Original paper

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

Vanni, Michael J.; Andrews, J. Scott Renwick; Gonzalez, Maria J.; Noble, Samanthia J.


We investigated the dynamics of nutrient and light limitation of phytoplankton in a reservoir ecosystem in relation to storm-mediated variation in stream discharge, 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 severity 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 severity 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 weaker at shorter and longer time scales. However, discharge events also flushed phytoplankton from the lake and enhanced light limitation, so it is not clear by which mechanism(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, phytoplankton were not more light limited near stream inflows than at the outflow; shallower depth near inflows compensated for higher turbidity, in terms of the light climate experienced by phytoplankton. Our results show that the mechanisms by which discharge events mediate phytoplankton resource limitation are complex and require further study in reservoirs as well as other aquatic systems subject to a high degree of temporal variation in discharge.