Grazing effects of a gammaridean Amphipoda, Ampithoe sp., on the seagrass, Syringodium isoetifolium , and epiphytes in a tropical seagrass bed of Fiji

In the pure stand of tropical seagrass, Syringodium isoetifolium , in a small oceanic island, Fiji, grazing effects of the seagrass‐associated gammarid, Ampithoe sp., on seagrass and epiphytes were assessed in October 1989, November 1991, November 1992. Density of the gammarid was estimated with two methods, mesh bag method and tuft method. During the three years surveyed the density of the gammarid increased remarkably from 1989 to 1991, with heavy epiphytism. Gut contents of the gammarid were examined. Grazing rates on seagrass leaf with and without epiphytic blue‐green algae were measured in a bottle experiment. Litter bag experiments were conducted using different mesh sizes each containing seagrass only and seagrass and gammarids. The seagrass leaf biomass in the litter bag reduced abruptly in both bags. After one week, 78–86% of seagrass biomass disappeared from the bags. Enhancement of decomposition of seagrass leaf by the gammarid grazing was observed. Oxygen consumption and ammonium excretion rates were measured simultaneously in bottle experiments. Carbon budget in the seagrass bed was estimated as follows: 0.9 gC m −2 day −1 in seagrass growth, gammarid grazing was about a half of it and further assimilated a half of it, about 0.1 gC m −2 day −1 , and more than half of it become CO 2 by respiration. Grazing effects on epiphyte and seagrass growth and production were discussed through the carbon budget and indirect interactions between seagrass, epiphytes and associated gammarids to explain the temporal change of seagrass and epiphyte dynamics.