The role of DON and the effect of N:P ratios on occurrence of cyanobacterial blooms:: Implications from the outgrowth of Aphanizomenon in Lake Kinneret

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.