The Extent and Controls on Ocean Acidification
in the Western Arctic Ocean and Adjacent Continental Shelf Seas
J. T. Mathis
School of Fisheries and Ocean Science & Institute of Marine Science,
University of Alaska Fairbanks, Fairbanks, AK, 99709
November 9, 2011
- Observations in the eastern Bering Sea have revealed aragonite undersaturation throughout the water column and areas of seasonal CaCO3 mineral suppression.
- The effects of ocean acidification in the Chukchi Sea, induced by the uptake of anthropogenic CO2 over the last century, are amplified by high rates of summertime phytoplankton primary production, which leads to increased seawater pCO2 and decreased pH of sub-surface waters, which become more corrosive to CaCO3.
It has been widely shown that the uptake of anthropogenic CO2 by the oceans (e.g. Sabine et al., 2004; Sabine et al., 2007) has a significant effect on marine biogeochemistry by reducing seawater pH (Feely et al., 2009; Caldiera et al., 2003) and the saturation states (Ω) of important calcium carbonate (CaCO3) minerals (Feely et al., 2004; Orr et al., 2005; Caldiera and Wickett, 2005) through a process termed "ocean acidification". Seawater exhibiting undersaturated conditions (i.e., Ω <1) are potentially corrosive for biogenic CaCO3 minerals such as aragonite, calcite and high-Mg calcite. The reduction of CaCO3 mineral saturation states in the surface ocean and along continental margins could have potentially negative consequences for benthic and pelagic calcifying organisms, and entire marine ecosystems (Fabry et al., 2008). Of even greater concern is the rate at which ocean acidification and CaCO3 mineral saturation state suppression are progressing, particularly in the high latitude Pacific-Arctic Region (PAR) (Fig. SIO15) (Byrne et al., 2010; Fabry et al., 2009), where mixing processes and colder temperatures naturally precondition the water column to have lower pH and Ω values compared to more temperate ocean environments (Fig. SIO16).
Fig. SIO15. Map of the Pacific Arctic Region (PAR) showing the major drainage basins and the associated biogeochemical properties that affect the carbon cycle. (1) Gulf of Alaska; (2) Bering Sea; and (3) Chukchi Sea. The color bars in the inset illustrate the various terrestrial drainage basins around North America and their predominant biogeochemical characteristics.
Fig. SIO16. Schematic of processes potentially influencing the inorganic carbon cycle and air-sea CO2 gas exchange in the western Arctic Ocean from the Bering Sea shelf through the Bering Strait, across the Chukchi Sea and northwards into the Canada Basin (from left to right) on "inflow" shelves of the Arctic (e.g., Barents and Chukchi seas). The two panels represent physical and biological processes likely operating during the summertime sea-ice free period (Panel a, top), and during the wintertime sea-ice covered period (Panel b, bottom). The processes are denoted by numeral with the caveat that the size of arrow does not necessarily reflect magnitude of flux, transport or transformation of CO2. The processes include: 1. northward transport of DIC; 2. air-sea gas exchange; 3. warming; 4. exposure of surface water to the atmosphere due to sea-ice retreat and melting; 5. localized air-sea gas exchange from surface water highly influenced by sea-ice melt; 7. air-sea gas exchange through sea-ice; 8. winter air-sea gas exchange in leads and polynyas; 9. inorganic carbon flux due to brine rejection during deep-water formation in fall and winter. 10. cooling of surface waters during northward transport of Atlantic or Pacific Ocean waters into the Arctic Ocean; 11. between-shelf transport of water and carbon; 12. redistribution of inorganic carbon between mixed layer and subsurface due to vertical diffusion and vertical entrainment/detrainment due to mixing; 13. shelf-basin exchanges of inorganic carbon (i.e., DIC) and organic carbon due to generalized circulation and eddy-mediated transport; 14. net uptake of CO2 due to phytoplankton photosynthesis or new production; 15. export flux of organic matter (OM) or export production; 16. remineralization of organic matter back to CO2 either in sub-surface waters or in sediments; 17. release of CO2 from sediments; 18. release of alkalinity from sediments due to anaerobic processes in sediments, and; 19. river runoff input. Adapted from Bates and Mathis, 2009.
