Accounting for parasites in freshwater benthic communities: Do trematodes modify snail-periphyton interactions?

Parasites are ubiquitous members of ecological communities, but have only recently been recognized as key players in broad ecological interactions and ecosystem dynamics.  I explored the effects of trematodes on freshwater snail-periphyton interactions with a field study of 50 sites in east-central Indiana and an outdoor mesocosm experiment.  Infection rates of trematodes on field-collected freshwater snails (Physa acuta) ranged from 2% to 36%.  Infected snails had significantly higher body N:P than uninfected snails.  Trematode rediae and cercariae N:P ratios were lower than snail N:P ratios but did not differ among trematode taxa. Additionally, periphyton N:P ratios were positively related to snail infection rates.  In outdoor mesocosms, experimentally-infected snails excreted higher N:P ratios than uninfected snails, resulting in significantly higher periphtyon N:P ratios in mesocosms with infected snails than in mesocosms with uninfected snails.  Thus, trematodes indirectly affected periphyton N:P by altering host snail excretion rates and content.  The indirect effects were stronger in more nutrient-limited mesocosms, illustrating the importance of ecological context on parasite-community interactions.  Overall, these results indicate that trematodes modify snail-periphyton interactions through a nutrient pathway and suggest that accounting for parasites may provide insight into important benthic processes.