Phytoplankton phenology is a crucial factor for understanding marine ecosystem health and chlorophyll-a is widely used as a proxy for this. This study investigates the phytoplankton seasonality in the northern Bay of Bengal (BoB) using 19 years (2004-2022) satellite-derived Chl-a concentration. Phenological characteristics, spatial and temporal variations across the five maritime zones were evaluated using relative threshold method. Over the study period, coastal, territorial, contiguous, offshore exclusive economic zone, and continental shelf limit exhibited 17, 18, 10, 2, and 6 bloom events, respectively. Bloom initiation and propagation showed consistent spatial patterns, originating near the coastal waters, then expanding through offshore waters. Eutrophic coastal and estuarine waters exhibited the highest Chl-a (>5 mg m 3) and most frequent blooms, while oligotrophic offshore regions showed lower concentrations (<0.5 mg m 3) but longer bloom durations, demonstrating a pronounced coastal-to-offshore productivity gradient. A highly productive phenoregion (~12,249 km2) was identified in the 21-22◦N region, which encompasses coastal, territorial, and contiguous waters. Seasonally, blooms occurred throughout the year, with pronounced peak during the pre-monsoon, monsoon, and post-monsoon periods. Continuous Wavelet Transform (CWT) analysis revealed a strong and persistent annual periodicity, confirming the dominance of seasonal forcing in bloom dynamics. The CWT also showed a notable interannual variability in the 2-4-year band, with the most pronounced signals during 2004- 2008 and after 2019. Thus, these results offer important implications for ecosystem-based fisheries management and marine spatial planning in the Bay of Bengal.
Publication
Evaluation of phytoplankton phenology in the northern Bay of Bengal using ocean color remote sensing
Phytoplankton phenology is a crucial factor for understanding marine ecosystem health and chlorophyll-a is widely used as a proxy for this. This study investigates the phytoplankton seasonality in the northern Bay of Bengal (BoB) using 19 years (2004-2022) satellite-derived Chl-a concentration. Phenological characteristics, spatial and temporal variations across the five maritime zones were evaluated using relative threshold method. Over the study period, coastal, territorial, contiguous, offshore exclusive economic zone, and continental shelf limit exhibited 17, 18, 10, 2, and 6 bloom events, respectively. Bloom initiation and propagation showed consistent spatial patterns, originating near the coastal waters, then expanding through offshore waters. Eutrophic coastal and estuarine waters exhibited the highest Chl-a (>5 mg m 3) and most frequent blooms, while oligotrophic offshore regions showed lower concentrations (<0.5 mg m 3) but longer bloom durations, demonstrating a pronounced coastal-to-offshore productivity gradient. A highly productive phenoregion (~12,249 km2) was identified in the 21-22◦N region, which encompasses coastal, territorial, and contiguous waters. Seasonally, blooms occurred throughout the year, with pronounced peak during the pre-monsoon, monsoon, and post-monsoon periods. Continuous Wavelet Transform (CWT) analysis revealed a strong and persistent annual periodicity, confirming the dominance of seasonal forcing in bloom dynamics. The CWT also showed a notable interannual variability in the 2-4-year band, with the most pronounced signals during 2004- 2008 and after 2019. Thus, these results offer important implications for ecosystem-based fisheries management and marine spatial planning in the Bay of Bengal.