Characterization of colored dissolved organic matter along the western continental shelf of India during the seasonal hypoxia

Abstract

In recent years the number of hypoxic zones in the world is rising, which has been attributed to the increase in nutrients and organic matter due to anthropogenic activities. Here we present results on the variations in absorption and fluorescent characteristics of colored dissolved organic matter (CDOM) along the western continental shelf of India (WCSI) during the southwest monsoon (SWM). WCSI hosts the world's largest, natural, seasonal hypoxic zone covering 180,000 km2. Water samples were collected along 4 transects off Kochi, Mangalore, Karwar, and Goa during SWM (June-July 2018) for CDOM and other hydrographic parameters. Also, the Goa transect was sampled during late SWM (September-November 2016 and September 2018) when the bottom waters experienced low oxygen conditions. The onset of upwelling was observed in the hydrographical parameters with low temperature and dissolved oxygen (DO) in the sub-surface waters along the Kochi transect. DO decreased with depth and was hypoxic to suboxic in the bottom waters at most of the stations.