The grazing effects of grass shrimp, Palaemonetes pugio, on epiphytic microalgae associated with Spartina alterniflora

Experiments were performed seasonally to estimate grass shrimp, Palaemonetes pugio, grazing on the epiphytic microalgae of cordgrass, Spartina alterniflora, and to determine if grass shrimp have the potential to regulate epiphyte abundance. Grass shrimp were given access to live culms with low and high epiphytic abundance and standing-dead culms collected from the streamside levee of a Louisiana salt marsh. Plexiglas frames were used to hold calms upright in aquaria and to restrict grass shrimp access to one half of each culm. We compared epiphyte biomass on the sides of culms exposed to shrimp with the corresponding unexposed sides. Epiphytes were removed from the lowest 10 cm. of culms on days 0, 3, and 10, and chlorophyll a (chl a) measurements on each culm half were made by fluorometry. Chl a biomass on culm halves not exposed to grass shrimp significantly increased over time. Percent reductions in chl a on culm halves exposed to grass shrimp (calculated by subtraction from the corresponding half not exposed to shrimp) significantly increased over time for at least one culm type in all seasons. Grass shrimp caused an average 30% reduction of epiphyte biomass over 3 d and a 40% reduction over 10 d, suggesting that grass shrimp have the capability of consuming a substantial proportion of the daily production of epiphytes. Epiphytes from standing-dead culms may be more important than those from live culms at the marsh edge to the diet of grass shrimp because chl a biomass was, on average, high, and standing-dead culms were seasonally abundant. Diagnostic photosynthetic pigments from selected culms, grass shrimp gut contents, and fecal pellets were identified by high performance liquid chromatography and were used to quantify the taxonomic groups of epiphytic microalgae. Results suggested that diatoms, brown algae, green algae, red algae, and cyanobacteria were present on all culms. Similarities in the pigment content of grass shrimp gut contents and fecal pellets suggested that all algal groups were ingested. Pigment data analysis could not detect a change in the composition of the microalgal assemblage associated with grass shrimp grazing. Assuming that the reduction in chl a was due exclusively to grazing, grass shrimp consumed an average of 0.5-1.5 g epiphyte carbon shrimp(-1) d(-1), suggesting that grass shrimp benefit significantly from the consumption of S. alterniflora epiphytic algae.