The Effect of Single Versus Dual Nutrient Control on Phytoplankton Growth Rates, Community Composition, and Microcystin Production in the Western Basin of Lake Erie
Dr. Thomas Johengen, Chair
Dr. Gregory Dick
Dr. Timothy Davis
The primary management strategy for minimizing harmful algal blooms (HABs) in Lake Erie has been to reduce springtime loading of phosphorus (P) to the lake. However, some studies have shown that the growth rate and yeild, particularly the HABs-causing cyanobacterium Microcystis, also respond to the availability of inorganic nitrogen (N). However, this evidence is based on either observational studies that correlate bloom development with changes in N concentrations in the lake, or experiments in which P and and/or N are added at concentrations in excess of those present in the lake. The goal of this study was to determine directly whether a combination of reduced N and P from ambient levels in Lake Erie could limit the development of HABs more than a reduction in P concentration only. To directly test the impact of P-only versus N and P reduction on phytoplankton in the western basin of Lake Erie, we evaluated changes in growth rate, community composition, and microcystin (MC) concentration through eight bioassay experiments performed from June through October, which included the normal HAB season. Our results showed that during the first five experiments covering June 25 to August 13, the –P and the –N-P treatment had similar effect, but later in the season when ambient N becomes scarce, the –N-P reductions resulted in negative growth rates for cyanobacteria whereas –P only reductions did not. During low ambient N conditions, dual nutrient reduction lowered the prevalence of cyanobacteria among the total phytoplankton community and decreased microcystin concentrations. The results presented here suggest that dual nutrient control could be an effective management strategy to decrease microcystin production during the bloom and even possibly diminish or shorten the bloom based on creating nutrient limiting conditions sooner in the HAB growing season.
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