Abstract
AbstractIn parts of the Baltic Sea, the phytoplankton spring bloom communities, commonly dominated by diatoms, are shifting toward the co‐occurrence of diatoms and dinoflagellates. Although phytoplankton are known to shape the composition and function of associated bacterioplankton communities, the potential bacterial responses to such a decrease of diatoms are unknown. Here we explored the changes in bacterial communities and heterotrophic production during the spring bloom in four consecutive spring blooms across several sub‐basins of the Baltic Sea and related them to changes in environmental variables and in phytoplankton community structure. The taxonomic structure of bacterioplankton assemblages was partially explained by salinity and temperature but also linked to the phytoplankton community. Higher carbon biomass of the diatoms Achnanthes taeniata, Skeletonema marinoi, Thalassiosira levanderi, and Chaetoceros spp. was associated with more diverse bacterial communities dominated by copiotrophic bacteria (Flavobacteriia, Gammaproteobacteria, and Betaproteobacteria) and higher bacterial production. During dinoflagellate dominance, bacterial production was low and bacterial communities were dominated by Alphaproteobacteria, mainly SAR11. Our results suggest that increases in dinoflagellate abundance during the spring bloom will largely affect the structuring and functioning of the associated bacterial communities. This could decrease pelagic remineralization of organic matter and possibly affect the bacterial grazers communities.
Highlights
The Baltic Sea is suffering from a progressive increase in surface water temperature, which has been suggested to alter the food web structure, favoring small flagellates and dinoflagellates 1
The NO2-+NO3- was depleted after the bloom with the exception of the P-limited BoB, where the NO2-+NO3- concentration was similar to the stations in the growth phase (Growth) phase and phosphate was low
The dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) were highest in the Archipelago Sea (ArS) in 2014, and higher in the Gulf of Finland (GoF) and the Gulf of Bothnia compared with the BP
Summary
The Baltic Sea is suffering from a progressive increase in surface water temperature, which has been suggested to alter the food web structure, favoring small flagellates and dinoflagellates 1. During the last decades, shifting phytoplankton spring bloom communities from diatom-dominated blooms towards higher abundances of dinoflagellates have been reported in some subbasins of the Baltic Sea milder winters structure and functioning of the associated bacterioplankton communities are still poorly understood. The nature and the timing of the spring bloom in the Baltic Sea vary between sub-basins 6. The bloom reaches the peak at the time when inorganic nutrients have been depleted; N-limitation prevails in most of the Baltic Sea except for the P-limited Bay of Bothnia sinking of the phytoplankton cells 10.
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