Phytoplankton dynamics as a response to physical events in the oligotrophic Eastern Indian Ocean

The Eastern Indian Ocean (EIO) is considered to exhibit low chlorophyll alpha (Chl alpha) concentrations. However, high Chl alpha levels were occasionally reported and attributed to physical events that frequently occur in this area. Because of limited in situ observations, the underlying formation mechanism of high Chl alpha through the responses of components within the ecosystem is not fully understood. In this study, we conducted a meridional cruise along the 88 degrees E longitude in the EIO (16.5 degrees N to 20 degrees S) during the fall intermonsoon period (November 8-28th, 2018). For the first time, surface phytoplankton growth and mortality rates were measured using dilution experiments. Sensitive nutrient analysis was also applied to determine the nanomolar nutrient concentrations. Our cruise captured high surface Chl alpha concentrations (similar to 298 ng L-1 at 10 m) in the Bay of Bengal, at the Equator, and in the southern EIO, even under consistent nitrogen depletion (dissolved inorganic nitrogen of similar to 58 nM). All the blooms observed were in the decay phase, as suggested by the satellite observations of Chl alpha distributions and the low or negative phytoplankton net growth rate. However, the phytoplankton growth rate was enhanced by similar to 4.9 times due to nutrient enrichment in dilution experiments, indicating that the bloom was initiated by nutrient enrichment and ceased by the severe nutrient limitation. The shipboard acoustic Doppler current profiler measurements and satellite observations of sea surface winds, surface currents, and sea surface height collectively suggested that phytoplankton blooms were triggered by nutrient enrichments that were possibly supplied by either the island effect near the Maldives or a cyclonic eddy in the southern EIO. In the former case, the Chl a rich water generated near the islands was then advected eastward toward the study area by surface currents associated with the Wyrtki Jet. Our study indicated that the EIO was not a stable oligotrophic marine ecosystem with consistently low Chl alpha concentrations, and was quite dynamic as affected by unique regional physical events.