Satellite retrieval of phytoplankton size class biomass: algorithm evaluation and long-term patterns in the northern Arabian Sea

Phytoplankton exist in different size classes (picophytoplankton (PP), nanophytoplankton (NP) and microphytoplankton (MP)). It’s important to study the dynamics of phytoplankton size classes (PSC) for a better understanding of the characteristics and effects on the ecosystem, as they are the key drivers of the food chain and trophic pathways in the marine ecosystem. This study evaluates the performance of algorithms (Sahay, Brewin) to retrieve PSC using total chlorophyll-a from the Moderate Resolution Imaging Spectroradiometer onboard Aqua satellite (MODISA). The validation of satellite-derived PSC with in situ data showed better statistical indicators for Sahay’s algorithm in retrieving PP (R2: 0.73, slope: 0.74, intercept: 0.10, RMSE: 0.08), NP (R2: 0.67, slope: 0.99, intercept: −.01, RMSE: 0.04), and MP (R2: 0.76, slope: 0.81, intercept: 0.08, RMSE: 0.12). Both algorithms performed similarly for NP and MP retrievals, while Sahay’s algorithm showed better accuracy for PP. Subsequently, the PSC retrieved from Sahay’s algorithm was analysed to understand their spatio-temporal variability with respect to the environmental conditions. The PSC concentration varied within a year in accordance with monsoonal changes in northern Arabian Sea. The temporal variation of PSC concentration showed an inverse relationship of MP with PP, which was attributed to the nutrient availability during the period. The spatial distribution exhibited relatively higher concentrations of MP in the north during the winter monsoon (January to March) and along the western and southeastern coasts of the Arabian Sea during the summer monsoon (June to August). MP dominated the offshore northern Arabian Sea during the winter monsoon, whereas the PP exhibited higher concentration throughout the summer. The outcomes of this study highlight the strong coupling between monsoon reversal and oceanographic processes, and PSC dynamics in northern Arabian Sea.