Unprecedented warming impacts on phytoplankton and special emphasis on diatom-diazotroph associations in the oligotrophic waters of the Eastern Arabian sea.

The impact of anomalous warming on the phytoplankton dynamics in the euphotic zone (0-60 m depth) of the eastern Arabian Sea (EAS) during the southwest monsoon (SWM-2020) was investigated along the 68°E transect from 8°N to 21°N. During SWM-2020, sea surface temperatures (SSTs) along the transect exceeded 28 °C, with temperatures of ∼29-30 °C recorded in the North of the Findlater Jet Axis (NFJA). The active Findlater Jet (FJ) modulated the thermocline, causing shallower thermocline in the North EAS (NEAS, 16-21°N) and deeper thermocline in the South EAS (SEAS, 8-15°N). The increased stratification due to warming reduced the vertical mixing and exacerbated the oligotrophic condition within the MLDs in the NEAS. High dissolved inorganic silicate and phosphorous with low nitrate concentrations favouring dinoflagellates and diatom-diazotroph associations (DDAs) within the MLDs, particularly in the NEAS. In the NEAS, heterocystous cyanobacteria Richelia was abundant as free-living and in endosymbiosis with Rhizosolenia and Hemiaulus, while coccoid cells of Crocosphaera, symbiotic with Climacodium sp., thrived in both SEAS and NEAS indicating a critical role of dissolved iron in distribution of DDAs. DDAs were abundant in subsurface waters (25-40 m) and rare or absent within the nutricline and subsurface chlorophyll maximum (SCM, 40-60 m depths). High dissolved ammonium concentration in the NEAS indicated that N fixed by DDAs could be supplied into N-limited waters. A high abundance of unhealthy and moribund host-symbiotic stages suggests extreme warming, and shallower nitracline could harm DDAs in the EAS. This study highlights the importance of DDAs in the EAS, which remains poorly understood, and whose underestimation in N and C fixation can impact the regional nitrogen and carbon budgets.