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

Abstract

<p><strong>Abstract.</strong> The stable isotopic composition of particulate organic carbon (δ<sup>13</sup>C<sub>POC</sub>) in the surface waters of the global ocean can vary with the aqueous CO<sub>2</sub> concentration ([CO<sub>2(aq)</sub>]) 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 δ<sup>13</sup>C<sub>POC</sub> is investigated in surface waters across the south subtropical convergence (SSTC) in the Atlantic Ocean, to determine carbon isotope fractionation (ε<sub>p</sub>) 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 δ<sup>13</sup>C<sub>POC</sub> across the Atlantic SSTC in summer. Combining cell size estimates with CO<sub>2</sub> concentrations, we can accurately estimate ε<sub>p</sub> within the varying surface water masses in this region. We further utilize these results to investigate future changes in ε<sub>p</sub> with increased anthropogenic carbon availability. Our results suggest that smaller cells, which are prevalent in the subtropical ocean, will respond less to increased [CO<sub>2(aq)</sub>] 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 CO<sub>2</sub> availability to the cell, but also if increased stratification leads to decreases in average community cell size. Coupled with decreasing δ<sup>13</sup>C of [CO<sub>2(aq)</sub>] due to anthropogenic CO<sub>2</sub> emissions, this change in isotopic fractionation and lowering of δ<sup>13</sup>C<sub>POC</sub> may propagate through the marine food web, with implications for the use of δ<sup>13</sup>C<sub>POC</sub> as a tracer of dietary sources in the marine environment.</p>

Trends in tuna carbon isotopes suggest global changes in pelagic phytoplankton communities

“…Such a reduction in the abundance of diatoms, a 13 C-rich carbon source in marine food webs (Fry & Wainright, 1991), is expected to decrease the δ 13 C values of consumers, and this diatom contribution has been emphasized in a recent model of phytoplankton δ 13 C variations in the global ocean (Magozzi et al, 2017). Tuerena et al (2019) also recently found that cell size was the primary determinant of δ 13 POC in the South Atlantic subtropical convergence zone and predicted that carbon isotopic fractionation will increase in the future, leading to lower δ 13 POC that may propagate through the food web. A decrease in the abundance of coccolithophores, another 13 C-rich carbon source, might also explain some of the tuna δ 13 C trend.…”