Abstract
In the Baltic Sea proper, N2-fixation contributes with a yearly N input larger than the entire riverine load. Summer blooms by filamentous, N2-fixing cyanobacteria are dominated by Nodularia sp., Aphanizomenon sp., and Anabeana sp. in the Baltic Sea. Aphanizomenon sp. blooms 2 to 3 months each summer. Using novel technology (combining nansocale secondary ion mass spectrometry, elemental analysis mass spectrometry, and microsensors) we recently demonstrated that this cyanobacterium may contribute between 50-100% of total yearly N2-fixation in the Baltic Sea of which at least one third is directly released as NH4+ to the surrounding water during N2-fixation. This cyanobacterium may, thus, supply other phytoplankton species and heterotrophic organisms with a substantial fraction of their N-demand during the whole summer season in the Baltic Sea. Other (so far unknown) organisms appeared to dominate N2-fixation and NH4+ release during darkness (Ploug and Musat et al., in press.). Using this forceful combination of novel high-technology, the present project aims at identifying the organisms responsible for N2-fixation and N-release in the Baltic Sea. Small-scale C- and N-fluxes associated with the different N2-fixing microorganisms in the Baltic Sea will be quantified during bloom and decay, and their respective contributions to overall C- and N-fluxes will be estimated in the Baltic Sea.
Sweden