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

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

doi:10.5194/bg-16-3621-2019

Abstract

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.

Highlights

The marine environment is undergoing rapid changes as atmospheric carbon dioxide increases, with the greatest change occurring in the upper ocean (Gruber et al, 1999; Sabine and Tanhua, 2010)
In the colder Subantarctic Surface Waters (SASW), located south of the south subtropical convergence (SSTC), concentrations of macronutrients are elevated and primary production is primarily limited by iron availability (Browning et al, 2014)
Higher δ13CCO2 is associated with lower [CO2(aq)] and warmer temperatures of the subtropical water masses (Fig. 1). δ13CCO2 is highest on the western boundary in the Brazil Current (BC) and in the Rio de la Plata outflow. δ13C of particulate organic carbon (δ13CPOC) across 40◦ S ranges from −25 ‰ to −20 ‰ indicating a predominantly marine source (e.g. Rau et al, 1989)

Summary

Introduction

The marine environment is undergoing rapid changes as atmospheric carbon dioxide increases, with the greatest change occurring in the upper ocean (Gruber et al, 1999; Sabine and Tanhua, 2010). Anthropogenic carbon inputs and the increase of greenhouse gases in the atmosphere are causing ocean warming (Cheng et al, 2019), changes to upper ocean stratification (Bopp et al, 2001; Capotondi et al, 2012), and altered distributions of nutrients and carbon (Khatiwala et al, 2013; Quay et al, 2003; Gruber et al, 2019). Alterations to phytoplankton diversity and/or productivity will likely have knock-on effects on marine food web dynamics. Investigating such changes in remote marine environments requires tracers that can pinpoint shifts in dietary sources. The δ13C of organic carbon in marine plants and animals can provide information on carbon sources to the base of the food web (Peterson and Fry, 1987)

Methods

Results

Conclusion