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

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

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摘要

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 km(2). 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. The Highest CDOM absorption was observed at the nearshore stations of the Goa transect during July 2018. The waters along the Goa transect were highly stratified during the late SWM (September 2018) with low saline waters at the surface (due to heavy rainfall), with the development of hypoxia in the bottom waters. High CDOM absorption (ag412 m-1) was observed in the bottom waters, which coincided with hypoxic/suboxic conditions. Spectral slope S250-600 also showed a significant difference between the oxygen rich and low oxygenated waters. Three fluorescent DOM components were identified from PARAFAC analysis. Component 1 (C1) and 2 (C2) were humiclike with emissions in the longer wavelength, while component 3 (C3) was protein-like with excitation and emission in the UV region. CDOM absorption (ag412) and humic-like fluorescent component (C1) showed a positive correlation with apparent oxygen utilization (AOU), indicating the role of microbial respiration for CDOM production. Laboratory experiments also shed light on the CDOM production in low oxygenated waters when sufficient organic matter is available. This study gives insight into understanding dissolved organic matter's cycling under varying oxygen conditions prevailing in these waters.

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