Limiting resources and the regulation of diversity in phytoplankton communities

Species diversity is a key concept in ecology, yet the mechanisms regulating diversity in most systems are not completely understood. To address this issue, we analyzed the relationship between phytoplankton diversity and limiting resources (N, P, Si, and light) over two summers in three lakes in the Yellowstone (Wyoming, USA) region. Diversity was highly variable along temporal and spatial axes within lakes. We discovered a strong positive correlation between diversity and the number of resources at physiologically limiting levels. Consistent with resource-competition theory, we found the highest diversity to occur when many resources were limiting. Conversely, the lowest diversity occurred when few resources were measured at limiting levels. Sensitivity analyses demonstrated that threshold levels of resources (below which growth is limited and above which there is saturation for the resource) appear to exist in the natural environment, and that diversity is regulated in part by absolute levels of resources available to phytoplankton in aquatic systems. Threshold levels are generally in agreement with those determined to be limiting in prior physiological experiments. Consistent with previous observations, among-lake comparisons yielded a significant negative relationship between species diversity and system productivity. This result tentatively supports the hypothesis that the proximity of individual phytoplankton plays a role in the strength of competitive interactions. Our results generally suggest that, even in dynamic environments, where equilibrium conditions are rare, resource competition among phytoplankton is a mechanism by which communities are continually structured.