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Although stable isotopes of organic carbon (δ 13 C) are typically used as indicators of ter- restrial, intertidal, and offshore organic carbon sources to coastal ecosystems, there is evidence that δ 13 C values are also sensitive to in situ ecosystem metabolism. To investigate this phenomenon, we examined δ 13 C values of filter-feeding hard clams Mercenaria mercenaria from 13 locations in Green- wich Bay, a sub-estuary of Narragansett Bay, Rhode Island (USA). The δ 13 C values of the clams showed a marked linear gradient of 2‰ over the 4 km length of Greenwich Bay (-19 to -17‰), from lower δ 13 C values in the inner bay to higher values at the mouth, where Greenwich Bay joins Narra- gansett Bay proper (R 2 = 0.94, p < 0.0001). This is in contrast to previous work that has shown that δ 13 C 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 salinity data from 2 fixed monitoring sta- tions were used to estimate aqueous CO2 (CO2(aq)) concentrations in the surrounding water. 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 δ 13 C values that are grown in a higher pH, more bicarbonate-rich environment. In con- trast, the inner Greenwich Bay clams may feed on phytoplankton grown in lower pH water with a greater availability of CO2(aq). The lower δ 13 C of CO2(aq) relative to HCO3 - is reflected in the phyto- plankton and in the clams that feed on them. Our work suggests that δ 13 C values may be sensitive to changes in inorganic C in estuarine systems, which may confound attempts to use stable isotopes to identify organic carbon sources.

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