Forty-eight different nutrient loading scenarios were developed using the calibrated Beaver Lake model. Nutrient (orthophosphorus, nitrite plus nitrate, and ammonia) concentrations were increased and decreased for the three main inflows to Beaver Lake (White River, Richland Creek, and War Eagle Creek) for the period of April 2001 to April 2003. Orthophosphorus concentrations were decreased by half and increased 2, 5, and 10 times the calibrated daily input concentrations in the three tributaries simultaneously and for each individual tributary. Daily input nitrite plus nitrate and ammonia concentrations also were decreased by half and increased 2, 5, and 10 times the calibrated daily input concentrations in the three tributaries simultaneously and independently. Phosphorus and nitrogen concentrations were increased and decreased independently and simultaneously. The response of Beaver Lake to changes in the nitrogen and phosphorus concentrations in the tributaries was analyzed by describing the changes in algal biomass (as measured by chlorophyll a concentrations), ammonia, nitrite plus nitrate, and orthophosphorus concentrations in Beaver Lake.

The greatest simulated increase in algal biomass in Beaver Lake occurred when nitrogen and phosphorus were increased simultaneously in the three main tributaries. When only additional nitrogen was introduced into Beaver Lake, algal biomass did not increase significantly (p>0.05) in most of the reservoir because algal growth remained limited by the less available phosphorus in the water column. However, when additional phosphorus was introduced into Beaver Lake independently from nitrogen, algal biomass increased significantly (p<0.05) compared to the calibrated concentrations. When additions of nitrogen and phosphorus were introduced simultaneously, algal biomass was much greater than when nitrogen or phosphorus was added independently. When both nitrogen and phosphorus were increased 10 times the calibrated daily input concentrations, chlorophyll a concentrations increased substantially in the downstream portion of the reservoir at depths of up to 12 meters in May of 2002. When nitrogen and phosphorus concentrations were increased simultaneously for the three main tributaries, the increase in chlorophyll a was the greatest in the late spring and summer of 2002. Increases in chlorophyll a concentrations probably were not as great in late spring and summer of 2001 because of the hydrologic conditions. April through November 2001 was a relatively dry period, which would mean that less mass (load) of nitrogen and phosphorus was delivered to Beaver Lake than in subsequent months. When water temperatures and water clarity were ideal for algal growth in late spring 2002, a relatively high amount of nutrients was readily available for algal uptake, resulting in significantly greater algal growth and higher chlorophyll a concentrations from May through November 2002.