The effects of nutrients and consumers on tidal pool communities

Patterns of community structure are thought to be driven by consumer processes in benthic marine systems. However, the conclusions of the qualitative and quantitative reviews presented in this dissertation suggest that nutrient variation may play a more important role than previously believed. The paucity of field experiments studying the role of nutrient variation in benthic marine communities is primarily due to the difficulties of manipulating nutrients in open systems. Tidepools along rocky shores provide a novel setting in which to study questions of trophic control, because they have discrete boundaries, which allow both nutrients and consumers to be manipulated easily. I studied the effect of nutrient enrichment in one trophic level (phytoplankton only) and two trophic level tidepool communities (phytoplankton and blue mussels, Mytilus edulis). Basic trophic interaction models predict that in one trophic level systems, nutrient enrichment will cause an increase in primary producer biomass, whereas in two trophic level systems, the models predict that herbivores will maintain low levels of producer biomass, resulting in no net increase in producer biomass. The results of a short time course experiment supported this model. Two subsequent experiments tested a modified model, which was developed to predict the effect of sustained nutrient enrichment over a summer season. The modified model predicted that primary producer biomass would increase with nutrient enrichment in both one and two trophic level systems. Furthermore, the modified model predicted that the increase in producer biomass in one trophic level systems would be due to an increase in large, competitively superior, phytoplankton cells, whereas the increase in producer biomass in two trophic level systems would be due to an increase in small phytoplankton cells that mussels are unable to capture. The experiments that tested the modified model produced mixed results, suggesting that basic trophic interaction models are an insufficient predictor of community structure in tidepool systems. More complex models that incorporate interactions between phytoplankton and bacterioplankton, seasonal variation in consumer-driven nutrient recycling, and variation in physical parameters that affect nutrient availability, are necessary to understand tidepool community structure