Do Baltic Sea diazotrophic cyanobacteria take up combined nitrogen in situ?

We used nitrogen stable isotopes to study the regulation of nitrogen fixation by filamentous cyanobacteria. Nitrogen fixation was found to be almost insensitive to combined nitrogen, along a gradient from the Himmerfjarden sewage treatment plant discharge to the open sea. We found similarly low cyanobacterial (mostly Aphanizomenon sp.) delta N-15-values at all stations, despite significant differences along the bay in both total nitrogen concentrations in water and delta N-15 in seston (particles <10 mu m), the latter used as a proxy for algae growing on combined nitrogen alone. Only late in the productive season, when cyanobacterial biomass was declining or already low, did elevated delta N-15 suggest uptake of combined nitrogen. However, this coincided with an increase in the contribution of Dolichospermum spp. to overall diazotrophic biomass and may indicate uptake of combined nitrogen by this species. These results indicate that almost all nitrogen used for growth by nitrogen-fixing cyanobacteria in the study area comes from nitrogen fixation, and very little from uptake of dissolved combined nitrogen. This study was part of a whole ecosystem experiment analyzing the effects of nitrogen removal in a sewage treatment plant discharging to the Himmerfjarden Bay, northern Baltic Sea Proper.