As atmospheric carbon dioxide concentrations increase, the ocean absorbs about 30 % of this additional CO2, leading to a decrease in seawater pH and the concentration of carbonate ions. Carbonate ions are important building blocks for organisms that build their shells or skeletons from calcium carbonate minerals, such as aragonite. Ocean acidification therefore poses major risks to calcifying organisms such as mussels, oysters, crabs, and sea snails, as they have difficulties building their shells and skeletons in waters with lower concentrations of carbonate ions, and as a result, may make them more vulnerable to predators. Other organisms, such as Coho salmon show impaired sensory systems as a result of ocean acidification and may have more difficulties to find their homing streams or escape predators. 

Because CO2 more readily dissolves in cold water, carbonate ion concentrations are naturally lower in polar than in temperate regions. High-latitude marine ecosystems are therefore naturally already closer to harmful thresholds. So, it only takes a little extra carbon dioxide emitted by humans to make the oceans around Alaska corrosive to shells.

With NSF support from this grant, the researchers from the University of Alaska Fairbanks developed a new modeling tool to study the changing chemical environment of the Gulf of Alaska. The researchers combined physical and biogeochemical ocean models, and a hydrological model to reproduce past Gulf of Alaska conditions from 1980 to 2013. The output can be easily accessed and used by the community interested in the seascape of the Gulf of Alaska through an online interactive modeling tool (Figure 1). With it, users can create maps, plot data and view statistics for over 100 ocean variables, including temperature, salinity, pH and carbon dioxide in the water. Many of the variables are linked to ocean acidification and climate change.

The researchers relied on available observations to thoroughly evaluate the model. The model has since been used to get a better understanding of the seasonal and interannual variability, and long-term trends of pH and the concentration of carbonate ions (building blocks) in the waters of the Gulf of Alaska in months of the year and areas that lack observations. For example, the model suggests that the majority of the near-bottom waters off the coastal town of Seward have such a low concentration of carbonate ions between June and January that this may be harmful for organisms dependent on these building blocks (Hauri et al., 2020).

The researchers also identified natural decadal fluctuations in chemical conditions that are driven by the strength of the North Pacific Subpolar Gyre (Hauri et al., 2021). This gyre is a large wind-driven system of circulating ocean currents affecting the Gulf of Alaska. When the gyre is strong it brings more deep water rich in carbon dioxide to the ocean's surface. This can accelerate ocean acidification creating extreme events that cause stress to sensitive organisms. However, when the gyre is weak less carbon is delivered to the surface which can dampen the observed ocean acidification effect or even reverse it (Figure 2).  

From 2011 to 2013, the model showed a strong phase of the gyre resulted in an ocean acidification extreme event in the center of the Gulf of Alaska. This event preceded the 2014-2016 "blob" of exceptionally warm water in the same region. Since the blob followed right after this very strong ocean acidification event, some organisms were probably first stressed because of ocean acidification and then they were hit with heat. In fact, a collaborative study (Bednarsek et al., 2021) shows that pteropods are already dissolving in the subpolar gyre region of the Gulf of Alaska. Pteropods are tiny seasnails that have a shell made out of the calcium carbonate mineral ?aragonite?. They are part of the zooplankton community and are an important food source for a variety of salmon species in Alaska. Pteropods that were collected from water with lowest concentrations of carbonate ions showed the highest degree of dissolution. The observational research was coupled with results from this modeling study that showed that the habitat with favorable conditions for pteropods has shrunk considerably since 1980. 

Another finding of this modeling work is that multiple decades of observational data are necessary to separate out the long term trend of ocean acidification from the natural variability driven by the strength of the Subpolar Gyre. This type of dataset does not currently exist for the Gulf of Alaska. 

Last Modified: 09/23/2021
Modified by: Claudine Hauri