Application of Stable Carbon Isotopes in a Subtropical North Atlantic MesocosmStudy: A New Approach to Assess CO2 Effects on the Marine Carbon Cycle

Mario Esposito,Eric P Achterberg,Lennart T Bach,Douglas P Connelly,Ulf Riebesell,Jan Taucher

doi:10.3389/fmars.2019.00616

Abstract

Stable isotope ratio analysis offers a unique opportunity to obtain information on ecosystem processes. The increase in atmospheric CO2 as a consequence of fossil fuel combustion and land-use change is altering the stable carbon isotope composition (δ13C) of the atmosphere and ocean. This work investigates the application of using δ13C measurements of seawater samples to explore the biogeochemical responses of marine ecosystems to anthropogenic CO2 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 pCO2 ranging from ~350 up to ~1,000 μatm during a phytoplankton succession. A clear CO2 response was detected in the isotopic datasets with 13C shifts of up to ~5%0, but increased CO2 levels only had a subtle effect on the concentrations of the dissolved and particulate organic carbon pools. Distinctive δ13C signatures of the particulate organic carbon pools in the water column and sediment traps were detectable for the different CO2 treatments after a nutrient stimulated phytoplankton bloom. These signatures were strongly correlated (p 13C signatures of the inorganic carbon but not with the δ13C of the dissolved organic carbon pools (p > 0.05). Fractionation of carbon isotopes in phytoplankton was positively affected (9.6 2 levels either because of the higher CO2 availability or because of a shift in phytoplankton community composition. Nevertheless, phytoplankton bloom intensity and development was independent of CO2 concentrations, and higher CO2 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.

Highlights

Since the beginning of the industrial revolution, the concentration of carbon dioxide (CO2) in the atmosphere has increased by circa 40% from about 280 ppm to values above 400 ppm
Attachment of the sediment trap and simultaneous pulling of the upper part of the bags above the sea surface level marked the beginning of the experiment (27th of September 2014) defined as time t-4 with t0 marking the day of the initial CO2 manipulation
A total of 9 mesocosms were deployed in Gando Bay, but only four of them (M5, M7, M8, and M9) were sampled for stable carbon isotope analysis

Summary

Introduction

Since the beginning of the industrial revolution, the concentration of carbon dioxide (CO2) in the atmosphere has increased by circa 40% from about 280 ppm to values above 400 ppm (http://www.esrl.noaa.gov). The flux of CO2 between the atmosphere and the ocean is mainly controlled by physical processes (Couldrey et al, 2016); biological processes affect the air-sea CO2 transfer: phytoplankton assimilate inorganic carbon dissolved in seawater and convert it into organic forms via photosynthesis. Surface water CO2 concentrations decrease promoting further CO2 transfer from the atmosphere. It is assumed that primary production and sinking of organic matter to depth (export) contribute to increase ocean CO2 sequestration from the atmosphere while community respiration tends to decrease it (Volk and Hoffert, 1985)

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Discussion

Conclusion