Abstract
Supply (or loading) ratios of biologically available nitrogen and phosphorus, N: P, have often been suggested as the major determinants for the presence or absence of N 2 fixing cyanobacteria in aquatic environments. Increasing evidence that some components of the dissolved organic nitrogen (DON) pool can play an active role in supplying N nutrition either directly or indirectly to phytoplankton implies that this source of N must be considered in any attempt to apply the N: P resource ratio approach to predict or explain phytoplankton population composition. For example, the unprecedented bloom of Aphanizomenon ovalisporum that occurred in Lake Kinneret from mid‐September through October 1994 derived most of the N required for growth directly or indirectly from DON rather than from N 2 ‐fixation. This would suggest that factors other than apparent low N: P ratios were important in causing the outgrowth of the cyanobacteria. The present analysis of the Aphanizomenon bloom in Lake Kinneret emphasizes (1) the need to include the DON pool as a potential source of available N for planktonic microbiota; (2) at least for some cyanobacteria, the presence of heterocysts does not necessarily imply active nitrogen fixation; and (3) the development of diazotrophic cyanobacterial blooms in nature is generally due to a multiplicity of environmental factors.