Limiting resources and the regulation of diversity in phytoplankton communities

Species diversity is a key concept in ecology, yet the mechanisms regulatin g diversity in most systems are not completely understood. To address this issue, we analyzed the relationship between phytoplankton diversity and lim iting resources (N, P, Si, and light) over two summers in three lakes in th e Yellowstone (Wyoming, USA) region. Diversity was highly variable along te mporal and spatial axes within lakes. We discovered a strong positive corre lation between diversity and the number of resources at physiologically lim iting levels. Consistent with resource-competition theory, we found the hig hest diversity to occur when many resources were limiting. Conversely, the lowest diversity occurred when few resources were measured at limiting leve ls. Sensitivity analyses demonstrated that threshold levels of resources (b elow which growth is limited and above which there is saturation for the re source) appear to exist in the natural environment, and that diversity is r egulated 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 w ith previous observations, among-lake comparisons yielded a significant neg ative 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. Ou r results generally suggest that, even in dynamic environments, where equil ibrium conditions are rare, resource competition among phytoplankton is a m echanism by which communities are continually structured.