Internal Phosphorus Loading and Potential Phytoplankton Availability in North Pipe Lake, Wisconsin

Abstract

North Pipe Lake, located in Polk County Wisconsin, is a small (66 acres), deep (11-m), dimictic lake that suffers from excessive algal growth. Because of this, the lake was recently added to the Wisconsin’s Impaired Waters List under the Federal Clean Water Act, Section 303(d). In 2019 a monitoring program was conducted to investigate the impact internal loading mechanisms are having on North Pipe Lake’s high algal population and degraded water quality. Throughout monitoring the lake exhibited strong stratification and hypolimnetic anoxia, leading to increased P and Fe concentrations below the metalimnion. By October, hypolimnetic P and Fe concentrations had increased to 0.498 mg/L and 6.09 mg/L respectively. However, even though hypolimnetic nutrient accumulation was high, it is unlikely that entrainment of hypolimnetic P into the epilimnion would occur due to the high Fe:P ratio (>3.6:1 molar) that persisted for the entirety of monitoring. Although fall turnover was never observed, the high hypolimnetic Fe:P ratio (>8:1 in October), would completely adsorb and precipitate the accumulated P pools during the chemical oxidation of the hypolimnion. This indicates that physical entrainment of internal P loading from bottom sediment was not a significant contributing factor to epilimnetic P concentrations. Phytoplankton vertical migration into the hypolimnion for P uptake and growth is still a possible recycling pathway that needs to be explored. Otherwise, it is likely that external loading of P from watershed runoff is the highest contributing factor to the increased algal blooms in North Pipe Lake.