The effects of a change in withdrawal operations on phytoplankton and nutrient dynamics in Eau Galle Reservoir, Wisconsin (USA)

Selective withdrawal through the surface outlet was employed in Eau Galle Reservoir to reduce phytoplankton populations by 1) strengthening thermal stability, thereby decreasing vertical entrainment of hypolimnetic phosphorus (P); and 2) increasing epilimnetic flushing rate and discharge of algae. In spite of substantial release of sediment P over the growing season, epilimnentic P concentrations were overwhelming dominated by external sources, and thus insensitive to changes in withdrawal depth. Nor did surface withdrawal increase flushing rate sufficiently to affect phytoplankton biomass. In one year, an increase in thermal stability, due in part to surface withdrawal, permitted the development of a metalimnetic chlorophyll maximum in a summer that also experienced relatively low populations of 'nuisance' cyanophytes, and can be regarded as a positive effect of the change in operations.