Dissolved inorganic carbon (DIC), total alkalinity (TAlk), pH, and dissolved oxygen (DO) were determined in the Mississippi River plume during five cruises conducted in the spring, summer, and fall. In contrast to many other large rivers, both DIC and TAlk were higher in river water than in seawater. Substantial losses of DIC, relative to TAlk, occurred within the plume, particularly at intermediate salinities. DIC removal was accompanied by high DO, high pH, and nutrient depletion, and was attributed to high phytoplankton production. As a result, the carbonate saturation in the plume became much higher than in ocean and river waters. A mixing model was used to determine DIC removal. We provide evidence that the use of a two-end-member (river and ocean) mixing model was valid during late summer and fall (low discharge period). However, for other periods we used salinity and TAlk to delineate a mixing model that included two river end members and an ocean end member. Net community production rates in the plume, estimated using a box model, peaked in the summer and were among the highest reported to date for large river plumes. In the summer and fall, biological production in the river plume consumed a majority of the available nutrients, whereas during the spring only a small fraction of the available nutrients were consumed in the plume. Biological production was the dominant process influencing pH and carbonate saturation state along the river-ocean gradient, whereas physicochemical dynamics of mixing played an important role in controlling the TAlk and DIC distributions of this large river plume.

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[1] To evaluate applicability of the end-member mixing model in assessment of input and transport of organic carbon in estuarine systems, we incubated marine diatom, land grass, and salt marsh plant in Altamaha estuarine water for two months. Chemical and isotopic parameters (bulk organic carbon/nitrogen contents, lipid compositions, stable C/N isotopes, and lipid stable carbon isotopic ratios) were analyzed for fresh and degraded materials. The results showed that although the C/N and d 15 N ratios of three materials varied similarly during degradation, the bulk d 13 C, lipid compositions, and lipid stable carbon isotopic compositions varied differently from material to material and from compound to compound, implying that applications of the end-member model should consider the diagenetic status of organic materials and the potential changes in chemical and isotopic signatures.

High‐molecular‐weight dissolved organic matter (HMW‐DOM, > 1,000 Daltons) is actively involved in the global biogeochemical cycling of many elements, but its carbon sources and detailed formation pathways are still not well understood. In this study, we measured bulk stable carbon and nitrogen isotopic ratios, lipid composition, and compound‐specific carbon isotopic ratios of HMW‐DOM samples collected from four U.S. estuaries (Boston Harbor/Massachusetts Bay, Delaware/Chesapeake Bay, San Diego Bay, and San Francisco Bay). Analytical results show (1) a fraction of HMW‐DOM (lipid associated) in estuarine and coastal waters is derived from bacteria and phytoplankton; (2) this fraction of HMW‐DOM is formed by various release processes of bacterial membrane components and bacterial reworking of phytoplanktonderived material; (3) this fraction of HMW‐DOM is generally present in all samples from different coastal systems despite variable organic matter inputs and environmental conditions, suggesting an important bacterial role in HMW‐DOM formation.

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