Strong effects of herbivorous amphipods on epiphyte biomass in a temperate seagrass meadow

The experimental manipulation of large marine herbivores (urchins, gastropods and fish) has repeatedly demonstrated their strong influence on the abundance and composition of benthic primary producers. However, the effects of smaller herbivores (amphipods, isopods and small gastropods) on community structure are not as well understood. We used a cageless technique (a slow-release insecticide) to exclude amphipods from seagrass meadows to test for their effects on epiphyte and seagrass biomass. Lower amphipod densities in a Posidonia sinuosa meadow after 7 wk were associated with 25% higher epiphyte biomass-evidence of a strong influence by amphipods on seagrass epiphytes. In an Amphibolis spp. meadow, lower amphipod densities showed a non-significant trend to increase leaf epiphytes, but did not affect stem epiphyte biomass. Effects of amphipod exclusion on seagrass biomass were not detected in either meadow. Our results indicate that natural densities of amphipods can reduce epiphyte biomass on seagrasses, but that their impacts may vary across seagrass meadows. A challenge for future research is to identify the conditions under which small marine herbivores are likely to influence the biomass of primary producers.