Facilitation of seagrass Zostera marina productivity by suspension-feeding bivalves

Abstract

Seagrasses and suspension feeders are both critical ecosystem engineers in estuaries. Seagrass beds are important structural habitats, and suspension feeders, when abundant, can regu- late phytoplankton densities. Furthermore, there may be mutual facilitation of growth and recruit- ment 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 produc- tivity was always significantly higher (on average, 48 ± 9.3% higher) in the treatments with the high- est density of bivalves compared to a control without bivalves (p < 0.05). The data indicate that clear- ance 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.