Amphipod control of epiphyte load and its concomitant effects on shoalgrass Halodule wrightii biomass

Abstract

We investigated the interaction between epiphyte-grazer abundance and eutrophication to assess the relative importance of top-down and bottom-up effects in subtropical seagrass meadows. In field experiments using a cageless technique to control amphipod abundance, we measured the effects of grazing and nutrient supply on the growth and productivity of shoalgrass Halodule wrightii and its epiphytes at both protected and wave exposed sites. Amphipod removal at the protected site resulted in 70% greater epiphyte loads on shoalgrass leaves and a 36% reduction in leaf biomass after 10 wk. At the wave-exposed site, where amphipod abundance was consistently low, we found no significant effects of grazer presence or nutrients on epiphyte load or leaf biomass. Average leaf length, however, was significantly reduced in nitrogen-enriched plots. Our results indicate that natural densities of amphipods can reduce seagrass loss by controlling epiphytes. © Inter-Research 2013.