Bioassay analysis of nutrient and Artemia franciscana effects on trophic interactions in the Great Salt Lake, USA

Abstract

14-day microcosm experiments demonstrated the strong interactions between bottom–up and top–down effects of nutrient addition (control, nitrogen, phosphorus, nitrogen + phosphorus) and Artemia franciscana grazing on algae in Great Salt Lake water from Gilbert Bay. Nitrogen addition increased phytoplankton chlorophyll concentrations, while phosphorus addition had no stimulatory effect. A combined N + P treatment was synergistic, increasing both phytoplankton and periphyton >10-fold above controls. Our results suggest that phytoplankton were primarily limited by nitrogen and secondarily limited by phosphorus and that periphyton was colimited by nitrogen and phosphorus. The grazing effect increased as A. franciscana grew from nauplii to adults and by the final day, A. franciscana had markedly reduced both phytoplankton and periphyton abundance in the Control, +N, and +P treatments. Grazing also significantly reduced periphyton in the N + P treatments. Due to high phytoplankton growth rates in the N + P treatment, A. franciscana grazing did not significantly reduce chlorophyll concentrations during the bioassay. However, A. franciscana in the N + P treatment was significantly larger and had greater reproductive output than in the controls, suggesting that the following generation might have exerted greater grazing pressure.