A marked gradient in delta C-13 values of clams Mercenaria mercenaria across a marine embayment may reflect variations in ecosystem metabolism

作者：Oczkowski Autumn J^*; Pilson Michael E Q; Nixon Scott W

来源：Marine Ecology Progress Series, 2010, 414: 145-153.

DOI：10.3354/meps08737

• Carbon
• Stable isotope
• Ecosystem metabolism
• pH
• Greenwich Bay
• Narragansett Bay

Although stable isotopes of organic carbon (delta C-13) are typically used as indicators of terrestrial, intertidal, and offshore organic carbon sources to coastal ecosystems, there is evidence that delta C-13 values are also sensitive to in situ ecosystem metabolism. To investigate this phenomenon, we examined delta C-13 values of filter-feeding hard clams Mercenaria mercenaria from 13 locations in Greenwich Bay, a sub-estuary of Narragansett Bay, Rhode Island (USA). The delta C-13 values of the clams showed a marked linear gradient of 2 parts per thousand over the 4 km length of Greenwich Bay (-19 to 17 %parts per thousand), from lower delta C-13 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 delta C-13 values of clams in Narragansett Bay proper (over 40 km long) are homogenous (mean +/- SD, -16.8 +/- 0.6 parts per thousand, n = 247). Mean daily pH, temperature, and salinity data from 2 fixed monitoring stations 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 delta C-13 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 CO2(aq). The lower delta C-13 of CO2(aq) relative to HCO3- is reflected in the phytoplankton and in the clams that feed on them. Our work suggests that delta C-13 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.

• 出版日期2010