Recent observations in the sub-Arctic north Pacific Ocean (Mathis et al., 2011) have already revealed areas of seasonal CaCO3 mineral Ω suppression. Aragonite undersaturation has been observed throughout the water column, while models project widening areas of aragonite undersaturation in the region during the next several decades (Steinacher et al., 2009). Undersaturation has potentially negative consequences for the region because the expansive continental shelf of the eastern Bering Sea sustains a commercially valuable fishery (Cooley and Doney, 2009; Cooley et al., 2009) that produces approximately 47% of the US fish catch by weight. This marine ecosystem is critical to both the regional and national economy as well as subsistence communities in Alaska that rely heavily on the seasonal fish catch as their primary source of protein. These new findings show that the eastern Bering Sea will likely be one of the first ocean acidification impact zones for US national interests. Therefore, it is critical to gain a better understanding of both the natural and anthropogenic controls on CaCO3 mineral suppression in the region.
As observed in several open-ocean time-series, the uptake of anthropogenic CO2 has already decreased surface water pH by 0.1 units. IPCC scenarios, based on present-day CO2 emissions, predict a further decrease in seawater pH by 0.3 to 0.5 units over the next century and beyond (Caldeira and Wickett, 2003). Ocean acidification and decreased pH reduces the saturation states of calcium carbonate minerals such as aragonite and calcite, with many studies showing decreased CaCO3 production by calcifying fauna (Buddemeier et al., 2004; Fabry et al., 2008) and increased CaCO3 dissolution. The PAR is particularly vulnerable to ocean acidification due to relatively low pH and low temperature of polar waters compared to other waters (Orr et al., 2005; Steinacher et al., 2009) and low buffer capacity of sea-ice melt waters (Yamamoto-Kawai et al., 2009).
In the high latitude PAR, the uncoupling of primary production and grazing leads to high export rates of organic matter to the bottom waters and the sediments. When this organic matter is remineralized back into CO2, it naturally decreases pH and suppresses carbonate mineral saturation states. However, the presence of anthropogenic CO2 in the water column has caused bottom waters over some parts of the PAR shelves to become undersaturated in carbonate minerals (mostly aragonite, but in some locations calcite undersaturations have been observed).
The Bering Sea
The eastern shelf of the Bering Sea (Fig. SIO15) is a highly dynamic area that is influenced by a number of terrestrial and marine processes (Fig. SIO17) that affect seawater carbonate chemistry with considerable spatial, seasonal and inter-annual variability in the saturation states of the two most biogenically important CaCO3 minerals: aragonite (Ωaragonite) and calcite (Ωcalcite) (Mathis et al., 2011). The springtime retreat of sea ice, coupled with warming and seasonally high rates of freshwater discharge create distinctive horizontal and vertical zones over the shelf, each with their own unique characteristics (Stabeno et al., 1999). The onset of stratification in surface waters stimulates an intense period of phytoplankton primary production (PP), particularly over the middle region of the shelf where the confluence of macronutrient-rich Bering Sea water and coastal water replete in micronutrients is highest (Agular-Islas et al., 2007). In this region, historically referred to as the "green belt", rates of PP or net community production (NCP) can exceed 480 mg C m-2 d-1 while average rates across the shelf are ~330 mg C m-2 d-1, making the eastern Bering Sea shelf one the most productive regions in the global ocean (Sambrotto et al., 2008; Mathis et al., 2010).
