Role of Phytoplankton Functional Groups on Biogeochemical changes

In an ecological perspective, relative abundance and the spatial distribution of phytoplankton functional groups (silicifiers, nitrogen fixers, and pico-autotrophs) mainly interrupt the ocean biogeochemical cycles. In addition to this concern, the seasonal changes are one more important factor responsible for the phytoplankton community distribution. The present study addresses the seasonal and biogeochemical changes based on the distribution of phytoplankton functional types (PFT) in the Arabian Sea Case-1 waters. First, we have estimated the PFT groups from MODIS AQUA Level 3 chlorophyll-a data using abundance based OC-PFT model. The retrieved PFT group gives the contribution of diatoms, dinoflagellates, prymnesiophytes, prokaryotes, green algae, Prochlorococcus and Synechococcus to the total chlorophyll-a concentration. Further, the phenology of PFT is estimated for 15 years (2003-2017) using a discrete Fast Fourier Transform to observe the seasonal trend. The estimated phenology result shows the increasing trend of phytoplankton functional group with the high dominance of diatoms (silicifers) that contributed about 50% to the total phytoplankton diversity. The prevalence of silicifiers is highly productive during winter monsoon (January to March) and considerably less during inter-monsoon period (April and May). Similarly, dinoflagellates contribute 45% to the overall phytoplankton diversity, with high abundance during winter monsoon and considerably less during inter-monsoon period. Nearly 2% is contributed by Prochlorococcus and Synechococcus (both nitrogen fixers and pico-autotrophs), mostly found in the nutrient-poor environment during inter-monsoon period, the remaining 3% is contributed by other phytoplankton such as prymnesiophytes, prokaryotes and green algae. The overall result highlights the dominance of silicifers over pico-autotrophs and nitrogen fixers in the northern Arabian Sea during winter monsoon due to the increased upwelling episodes and the dominance of pico autotroph and nitrogen fixers which are high in the central Arabian Sea during the inter-monsoon period due to the prevalence of oligo-trophic condition.