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Uta Passow


Uta Passow, Marine Science Institute, University of California- Santa Barbara
Phone Number:  805-893-2363 


Ocean acidification Research on Carbon Cycling

The marine carbon cycle is an important component of the global carbon cycle and the ocean has to date taken up a third to a fourth of the CO2 generated by fossil fuel burning. It is important to know if the future ocean will continue to act as a carbon sink, if we want to make predictions regarding future atmospheric CO2 and global change. 

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The atmosphere and the surface layer of the ocean exchange carbon dioxide. Increasing concentrations of carbon dioxide in the atmosphere due to fossil fuel burning thus lead to an increase of inorganic carbon in the surface ocean. Carbon dioxide reacts with water forming bicarbonate, carbonate and hydrogen ions (see figure). Consequently, elevated atmospheric pCO2 concentrations result in an increase in total inorganic carbon, in a shift in the relative concentrations of the different inorganic carbon species and in a simultaneous decrease in pH in surface waters. These changes are termed ocean acidification. Already a pH of 7.6 can be observed in surface waters off the Californian coast (pre-industrial world avg. 8.2 pH units). Ocean acidification effects the formation of calcium carbonate shells, which dissolve more easily in more acidic conditions. But ocean acidification also impacts reproduction, growth, photosynthesis and nitrogen fixation rates. Shifts in the speciation of inorganic carbon impact the uptake of carbon during photosynthesis, as energetically the cellular uptake of bicarbonate ions differs from that of carbonate ions. Most likely ocean acidification will also impact the chemical behavior and cycling of organic carbon. A lower pH is expected to impact all parts of the carbon cycle, including production, exudation, utilization, aggregation and sedimentation of organic matter, impacting the efficiency of the biological pump (, and the sequestration rate of carbon by the ocean.

One of our projects concentrates on the impact of ocean acidification on aggregation, which drives carbon flux and the biological pump. Possibly a lower pH will lead to a decrease in aggregation and subsequently sedimentation rates (for more details: In another of our ocean acidification projects we and our collaborators investigate potential shifts in the quantity and quality of organic matter produced by phytoplankton under future CO2 regimes. We hypothesize that under future conditions the cycling of the dissolved carbon that is released by phytoplankton will change. Under future CO2 regimes organic matter exuded by phytoplankton is expected to be more carbon rich per nitrogen. If this material is prone to rapid microbial degradation, it would re-enter the atmosphere and be lost to the biological pump. If, instead, it accumulates in surface ocean it will eventually be transported to depths of the ocean by physical processes, removing this carbon from the atmosphere. Alternatively, this material could form transparent exopolymer particles and enhance sedimentation. 

To learn more about ocean acidification or find presentations, actividoplphinties and lesson plans go here: Enjoy the dolphin my daughter drew.

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