Over recent decades, the Bothnian Sea has had an increase in filamentous cyanobacteria abundance. This project will focus on the effect on the nitrogen cycling in the region since these organisms potentially perform high rates of nitrogen fixation which counteract eutrophication mitigation strategies.
Dense blooms of filamentous cyanobacteria occur annually in the Baltic Proper, while the Bothnian Sea has been overlooked in this regard. However, with climate change already in place, bloom formations are now increasingly common.
The project will perform the first ever nitrogen fixation measurements in the Bothnian Sea by using a combination of single-cell measurements (Secondary Ion Mass Spectrometry; SIMS) and bulk measurements (Elemental Analyzer Isotope Ratio Mass Spectrometry; EA-IRMS) to decipher the relative contribution by the taxa living here. In addition, ammonium and nitrate measurements will be performed, and nitrogen utilization by surrounding heterotrophic bacteria will be traced using nanoSIMS. The experimental design is layouted in the schematic below.
Schematic drawing of the experimental design during stable isotope incubations in the field.
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We will also use molecular tools (nifH-gene abundance, metagenomics, and metatranscriptomics) to gain understanding in the diversity of nitrogen fixers in the region and the adaptation strategies by the filamentous strategies in the changing environment.
The outcome of the project will be highly relevant for policy makers that can use this knowledge when setting the nitrogen budget and suggesting limits on nitrogen loads. The results will later on also be used in collaboration with the Swedish Meteorological and Hydrological Institute (SMHI) where biogeochemical models can be validated by the measurements and monitoring observations.
Cyanobacteria in the Baltic Sea
The project is funded by ÅForsk Repatriation Grant 2021 and BalticWaters2030 during 2022-2024.