Epilithon is considered a great bio-indicator of trophic state in freshwaters and this “community” is mainly composed by algae, cyanobacteria, heterotrophic microbes and detritus. Snail grazing is a main regulator of standing crop of algae and, thus, influences primary productivity in aquatic systems. Our objective was to determine the composition of the epilithon in areas dominated by freshwater snails in a subtropical stream: Quebrada de Oro at Mayagüez, Puerto Rico. This subtropical urban stream was selected because the main grazing snail, Thiara granifera, is an introduced species with clinical relevance and has been related also to nutrient fluctuations and habitat degradation. Three stations were selected and within each one, two transects were set to determine the mean density of the snails and measure abiotic factors, such as temperature and turbidity. We obtained two composite samples by scrapping the epilithon from 4-5 rocks per station, preserved them with 10% formalin, and performed a taxonomic analysis of each sample in the laboratory.
T. granifera dominated the snail community in all stations but was more abundant downstream. Results suggest that cyanobacteria abundance is directly related to T. granifera. However, factors such as turbidity and light exposure was also related to algal density; thus the effects on algal standing crop due to the interactions between grazing and other parameters require further studies.