Colored dissolved organic matter signature and phytoplankton response in a coastal ecosystem during mesoscale cyclonic (cold core) eddy

•A cyclonic eddy caused distinct changes in the ecosystem of the NW Bay of Bengal.•The cyclonic eddy-driven ecosystem supported pennates better than centric diatoms.•Strong enrichment of B fluorescence occurred during the cyclonic eddy.•A red-shifted T (TU), in a new fluorescence component, is likely unfolded protein.•Petroleum hydrocarbon-like P fluorescence, land derived, is detected.

Chromophoric dissolved organic matter (CDOM) and hydrochemical parameters were measured in the nearshore region of the western Bay of Bengal with and without significant terrestrial influence. A meso-scale cyclonic eddy that occupied the northern part of the study area set up a nutrient enriched distinct ecosystem in April (premonsoon) attended with increased levels of DOM fluorescence, particularly the protein tyrosine like fluorescence (B). A new (minor) fluorescence component, attributed to land source was revealed which contained two fluorophores, the red-shifted tryptophan-like (TU) hypothesized as the “unfolded protein” and the petroleum hydrocarbon-like (P). During the eddy, pennate diatom population increased, bringing the centric:pennate diatom ratio to half of what it was during the remaining period (monsoon season). The nutrients distribution suggested that when pennates are favored (premonsoon), orthophosphate and silicate are the limiting nutrients and that when centric diatoms are favored (monsoon season), the limitation is by nitrate.