Phytoplankton community dynamics in response to ESCS Fani: a multivariate assessment from the surf zone of south-central Odisha, east coast of India

Tropical cyclones profoundly modify surf zone ecosystems by altering hydrography, nutrient cycling, stratification, and phytoplankton community composition. The present study evaluated the impact of Extremely Severe Cyclonic Storm (ESCS) Fani, which made landfall on 3rd May 2019 along the south-central Odisha coast, using in situ observations of nutrients, Chl-a, phytoplankton assemblages, and Ocean Moored buoy Network for Northern Indian Ocean (OMNI) buoy-derived vertical temperature and salinity data. Post-cyclone conditions exhibited marked nutrient enrichment driven by intense vertical mixing and upwelling, with nitrate, nitrite, ammonia, phosphate, and silicate increasing by 90.87%, 68.24%, 12.87%, 68.24%, and 146.53%, respectively. OMNI buoy observations further revealed rapid collapse of water-column stratification followed by weak stratification with intermittent upwelling, confirming vigorous vertical mixing and nutrient entrainment into the euphotic layer. Despite elevated nutrient availability, Chl-a declined by 28.6%, while, phytoplankton abundance declined by 59.78% in the NE sector and showed a slight increase (+ 5.66%) in the SW sector, likely due to intense turbulence, biomass dilution, and physical displacement of phytoplankton populations. Phytoplankton communities remained diatoms-dominated (> 95%), suggesting short-term ecological resilience, though increased diversity and evenness. The total phytoplankton abundance decreased by 52%, accompanied by a decline in dominance from 71% to 41% and concurrent increases in diversity, species richness, and evenness. Multivariate analyses (NMDS, ANOSIM, SIMPER, CCA) revealed significant restructuring of phytoplankton assemblages. ANOSIM revealed significant temporal variation between pre- and post-Fani assemblages (R = 0.415, p < 0.05) and marked post-cyclone spatial heterogeneity (R = 0.32, p = 0.001). Canonical Correspondence Analysis identified salinity, dissolved oxygen, and nitrate as the principal environmental drivers controlling phytoplankton distribution, with nitrate showing strong correlations with Chl-a before (r = 0.886, p < 0.01) and after the cyclone (r = 0.651, p < 0.05). The weakened post-cyclone species-environment relationship suggested a transition from biological regulated to physically dominated ecosystem processes. Overall, the study demonstrates that cyclone-induced mixing enhanced nutrient availability, but immediate biological responses were suppressed by hydrodynamic disturbance, with cyclone translation speed playing a key role in regulating post-cyclone phytoplankton variability.