RECENT CHANGES OF N-2-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 annu al total N and total P concentrations and changes in phytoplankton spe cies composition. Summer phytoplankton occasionally shifted from N-2-f ixing cyanobacteria to non-N-2-fixing forms. In 1990, additions of N f ertilizer were stopped in order to test the hypothesis that low N:P lo ading rates cause selection for heterocystous cyanobacteria and stimul ate N-2-fixation. Five years of phytoplankton nutrient-status measurem ents, together with estimates of N-2-fixation before and after lowerin g the N:P loading ratio, verified the importance of this ratio in prod ucing blooms of N-2-fixing cyanobacteria. Nitrogen input via biologica l N-2 fixation increased, and summer dominance by heterocystous cyanob acteria returned as a consequence of lowering the N:P ratio of externa l inputs. Seasonally, N return from the epilimnetic sediments increase d after a period of high planktonic N-2-fixation rates. This regenerat ed N reduced the abundance of heterocystous cyanobacteria and lowered late summer N-2-fixation rates. Over several years, preferential regen eration of N relative to P maintained the N:P ratio of the internally cycling nutrient pool, despite the reduction in the external N:P loadi ng ratio.