Isotopic and elemental characterization of colloidal organic matter from the Chesapeake Bay and Galveston Bay

In order to investigate sources and turnover rates of dissolved organic matter from Chesapeake Bay and Galveston Bay, colloidal organic matter (COM) was isolated using cross-flow ultrafiltration and subsequently characterized for its elemental (C, N, and S) and isotopic (13C and 14C) composition. Distributions of dissolved organic carbon (DOC) in Chesapeake Bay showed a non-systematic variation, while in Galveston Bay, a non-conservative behavior of DOC with source inputs in the low salinity region was observed. Results of size fractionation of total organic carbon (TOC) revealed that, on average, paniculate organic carbon (POC) comprised ˜ 12% and 39% of the TOC pool in Galveston Bay and the Chesapeake Bay, respectively. Colloidal organic carbon (COC) between 1 kDa and 0.2 μm (COC1) constituted ˜ 53% and 35%, respectively, with 6–7% of TOC in the HMW fraction (10 kDa to 0.2 μm, COC10), and only ˜ 34% and 25%, respectively, of the TOC in the < 1 kDa dissolved fraction. Values of Δ14C and C/N ratios of COM, in general, decreased from river to coastal seawater whereas δ13C values increased with increasing salinity, indicating that organic carbon sources changed from more terrestrial components to phytoplankton-derived sources during estuarine mixing. The distinct isotopic signature and elemental composition of riverine and estuarine COM also suggest that most riverine HMW COM could be removed or decomposed rapidly within the estuary. The fact that values of C/N ratios increased from particulate to HMW to medium MW COM suggest that reactivities of organic matter decrease with reducing size. While Δ14C values of COM1 were generally equivalent to contemporary ages, they were consistently lower for the COM10. Lower Δ14C values and lower C/N ratios in the COM10 than in the COM1 suggest that most of the estuarine HMW COM is from older and more proteinaceous sources within the estuaries. We hypothesize that resuspended sedimentary organic matter or recycled older DOM is likely the source for COM10.