Does excessive phosphorus necessarily cause increasing biomass of diazotrophic cyanobacteria?

Abstract

The influence of increasing phoshporus (PO[SUB4]-P) on the biomass and composition of diazotrophic (N[SUB2]-fixing) cyanobacteria was evaluated on the ground of 6-year (1997-2002) intensive monitoring data, collected from the ship route between Tallinn and Helsinki (central Gulf of Finland). Phytoplankton biomass estimations were performed weekly from 15 June to 31 July to obtain the magnitude of cyanobacterial dominance, and the results were related to inorganic nutrient residual concentrations left from spring bloom. We could not find any strong relationships between cyanobacterial biomass and pre-bloom PO[SUB4]-P concentrations or the N: P ratio. In 1997 and 2002, the years of exceptional summer blooms, the mass development of Aphanizomenon flos-aquae was likely a response to short-time nutrient pulses accompanied with upwelling and/or other hydrodynamical forcing. Nodularia spumigena formed only local short-time blooms in 1999 and 2001, when the phosphorus concentration was at its lowest (<0.1 μM). It is the favourable nutrient conditions preceding the most intensive development of cyanobacterial bloom that determine the magnitude of biomass growth rather than the situation after the spring bloom decline. [ABSTRACT FROM AUTHOR]