Carbonate chemistry in the coastal zone responds more strongly to eutrophication than ocean acidification

The accumulation of anthropogenic CO2 in the ocean has altered carbonate chemistry in surface waters since preindustrial times and is expected to continue to do so in the coming centuries. Changes in carbonate chemistry can modify the rates and fates of marine primary production and calcification. These modifications can in turn lead to feedback on increasing atmospheric CO2. We show, using a numerical model, that in highly productive nearshore coastal marine environments, the effect of eutrophication on carbon cycling can counter the effect of ocean acidification on the carbonate chemistry of surface waters. Also, changes in river nutrient delivery due to management regulation policies can lead to stronger changes in carbonate chemistry than ocean acidification. Whether antagonistic or synergistic, the response of carbonate chemistry to changes of nutrient delivery to the coastal zone (increase or decrease, respectively) is stronger than ocean acidification. The open ocean is a major sink of anthropogenic CO2 (Sabine et al. 2004); however, the accumulation of anthropogenic CO2 has altered carbonate chemistry in surface waters since preindustrial times, and is expected to continue to do so in the coming centuries (Orr et al. 2005). Ocean acidification of surface waters corresponds to the increase of [CO2] and of [H+], the decrease of pH and of [CO 2{ 3 ], and of the saturation state of calcite (Vca) and aragonite (Var), all related to shifts in thermodynamic equilibria in response to the input of anthropogenic CO2 from the atmosphere. Changes of the carbonate chemistry of surface waters related to ocean acidification can alter the rates and fates of primary production and calcification of numerous marine organisms and communities (Kleypas et al. 2006; Doney et al. 2009). Such changes can provide either positive or negative feedback on increasing atmospheric CO2 by modifying the flux of CO2 between the ocean and the atmosphere. The increase of [CO2] can favor the availability of inorganic carbon for primary producers, whereas the decrease of [CO 2{ 3 ], Vca, and Var make the precipitation of CaCO3 thermodynamically less favorable (or impossible spontaneously in CaCO3-undersaturated conditions). An increase in primary production associated with efficient organic carbon export would induce a negative feedback on increasing atmospheric CO2, according to: CO2zH2O?CH2OzO2 ð1Þ

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