Isotopic fractionation of carbon during uptake by phytoplankton across the South Atlantic subtropical convergence

Tuerena, Robyn E.; Ganeshram, Raja S.; Humphreys, Matthew P.; Browning, Thomas J.; Bouman, Heather; Piotrowski, Alexander P.

The stable isotopic composition of particulate organic carbon (inline-formulaδ13CPOC) in the surface waters of the global ocean can vary with the aqueous inline-formulaCO2 concentration ([inline-formulaCO2(aq)]) and affects the trophic transfer of carbon isotopes in the marine food web. Other factors such as cell size, growth rate and carbon concentrating mechanisms decouple this observed correlation. Here, the variability in inline-formulaδ13CPOC is investigated in surface waters across the south subtropical convergence (SSTC) in the Atlantic Ocean, to determine carbon isotope fractionation (inline-formulaεp) by phytoplankton and the contrasting mechanisms of carbon uptake in the subantarctic and subtropical water masses. Our results indicate that cell size is the primary determinant of inline-formulaδ13CPOC across the Atlantic SSTC in summer. Combining cell size estimates with inline-formulaCO2 concentrations, we can accurately estimate inline-formulaεp within the varying surface water masses in this region. We further utilize these results to investigate future changes in inline-formulaεp with increased anthropogenic carbon availability. Our results suggest that smaller cells, which are prevalent in the subtropical ocean, will respond less to increased [inline-formulaCO2(aq)] than the larger cells found south of the SSTC and in the wider Southern Ocean. In the subantarctic water masses, isotopic fractionation during carbon uptake will likely increase, both with increasing inline-formulaCO2 availability to the cell, but also if increased stratification leads to decreases in average community cell size. Coupled with decreasing inline-formulaδ13C of [inline-formulaCO2(aq)] due to anthropogenic inline-formulaCO2 emissions, this change in isotopic fractionation and lowering of inline-formulaδ13CPOC may propagate through the marine food web, with implications for the use of inline-formulaδ13CPOC as a tracer of dietary sources in the marine environment.



Tuerena, Robyn E. / Ganeshram, Raja S. / Humphreys, Matthew P. / et al: Isotopic fractionation of carbon during uptake by phytoplankton across the South Atlantic subtropical convergence. 2019. Copernicus Publications.


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