Rapid response of centric diatoms to nitrogen pulses in a warming Arabian Sea

Climate change is imposing escalating stress on marine ecosystems through ocean warming, acidification, and altered nutrient dynamics. Among marine primary producers, microphytoplanktonic forms, especially diatoms, play a pivotal role in supporting food webs and regulating global carbon cycles. However, their responses to nutrient limitation under shifting oceanographic regimes remain poorly constrained. Here, we investigate the impacts of nitrogen availability on diatom community structure in the south-eastern Arabian Sea, using a combination of in-situ observations and controlled enrichment experiments. Field data revealed chronic nitrogen limitation, with N:P and N:Si ratios significantly below Redfield stoichiometry, indicative of nutrient stress. The onset of winter monsoon-driven convective mixing induced episodic nitrogen enrichment, which coincided with a pronounced shift in community composition: centric diatoms increased in relative abundance from 17 % to 83 %, with Chaetoceros sp. dominating the assemblage. Parallel microcosm experiments confirmed that nitrogen addition selectively enhanced centric diatoms proliferation, with their contribution rising from 33 % to 100 %. These results demonstrate that centric diatoms are more responsive to nitrogen pulses than pennate forms. Under future climate scenarios characterized by intensified stratification and nutrient depletion, such shifts in community structure may favor pennate diatoms, potentially altering the efficiency of the biological carbon sequestration.