Factors Terminating the Subarctic Spring Bloom : An Effect of Temperature, Nutrients or a Combination of Both?

The phytoplankton spring bloom is a key ecological event in the Baltic Sea, providing primary produced biomass that supports higher trophic levels and influences nutrient and carbon cycling. However, the mechanisms that govern spring bloom termination remain unclear. This study investigates the effects of temperature, nutrients (cell extract), and their interaction on the termination of the spring bloom. A mesocosm experiment simulating Baltic Sea conditions was conducted with four treatments: low and high temperatures (1°C and 10°C), with and without the addition of labile organic carbon and associated nutrients (cell extract). The results showed that elevated temperature significantly reduced phytoplankton biomass and diatom abundance, while simultaneously promoting bacterioplankton growth and activity. The addition of cell extract had a stronger inhibitory effect under low-temperature conditions, likely due to intensified competition for inorganic nutrients between phytoplankton and bacterioplankton. While temperature emerged as the dominant factor in bloom termination, the combined effect of warming and organic matter addition further suppressed phytoplankton through nutrient competition. These findings highlight the role of bacterioplankton in regulating bloom dynamics, complementing the known influence of zooplankton grazing. As climate change alters both temperature regimes and organic matter inputs in coastal ecosystems, the combined effects of these changes may lead to earlier and more abrupt bloom terminations, with implications for food web structure, function and biogeochemical cycling in the Baltic Sea.