Decadal changes in global phytoplankton compositions influenced by biogeochemical variables.

The global environmental changes owing to natural and anthropogenic influences are challenging the structure and functioning of the ocean ecosystem. The complex processes interacting within the physical, chemical, and biological environment at different spatio-temporal scales and their impact on the ocean ecosystem processes are yet to be investigated. A long term trend on phytoplankton biomass in terms of Chlorophyll-a concentration (Chl-a), phytoplankton compositions and the processes that control the variability is required for understanding the ocean ecosystem. This study investigated decadal trends (2002-2015) of phytoplankton composition and biogeochemical parameters over the Global Ocean (GO), Southern Ocean (SO), and the Arctic Ocean (AO) using ocean color remote sensing and assimilated data from the National Aeronautics and Space Administration (NASA) Ocean biogeochemical model. The results revealed the dominance of larger cell phytoplankton mainly diatoms throughout the SO and AO; however, the coccolithophores dominate in the remaining part of the GO. Analysis of nutrients showed that nitrate is not a limiting factor for the variability of phytoplankton biomass in the SO and AO. The low nitrate concentration influenced in the rest of the GO. The photosynthetically available radiation (PAR) limiting the phytoplankton biomass and composition in the SO and AO. Although the SO is known as the high nutrient low chlorophyll (HNLC) region of the GO, the low iron concentration along with the PAR co-limits the growth of phytoplankton biomass. Trend analysis showed that an increase in Chl-a and diatoms in the SO and AO. In contrast, it declined significantly in the other regions of the GO, in response to the consistent increase in sea surface temperature. The results indicated that, shifting of phytoplankton community from regional to global scale have a greater implication for climate change and marine ecosystem.