Recent Changes of N₂-Fixation in Lake 227 in Response to Reduction of the N:P Loading Ratio

Continuous artificial additions of nitrogen (N) and phosphorus (P) to Lake 227 at a loading ratio of 11:1 (atom: atom; 5:1 by mass) from 1975 to 1989, combined with natural inputs, resulted in variable mean annual total N and total P concentrations and changes in phytoplankton species composition. Summer phytoplankton occasionally shifted from N 2 -fixing cyanobacteria to non-N 2 -fixing forms. In 1990, additions of N fertilizer were stopped in order to test the hypothesis that low N:P loading rates cause selection for heterocystous cyanobacteria and stimulate N 2 -fixation. Five years of phytoplankton nutrient-status measurements, together with estimates of N 2 -fixation before and after lowering the N:P loading ratio, verified the importance of this ratio in producing blooms of N 2 -fixing cyanobacteria. Nitrogen input via biological N 2 fixation increased, and summer dominance by heterocystous cyanobacteria returned as a consequence of lowering the N:P ratio of external inputs. Seasonally, N return from the epilimnetic sediments increased after a period of high planktonic N 2 -fixation rates. This regenerated N reduced the abundance of heterocystous cyanobacteria and lowered late summer N 2 -fixation rates. Over several years, preferential regeneration of N relative to P maintained the N:P ratio of the internally cycling nutrient pool, despite the reduction in the external N:P loading ratio.