Seasonal dynamics and nutrient controls of biogenic silica in Baltic Sea surface microplankton and picoplankton communities.

In recent years, new contributors to the marine silica cycle have emerged, including pico-sized phytoplankton (<2-3 µm in size) such as and picoeukaryotes. Their contribution and relevance to silica cycling are still under investigation. Field studies reporting the biogenic silica (bSi) standing stock in the pico-sized fraction are limited to silica-poor oligotrophic environments, and the mechanism of bSi accumulation in picoplankton remains unknown. We investigated the variability of bSi standing stocks in two size fractions (picoplankton, 0.22-3 µm and microplankton, >3 µm) in the dissolved silica-replete Baltic Sea via biweekly time series samplings spanning 2 years. Time series data showed that the large changes in bSi standing stock in the Baltic Proper were primarily related to microplankton biomass and community composition. Meanwhile, picoplankton were, at times, surprisingly high contributors to total bSi year-round (up to 21.6%). Simultaneously, we performed microcosm incubation experiments with natural phytoplankton communities in each season to examine how nutrient additions affected bSi concentrations. In these experiments, increases in microplankton bSi were directly correlated to increases in diatom biomass, highlighting their influential role in the Baltic Sea silica cycle. Meanwhile, phosphorus additions triggered an increase in picoplankton bSi accumulation in all experiments. This uncovers a potential control of bSi accumulation in picoplankton, which can help identify the cellular mechanisms behind this process and uncover their role in silica cycling. The results link phytoplankton community composition and silica cycling, which is important for understanding the consequences of organism shifts due to climate change.IMPORTANCEThe marine carbon and silica cycles are tightly intertwined and largely controlled by diatoms. Nevertheless, recent studies, mostly in oligotrophic waters, have proposed new contributors to the marine silica cycle: picoplankton. Here, we report the first study of seasonal dynamics of biogenic silica (bSi) standing stock in microplankton and picoplankton in the silica-replete Baltic Sea. Microplankton bSi dynamics were correlated with changes in composition and biomass. Picoplankton were consistent contributors to bSi, and for the first time in diverse natural communities, we found a direct correlation between phosphorus and bSi accumulation. The results are important for understanding how climate change-predicted phytoplankton composition shifts will affect carbon and silica cycling and provide a direction for future research on nutrient controls of silica accumulation in picoplankton.