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

Abstract

Summary 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.

Differences in phytoplankton dynamics and community structure between a wet year and dry year in the Zhoucun Reservoir

“…Previous studies have suggested that N-fixing cyanobacteria can outcompete non-N-fixing cyanobacteria only under sufficient P levels (Piehler et al 2009;Vahtera et al 2010). The concentration of SRP was 2À8 mg/L in the summer of 2014 in the Zhoucun Reservoir and may be inadequate for an N-fixing cyanobacteria bloom (Vrede et al 2009). …”

Human-induced nitrogen–phosphorus imbalances alter natural and managed ecosystems across the globe

“…The changes in community composition in response to altered N-P availability by nitrogen enrichment can result from multiple processes. These are some reported examples: First, the N 2 -fixing community loses its competitive advantage over other species and decreases in abundance 39 . Second, increased N:P availability ratio in soils and waters favours the slow-growing species with high optimal N:P ratios at the expense of the fast-growing species with lower optimal N:P ratio 34 .…”

Effects of nutrients and physical lake characteristics on bacterial and phytoplankton production: A meta‐analysis

“…Research has shown that in areas with low atmospheric N deposition (such as at high latitudes) N strongly limits PPr, especially in unproductive lakes (Vitousek and Howarth 1991;Bergström and Jansson 2006;Elser et al 2009). In productive lakes N fixation by cyanobacteria can potentially compensate for N deficiency, as N-fixing cyanobacteria often become dominant at low N : P ratios, thus alleviating N limitation of PPr (Vrede et al 2009). However, in nutrient-rich lakes, N limitation is also common (Downing and McCauley 1992;Elser et al 2007), and in some cases N fixation may not totally compensate for N limitation of PPr (Scott and McCarthy 2010).…”