Nutrient and algal responses to a managed drawdown in an agricultural riverine lake

Water level of a shallow impounded riverine lake was lowered to help improve downstream flows and water quality within the Big Sunflower River in the agriculturally dominated Mississippi Delta, Mississippi, USA. A study was conducted to better understand how nutrient and algal processes within shallow lakes respond to relatively rapid water level changes to guide water resource management decisions. The drawdown was initiated in late summer and completed within six days, ending when outflow was <5% of peak flows. Water depths were reduced from 28-82%. Nutrients (phosphorus, nitrogen and carbon) and water column algal measurements (chlorophyll a, in-vivo chlorophyll, in-vivo phycocyanin, chlorophyll:phycocyanin ratio, and photosynthetic activity) were measured at three sites on days -7, -1, 0, 1, 2, 3, 6, 13, 20, 28, and 34, relative to the start of the drawdown. Nitrogen and phosphorus responses were bimodal, showing a decrease during the initial drawdown phase followed by increasing levels in the ensuing weeks. Algal responses showed complex interactions between nutrients and changes in light availability that were often indirectly related to hydrologic changes. Late summer to early fall drawdowns in small impoundments used to improve downstream flows can have minimal water quality impacts on the upstream impounded systems.