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.

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Shallow, coastal lagoons are vulnerable to eutrophication and harmful algal blooms, often due to the loss of benthic suspension feeders. Florida Bay, USA, is a sub-tropical lagoon that has suffered from a series of ecological disturbances, including cyanobacterial blooms, loss of seagrass habitat, and widespread sponge mortality. A field study was executed at sites across Florida Bay to investigate effects of cyanobacterial blooms of the genus Synechococcus on the suspensionfeeding loggerhead sponge Spheciospongia vesparium. In situ measurements of loggerhead sponge survival, water pumping rates, and particle retention were made over the course of a year under both bloom and non-bloom conditions and for naturally-occurring and transplanted sponges. The mortality of transplanted loggerhead sponges significantly increased following cyanobacterial blooms. Sponge water pumping rates, particle retention, and filtration rates were depressed in areas where cyanobacteria blooms were present. When loggerhead sponge community filtration rates were compared with cyanobacteria intrinsic growth rates, sites with low abundances of sponges had persistent, dense blooms and harbored positive net growth rates for cyanobacteria (0.78 to 1.45 d −1 ). In contrast, sites with high abundances of sponges had few or no blooms and net growth rates for cyanobacteria that were slow or negative (−0.17 to 0.24 d −1 ). This suggests that the loss of filtration from sponge mortality in Florida Bay has contributed to the persistence of algal blooms. Restoration of benthic suspension feeders such as sponges could be an important management tool to mitigate algal blooms in shallow coastal lagoons, although survival of transplanted populations will likely require concurrent improvement of water quality by other means since blooms inhibit sponge pumping, particle retention, and filtration.KEY WORDS: Florida Bay · Sponges · Spheciospongia vesparia · Loggerhead sponge · Cyanobacteria · Synechococcus · Harmful algal bloom Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 451: [31][32][33][34][35][36][37][38][39][40][41][42][43] 2012 2001), the loss of benthic suspension feeders from such a system likely represents a more significant loss of top-down control than for deeper, well-flushed ecosystems.Florida Bay, USA, is a shallow, sub-tropical lagoon between mainland Florida and the Florida Keys; it is the largest estuary in Florida, valuable for recreation and fisheries, and adjacent to the sensitive habitats of the Florida Keys National Marine Sanctuary and Everglades National Park. Since the late 1980s, Florida Bay has been affected by a series of ecological disruptions, including sponge die-offs (Butler et al. 1995), blooms of the cyanobacteria Synechococcus spp. (Phlips et al. 1999), and seagrass mortality (Robblee et al. 1991). The root cause of these disturbances may be linked to human alterations of the freshwater flow in the Everglades, which is upstream of Florida Bay (Phlips e...

While many coastal ecosystems previously supported high densities of seagrass and abundant bivalves, the impacts of overfishing, eutrophication, harmful algal blooms, and habitat loss have collectively contributed to the decline of these important resources. Despite improvements in wastewater treatment in some watersheds and subsequent reduced nutrient loading to neighboring estuaries, seagrass and bivalve populations in these locations have generally not recovered. We performed three mesocosm experiments to simultaneously examine the contrasting effects of nutrient loading and historic suspensionfeeding bivalve densities on the growth of eelgrass (Zostera marina), juvenile bivalves (northern quahogs, Mercenaria mercenaria; eastern oysters, Crassostrea virginica; and bay scallops, Argopecten irradians), and juvenile planktivorous fish (sheepshead minnow, Cyprinodon variegatus). High nutrient loading rates led to significantly higher phytoplankton (chlorophyll a) levels in all experiments, significantly increased growth of juvenile bivalves relative to controls with lower nutrient loading rates in two experiments, and significantly reduced the growth of eelgrass in one experiment. The filtration provided by adult suspension feeders (M. mercenaria and C. virginica) significantly decreased phytoplankton levels in all experiments, significantly increased light penetration and the growth of eelgrass in one experiment, and significantly decreased the growth of juvenile bivalves and fish in two experiments, all relative to controls with no filtration from adult suspension feeders. These results demonstrate that an appropriate level of nutrient loading can have a positive effect on some estuarine resources and that bivalve filtration can mediate the effects of nutrient loading to the benefit or detriment of different estuarine resources. Future ecosystem-based approaches will need to simultaneously account for anthropogenic nutrient loading and bivalve restoration to successfully manage estuarine resources.

Florida Bay is Florida's (USA) largest estuary and has experienced harmful picocyanobacteria blooms for nearly two decades. While nutrient loading is the most commonly cited cause of algal blooms in Florida Bay, the role of zooplankton grazing pressure in bloom occurrence has not been considered. For this study, the spatial and temporal dynamics of cyanobacteria blooms, the microbial food web, microzooplankton and mesozooplankton grazing rates of picoplankton, and the effects of nutrients on plankton groups in Florida Bay were quantified. During the study, cyanobacteria blooms (>3×10 5 cells mL −1 ) persisted in the eastern and central regions of Florida Bay for more than a year. Locations with elevated abundance of cyanobacteria hosted microzooplankton grazing rates on cyanobacteria that were significantly lower (p<0.001) and less frequently detectable compared to sites without blooms. Consistent with this observation, cyanobacteria abundances were significantly correlated with ciliates and heterotrophic nanoflagellates at low cyanobacteria densities (p<0.001) but were not correlated during bloom events. The experimental enrichment of mesozooplankton abundance during blooms yielded a significant decrease in the net growth rate of picoplankton but had the opposite effect when blooms were absent, suggesting that the cascading effect of mesozooplankton grazing on the microbial food web was also altered during blooms. While inorganic nutrient enrichment significantly increased the net growth rates of eukaryotic phytoplankton and heterotrophic bacteria, such nutrient loading had no effect on the net growth rates of cyanobacteria. Hence, this study demonstrates that low rates of zooplankton grazing and low rates of inorganic nutrient loading contribute to the persistence of cyanobacteria blooms in Florida Bay.