With or Without Nutrients, Sponges Are Boring: The Effects of Eutrophication on Bioerosion

Eutrophication, largely due to coastal development and human activities, is an ongoing issue for coastal ecosystems. Excess nutrients can impact ecosystem processes, promote the occurrence of algal blooms, and decrease the overall water quality. One process that may be impacted by eutrophication is the biological breakdown of carbonate material, or bioerosion. Correlative evidence suggests that bioeroding sponges increase in size and abundance along nutrient gradients, leading to increased bioerosion rates. However, there have been no experimental studies investigating the direct effects of increased nutrients on sponge bioerosion rates. To determine whether sponge bioerosion of carbonate material is enhanced under eutrophied conditions, we performed both in situ and laboratory studies of oyster shell bioerosion by clionaid sponges. Using Osmocote fertilizer, we artificially increased localized water column nutrients in both a mesocosm and field experiment. In each experiment, initial and final buoyant weights of oyster shells infested with Cliona spp. were compared to quantify sponge bioerosion rates over a 12-week period. Our results indicate that sponge bioerosion rates are not directly affected by increased inorganic nutrients. This study is the first to present experimental evidence suggesting eutrophication does not impact sponge bioerosion.