Long-term phosphorus reduction and phytoplankton responses in an urban lake(USA)

Eutrophication is one of the primary factors causing harmful cyanobacteria blooms in freshwater lakes.This study investigated the long-term changes in water quality and summer phytoplankton assemblages in Oswego Lake,OR,USA,in relation to phosphorus reduction through hypolimnetic aeration and alum applications.Both water quality and phytoplankton assemblages were sampled biweekly during the summers from 2001 to 2013.The concentrations of total phosphorus,soluble reactive phosphorus,and total nitrogen decreased 66%,93%and 31%,respectively,in response to the hypolimnetic aeration and alum treatments since 2005.Summer phytoplankton assemblages showed a 62%reduction of cyanobacteria biovolume and a shift from cyanobacteria dominance(2001–2005)to diatom and chlorophyte dominance(2006–2013).Cluster analysis identified four statistically different groups of summer phytoplankton assemblages(denoted Groups 1–4).Nonmetric multidimensional scaling analysis indicated that the four groups were associated with different water quality conditions.Group 1 occurred prior to hypolimnetic aeration and was primarily comprised of cyanobacteria,associated with water conditions of high nutrients and high primary production.Group 2,dominated by cyanobacteria and chlorophytes,occurred between hypolimnetic aeration and alum surface application.Group 3 was dominated by diatoms and occurred after alum surface application.Group 4 included R-strategist phytoplankton that quickly respond to environmental changes and occurred in the years following alum injection,drawdown,and inflow alum treatment.Both Group 3 and 4 were associated with reduced nutrients in the lake.We conclude that these lake management practices had strong effects on both production and community compositions of phytoplankton,and advocate for future studies on large-scale climate impacts on lake ecosystems and to identify corresponding best management practices.