Establishing a Chlorophyll a Goal for a Run-of-the-river Reservoir

ABSTRACT

Lake Pepin, a 100 km² run-of-the-river reservoir, is located on the Mississippi River about 80 km downstream of the Twin Cities metropolitan area on the border between Wisconsin and Minnesota. Amajor inter-agency study of Lake Pepin and the Mississippi River has been underway since 1990 for the purposes of determining the impacts of the effluentfrom the Metropolitan Waste Control Commission's Metropolitan Wastewater Treatment Facility on Lake Pepinand to predict the benefits of reducing effluent phosphorus levels to 1 mg L⁻¹ or lower. Severe nuisance algal blooms and fish kills during the low flows of 1988 prompted this study.

Understanding the reservoir limnology and factors contributing to user perception of “nuisance algal blooms” (in terms of chlorophyll a concentration or phytoplankton species composition), are important steps in developing a chlorophyll a goal for Lake Pepin. Based upon analyses of chlorophyll a data, phytoplankton composition, and user perception information, a summer mean chlorophyll a concentration of 30 mg m⁻³ is recommended as a water quality goal for Lake Pepin. Nutrient-mass balance modeling suggests that a dramatic reduction in the inflow phosphorus concentration and in the overall in-lake phosphorus concentration (including internal loading) will be required to achieve this goal during low-flow summers.

Key Words:

• phosphorus
• reservoir
• goal setting
• chlorophyll a
• hydraulic residence time
• user perception