained by the loss of the dominant suspension feeder in this system, and there is no need to invoke a new addition of nutrients within this region for the blooms to occur.

In shallow coastal waters, suspension-feeding bivalves often dominate the benthos in numbers as well as biomass. In the Gulf of Mexico and the Caribbean Sea, these filter feeders are usually associated with seagrass. Two simultaneously conducted field experiments (a mussel density manipulation and a mussel predation experiment) were conducted to examine the potential positive interactions between the suspension-feeding mussel Modiolus americanus (Leach) and the seagrass Thalassia testudinum Banks ex König. The mussel density manipulations resulted in a doubling of the total nitrogen and total phosphorus levels of sediments, and a significant reduction in leaf tissue C:N, N:P and C:P ratios, demonstrating that the mussels increased the sediment nutrient content and that these increased nutrients were biologically available to the plant. T. testudinum responded to the presence of mussels by significantly increasing leaf widths and lengths. In addition, productivity significantly increased in the mussel-addition treatments. Another response to the presence of mussels included a significantly reduced epiphytic load on the seagrass leaves. The mussel predation experiment evaluated the effects of seagrass on the survivorship of the associated mussel, M. americanus. Mean survival was significantly greater in vegetated habitats than in unvegetated sediments. Consequently, when mussels are present in seagrass meadows, they elevate seagrass productivity through either increased nutrient resource pools or reduced epiphytic loads on the leaves, while the seagrass increases mussel survivorship. Thus, this study demonstrates the reciprocal positive interactions of these organisms when associated and suggests that seagrass meadows may exist as a mosaic of nutrient and productivity 'hot spots' when suspension-feeding organisms are present. These positive interactions may have important consequences in the development, structure and organization of seagrass communities.KEY WORDS: Bentho-pelagic couple · Habitat complexity · Plant-animal interactions · Seagrass · Thalassia testudinum · Suspension-feeding bivalves · Modiolus americanus · Facultative mutualism 213: 143-155, 2001 loops in which particulate nitrogen (phytoplankton) consumed by the suspension-feeding bivalves is rapidly remineralized into ammonium (NH 4 ), which is then available for plant growth. Such a feedback mechanism has been envisioned as both a nitrogen retention mechanism as well as a process that accelerates the nitrogen cycle (Dame et al. 1984(Dame et al. ,1989. In addition, organic deposits associated with suspension-feeding bivalves are of special importance for the remineralization of nutrients because processes of phosphorus release are promoted in anaerobic environments (Jansson & Wulff 1977, Holm 1978, Nixon et al. 1980. Previous studies have demonstrated that sedimentation of fecal organic material provides a considerable input of phosphorus to the sediment surrounding suspension-feeders (Sornin et al. 1986, Dame et al. 1989). This phosp...

Seagrasses and suspension feeders are both critical ecosystem engineers in estuaries. Seagrass beds are important structural habitats, and suspension feeders, when abundant, can regulate phytoplankton densities. Furthermore, there may be mutual facilitation of growth and recruitment between seagrasses and suspension-feeding bivalves. In a series of mesocosm experiments, the effects of environmentally realistic densities of 3 different suspension-feeding bivalves (Mercenaria mercenaria, Crassostrea virginica, Mytilus edulis) on the growth of eelgrass Zostera marina in a eutrophied environment were examined. Experimental treatments with bivalves consistently yielded significantly lower chlorophyll a concentrations (p < 0.05), and most bivalve treatments also showed significant increases in light penetration (p < 0.05). Eelgrass productivity was measured by leaf area growth, and varied from 0.318 ± 0.018 to 0.832 ± 0.036 cm 2 shoot -1 d -1 (mean ± SE); leaf area productivity was always significantly higher (on average, 48 ± 9.3% higher) in the treatments with the highest density of bivalves compared to a control without bivalves (p < 0.05). The data indicate that clearance of the water column, and the subsequent increase in light penetration, was the primary mechanism by which suspension-feeding bivalves facilitated the growth of eelgrass. These findings suggest that healthy populations of suspension-feeding bivalves can mitigate the effects of estuarine eutrophication and can facilitate the growth of seagrass in degraded, light-limited habitats.