Sea ice bacterial growth rate, growth efficiency and preference for inorganic nitrogen sources in the Baltic Sea

Seasonal Baltic Sea ice is structurally similar to polar sea ice and provides habitats for diverse ice organism assemblages that are integral to the biogeochemistry and ecology of the sea during winter. Temperature and inorganic nitrogen sources have been suggested to control bacterial growth, with increasing dependence on ammonium at low temperatures. To study the bacterial growth and preference for the nitrogen source, we conducted experiments at 0 and 4°C, using ammonium and nitrate as nitrogen sources at two coastal fast-ice stations in the Gulf of Finland and in the Gulf of Bothnia during three successive winters. The two study sites differ markedly in relation to the allochthonous dissolved organic matter supply from the catchment area. High levels of bacterial growth were recorded at both study sites, with community generation times of 15–37 h. The measured bacterial growth efficiencies of 20–58% suggest that the Baltic sea ice brines provide a rich medium for bacterial growth and efficient functioning of bacteria-based food webs. Our experiments with sea ice samples showed a preference for ammonium at both temperatures and high potential growth in both types of nitrogen supplies. No major differences in phosphorus depletion rates were found at the two temperatures, but rates were always highest when ammonium was added to the experiments. These experiments point out that ice maturity, presumably through changes in bacterial community structure, impacts nitrogen processes and that these processes are pronounced prior to melting of the ice.