Winter phytoplankton size classes in the Northeastern Arabian Sea based on in-situ and remote sensing methods

Phytoplankton size classes (PSCs) are important in marine ecosystems because they organise the food chain and trophic pathways, which determine the overall biological environment. Based on three FORV Sagar Sampada cruises, the current study provides changes in PSCs in the Northeastern Arabian Sea (NEAS; north of 18 N) during different phases of the Northeast Monsoon [NEM (November-February)]. During all three phases of NEM such as early (November), peak (December), and late (February), in-situ chlorophyll-a fractionation data revealed that nanoplankton (2-20 μm) predominated, followed by microplankton (>20 μm) and picoplankton (0.2-2.0 μm). This was primarily because winter convective mixing in the NEAS maintains only a moderate level of nutrients in the surface mixed layer, which is more conducive to the dominance of nanoplankton. Brewin et al. (2012) and Sahay et al. (2017) have satellite-based PSC estimation algorithms; the former was developed for the entire Indian Ocean, while the latter is a modification of the former for the Noctiluca bloom-infested NEAS, with a claim that such blooms are typical of the NEM. When current in-situ PSCs data were compared to algorithm-based NEM data, Brewin et al. (2012) revealed a more realistic PSCs contribution pattern, especially in oceanic waters, with nanoplankton predominating except during early NEM. But the PSCs data from Sahay et al. (2017) showed a high degree of variation from the in-situ data, demonstrating the dominance of pico- and microplankton and a notably small contribution from the nano phytoplankton. The current study showed that Sahay et al. (2017) is inferior to Brewin et al. (2012) at quantifying PSCs in the NEAS without Noctiluca blooms, and provided evidence to show that Noctiluca blooms are not a typical feature of the region during the NEM.