Variations in the isotopic composition of particulate organic carbon in surface waters along an 88°W transect from 67°S to 54°S

Abstract

The concentration and isotopic composition of carbon was measured in suspended particulate organic matter in surface waters of the Southern Pacific Ocean along an 88°°W transect from 67−54°S during the Austral summer. Concurrent measurements of total dissolved inorganic carbon (ΣCO 2), pCO 2, particulate organic carbon (POC), salinity, chlorophyll and temperature provide an opportunity to study the covariance of these parameters with changes in the isotopic composition of particulate organic carbon (δ 13C-POC). The south-north transect did not show any significant changes in POC (10–19 mmol m −3) or chlorophyll (0.5–1 mg m −3), while δ 13C-POC variations of approximately 4%o were recorded. Although surface water CO 2(aq) is substantially out of equilibrium with the local atmosphere, the isotopic changes are negatively correlated with the concentration of dissolved carbon dioxide along the transect, and follow the general trends predicted from previously determined relationships. Estimates of ε p ( ε # p ) are less well constrained and show a weaker correlation with [CO 2 (aq)]. Where ε # p varies independently of [C0 2(aq)] small changes in the concentration gradient between extra- and intra-cellular CO 2 can be predicted. The difficulty in obtaining independent measures of growth rate and matching these measurements to the timescales over which the 613C of the phytoplankton is integrated is examined.