Picophytoplankton phenology in the global ocean assessed by quantitative niche models

Phytoplankton phenology, connected to the seasonal variation in physiology, populations, community composition or whole community aggregated and biological interactions, responds to downwelling irradiance, temperature and nutrient supply. Quantitative niche models estimate picophytoplankton cell abundance based on photosynthetic active radiation (PAR), temperature and nitrate and thus can be used to study picophytoplankton phenology. We used quantitative niche models to estimate the abundance of picophytoplankton groups for the global ocean in 1-degree grids and the upper 200 m of the water column. The inputs of the models were the monthly climatology of temperature and nitrate from the World Ocean Atlas (WOA) and PAR from the Moderate Resolution Imaging Spectroradiometer (MODIS-Aqua). We aimed to study phenological cycles of picophytoplankton and establish a mean baseline to contrast changes in the distribution of Prochlorococcus, Synechococcus and picoeukaryotic phytoplankton. Prochlorococcus presented high abundances in ≥ 9 months in an uninterrupted area between 20ºN-S, whereas Synechococcus and picoeukaryotic phytoplankton showed high abundances in three areas: between 20ºN-S, and around 45ºN and 45ºS. Abundance seasonal change was 7, 15 and 28% around the annual mean, respectively, and the three groups presented two peaks. The seasonality of picophytoplankton was among the lowest compared to other phytoplankton groups and processes in the global ocean. The combined effects of groups’ ecological sensitivity captured by the niche models and basins environmental variability provided by the WOA shaped the uneven phenological cycle of picophytoplankton groups.