Abstract. The stable isotopic composition of particulate organic carbon
(δ13CPOC) in the surface waters of the global ocean can
vary with the aqueous CO2 concentration ([CO2(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 δ13CPOC is investigated in surface waters across the south
subtropical convergence (SSTC) in the Atlantic Ocean, to determine carbon
isotope fractionation (εp) 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 δ13CPOC across the Atlantic SSTC in summer. Combining
cell size estimates with CO2 concentrations, we can accurately estimate
εp within the varying surface water masses in this region.
We further utilize these results to investigate future changes in
εp with increased anthropogenic carbon availability. Our
results suggest that smaller cells, which are prevalent in the subtropical
ocean, will respond less to increased [CO2(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 CO2 availability to the cell, but also
if increased stratification leads to decreases in average community cell
size. Coupled with decreasing δ13C of [CO2(aq)] due to
anthropogenic CO2 emissions, this change in isotopic fractionation and
lowering of δ13CPOC may propagate through the marine food
web, with implications for the use of δ13CPOC as a tracer
of dietary sources in the marine environment.
Benthic-pelagic trophic coupling in an Arctic marine food web along vertical water mass and organic matter gradients
