Sources and compositional characterization of chromophoric dissolved organic matter in a Hainan tropical mangrove-estuary

Abstract

Chromophoric dissolved organic matter (CDOM) is a major optically active component of DOM in estuaries that plays a significant part in maintaining the ecosystem sustainability of mangrove forests via affecting nutrient and micronutrient ((micro)nutrient) biogeochemical processes and pathways. This study was conducted in Hainan Dongzhai Harbor to better understand the sources, composition, and behaviors of CDOM in a tropical mangrove estuary. Water samples were collected from the upper fluvial mangrove zones to the lower marine endmember along the estuarine salinity gradient and analyzed by absorption and fluorescence spectroscopy combined with parallel factor analysis (PARAFAC). DOC, TDN, TDSi, Mn, Ba, and optical characteristics, i.e., a254, SUVA, HIX, and the abundance of humic-like CDOM, are negatively correlated with salinity. Terrestrial/anthropogenic inputs, riverine runoff, and mangrove porewater exchange are sources of (micro)nutrients and aromatic CDOM into the estuary. In contrast, pH, DO, TDP, SR, BIX, and tryptophan-like CDOM are positively correlated with salinity. This indicates saltwater dilution, tidal mixing, photo-oxidation, and microbial degradation processes promote the enrichment of low MW autochthonous CDOM and TDP, together with the removal of humic-like substances and other (micro)nutrients in the lower estuary. Overall, these findings have important implications for biogeochemical cycling processes driven by CDOM composition and sources in mangrove estuaries.