Disentangling environmental effects on picophytoplankton communities in the Eastern Indian Ocean.

Photosynthesis by picophytoplankton provides energy for higher organisms and is essential in the food chain and global carbon cycle. In 2020 and 2021, we investigated the spatial distribution and vertical changes of picophytoplankton in the euphotic layer of the Eastern Indian Ocean (EIO) and estimated their carbon biomass contributions through two cruise surveys. The abundance of picophytoplankton was composed of Prochlorococcus (69.94%), Synechococcus (22.21%) and picoeukaryotes (7.85%). Synechococcus was mainly found in the surface layer, while Prochlorococcus and picoeukaryotes had high abundances in the subsurface layer. The surface picophytoplankton community was greatly affected by fluorescence, the middle layer was significantly regulated by temperature and dissolved oxygen concentration, and the lower layer was dominated by nutrients and apparent oxygen utilization (AOU). Aggregated boosted tree (ABT) and Generalized Additive models (GAM) indicated that temperature, salinity, AOU, and fluorescence were strong influencing factors of picophytoplankton communities in EIO. The mean carbon biomass contribution of picophytoplankton in the surveyed area was 0.565 μg C/L, which was contributed by Prochlorococcus (39.32%), Synechococcus (38.88%) and picoeukaryotes (21.80%). These findings contribute to our understanding of the effects of different environmental factors on picophytoplankton communities and the influence of picophytoplankton contributions to the carbon pools of the oligotrophic ocean.