Interactive effects of N:P ratios and light on nitrogen‐fixer abundance

Nitrogen‐fixers can contribute significant amounts of nitrogen (N) and impact ecosystem functioning in diverse aquatic and terrestrial ecosystems. What determines N‐fixer abundance still remains poorly understood. Here we experimentally investigate major environmental controls on the abundance of N‐fixers: nitrogen to phosphorus (N:P) ratio and light. We grew a N‐fixer, cyanobacterium Anabaena flos‐aquae, in a multispecies community of freshwater phytoplankton in replicated factorial design treatments with two N:P ratios and two light levels. We show that low N:P ratios promote the dominance of the N‐fixer in the community, but only under high light. Under low light, N:P ratio did not have a significant effect on the abundance of the N‐fixer. N fixation occurred at low N:P only and increased with increasing light. In contrast, the density of non N‐fixing cyanobacteria did not depend on N:P ratios. Green algae dominated under high N:P and high light only, exhibiting the opposite pattern of dominance to N‐fixers. These results are consistent with patterns observed in nature and help explain the N‐fixer distribution along the environmental gradients of nutrients and light.