Algal Blooms (AB), characterized by the rapid proliferation of algae, are a significant environmental concern in the Arabian Sea, especially during the winter monsoon. Driven by monsoonal winds and seasonal upwelling, these blooms, often dominated by species like Noctiluca scintillans, can lead to oxygen depletion, hypoxic "dead zones" and disruptions to the marine food web. While optical sensors traditionally monitor Chlorophyll-a (Chl-a) as a proxy for phytoplankton biomass, they are often hindered by persistent cloud cover and lack the spatial resolution necessary to capture sub-mesoscale features (oceanic structures smaller than 50 km).

Using the synergetic integration of ISRO’s EOS-04 C-Band Synthetic Aperture Radar (SAR) with optical data from EOS-06 OCM-3 and Sentinel-3 OLCI, we have demonstrated a robust, all-weather monitoring approach for these events. Unlike optical sensors, SAR operates at microwave frequencies, providing high-resolution (33m), day-and-night data regardless of weather conditions. Algal blooms act as natural surfactants that dampen small-scale surface waves, causing them to appear as distinct dark signatures in SAR imagery. This study analyzed two major events (9 February 2023 and 18 February 2024) and validated the observations using in-situ Bio-Argo float data. The study revealed a strong correlation between SAR backscatter and optical Chl-a concentrations. In the February 2023 case, SAR captured a network of interconnected cyclonic spiral eddies ranging from 1.93 km to 27.97 km in diameter, which matched the high Chl-a patterns observed by OCM-3 and OLCI. The February 2024 case further highlighted how SAR can provide the "finest details" of bloom organization that appear broadened in coarser optical data. Furthermore, Principal Component Analysis (PCA) demonstrated that stacking SAR data with specific optical bands (510 nm, 555 nm, and 670 nm) explains 75% of the data variance, significantly enhancing the ability to characterize bloom intensity. This synergetic approach offers a comprehensive and reliable monitoring system, essential for managing coastal health and understanding the evolving dynamics of the Arabian Sea.

More details can be found in paper: Swapna Mulukutla, Rajesh Sikhakolli, Devi Vara Prasad Thumu, Suhail Mohammed, Nagamani Pullaiahgari Venkata, Srinivasa Rao Goru, Rajashree Vinod Bothale & Prakash Chauhan, Unravelling Algal Bloom Signatures: Synergetic Use of EOS-04 C-Band SAR, OCM3, OLCI and In Situ Observations. J Indian Soc Remote Sens (2025). Visit here.