Controls on the carbon isotopic composition of Southern Ocean phytoplankton

• Published in: Global Biogeochemical Cycles
• Main Authors: Popp, BN, Trull, T, Kenig, F, Wakeham, SG, Rust, TM, Tilbrook, B, Griffiths, FB, Wright, SW, Marchant, HJ, Bidigare, RR, Laws, EA
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Geophysical Union 1999
• Subjects: Biological Sciences; Ecology; Marine and Estuarine Ecology (incl. Marine Ichthyology); Princess Elizabeth Trough; Prydz Bay; Southern Ocean; Antarc*; Antarctica
• Online Access: https://doi.org/10.1029/1999GB900041; http://ecite.utas.edu.au/17008

Carbon isotopic compositions of suspended organic matter and biomarker compounds were determined for 59 samples filtered from Southern Ocean surface waters in January 1994 along two north-south transects (WOCE SR3 from Tasmania to Antarctica, and across the Princess Elizabeth Trough (PET) east of Prydz Bay, Antarctica). Along the SR3 line, bulk organic matter show generally decreasing 13C contents southward, which are well correlated with increasing dissolved molecular carbon dioxide concentrations, CO 2(aq). This relationship does not hold along the PET transect. Using concentrations and isotopic compositions of molecular compounds, we evaluate the relative roles of several factors affecting the 13C of Southern Ocean suspended particulate 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 demand model for the extent of cellular CO 2 utilization and its associated linear dependence of isotopic fractionation (p) on the reciprocal of CO 2(aq). An equally important factor appears to be changes in algal assemblages along the SR3 transect, with their contribution to isotopic fractionation also well described by the supply and demand model, when formulated to include the cell surface/volume control of supply. Changes in microalgal growth rates appear to have a minor effect on p. Along the PET transect, algal assemblage changes and possibly changes in microalgal growth rates appear to strongly affect the carbon isotopic variations of suspended organic matter. These results can be used to improve the formulation of modern carbon cycle models that include phytoplankton carbon isotopic fractionation.