High-resolution dynamics of the spring bloom in the Gulf of Finland of the Baltic Sea

- The ratio of nutrient consumption was close to the Redfield ratio during the growth phase of the spring bloom.
- The phytoplankton-bound C stock increased with the transition from diatom to dinoflagellate dominance in the community.
- Diatoms exhibit a relatively narrow period of high biomass that can be overlooked by traditional monitoring.
- The observed dominance of dinoflagellates and M. rubra increases the retention time for newly produced organic material.

During the period from March to the end of May in 2009 and 2010, intensive measurements and sampling were undertaken in the Gulf of Finland. The compiled results indicate a high variability of the phytoplankton distribution both temporally and spatially. The spring bloom dynamics and heterogeneity was influenced by physical forcing, such as prevailing circulation in the surface layer and the development of stratification, including the upward and downward movement of the seasonal thermocline. The estimated ratio of nitrogen to phosphorus consumption during the growth phase of the spring bloom was close to the Redfield ratio during both springs. The maximum phytoplankton carbon biomass was observed after the depletion of inorganic nitrogen from the surface layer, which coincides with the transition in the community dominance from diatoms to dinoflagellates. Diatoms exhibited a short, well-defined period of high biomass, and we argue that measurements with low temporal resolution can overlook this period of diatom dominance in the Gulf of Finland. The observed dominance of dinoflagellates (Peridiniella catenata and the Scrippsiella/Biecheleria complex) and the ciliate Myrionecta rubra might have a substantial biogeochemical impact because these species increase the retention time of newly produced material in the nutrient-limited surface layer in late spring.