Nutrient Availability for Phytoplankton Production in a Multiple-Impoundment Series

We investigated the availability of nitrogen (N) and phosphorus (P) for phytoplankton production and the importance of inter-reservoir nutrient transfers in a headwater-to-mainstream series of three hypolimnetic-discharge impoundments during June–December 1982. Dissolved nutrient analyses and enrichment experiments indicated that P, rather than N, was the primary limiting nutrient for plankton growth throughout the reservoir series. Phytoplankton P deficiency, as reflected by chlorophyll-specific alkaline phosphatase activity (APA), peaked in late August, and then decreased during the fall and winter. Late in the growing season, APA levels increased downstream within each reservoir, reflecting uplake-to-downlake decreases in P availability, but decreased downstream from reservoir to reservoir, indicating increased P availability down the impoundment series. In this reservoir series, hypolimnetic releases of water and nutrients from upstream impoundments during the growing season appear to enhance nutrient availability for phytoplankton production in impoundments downstream. This result suggests that the productivity of reservoirs located in multiple-impoundment series should be viewed in the context of the limnological dynamics resulting from river basin-level reservoir operations.