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

Abstract

Although stable isotopes of organic carbon ( 13 C) are typically used as indicators of terrestrial, intertidal, and offshore organic carbon sources to coastal ecosystems, there is evidence that 13 C values are also sensitive to in situ ecosystem metabolism. To investigate this phenomenon, we examined 13 C values of filter-feeding hard clams Mercenaria mercenaria from 13 locations in Greenwich Bay, a sub-estuary of Narragansett Bay, Rhode Island (USA). The 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 13 C values in the inner bay to higher values at the mouth, where Greenwich Bay joins Narragansett Bay proper (R 2 = 0.94, p < 0.0001). This is in contrast to previous work that has shown that 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 stations were used to estimate aqueous CO 2 (CO 2(aq) ) concentrations in the surrounding water. CO 2(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 13 C values that are grown in a higher pH, more bicarbonate-rich environment. In contrast, the inner Greenwich Bay clams may feed on phytoplankton grown in lower pH water with a greater availability of CO 2(aq) . The lower 13 C of CO 2(aq) relative to HCO 3 -is reflected in the phytoplankton and in the clams that feed on them. Our work suggests that 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.