The waters off the west coast of the USA are known for their particularly large wealth of fauna and flora. This is due to the alongshore wind driving the surface water away from the coast, causing the upwelling of nutrient-rich water from depths. However, this upwelling process makes the nearshore waters also generally low in pH (higher acidity). As a result, these systems are particularly prone to the worldwide phenomenon of ocean acidification. The acidification of the seawater is a direct consequence of the increased CO2 concentration in the atmosphere, since oceans absorb about one third of the carbon dioxide produced by mankind. They thus function as an important carbon sink. However, the CO2 affects the chemical composition of the oceans. It dissolves in seawater and increases its acidity, thereby reducing the saturation level with regard to the mineral calcium carbonate. If the water is undersaturated, calcium carbonate spontaneously dissolves and new calcium carbonate can be formed only if it is protected from the surrounding sea water. This could have serious consequences for many organisms, particularly for those that form shells or skeletons out of calcium carbonate, such as mussels and corals.
Scientists working with Nicolas Gruber, a professor at the Institute of Biogeochemistry and Pollutant Dynamics at ETH Zurich, have now investigated how the acidity and the degree of carbonate saturation along the west coast of the USA will develop in the future. Using a high-resolution model, they first simulated the circulation of the area, in particular the upwelling processes. Then they combined this physical model with models of the ecosystems and of the carbon cycle, taking into consideration especially the exchange of CO2 with the atmosphere. By doing so, they were able to create projections for different atmospheric CO2 scenarios until the year 2050. They are presenting their findings in the current issue of Science magazine.
The simulations show that even in an optimistic CO2 scenario, the saturation state of carbonate drops rapidly in the regions studied, passing the important threshold where waters go from super- to undersaturated conditions. While today the water masses in the top 200 meters are nearly always supersaturated, in 20 to 30 years, such water masses will only be found in the top 60 meters during the summer. In 2050, the seawater will no longer have a year-round sufficient saturation state. This is especially concerning since many organisms live in the top 100 meters of the water.
The acidification is most clearly manifesting itself in the nearshore within 10 kilometres of the coast. There, the pH value will drop to 7.8 by 2050. "Considerable changes in the ecosystem along the west coast of the USA are bound to occur," explains Nicolas Gruber. However, the scientists can't predict yet how these ecosystem changes will look like in detail. Although they are able to calculate the chemical and physical aspects of ocean acidification very precisely, not enough is known about the sensitivity of the different organisms. Not all organisms suffer equally from the ocean acidification, and a few species may even profit from it. Nevertheless, mussels, particularly in their early stages appear to be among those most strongly impacted.
The scientists are concerned that ocean acidification is increasing so rapidly. Nicolas Gruber thinks that the undersaturation limit will be reached within the next 20 to 30 years. Considering how much the CO2 emissions have increased in the last years, the development would be hard to stop: "Our study is an example of how mankind is about to exhaust the limits of what an ecosystem can tolerate."
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