Calcium carbonate saturation and ocean acidification in Tokyo Bay, Japan

Abstract

From April 2011 to January 2012, seasonal variation of the aragonite saturation state (Ωar) was observed for the first time in Tokyo Bay, in order to understand the current state of ocean acidification in a highly eutrophicated bay in Japan. Ωar in the bay ranged between 1.55 and 5.12, much greater than observed in offshore waters. At the surface, Ωar was high during summer as a result of photosynthesis with some conflicting effect of freshwater input. At the bottom, Ωar was low during summer due to remineralization of organic matter. Based on an assumption that our observations represent current conditions in Tokyo Bay, it is estimated that the emission of anthropogenic CO2 has already decreased Ωar by 0.6 since the preindustrial period and will further decrease by 1.0–1.6 by the end of this century if emission of CO2 is continued at a high level [representative concentration pathway (RCP) 8.5 scenario]. With other conditions remaining the same, bottom waters of the bay will reach seasonal aragonite undersaturation by 2060–2070. However, because coastal regions have a large interannual variability, we need further observations to evaluate our estimations and future predictions presented here. Nevertheless, it should be safe to say that the larger seasonal variation in Ω causes the Tokyo Bay to reach aragonite undersaturation earlier than offshore regions and such conditions have negative consequences on the variety of calcifying organisms living in Tokyo Bay. Ocean acidification could thus give an additional stress to the ecosystem of the bay, which is now suffering from eutrophication and hypoxia.