Phytoplankton nutritional ecology in a piedmont reservoir

Abstract

Many potable source-waters in the U.S. are sustaining cultural eutrophication from elevated nutrient supplies concomitant with major shifts in nutrient ratios away from healthy conditions as indicated by Redfield proportions (16:1, molar). Falls Lake is a representative eutrophic impoundment (reservoir; length ~38 km, width ~0.2-2.9 km) in a rapidly urbanizing watershed in the southeastern U.S. The shallow upper reservoir is especially prone to algal blooms in comparison to the deeper lower reservoir (mean depth 5 m and 14 m, respectively). Using a long-term water quality dataset provided by the Center for Applied Aquatic Ecology (CAAE; 2011-2019, biweekly to monthly samples), we compared the upper and lower reservoir over time for nitrogen (N) and phosphorus (P) nutrient regimes, phytoplankton biomass (as chlorophyll a [chla] concentrations), and phytoplankton assemblage composition during bloom conditions defined as > 40 µg chla/L, the state water quality standard. In the upper reservoir, TP and inorganic N (Ni) were chronically elevated and chla commonly exceeded the state standard, with noxious cyanobacteria dominant in cell number. In contrast, the lower reservoir was generally characterized by moderate nutrient regimes except for elevated NH4 + (up to ~400 µg/L) in surface waters during fall-winter. Short-term experiments were conducted to gain further insights about assemblage responses to changing nutrient regimes during two summers that were planned to be replicate seasons; however, one summer had average precipitation (considering the past decade; 127 mm) whereas the other had much higher precipitation (170 mm). Microcosm experiments in situ (duration, 5 days) were used to assess reservoir phytoplankton assemblage responses to inorganic N form (Ni) + inorganic P (Pi) enrichment, and to Ni:Pi ratios. The abundance and composition of phytoplankton functional groups were similar reservoir-wide in both summers, except for higher relative abundance of cyanobacteria in the upper region under average precipitation. In both summers, there was a positive relationship between chla and Ni concentrations (both forms), with or without Pi enrichment, and a stronger relationship between chla and NH4 + than between chla and NO3 - . Also in both summers, the eutrophic lower region assemblage responded more strongly to nutrient enrichment. Maximal final phytoplankton biomass as chla was attained, for the upper region assemblage, with Ni + Pi enrichment as NO3 - . For the lower region assemblage, maximum chla occurred with Ni + Pi enrichment as NH4 + . The toxigenic cyanobacterium Cylindrospermopsis raciborskii was the most abundant taxon initially reservoir-wide; it was stimulated by enrichment with either Ni form, especially along with Pi enrichment. Similar responses in the two summers likely occurred because the precipitation/dilution/washout regimes would only have affected the phytoplankton before they were placed into closed microcosms in the experiments. The precipitation differential during the two summers, although substantial, probably was not enough to cause major changes in these resilient reservoir. Overall, this study contributed species-level insights about seasonal influences of chronic cultural eutrophication on reservoir phytoplankton blooms. The findings indicate that reservoir assemblages are well-adapted to variable precipitation/hydrologic changes, and that cyanobacteria will continue to be favored under warming temperatures and high N/P enrichment. This study also supports co-managed reductions of both N and P supplies, as in the Falls Lake Rules, to minimize noxious algal blooms in this important, representative potable source-water reservoir of the North Carolina Piedmont.