Early trophic responses to nutrient addition in Dworshak Reservoir, Idaho

Abstract

ABSTRACTWilson SM, Brandt DH, Corsi MP, Dux AM. 2017. Early trophic responses to nutrient addition in Dworshak Reservoir, Idaho. Lake Reserv Manage. 34:58–73.Dworshak Reservoir, Idaho, responded favorably across multiple trophic levels after 4 yr of nutrient (primarily ammonium nitrate) additions. Nitrogen (N) and phosphorus (P) concentrations in the epilimnion did not increase, suggesting rapid biological uptake. Secchi depth was not significantly lower during the treatment period and chlorophyll a concentrations were unchanged. At the lowest trophic level, densities of heterotrophic bacteria increased by 109% and pico-cyanobacteria increased by 60% for the treatment period. While we did not observe a significant increase in the mean biovolume of phytoplankton, the proportion of Dolichospermum, an inedible taxa, in the phytoplankton community declined by 74%, coupled with a 94% increase in the proportion of the phytoplankton biovolume that was edible to zooplankton. The mean density of Daphnia for the treatment period was 70% higher than the untreated period. In the fourth year of treatment, kokanee (Oncorhynchus nerka) biomass increased to 233% of the untreated mean and 143% of the highest untreated estimate. Our observations are consistent with reports from similar projects throughout British Columbia, Alaska, and Scandinavia, which suggests ammonium nitrate enrichment should be further explored as a management strategy for mitigating oligotrophication in N limited waters.