A marked gradient in δ13C values of clams Mercenaria mercenaria across a marine embayment may reflect variations in ecosystem metabolism

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Although stable isotopes of organic carbon (δ13C) are typically used as indicators of terrestrial, intertidal, andoffshore organic carbon sources to coastal ecosystems, there is evidence that δ13C values are also sensitive to in situecosystem metabolism. To investigate this phenomenon, we examined δ13C values of filter-feeding hard clams Mercenaria mercenaria from 13 locations in Greenwich Bay, a sub-estuary of Narragansett Bay, Rhode Island (USA). The δ13C values ofthe clams showed a marked linear gradient of 2‰ over the 4 kmlength of Greenwich Bay (-19 to -17‰), from lower δ13C values in the inner bay to higher values at the mouth, where Greenwich Bay joins Narragansett Bay proper (R2 = 0.94, p < 0.0001). This is in contrast to previous work that has shown that δ13C values of clams in Narragansett Bay proper (over 40 km long) are homogenous (mean × SD, -16.8 × 0.6 ‰, n = 247). Mean daily pH, temperature, and salinitydata from 2 fixed monitoring stations were used to estimate aqueous CO2(CO2(aq)) concentrations in the surroundingwater. CO2(aq) concentrations were higher in inner Greenwich Bay than immediately outside of the bay, suggesting that the dissolved inorganic carbon sources supporting phytoplankton production are quite different across the bay. The outer Greenwich Bay clams appear to feed on Narragansett Bay phytoplankton with higher δ13C values that are grown in a higher pH, more bicarbonate-rich environment. In contrast, the inner Greenwich Bay clams may feedon phytoplankton grown in lower pH water with a greater availabilityof CO2(aq). The lower δ13C of CO2(aq) relative to HCO3 - is reflected in the phytoplankton and in the clams that feed on them. Our work suggests that δ13C values may be sensitive to changes in inorganic C in estuarine systems, which may confound attempts to use stable isotopesto identify organic carbon sources. © Inter-Research 2010.

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Marine Ecology Progress Series