Acidification in the Sea of Japan between 1965 and 2011

Oceans absorb approximately 26% of carbon dioxide emitted to the atmosphere owing to fossil fuel burning, cement production and clearing of forests. Consequently, the pH value of seawater may be decreasing at a rate not seen for at least the last 30 million years. As is well known, such ocean acidification significantly impacts the ability of many major marine organisms (e.g., calcareous phytoplankton) to build their outer shells as well as inner skeletal structures, subsequently reducing the growth and survival of early life stages of some species.

The rate of acidification is generally diminished with an increasing depth. Slowing down the thermohaline circulation due to global warming could reduce the pH in the deep oceans. The effect of deep oceans acidification has not been discussed. As a miniature ocean with its own deep and bottom water formations, the Sea of Japan provides an insight into how future global warming can alter the deep oceans acidification.

The data collected between 1965 and 2011 from the Sea of Japan have been archived by the Japan Meteorological Agency (JMA). To compare with the observed data, we considered three factors which affect ocean acidification: (1) global warming; (2) penetration of anthropogenic CO2 and (3) reduced ventilation. The observed data shows that the acidification rates have the maximum of -0.00236 pH units yr-1 at a depth of 300m. The rates increase from -0.00129 pH units yr-1 at 1000m to -0.00303 pH units yr-1 at 2500m. In our estimation, the acidification rate due to (1) global warming only affects the upper 100m seawater; (2) penetration of anthropogenic CO2 in the Sea of Japan is faster than that of the open ocean at the same depth, owing to the younger Sea of Japan water; (3) reduced ventilation has accumulated more CO2 and acid by the decomposition of organic matter. This factor is more effective than anthropogenic CO2 penetration. To sum up the three factors above, the estimated rates at the depths below 1500m are similar to those observed rates.

Our result shows that the effect of reduced ventilation should be considered in the ocean acidification studies. The ocean ecosystem could be impacted earlier and more severely than only considering the penetration of anthropogenic CO2.

Hsieh C.-H., 2013. Acidification in the Sea of Japan between 1965 and 2011. MSc thesis, National Sun Yat-sen University. Thesis (restricted access).

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