Vertical distribution of optically active substances (OAS) in the coastal sea off Indian Sundarban during post monsoon

Abstract

ABSTRACTOptically active substances (i.e. chromophoric dissolved organic matter (CDOM), total suspended matter (TSM), chlorophyll-a (Chl-a)) and other physico-chemical parameters were measured in the coastal sea off Indian Sundarban during October 2016–January 2017. The present study, firstly, portrayed the vertical distribution of optically active substances (OAS) in the study zone. The salinity ranged from 18.2–32.1 and 18.3–30.1 in the surface water as well as bottom water respectively. Higher values of CDOM were observed near the Hugli River confluence in both surface and bottom water and a gradual decrease towards the offshore was observed except in January. The aCDOM(440) varied between 0.1031–0.4321 m−1 and 0.0265–0.4221 m−1 in the surface water and bottom water, respectively, during the study period. Moreover, CDOM decreased towards the bottom of the water column in all three stations throughout the study periods except in October in the inshore sampling station (A1). CDOM also showed a significant temporal variation in the inshore in parity with the dynamics of TSM. Average Chl-a concentration was higher (<2 mg m−3) in the surface water than the bottom water (<1 mg m−3). During January, when favourable conditions for photosynthetic activity prevailed at the water surface, an increase in Chl-a was observed in the offshore along with a concurrent mild increase in CDOM but there was no similar trend in the bottom water. Overall, vertical distribution of OAS variability in this study area is principally dominated by the freshwater flow of the Hugli River.