DOC cycling in a temperate estuary: A mass balance approach using natural 14C and 13C isotopes

We measured dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and their corresponding D 14 C and d 13 C values in order to study the sources and fates of DOC in the York River Estuary (Virginia, U.S.A.). The D 14 C and d 13 C values of DOC and DIC at the freshwater end-member indicate that during periods of moderate to high flow, riverine DOC entering the York was composed of decadal-aged terrestrially organic matter. In nearly all cases, DOC concentrations exceeded conservative mixing lines and were therefore indicative of a net DOC input flux from within the estuary that averaged 1.2 m ML 21 d 21 . The nonconservative behavior of DOC in the York River Estuary was also apparent in carbon isotopic mixing curves and the application of an isotopic mixing model. The model predicted that 20‐38% of the DOC at the mouth of the estuary was of riverine (terrestrial 1 freshwater) origin, while 38‐56% was added internally, depending on the isotopic values assigned to the internally added DOC. Measurements of D 14 C and d 13 C of DOC and DIC and marsh organic matter suggest that the internal sources originated from estuarine phytoplankton and marshes. The isotopic mixing model also indicates a significant concomitant loss (27‐45%) of riverine DOC within the estuary. Changes in DOC concentration, D 14 C-DOC, and d 13 C-DOC were also measured during incubation experiments designed to quantify the amounts, sources, and ages of DOC supporting the carbon demands of estuarine bacteria. Results of these experiments were consistent with an estuarine source of phytoplankton and marsh DOC and the preferential utilization of young ( 14 C-enriched) DOC in the low-salinity reaches of the York. However, the average removal of riverine DOC by bacteria accounts for only ;4‐19% of the riverine pool; therefore, other significant sinks for DOC exist within the estuary.

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