26In the Baltic Sea, climate change has caused shifts in the phytoplankton spring bloom 27 communities with co-occurrence of diatoms and dinoflagellates. Such changes likely affect the 28 composition and function of associated bacterioplankton, key members of the carbon cycling, 29 although the actual effects are unknown. To understand how changes in phytoplankton impact 30 on bacterioplankton composition and function, we analysed bacterioplankton communities and 31 their production during different phases of the spring bloom in four consecutive years across 32 the Baltic Sea, and related them to environmental variables. Phytoplankton communities varied 33 largely in composition, modifying the taxonomic structure and richness of the associated 34 bacterioplankton assemblages. In presence of certain diatoms (Achnanthes taeniata, 35 Skeletonema costatum and Chaetoceros spp.), bacterial production and diversity were high and 36 with more relative abundance of Flavobacteriia, Gammaproteobacteria and Betaproteobacteria. 37This bacterial community structure correlated positively with high diatom biomass and with 38 high bacterial production rates. In contrast, during dinoflagellate-dominated blooms or when 39 the diatom Thalassiosira baltica was abundant, both bacterial production rates and diversity 40 were low, with bacterial communities dominated by SAR11 and Rhodobacteraceae. Our results 41 demonstrate that, changes in the phytoplankton spring bloom will have profound consequences 42 on bacterial community structure and their role in carbon cycling. 43 44 45 46 47 48 49 50 51The Baltic Sea is suffering from a progressive increase in surface water temperature, which has 52 been suggested to alter the food web structure, favoring small flagellates and dinoflagellates 1 . 53During the last decades, shifting phytoplankton spring bloom communities from diatom-54 dominated blooms towards higher abundances of dinoflagellates have been reported in some 55 subbasins of the Baltic Sea 2-4 , likely due to the increase in frequency of milder winters 1,5 . 56However, the consequences of these changes in spring blooms on the structure and functioning 57 of the associated bacterioplankton communities are still poorly understood. 58The nature and the timing of the spring bloom in the Baltic Sea vary between sub-basins 6 . 59The bloom typically starts with the light promoting net primary production in the southernmost 60 Baltic Sea in February/March, reaching the Gulf of Finland in April, and the Gulf of Bothnia 61 in May (Map in Fig. 1). The bloom reaches the peak at the time when inorganic nutrients have 62 been depleted; N-limitation prevails in most of the Baltic Sea except for the P-limited Bay of 63 Bothnia 7-9 . The subsequent decline phase is characterized by rapid sinking of the 64 phytoplankton cells 10 . 65 Diatoms and dinoflagellates have different ecological features, as diatoms grow faster than 66 co-occurring dinoflagellates 11 , they also differ in terms of quality and quantity of dissolved 67 organic matter (...