Using stable isotope analyses to assess carbon dynamics in a shallow subtropical estuary

The sources of carbon, which fuel water column respiration, remain unresolved for most estuaries; our objective was to examine carbon dynamics in a shallow subtropical estuary. We sampled the Sabine-Neches estuary, Texas, during low (November 1999) and high (May 2000) freshwater inflow and measured stable carbon isotope ratios of the dissolved inorganic and organic carbon (delta(13)C-DIC, delta(13)C-DOC), as well as quantifying accessory parameters (salinity, nutrients, total suspended solids, and photosynthetic pigments). Pigment analysis indicated that diatoms were the predominant phytoplankton. Data from the May 2000 sampling event exhibited conservative mixing, indicating that the system was acting as a conduit between the watershed and the Gulf of Mexico. During November, mixing was generally nonconservative indicating extensive recycling of allochthonous and autochdionous carbon sources. Our data imply that both carbon sources had similar isotope ratios that made it impossible to unambiguously determine the dominant source supporting respiration. The nonconservative DIC concentration data indicating an autotrophic sink as well as the strong relationship between delta(13)C-DOC and chlorophyll a, suggest that in situ production was an important component of the DOC pool. We hypothesize that uncharacteristically calm wind conditions during sampling may have promoted phytoplankton settling, removing autotrophs from the water column, but leaving behind a dissolved biogeochemical signature. Interpretation of carbon dynamics may be confounded by spatial and temporal decoupling of producers and consumers from biogeochemical indicators.