Effect of reducing allochthonous P load on biomass and alkaline phosphatase activity of phytoplankton in an urbanized watershed, Michigan

Lehman JT, Doubek JP, Jackson EW. 2013. Effect of reducing allochthonous P load on biomass and alkaline phosphatase activity of phytoplankton in an urbanized watershed, Michigan. Lake Reserv Manage. 29:116–125. Phosphorus (P) concentrations have declined in the Huron River in southeast Michigan subsequent to first a municipal ordinance, and later state legislation, restricting the application of commercial fertilizers containing phosphate. One management objective of the environmental policy was to reduce eutrophication of surface waters and nuisance blooms of cyanobacteria, particularly in large impoundments along the Huron River. Alternative management theory contended that nuisance algal conditions resulted from internal loading of P within the lakes rather than the allochthonous nutrient load. In summer 2012 during conditions of relative drought, we had an opportunity to test whether reduced river P was intensifying P limitation among phytoplankton in impoundments receiving the river water. We compared chlorophyll concentrations and alkaline phosphatase activity measured in 2012 to measurements at the same sites obtained using identical methods in 2004 and 2005, before P reductions were implemented. A large bloom of Aphanizomenon developed in 2012 after a polymictic episode mixed high concentrations of P from the anoxic hypolimnion throughout the lake. Algal biomass was higher and alkaline phosphatase activity levels in 2012 were significantly lower at all sites than in 2004 and 2005, despite reduced supply rate of riverine P. The results confirm that reducing nonpoint source P runoff in the upstream catchment alone is not an efficacious management strategy for these lakes.