Effects of grass shrimp versus nutrient addition on epiphytic algae associated with the ephemeral widgeongrass Ruppia maritima

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Grazers can reduce epiphytic algal loads on submerged aquatic vegetation (SAV) and indirectly alter biotic interactions among epiphytic algal groups. In the present study, we elucidate responses by 3 major epiphytic functional groups (diatoms, rhodophytes, and chlorophytes) to grass shrimp Palaemonetes spp. addition and nutrient addition during 3 growth stages of the ephemeral estuarine SAV, Ruppia maritima. Three field experiments conducted in the Grand Bay National Estuar- ine Research Reserve, Mississippi, USA, revealed that grass shrimp altered the community composition of epiphytic algae mainly in the mid- and late-growth stages by selectively grazing red algal epiphytes. Corresponding diatom and green algal responses likely reflect release from biotic interactions, or possibly secondary shrimp feeding preferences. Except for a modest but significant increase in red al- gae relative to shrimp addition, which did not reflect effects of grazing, epiphyte groups did not respond noticeably to shrimp addition in the early stage experiment. Grass shrimp addition appeared to be more important in shaping epiphytic algal composition than nutrient addition. Enclosure effects possibly in- fluenced algal growth and shrimp addition effects. Changes in the composition of the epiphyte commu- nity relative to nutrient addition were modest and variable. The lack of significant shrimp-nutrient in- teractions across all 3 experiments reflected primarily independent effects of both factors on epiphyte composition. Disproportionate effects of grass shrimp on the SAV epiphyte community, as expressed within the context of the inherently plastic and complex trophic role played by these facultative grazers, should help maintain the diversity and function of structured SAV habitats.