The source and distribution of dissolved and particulate organic matter in the Bay of St. Louis, northern Gulf of Mexico

Dissolved (DOM) and particulate organic matter (POM) samples were collected from the Bay of St. Louis (BSL) in the northern Gulf of Mexico during March–September, 2007 for chemical and isotopic characterization to examine the distribution and sources of organic matter species in the estuarine environment. Similar to the variations in hydrographic parameters and nutrients, concentrations of organic C, N, and P and stable isotopic composition show large spatial and seasonal variations during the sampling period. Concentrations of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) decreased with increasing salinity while δ13C-DOC increased with salinity from −30.23‰ to −19.04‰, suggesting a shift of DOC sources from terrestrial- to marine-dominated inputs during estuarine mixing. In contrast to both DOC and DON, the concentration of dissolved organic phosphorus (DOP) increased with increasing salinity, indicating additional DOP sources at higher salinity stations. Concentrations of particulate organic carbon (POC) and nitrogen (PN) decreased with increasing salinity, showing a negative correlation with the concentration of suspended particulate mater. Both POC/PN ratio (8.6–19.6) and δ13C-POC (−28.51‰ to −23.79‰) increased in general with increasing salinity, indicating the predominance of terrestrially derived organic matter in the upper bay and increasing diagenetically altered marine POM component in the lower bay. Fresh microalgae might account for about one third of POM in the BSL as estimated from POC/Chl a ratio. Overall, the terrestrial inputs, in situ primary production and diagenetically altered marine POM mostly from sediment resuspension are the major sources of POM in the BSL.