Spring phytoplankton blooms contribute significantly to global marine primary production. A large fraction of the bloom derived organic matter is available to heterotrophic bacteria in the form of polysaccharides. We analyzed changes in the modes of polysaccharide utilization (selfish uptake and extracellular hydrolysis) during a spring phytoplankton bloom using fluorescently labelled polysaccharide incubations coupled with 16s rRNA sequencing and fluorescence in situ hybridization. We found that in the early bloom phases there was high selfish activity of simple polysaccharides (laminarin) and low extracellular hydrolysis rates of a limited range of polysaccharides. During the course of the bloom both the selfish uptake and extracellular hydrolysis rates increased but only for a limited range of substrates. At the late bloom phase a wide range of substrate was extracellularly hydrolyzed and the level of selfish uptake decreased. We found that during a spring phytoplankton bloom the mode of substrate utilization depended on both the substrates structural complexity and the composition of the heterotrophic community related to the bloom phase.