Application of stable carbon isotopes in a subtropical North Atlantic mesocosm study: a new approach to assess CO 2 effects on the marine carbon cycle

• Published in: Frontiers in Marine Science
• Main Authors: Esposito, M, Achterberg, EP, Bach, LT, Connelly, DP, Riebesell, U, Taucher, J
• Format: Article in Journal/Newspaper
• Language: English
• Published: Frontiers Research Foundation 2019
• Subjects: Earth Sciences; Oceanography; Biological Oceanography; North Atlantic
• Online Access: https://doi.org/10.3389/fmars.2019.00616; http://ecite.utas.edu.au/136056

Stable isotope ratio analysis offers a unique opportunity to obtain information on ecosystem processes. The increase in atmospheric CO 2 as a consequence of fossil fuel combustion and land-use change is altering the stable carbon isotope composition (δ 13 C) of the atmosphere and ocean. This work investigates the application of using δ 13 C measurements of seawater samples to explore the biogeochemical responses of marine ecosystems to anthropogenic CO 2 perturbations. The combination of isotopic and non-isotopic measurements from a subtropical North-Atlantic mesocosm experiment provided a holistic view of the biogeochemical mechanisms that affect carbon dynamics under a gradient of pCO 2 ranging from ~350 up to ~1,000 μatm during a phytoplankton succession. A clear CO 2 response was detected in the isotopic datasets with 13 C shifts of up to ~5%0, but increased CO 2 levels only had a subtle effect on the concentrations of the dissolved and particulate organic carbon pools. Distinctive δ 13 C signatures of the particulate organic carbon pools in the water column and sediment traps were detectable for the different CO 2 treatments after a nutrient stimulated phytoplankton bloom. These signatures were strongly correlated ( p < 0.05) with the δ 13 C signatures of the inorganic carbon but not with the δ 13 C of the dissolved organic carbon pools ( p > 0.05). Fractionation of carbon isotopes in phytoplankton was positively affected (9.6 < ε < 16.5%0) by high CO 2 levels either because of the higher CO 2 availability or because of a shift in phytoplankton community composition. Nevertheless, phytoplankton bloom intensity and development was independent of CO 2 concentrations, and higher CO 2 levels had no significant effect on inorganic nutrient uptake. Results from this mesocosm experiment showed that variations in the carbon isotopic signature of the carbon pools depend on both physical (air-sea exchange) and biological (community composition) drivers opening the door to new approaches for investigations of carbon cycling in marine ecosystems.