Rising CO2 levels in the atmosphere and ocean have lead to an anthropogenically induced acidification phenomenon in high latitude seas. These areas are projected to become persistently undersaturated with respect to important carbonate minerals as early as mid-century and seasonal aragonite undersaturations have already been observed in surface and shallow subsurface waters over of the continental shelf seas surrounding Alaska. Some calcifying marine organisms, including pteropods, foraminifers, mollusks, and coralline algae that could be susceptible to reduced calcification rates under increasing ocean acidity are keystone species in the Pacific-Arctic region. Recent observations along the only long term time-series in the northern Gulf of Alaska found that the high seasonal and spatial variability of the carbonate parameters are largely controlled by physical circulation and glacial discharge. In general, surface DIC and TA concentrations decreased between May and September due to primary production and dilution from the region’s numerous glacial sources. Conversely, concentrations of DIC and TA increased in the bottom waters of the inner shelf between May and September likely due to a combination of remineralization of exported organic matter and seasonally induced upwelling. Analysis of the calcite and aragonite saturation states (Ω) showed an increase in the surface layer from May to September. However, in the bottom waters over the inner shelf the Ω of calcite and aragonite was suppressed and aragonite undersaturations were observed within 50 m of the surface. In the Bering Sea, prior to sea ice retreat in 2008, calcite and aragonite Ω ranged from 1.3 to 3.2 and 0.8 to 2.0 respectively in the upper 30 m over the shelf. Two inshore stations likely impacted by the outflows of the Yukon and Kuskokwim Rivers showed aragonite undersaturation (0.91 – 0.84) from the surface to the bottom. In summer, DIC concentrations in the upper 30 m were drawn down by primary production and diluted by sea ice melt. At most locations, calcite and aragonite Ω had increased compared to spring. However, beneath the mixed layer (30 – 150 m), DIC concentrations increased between spring and summer likely due to the remineralization of exported organic matter. This increase in DIC caused a suppression of the carbonate mineral Ω near the bottom where calcite and aragonite Ω as low as 1.08 and 0.68, respectively were observed. Biological amplification of ocean acidification effects in subsurface waters could reduce the ability of some calcifying species to produce shells or tests with profound implications for Bering Sea benthic ecosystems, including the commercially valuable crab fishery. In both the northern Gulf of Alaska and the eastern Bering Sea, the observed aragonite undersaturations were directly related to the intrusion of anthropogenic CO2 indicating that these are likely recent occurrences.
Mathis, J. T., Cross, J. N. & Shake, K. L., 2010. Coupling physics, biology and terrestrial runoff to ocean acidification and carbonate mineral suppression in the Pacific-Arctic region. American Geophysical Union, Fall Meeting 2010, abstract #OS32B-02. Abstract.
