SONaR-CO2

Introduction

The impact of anthropogenic ocean acidification on calcifying organisms is expected to be imminent, particularly in high latitude ecosystems. The Southern Ocean absorbs about 40% of the global oceanic uptake of anthropogenic CO₂ with much of the drawdown occurring in the Sub-Antarctic Zone or SAZ. This large inventory of anthropogenic CO₂ makes this region an ideal setting to assess the response of marine calcifying plankton to increasing anthropogenic CO₂ levels in their natural habitat. Indeed, there is evidence that the ongoing ocean acidification in the SAZ is already affecting the calcification of key calcifying plankton such as planktonic foraminifera and pteropods.

Coccolithophores, unicellular eukaryotic algae that secrete calcite plates (coccoliths; Figure 1), are the most abundant marine calcareous phytoplankton and play an important role in the marine carbon cycle by contributing to the oceanic pumps of organic matter and carbonate (Figure 2). The cosmopolitan coccolithophore Emiliania huxleyi is known to develop large-scale blooms in high latitudes systems where the overproduction and shedding of coccoliths turn the surface waters a milky-turquoise appearance allowing their detection from satellites.

Figure 1: Complete cell (coccosphere) (a) and detached coccoliths (b) of the coccolithophore Emiliania huxleyi. Scale bar 1 µm.

Satellite imagery reveals extensive coccolithophorid blooms in the Southern Ocean resulting in high concentrations of particulate inorganic carbon (PIC) that extend along the circumpolar Subtropical, Subantarctic and Polar Fronts during austral summer, a feature referred to as the “Great Calcite Belt” (Click here to see animation). Changes in coccolithophore abundance, composition and degree of calcification may potentially impact the entire marine ecosystem and ocean chemistry, ultimately affecting the climate.

The Marie Curie-funded project SONaR-CO2 aims to shed light on the ongoing debate whether or not ocean acidification will lead to a replacement of heavily-calcified coccolithophores by lightly-calcified ones in subpolar ecosystems. SONAR-CO2 aims to answer the call by the Southern Ocean Observing System Science Strategy for urgent and increased effort in research initiatives that address the impacts of ocean acidification on marine and coastal ecosystems and resources. Moreover, long-term studies on key environmental variables and organisms are crucial for establishing a baseline against which projected changes can be assessed.

Figure 2: A schematic representation of the oceanic carbon pumps: The production of organic matter through photosynthesis in the photic zone and its subsequent transport to depth in settling particles is referred to as the “biological pump” or “soft tissue pump”. This process lowers atmospheric CO₂ concentration. In contrast, the carbonate counter pump counteracts the effect of the biological pump in terms of its influence on atmosphere-ocean CO₂ exchange. Marine plankton such as coccolithophores and foraminifera precipitate CaCO₃ shells, removing alkalinity and increasing the CO₂ concentration in the surface ocean, thereby driving a CO₂ flux into the atmosphere. The relative strengths between the biological and carbonate pumps is referred to as the “rain ratio” and largely determines the CO₂ exchange between the atmosphere and the ocean. Modified from Rost and Riebesell (2004).