Facilitation of seagrass Zostera marina productivity by suspension-feeding bivalves

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 cm2 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.