Increasing atmospheric CO2, ocean acidification and pelagic ecosystems

Gangstø, Reidun (2009). Increasing atmospheric CO2, ocean acidification and pelagic ecosystems (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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This thesis explores the impacts of rising atmospheric CO2 and the subsequent ocean acidification on pelagic calcifiers.
The ocean plays a major role in the global carbon cycle and in regulating the atmospheric concentrations of CO2. About one third of the CO2 emitted to the atmosphere each year by human activities is taken up by the ocean. This leads to a reduction in pH (a measure of acidity) and a decrease in CaCO3 saturation state, which makes the ocean more corrosive to the mineral CaCO3. The pH of the oceans has already dropped by 0.1 units since pre-industrial times, and the current rate at which this process is occurring will likely have large biological consequences for ocean ecosystems within the near future. Plankton which build shells out of CaCO3 (mainly coccolithophores, foraminifers, and pteropods) might have difficulties producing and maintaining their shells. Two important forms of CaCO3 in the ocean are calcite and aragonite, the latter being 50% more soluble. Pteropods, which produce their shells of aragonite, might thus be especially vulnerable to increasing atmospheric CO2. These species are an important element of the food web in high-latitude surface waters and changes in their abundance will affect the whole food chain up to higher levels. A decrease in CaCO3 production would also provide a negative feedback on atmospheric CO2, with an additional CO2 uptake by the
oceans.
A prerequisite to address the impact of ocean acidification on pelagic ecosystems is a quantitative understanding of the evolution of the aragonite and calcite saturation states. In order to project this evolution, evaluate the global and regional impacts on the carbonate budget and determine the associated feedbacks, global biogeochemical models are essential. The studies performed in this thesis are all based on the biogeochemical model PISCES, which simulates the marine biological productivity and the biogeochemical cycles of carbon and main nutrients. This model is used in combination with two global dynamical models of different complexity: the NEMO/OPA model and the Bern3D model. The NEMO/PISCES model has a relatively high spatial resolution, which allows for an advanced representation of ocean dynamics and ecosystems. The Bern3D/PISCES model has a comparatively lower spatial resolution, which makes it more cost-efficient and ideally suited for sensitivity studies and multi-scenario analyzes.
The thesis starts with an introduction to the global carbon cycle and its changes due to increasing atmospheric CO2, with a special focus on the marine carbon cycle.
Chapter 2 includes descriptions of the NEMO/PISCES and the Bern3D/PISCES models. Implementations and changes done to the PISCES model in order to better simulate the global carbonate system are presented. Furthermore, a short overview of my work related to implementing the PISCES model into the Bern3D model is given, including a small sensitivity study on the most important tuning parameters. Finally, model results of the new Bern3D/PISCES model are shown and discussed in order to verify the functioning of the model.
In Chapter 3, a model study with the NEMO/PISCES model is presented (Gehlen et al. 2007), investigating the changes in CaCO3 production and dissolution in response to increasing atmospheric CO2 concentrations. In this study, CaCO3 is modeled as calcite only. A dependency of the calcite production and dissolution of saturation state was implemented. This was based on laboratory and mesocosm studies on the evolution of particulate inorganic and particulate organic carbon in the coccolithophorid Emiliana huxleyi as a function of increasing atmospheric CO2. The model predicts global values of CaCO3 production and dissolution in line with recent estimates. The effect of rising pCO2 on the CaCO3 cycle was quantified by means of model simulations forced with atmospheric CO2. In addition to addressing changes in calcite production and dissolution, the CO2-calcification feedback was estimated.
Chapter 4 presents a second model study with the NEMO/PISCES model (Gangstø et al. 2008). This time aragonite was implemented as a new tracer. The production of aragonite is assigned to mesozooplankton as a function of biomass and seawater aragonite saturation state in an equivalent way as it was done for calcite (Gehlen et al. 2007). Observation-based estimates of marine carbonate production and dissolution are well reproduced by the model. Our model results highlight the contribution of aragonite dissolution to the total CaCO3 dissolution flux above 2000 m. They suggest that the aragonite cycle should be included in biogeochemical models for a realistic representation of shallow CaCO3 dissolution and alkalinity. The study includes an exploration of the response of calcite and aragonite production to decreasing CaCO3 saturation states under the SRES A2 CO2 scenario. Geographically, the effect from increasing atmospheric CO2, and the subsequent reduction in saturation state, is largest in the subpolar and polar areas, with especially large reductions in aragonite production projected by the year 2100.
Chapter 5 includes a study with the new Bern3D/PISCES model (Gangstø et al., manuscript in preparation). The model reproduces observation-based estimates of physical and biological fields. Estimates of marine carbonate production and dissolution are also well represented. The model results confirm that including aragonite improves the modeled alkalinity and carbonate cycle, in line with previous studies (Gangstø et al. 2008). The Bern3D/PISCES model and the NEMO/PISCES, which have different physics and resolution, estimate comparable responses of CaCO3 production and related feedbacks to increasing atmospheric CO2. The effect of using various parameterizations of the response of CaCO3 production to decreasing saturation state is explored for nine different model versions, where the parameterizations are fitted to available laboratory and field experiments. Calcite and aragonite may be produced by auto- and heterotrophic plankton groups and various IPCC CO2 scenarios are used. Under the business-as-usual scenario, the CaCO3 production of all the model versions decreases from 1 Pg C y−1 to between 0.36 and 0.82 Pg C y−1 by the year 2100. Despite the wide range of parameterizations and model versions that are included in our study, the changes in CaCO3 production and dissolution following from ocean acidification provide only a small overall feedback on the atmospheric CO2 of 6-29 Pg C by the year 2100. Undersaturation in the surface area of the Arctic and Southern Ocean are projected within a few decades, under the business-as-usual scenario. Assessments of the legacy of atmospheric CO2 emissions up to the year 2500 conclude that undersaturation in the Arctic is maintained over several centuries, even under a moderate CO2 scenario. A larger reduction in aragonite than calcite production with increasing atmospheric CO2 buffers the effect of decreasing saturation state and speeds up the recovery of the ocean carbonate system after the CO2 perturbation. While the CO2-calcification feedback on atmospheric CO2 remains small for all the studied cases, the potential for major impacts on the marine ecosystem must not be excluded.
An outlook with future research topics is given in Chapter 6. Finally, the appendix provides additional information about the PISCES model and how to use the Bern3D/PISCES model.

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