Understanding seasonal phosphorus dynamics to guide effective management of shallow, hypereutrophic Grand Lake St. Marys, Ohio

Abstract

Grand Lake St. Marys, hereafter Grand Lake, is a large shallow reservoir in western Ohio that has become hypereutrophic in recent decades owing to external phosphorus (P) loading. To restore water quality in Grand Lake, managers must decide whether to reduce external P loading from tributaries or continue to reduce internal P loading from sediments using aluminum sulfate additions. To help managers identify the most effective strategies for water quality improvements in Grand Lake, we quantified the reservoir's seasonal P dynamics using tributary and in-reservoir P measurements during 2009–2011. We also tested the short-term effects of reduced external P loading on Grand Lake's water quality using a pond experiment performed at the adjacent St. Marys State Fish Hatchery. The highest P concentrations in Grand Lake (1660, 391, and 311 μg/L in 2009, 2010, and 2011, respectively) occurred in summer when external P loads from tributaries were minimal, suggesting internal P release from sediments was the major source of P to fuel cyanobacterial blooms. Results from our pond experiment showed that decoupling the reservoir from external P loading during summer months does not improve short-term water quality. We conclude that external P loading during winter and spring, coupled with internal P loading from sediments during summer, drive the extremely high P concentrations in summer that support dense cyanobacterial blooms. Long-term improvement in Grand Lake water quality will first require major reductions to the external P loads from tributaries, followed by manipulations to reduce internal P loading from sediments.