Excess CO2 making West Coast waters too acidic for marine organisms with shells

       Scientists have long expected that recent high atmospheric CO2 levels will make ocean water more acidic, but the impact was expected to be small. But a Swiss team suggests that by 2050, more than half of the surface waters in the California current off the US west coast will become too acidic for shell-building marine life to thrive or survive—at least at certain times of the year. When that eventually happens, essentially all of the water in the bottom layer will not be capable of supporting oysters and other shell-building organisms.

       When CO2 is dissolved in water, it creates a weak acid with some hydrogen ions. Acidity measures how many hydrogen ions are present; and when more are present, an organism expends more work to build a shell. Many marine organisms, including oysters, crabs, lobsters, periwinkles, and corals, use calcium carbonate (CaCo3) to build shells, which are essential for their life. Above a certain level of acidity, it is no longer possible for them to build shells, and in fact shells may dissolve.

       The atmospheric level of CO2 has become higher over the last century. The impact on the oceans is still small, but in certain areas where deep water wells up to the surface, CO2 is already naturally high. In these areas the CO2 level in the water is closer to the “saturation point” of carbonate.

       In the California current, as in west-coast currents around the world, deep cold water rises up to the surface. The water is rich in nutrients from organic matter that decomposed in the depths or on the bottom. Decomposition releases CO2, which makes water more acidic and closer to being undersaturated. There have been incidents of so-called “corrosive” undersaturated water on the Oregon coast that led to failures of oyster hatcheries (see one report here1). To date, excess atmospheric CO2 has reduced oceanic pH by about 0.1 unit and the saturation state of carbonate by 0.4 units for the ocean as a whole.

       A team from ETH in Zurich, Switzerland, explored where and when acidification might result from elevated CO2 levels. They simulated the ocean chemistry of the California current to the year 2050, in a model based on two common scenarios of CO2 emissions from economic activities. Their model was able to depict details of the current and the nearshore waters, unlike global ocean models which are too coarse to depict such fine details.

Continues . . .

       The Swiss team reported2 that large areas (about one-half) of California surface waters are expected to become undersaturated, and thus “acidic” in the summer, by 2050. If this happens, sensitive organisms would be forced to live in less than half of their current habitat area. Water becomes more acidic the deeper one goes: in twenty to thirty years from now, “essentially all the waters above the shelf sediments will be undersaturated” and water on the sea floor would be undersaturated all year long.

       The impact of acidification depends very much on the atmospheric level of CO2. When the level reaches 400 parts per million (ppm) (the current level in 2012 is 390 ppm), “substantial parts of the twilight zone . . . and habitats along the sea floor on the shelf become undersaturated.” Many scientists expected that the Arctic Ocean would be the first region to experience profound acidification, but the California waters will become acidic before the Arctic Ocean will. When the atmospheric CO2 reaches 500 ppm, surface waters in the sunlit zone would start to be acidic for extended periods.

       The Swiss cautioned that organisms differ greatly in their tolerance of acidity: for some, undersaturated water is not a problem. And some species have evolved to tolerate acidic water for certain periods of time.

       The authors conclude that the California current is rapidly moving toward conditions “well outside the natural range” with frequently undersaturated water. The impact of this can be magnified if further warming or low oxygen levels (“dead zones”) accompany the acidic water. Both have been noted in the California current.

       Jane Lubchenco, administrator of the US National Oceanic and Atmospheric Administration (NOAA), remarked to the Associated Press that the speed by which the oceans have become more acid caught scientists off-guard, and the problem is now considered to be the “equally evil twin” of climate change in the atmosphere.

CITATIONS

1. “Rising acidity brings an ocean of trouble,” news focus, Science, vol. 337, 146-148, 13 July 2012.

2. “Rapid progression of ocean acidification in the California Current system” by N. Gruber, C. Hauri, Z. Lachkar, D. Loher, T. Frolicher, G-K. Plattner, Science, vol. 337, 220-223, 13 July 2012.

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