Seagrass beds versus sand bottoms: the trophic importance of their associated benthic invertebrates.

Seagrass beds in the Indian River lagoon, Florida, had 3 times the density of
macrobenthic invertebrates found in unvegetated sediments a few meters away. Epifaunal abundance
was 13 times greater in seagrass than sand. Most epifaunal species (e.g., amphipods,
isopods, tanaids, gastropods and shrimps) were uncommon in the unvegetated sediments,
presumably due to their dependence on sea grass for providing habitat, food, nursery area, and/or
protection from predators. Not only were macrofaunal densities much higher in natural seagrass
than in sand, those animals which were more abundant (primarily the epifauna) were also more
heavily preyed upon and thus are trophically more important than infauna. Four 3-mm mesh
cages were set up, each within a larger 12-mm mesh cage, 2 in seagrass and 2 in sand to test 1) the
importance of associated macrobenthos to the local food web and 2) the effect of small decapod
predators. After 2 mo the inner cages had the lowest macrobenthos density and the highest density
of decapod crustaceans, the animals we intended to exclude. The only animals more abundant
in the inner cages were the decapod crustaceans. Because cages with mesh sizes even smaller
than 3 mm do not effectively exclude many crustacean predators, we conclude that it is nearly
impossible to exclude predators from local seagrass meadows. The grazing epifauna of seagrass
meadows forms a major trophic pathway to higher predators via the decapods; such a pathway is
lacking in sand bottom communities.