How were the changes in nutrient concentrations related to the growth of algae in the reservoirs?
The amount of algae (indicated by the concentration of chlorophyll-a, a green pigment in algae) increased during the summer in all reservoirs (table 2). Because algae used nitrogen for growth, the increase in the amount of algae generally corresponded to the decrease in nitrogen concentrations during the summer. In Jackson Reservoir, excessive algal growth near the water surface during July may have blocked the light necessary to sustain growth from reaching underlying water, leading to a drop in chlorophyll-a concentrations by September. Chlorophyll-a concentrations also provided an indication of the trophic state of each reservoir, a relative classification that reflects the nutrient enrichment in a water body. Higher average concentrations of chlorophyll-a in the range from 2.7 to 150 µg/L are characteristic of eutrophic reservoirs with high nutrient concentrations, low light penetration, high amounts of algae, and frequent algal blooms (Organization for Economic Cooperation and Dev
