Limiting resources, disturbance, and diversity in phytoplankton communities

Phytoplankton diversity, limiting resources, and disturbance were studied in two reservoirs, Eagle Mountain Lake (EML) and Joe Pool Lake (JPL), in north Texas, USA, for three summer growing seasons and two winters. Availabilities of phosphorus, nitrogen, and silicon were measured as chemical concentrations, and availability of light was measured as irradiance during the photoperiod, averaged over the depth of the surface mixed layer. A resource was defined as limiting whenever its availability fell below a threshold, treated as a parameter to be fitted. Depth of the surface mixed layer and wind speed were taken as indices of disturbance associated with episodic mixing. Hydrological disturbance was gauged by rainfall, inflow, and the variation of lake level. We found that diversity was strongly and significantly correlated with the number of limiting resources in one lake (JPL), but not the other (EML). The onset of nitrogen and phosphorus limitation during the summer growing season is associated with increased phytoplankton diversity in JPL. Regression modeling detected decreasing relationships between diversity and disturbance indices in one lake (JPL), but concave-down relationships in the other (EML), though the peaks of these relationships did not lie within the usual range of disturbance. Predictive regressions for diversity incorporating both limiting resources and disturbance were partitioned into unique and shared effects of these factors. There was a large unique effect of the number of limiting resources in JPL, but otherwise the shared effects were large relative to unique effects.