Effects of N: P loading ratios on phytoplankton community composition, primary production and N fixation in a eutrophic lake

1. The aim of this study was to assess the effects of different nitrogen (N) to phosphorus (P) loading ratios on phytoplankton community composition and primary production in a naturally eutrophic lake. Furthermore, the sources of N fuelling primary production were estimated using ¹⁵N stable isotope tracers. 2. A mesocosm experiment was performed with the same amount of P added to all mesocosms (similar to internal loading rates) but with a range of N additions (0-86 μm N), resulting in a gradient of N : P supply ratios. 3. Low N : P supply ratios resulted in a significant shift in the phytoplankton assemblage to a community dominated by N-fixing cyanobacteria and a supply of atmospheric N₂ estimated to be up to 60% of total supply. 4. The N : P loading ratio had no significant effect on primary production, total nitrogen (TN) concentration or particulate N concentration. 5. Our results imply that a reduced N : P ratio of the nutrient load does not necessarily result in a lower TN concentration and downstream N export due to compensation by N-fixing cyanobacteria.