Nutrient and light limitation of reservoir phytoplankton in relation to storm-mediated pulses in stream discharge

We investigated the dynamics of nutrient and light limitation of phytoplank- ton in a reservoir ecosystem in relation to storm-mediated variation in stream dis- charge, and how dynamics differed at a shallow site near stream inflows versus one in deep water near the lake outflow. Storm-mediated discharge events reduced the sev- erity of nutrient limitation and increased the severity of light limitation, as predicted by a model of reservoir resource limitation developed by Kimmel et al. (1990). The sev- erity of nutrient limitation was negatively correlated with discharge to the lake; the correlation was strongest with discharge over the preceding 10-14 day period and wea- ker at shorter and longer time scales. However, discharge events also flushed phyto- plankton from the lake and enhanced light limitation, so it is not clear by which mech- anism(s) discharge events mediate phytoplankton resource limitation. Phytoplankton near stream inflows were less nutrient limited than phytoplankton at the lake outflow, consistent with predictions of the Kimmel et al. (1990) model. However, this was true even when streamflow was negligible, suggesting alternative mechanisms for reduced nutrient limitation near stream inflows. In contrast to predictions of the model, phyto- plankton were not more light limited near stream inflows than at the outflow; shallo- wer depth near inflows compensated for higher turbidity, in terms of the light climate experienced by phytoplankton. Our results show that the mechanisms by which dis- charge events mediate phytoplankton resource limitation are complex and require fur- ther study in reservoirs as well as other aquatic systems subject to a high degree of temporal variation in discharge.