Smaller phytoplankton size‐groups control the stoichiometry of the autotrophic community

In the marine environment, the prevailing paradigm is that larger organisms like diatoms are primary contributors to phytoplankton stoichiometry. Numerous studies investigated the stoichiometry of phytoplankton groups or total community but its dynamics among different size‐groups are not resolved. In exploring the influence of phytoplankton community composition and succession on seasonal stoichiometry in the Baltic Sea, our study reveals that smaller size‐groups, such as nanoplankton and picoplankton, play a more significant role than traditionally thought. During seasonal transitions in nutrient availability—from nutrient‐rich spring conditions favoring diatoms and dinoflagellates to nitrogen‐limited summer conditions favorable for cyanobacteria—the Baltic Proper exhibits marked shifts in community structure and offers a unique system to investigate stoichiometric dynamics. Our yearly sampling at an offshore station using a size‐fraction protocol unveils that the stoichiometry within larger size fractions (> 20 μm) does not reflect the overall community's stoichiometry. Instead, nanoplankton and picoplankton dominate nutrient cycling processes despite their smaller size. On any occasion, they represent between 55% and 90% of the biomass making them critical for nitrogen and phosphorus uptake and photosynthetic carbon fixation. These findings challenge the plankton stoichiometry paradigm and highlight the necessity to include these smaller phytoplankton groups into future climate change models to improve predictions regarding ecosystem responses to eutrophication and environmental changes.