Impact of seasonal anoxic events on phytoplankton dynamics over the Western Indian shelf: An intra- and inter-annual study.

Coastal deoxygenation impacts phytoplankton communities crucial for marine productivity. The inter- and intra-annual variability in phytoplankton communities at a shallow (27 m) station over the Western Indian Shelf (CaTS site, off Goa) during deoxygenation events of the late southwest monsoon (LSWM September-October) were studied from 2020 to 2023. The water column (0-27 m depth) experienced seasonal hypoxia/anoxia at subsurface depths (0-1.4 mL L O, 18-27 m) with the presence of hydrogen sulfide in the bottom waters during LSWM of all years. The stratified, deoxygenated, near-bottom waters were cold, highly saline, and nitrate-depleted, whereas the upper layers of the water column (<18 m) retained warmer temperatures and well‑oxygenated conditions, with relatively lower salinity. The phytoplankton abundance and species composition showed inter and intra-annual variations. Interestingly, higher phytoplankton abundance found in the anoxic waters (>18 m) was dominated by diatoms, especially pennate forms, in the dark zone (i.e., photosynthetically active radiation (PAR) <1 μE m s). Experiments on the effect of dark, anoxic conditions on phytoplankton further revealed that certain pennate diatoms, such as Fragilaria sp., Pleurosigma sp., and Navicula sp., exhibit greater resilience and survival under dark, anoxic conditions compared to other species. The oxic surface waters were chiefly dominated by centric diatoms. At times, oxic surface waters were also dominated by dinoflagellates, while they were relatively less abundant in low-oxygen waters in the study area. In the climate change scenario, future studies are required to understand the expansion and magnitude of the deoxygenation zones in the coastal waters and its effect on phytoplankton in the marine ecosystems.