Abstract

Sources and processing of carbon in the hypoxic St. Lawrence Estuary are investigated via a coupled bulk (stable carbon isotope and elemental quantitative) and molecular (polycyclic aromatic hydrocarbons and lignin phenols) approach. In order to analyze the stable carbon isotope ratio of dissolved organic carbon (e 13 C DOC ), a novel method was developed. coupling a total organic carbon (TOC) analyzer to an isotope ratio mass spectrometer allowing the routine analysis of e 13 C DOC in marine waters. Bulk analyses of five pools of carbon (sedimentary, pore-water dissolved organic, water column dissolved inorganic and organic, and particulate organic) distributed through the estuarine transition zone are analyzed to gain further insight into the spatial variability of carbon source(s) and cycling in the system. with particular insight gleaned into the water column. The system identifies with other non-anthropogenically perturbed systems by all indicators, despite its proximity to major industrial centers and one of the historically heaviest polluted watersheds in North America. The extensive elemental and isotopic analyses of five pools of carbon show; (1) a clear spatial shift in trophic state between with Upper estuarine net heterotrophy and primary production resulting in lower Estuarine net autotrophy; (2) the dissolved and particulate organic pools do share sources, but appear decoupled because physical mixing outpaces transfer; (3) approximately 90 % of primary produced particulate carbon and nitrogen is consumed prior to sedimentary deposition in the Lower Estuary; (4) stable isotopes indicate the export of riverine dissolved organic carbon is more efficient than estuarine systems from lower latitudes perhaps making similarly located estuaries important sources of terrestrial dissolved organic carbon to the world's oceans; (5) the benthic nepheloid layer may be a site of microbial activity in the hypoxic Lower Estuary; and (6) isotopic fractionation between pore water dissolved organic carbon and sedimentary organic carbon implies compositionally-driven in situ sedimentary fractionation. To better understand estuarine processes, directed experiments investigating the role of UV oxidation coupled to microbial respiration and particle dynamics should be conducted in the future. To better understand where and how carbon is consumed in the hypoxic zone the role of benthic layer cyanobacteria should be a primary focus.