Nutrient control of cyanobacterial blooms in the Baltic Sea

Abstract

Cyanobacterial blooms in the Baltic Sea were investigated with respect to growth limitation and nitrogen fixation. The community was composed predominantly of Synechococcus spp., and large, heterocystous, nitrogen-fixing cyanobacteria (Aphanizomenon spp. and Nodularia spp.), that usually formed buoyant macroscopic aggregates. Although conspicuous, these aggregates often represented less than 20 to 30% of the total chlorophyll a. Nitrogenase activity was not limited by molybdate availability, but, instead, by high concentrations of sulfate. This may explain inhibition of nitrogenase activity at high salinities. Inhibition of nitrogenase activity at high salinity did not occur when sulfate concentration was kept low. Nitrogen fixation and growth of the diazotrophic cyanobacteria were limited by iron. Synechococcus spp. was primarily nitrogen limited but iron appeared to be the secondary limiting substrate, particularly when these organisms depended on nitrate as the source of nitrogen. Nutrient limitation of the picoplanktonic community was particularly apparent when a wind-induced mixing event occurred. These organisms responded by a subsequent doubling of their biomass within 24 h. Mixing of the water column apparently transported nutrients from greater depth into the euphotic zone, causing a temporary relieve of nitrogen limitation.