Fig. SIO17. Generalized description of the processes affecting the carbonate chemistry of the eastern Bering Sea shelf. The influx of runoff from the coast delivers water with high pCO2, low TA, and moderate concentrations of dissolved organic matter (OM). The high pCO2 of the water creates a seasonal source of CO2 to the atmosphere while reducing carbonate mineral saturation states. Offshore, the upper water column is dominated by sea ice melt in late spring and summer that creates a highly stratified surface layer where primary production is controlled by the confluence of coastal waters rich in micronutrients and basin water replete in macronutrients. Seasonally high rates of NCP lead to a rapid drawdown of CO2 at the surface creating a strong seasonal sink for atmospheric CO2. In 2009, coccolithophore (Cocc.) blooms were observed in the intermediate shelf waters and lowered TA concentrations at the surface. The varying degree of export production at the surface determined the amount of remineralization that occurred at depth, which ultimately controlled saturation states. This PhyCaSS interaction can be observed to varying degrees across the shelf (adapted from Mathis et al., in press).
On the eastern Bering Sea shelf, a seasonal divergence in pH and Ω is observed between surface and sub-surface waters, driven primarily by the biology of the system (Mathis et al., 2010). During the spring phytoplankton bloom, high rates of NCP effectively remove CO2 from the surface waters creating a strong seasonal disequilibrium with the atmosphere (Bates et al., 2010), but also increasing pH and Ω values by ~0.1 and ~1, respectively (Mathis et al., 2011). However, in sub-surface waters the opposite is observed, with pH and Ω values decreasing significantly (~0.3 and ~0.2, respectively) (Mathis et al., 2011). Much of the organic matter that is produced during the spring phytoplankton bloom is exported vertically out of the mixed layer. By mid-summer, the water-column becomes highly stratified and bottom waters are effectively isolated from surface waters over much of the shelf. The vertical export of organic matter and its subsequent seasonal remineralization at depth induces a significant build-up of CO2 in bottom waters (i.e. pCO2 increases) and concurrent suppression of CaCO3 mineral Ω values (Mathis et al., 2010; Mathis et al., 2011). The seasonal divergence of pH and Ω in surface and sub-surface waters has been described in terms of a "Phytoplankton-Carbonate Saturation State" (PhyCaSS) (Bates et al., 2009). In 2008, sub-surface waters of the eastern Bering Sea shelf became undersaturated with respect to aragonite (but not calcite) (Mathis et al., 2011). It has also been shown that the addition of anthropogenic CO2 to the ocean augments this natural seasonal interaction between ocean biology and seawater carbonate chemistry, tipping sub-surface waters below the saturation state threshold (Ωaragonite = 1) for aragonite. Increasing levels of atmospheric CO2 could push the Bering Sea closer to a tipping point that could be detrimental for calcifying organisms.
The Western Arctic Ocean
In the Arctic Ocean, potentially corrosive waters are found in the sub-surface layer of the central basin (Jutterstrom and Anderson, 2005; Yamamoto-Kawai et al., 2009; Cheirici and Fransson, 2009), on the Chukchi Sea shelf (Bates et al., 2009) and in outflow waters of the Arctic found on the Canadian Arctic Archipelago shelf (Azetsu-Scott et al., 2010). In the Chukchi Sea, waters corrosive to CaCO3 occur seasonally in the bottom waters, with unknown impacts on benthic organisms. The seasonally high rates of summertime phytoplankton primary production in the Chukchi Sea (see the essay on Arctic Ocean Primary Productivity) drives a downward export of organic carbon, which is remineralized back to CO2, which in turn increases seawater pCO2 (and decreasing pH) of sub-surface waters. Such a seasonal biological influence on the pH of sub-surface waters amplifies existing effects of ocean acidification induced by the uptake of anthropogenic CO2 over the last century (Bates et al., 2009). Given the scenarios for pH changes in the Arctic, the Arctic Ocean and adjacent Arctic shelves, including the western Arctic, will be increasingly affected by ocean acidification, with potentially negative implications for shelled benthic organisms as well as those animals that rely on the shelf seafloor ecosystem.
Note: Acidification in the Canada Basin of the Arctic Ocean is described in the essay on Ocean Biogeophysical Conditions.
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