Controls on the carbon isotopic composition of Southern Ocean phytoplankton

Carbon isotopic compositions of suspended organic matter and biomarker comp ounds were determined for 59 samples filtered from Southern Ocean surface w aters in January 1994 along two north-south transects (WOCE SR3 from Tasman ia to Antarctica, and across the Princess Elizabeth Trough (PET) east of Pr ydz Bay, Antarctica). Along the SR3 line, bulk organic matter show generall y decreasing C-13 contents southward, which are well correlated with increa sing dissolved molecular carbon dioxide concentrations, CO2(aq). This relat ionship does not hold along the PET transect. Using concentrations and isot opic compositions of molecular compounds, we evaluate the relative roles of several factors affecting the delta(13)C of Southern Ocean suspended parti culate organic matter. Along the WOCE SR3 transect, the concentration of CO 2(aq) plays an important role. It is well described by a supply versus dema nd model for the extent of cellular CO2 utilization and its associated line ar dependence of isotopic fractionation (epsilon(P)) on the reciprocal of C O2(aq). An equally important factor appears to be changes in algal assembla ges along the SR3 transect, with their contribution to isotopic fractionati on also well described by the supply and demand model, when formulated to i nclude the cell surface/volume control of supply. Changes in microalgal gro wth rates appear to have a minor effect on epsilon(P). Along the PET transe ct, algal assemblage changes and possibly changes in microalgal growth rate s appear to strongly affect the carbon isotopic variations of suspended org anic matter. These results can be used to improve the formulation of modern carbon cycle models that include phytoplankton carbon isotopic fractionati on.