Spatiotemporal variations in chromophoric dissolved organic matter in the Saint Louis Bay, northern Gulf of Mexico

Water samples were collected from the Saint Louis Bay (SLB), a semi-closed estuary, in the Northern Gulf of Mexico from May to November 2012, for the measurements of bulk dissolved organic carbon (DOC) concentration, UV-vis absorbance, and fluorescence excitation emission matrix (EEM) to examine the abundance, optical properties, composition, and biogeochemical cycling of dissolved organic matter (DOM). Concentrations of DOC ranged from 277 to 1618 µmol/L in the SLB, with a consistent decrease from stations near the river to the most offshore station in the lower estuary, showing a major DOM source from river. Absorption coefficient at 254nm (a₂₅₄) showed similar spatial distributions as DOC and had a significant positive correlation with DOC. Average percentage of non-chromophoric DOC in the bulk DOC was 36%, indicating the predominance of chromophoric DOM and its potential role in the ecosystem function and biogeochemical processes in the SLB. In addition, values of specific UV absorbance (SUVA₂₅₄) were less variable, although following the same trend as DOC and a₂₅₄, supporting a consistent DOM source in the bay. PARAFAC analysis on EEM data revealed one UV humic-like and one terrestrial humic-like DOM components in the SLB. Although a protein peak (peak T) can be seen for some offshore samples, the absence of protein-like DOM as the major fluorescent components suggests that DOM sources are dominated by allochthnous ones in the bay. Spatial variations in the components again feature the decreasing trend of terrestrial humic-like DOM from rivers to lower estuarine waters. Seasonal variations in DOM characteristics were related to riverine discharge, water column hydrographic conditions, and biological activities.