Controls on algal abundance in a eutrophic river with varying degrees of impoundment (Kalamazoo River, Michigan, USA)

This study examined how nutrients and hydraulic flushing interact to regulate growth of phytoplankton in the Kalamazoo River (Michigan, USA), which has seven reservoirs with summer residence times ranging from < 1 to 12 days. The largest reservoir, Lake Allegan, suffers from eutrophication and resultant impairments of beneficial uses, problems being addressed by a TMDL focused on control of phosphorus. Water residence time was the most important control on algal growth in the various impoundments, including Lake Allegan, where residence time remained < 12 days through the summer. Based on longitudinal surveys, free-flowing river reaches appeared to remove phytoplankton, whereas a series of old decommissioned dams above Lake Allegan evidently contributed to algal biomass accumulation in the river. Nutrient concentrations were generally high throughout the river system; thus, algal growth may not be nutrient-limited at present. Phytoplankton in the two largest reservoirs was dominated by diatoms and green algae during late summer, despite nutrient concentrations that would tend to favor cyanobacteria in lakes. The relative availability of phosphorus (P), nitrogen (N), and silicon (Si) can indicate how algal growth may respond if nutrient concentrations were to decrease in the future. Nutrient ratios suggest that N and Si could be important in addition to P, depending on the reservoir and the season. In reservoirs with short water residence times, strategies to control eutrophication by reducing phosphorus loading may not yield results as readily as they do in lakes; hydraulic flushing, other nutrients, and upstream impoundments must also be considered.