Nitrogen inputs enhance phytoplankton growth during sediment resuspension events: a mesocosm study

Abstract

Nitrogen (N) and phosphorus (P) are key factors that limit phytoplankton growth in freshwater lakes. Controlling external P loading is a widely recognized measure for reducing eutrophication, but the usefulness of limiting N inputs remains under debate. Shallow lakes with frequent sediment resuspension events exacerbate internal P loading and limit the potential for in-lake P reduction. A two-way factorial mesocosm experiment was carried out using two levels of N inputs (with and without addition of potassium nitrate as N source) and sediment resuspension (with and without resuspension) for 35 days in order to examine the effect of N inputs on phytoplankton growth during sediment resuspension events. N addition alone did not affect phytoplankton biomass in mesocosms in treatments without sediment resuspension, likely due to the P limitation. However, internal P loading, via sediment resuspension, shifted mesocosms from P- to N-limited systems in which N addition significantly boosted phytoplankton biomass. Our results suggested that external loading comprising N alone may enhance phytoplankton growth in shallow, eutrophic lakes with frequent resuspension events like Lake Taihu, China. In such lakes, P-only management is unlikely to usefully reduce eutrophication, and a strategy to reduce both N and P is needed.