Managing internal phosphorus loading and vertical entrainment in a weakly stratified eutrophic lake

Abstract

Unsuccessful control of internal phosphorus (P) loading in Cedar Lake, Wisconsin, via artificially induced destratification led to reexamination of management strategies and linkages between profundal sediment diffusive P flux, P entrainment, and cyanobacterial bloom development. The destratification system, operated since 1991, was suspended in 2009-2010 and turned on in 2011 to evaluate P and phytoplankton dynamics under stratified and destratified conditions. In 2009-2010, the lake was susceptible to wind-induced mixing and P exchange into the epilimnion due to its shallow morphometry, long fetch, and low Osgood Index (2.7). Anaerobic diffusive P flux from bottom sediment was high at 12 mg/m2/d. While hypolimnetic soluble reactive P (SRP) exceeded 0.8 mg/L, dissolved iron (DFe) was low, resulting in a low DFe:SRP mass ratio of 0.9:1 despite high sediment Fe concentrations. Incomplete Fe binding control of P during entrainment events may have been due to Fe reaction to an inert form (chelation with organic carbon and reaction with sulfur) in sediment and removal from recycling. High anaerobic diffusive P flux coupled with entrainment resulted in potentially toxin-producing cyanobacteria blooms in excess of 50-100 g/L chlorophyll that were sustained from late August through October. Operation of the destratification system in 2011 did not prevent anoxia and actually exacerbated diffusive P flux, resulting in >0.1 mg/L SRP in the epilimnion and nitrogen-limited phytoplankton growth. Fe-P linkages and Fe:P ratio stoichiometry need to be considered in management and control of internal P loading. For Cedar Lake, internal P loading management should consider application of materials that can irreversibly bind sediment P instead of destratification.