THE RELATIVE IMPORTANCE OF AMPHIPOD AND GASTROPOD GRAZERS IN POSIDONIA-SINUOSA MEADOWS

The relative importance of amphipod and gastropod grazers in controlli ng periphyton and epiphytes of the seagrass Posidonia sinuosa Cambridg e and Kuo, was assessed using exclosure chambers in the field with an amphipod assemblage and the herbivorous gastropod, Thalotia conica Gra y. Periphyton were considered to be bacteria, diatoms and algal propag ules whereas epiphytes were defined as larger algal forms visible to t he naked eye. Subtle grazing effects were detected after 14, 21 28 and 35 days despite chamber artefacts. Seagrass mortality was three times lower in the ''no-chamber'' controls than inside the chambers. It was 24% lower in the gastropod-inclusion chambers, suggesting that gastro pod grazing enhanced seagrass leaf survival. Grazing effects on periph yton were varied. Amphipods had no significant effect on periphyton bi omass or chlorophyll a, but they reduced the ratio of dry weight to as h weight by 54% and the taxonomic richness by 12%, suggesting an activ e selection of taxa during grazing. Gastropods reduced periphyton chlo rophyll a levels by 51% after 35 days although they had no clear effec ts on periphyton biomass, Gastropods also reduced the ratio of chlorop hyll a to ash-free dry weight by 99% which indicated a change in the p eriphyton community composition. Grazing impacts on epiphytes also dif fered with the different grazers. Gastropods reduced epiphyte biomass by 44% but had no effect on the number of taxa, while amphipods increa sed the number of taxa by 29% but had no impact on epiphyte biomass. T here was no evidence of larger grazing impacts on periphyton or epiphy tes when both grazers were present. Gastropods are more efficient but less selective grazers than amphipods. However, because amphipods are highly mobile with rapid production rates their impact on epiphytes ma y be important, particularly in affecting species composition rather t han biomass. However, spatial and temporal patchiness in abundance, wh ich was not examined in the present study may also play a role in dete rmining the relative effectiveness of these two types of grazers.