Influence of environmental parameters on bio-optical characteristics of colored dissolved organic matter in a complex tropical coastal and estuarine region

Abstract

Dissolved organic matter (DOM) is an important source of carbon in aquatic ecosystems, and colored DOM (CDOM), which is smaller than 0.2 μm and interacts with ultraviolet (UV) and visible light, affects the spectral quality and quantity of light in water. In this study, the spatial and temporal variations of CDOM with changes in environmental conditions were investigated from March 2014 to May 2017 in the coastal waters and two estuaries (Zuari and Mandovi) of Goa, western India, and the major sources and sinks controlling the optical properties of these waters were identified. The CDOM absorption in the estuaries was two times higher than that of the coastal waters. It was also determined that the CDOM absorption at 412 nm (ag412) in the coastal and estuarine waters significantly varied between seasons. The ag412 was found to be higher in the coastal waters during the spring inter-monsoon (SIM) and fall inter-monsoon (FIM) than during the northeast monsoon (NEM). The high absorption during the SIM was of autochthonous origin, while terrigenous DOM was the primary contributor mainly during the FIM. The photobleaching of CDOM was highest during the SIM, resulting in the predominance of low-molecular-weight DOM in the coastal waters. This photobleaching of DOM also resulted in deeper UV light penetration, as indicated by the diffuse attenuation coefficient Kd at 350 nm. In the Mandovi and Zuari estuaries, higher levels of CDOM were observed during the southwest monsoon (SWM) and SIM than the FIM and NEM. The terrigenous DOM contribution was higher in the estuaries during the SWM, while phytoplankton contributed to a higher level of CDOM during the SIM. CDOM exhibited non-conservative mixing behavior in the study region, as it decreased in estuaries with lower salinities and increased at salinities between 20 and 31. Considering the importance of CDOM in the carbon cycle, this study highlights the various sources and sinks of CDOM controlling the optical properties and biogeochemical processes of coastal and estuarine waters.