Impact of differential predation potential on eelgrass (Zostera marina) faunal community structure

The trophic structure of fauna within eelgrass beds (Zostera marina) was assessed at two sites in Little Egg Harbor, New Jersey, USA during the summer of 1999. Although the sites were similar with respect to both Z. marina shoot density and plant biomass, they differed significantly in the relative distribution of large predatory fish (e.g., Cynoscion regalis, Paralichthys dentatus, Morone saxatilis). Site One, Marsh Elder, was characterized by a significantly greater catch per unit effort for large predators than Site Two, Shelter Island. Gut content analysis provided direct evidence of trophic linking and significant declines between these fish and four of the five most abundant organisms collected in throw traps used to analyze the density of large benthic prey/small predators. The densities of grass shrimp (Palaemonetes spp. Hippolyte zostericola), blue crab (Callinectes sapidus), and small predatory fish (e.g., Syngnathus fuscus, Opsanus spp., Tautoga onitus) were significantly reduced at Marsh Elder, potentially as a direct impact of large predatory fish. In turn, the differences in the density of small predators observed between sites produced either a significant positive or negative effect on the distribution of small benthic prey (e.g., polychaetes, amphipods), resulting in a two-step trophic cascade within the system. Additionally, an analysis of similarities defined each site independently for both large prey/small predators and small benthic prey community structure. Although the mechanism which produced the differences in the distribution of large predatory fish remains unknown, their impact on faunal community structure mediated not only the distribution of their potential prey, but also subsequent lower trophic levels.