| Freshwater sponges are often an abundant and prolific member of benthic communities. They are capable of filtering a wide range of particulate food resources including bacteria, phytoplankton, and colloidal organic matter. In many individuals growth is enhanced by the photosynthetic products of endocellular algal symbionts. While it is known that freshwater sponges are capable of utilizing many resources, little work has been done to identify their primary energy source and relative trophic position within a benthic community. This task is made difficult by the cryptic nature of sponge trophic processes and their apparent intolerance for lab settings. In this study, carbon and nitrogen stable isotope analyses are used to infer the primary energy source and relative trophic position of the cosmopolitan freshwater sponge Spongilla lacustris at two sites. Data from sponges and sponge predators are compared to other species for which food source is well known. In addition, data from sponges with and without symbiotic algae are compared. At one lotic site, carbon data imply that S. lacustris is energetically linked to primary production that occurs in the pelagic zone of its source lake. The presence of algal symbionts was found to have a significant effect on the carbon and nitrogen signatures of S. lacustris and possible mechanisms for these effects are discussed. I also present evidence that symbiotic algae are important dietary constituents for two sponge predators. |
