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Abstract
In an attempt to reduce the threat of global warming, it has been proposed that the
rise of atmospheric carbon dioxide concentrations be reduced by the ocean disposal
of CO2 from the flue gases of fossil fuel-fired power plants. The release of large
amounts of CO2 into mid or deep ocean waters will result in large plumes of acidified
seawater with pH values ranging from 6 to 8. In an effort to determine whether these
CO2-induced pH changes have any effect on marine nitrification processes, surficial
(euphotic zone) and deep (aphotic zone) seawater samples were sparged with CO2 for
varying time durations to achieve a specified pH reduction, and the rate of microbial
ammonia oxidation was measured spectrophotometrically as a function of pH using an
inhibitor technique. For both seawater samples taken from either the euphotic or aphotic
zone, the nitrification rates dropped drastically with decreasing pH. Relative to
nitrification rates in the original seawater at pH 8, nitrification rates were reduced
by ca. 50% at pH 7 and more than 90% at pH 6.5. Nitrification was essentially completely
inhibited at pH 6. These findings suggest that the disposal of CO2 into mid or deep
oceans will most likely result in a drastic reduction of ammonia oxidation rates within
the pH plume and the concomitant accumulation of ammonia instead of nitrate. It is
unlikely that ammonia will reach the high concentration levels at which marine aquatic
organisms are known to be negatively affected. However, if the ammonia-rich seawater
from inside the pH plume is upwelled into the euphotic zone, it is likely that changes
in phytoplankton abundance and community structure will occur. Finally, the large-scale
inhibition of nitrification and the subsequent reduction of nitrite and nitrate concentrations
could also result in a decrease of denitrification rates which, in turn, could lead
to the buildup of nitrogen and unpredictable eutrophication phenomena. Clearly, more
research on the environmental effects of ocean disposal of CO2 is needed to determine
whether the potential costs related to marine ecosystem disturbance and disruption
can be justified in terms of the perceived benefits that may be achieved by temporarily
delaying global warming.