2026/05/07 by R. Chandrasekhar Naik, AA Lotliker, Sudarsana Rao Pandi +4 · 1 voice
Earth and Planetary Sciences · Environmental Science · #Marine and coastal ecosystems #Marine and fisheries research #Oceanographic and Atmospheric Processes
paper · pdf · doi:10.3390/rs18101451
openalex publication_date 2026/05/07 · openalex created_date 2026/05/08 · openalex updated_date 2026/07/22
Phytoplankton size classes (PSCs) fundamentally regulate ocean productivity, biogeochemical cycling, and carbon export, yet their distribution and optical variability across the Arabian Sea remain poorly constrained. This study develops and validates a regionally tuned absorption-based approach for phytoplankton size class estimation using in situ phytoplankton absorption spectra (aph(λ)) collected during six research cruises between 2016 and 2024. A significant power-law relationship between aph(443) and the spectral slope (S443–510) (R2 = 0.963, p < 0.001) provided a consistent optical basis for distinguishing PSCs. Co-located HPLC pigment data were used to derive empirical aph(443) thresholds for pico- (≤0.011 m−1), nano- (0.011–0.059 m−1), and micro-phytoplankton (>0.059 m−1). Class-specific mean spectra showed clear optical distinctions consistent with size-dependent pigment packaging. Model evaluation showed reduced error and improved regression agreement relative to existing aph- and chl-a-based models when applied to the Arabian Sea dataset, with regression slopes close to unity (0.78–0.81) across all PSCs. This regional model also improved representation of transitional nano communities, which are commonly associated with higher uncertainties in global models. The empirical relationships developed in this study were applied to VIIRS Level 3 aph(443) data for 2024 to generate PSC distributions. Satellite-derived PSC fields revealed pronounced spatial gradients and regional contrasts across the Arabian Sea, including micro-phytoplankton blooms in the northern Arabian Sea and mixed nano-dominated communities along the western Arabian Sea (Somali coast). Pico-phytoplankton dominated the low-absorption oligotrophic offshore waters, while nano-phytoplankton were most common in transitional regions influenced by moderate nutrient inputs. Taken together, these results demonstrate that the combined aph(443)-S443–510 framework provides a practical, regionally optimized method for retrieving PSCs at synoptic scales across the Arabian Sea, supporting improved bio-optical modelling and satellite-based monitoring of phytoplankton community structure in this region.