Nutrient availability for phytoplankton production along a headwater-to-mainstream reservoir gradient

The occurrence of phytoplankton nutrient deficiency in a headwater-to-mainstream series of three reservoirs was examined using analyses of dissolved nutrients, nutrient enrichment experiments and an alkaline phosphatase bioassay. Size-fractionation of APA showed it to be primarily (75%) associated with >1.0-µm particles, and enzyme suppression experiments demonstrated that, while APA was suppressed by phosphorus enrichment, a residual amount was insensitive to elevated P0₄-P. Nutrient analyses and nutrient enrichment experiments indicated potential phosphorus limitation of phytoplankton growth in the series, but comparisons of nutrient analyses, enrichment experiments, and APA patterns emphasized the importance of multiple limiting factors in situ. Specific APA increased moving downlake within individual reservoirs in a manner consistent with the river-lake hybrid analogy. A pattern of decreasing phosphorus deficiency moving downstream in the series agreed with predictions based on possible effects of hypolimnetic releases on downstream nutrient availability, but other factors may have contributed to this pattern.