What is the variability and trend in ocean pH? How does the changing carbonate saturation state affect biogeochemical processes?

Seawater pH is one of the fundamental chemical properties of the ocean. It is affected by a variety of natural processes such as net primary production and respiration, formation and dissolution of biogenic calcium carbonate minerals, net air-sea CO2 exchange, as well as mixing and circulation. In addition, the pH of seawater is being reduced over the upper layer of the oceans as a consequence of anthropogenic CO2 invasion into the ocean, which reacts to form carbonic acid. This process is termed ocean acidification. The effect of ocean acidification has already been observed in marine and coastal organisms such as corals in low latitude, bivalves in mid-latitude, and pteropods and krill in high latitudes. At the current rate of anthropogenic CO2 emission, a wide variety of organisms that produce CaCO3 skeletons may be at an existential risk by the end of this century with subsequent impacts on ecosystems and the services they provide [Gattuso et al., 2015]. The serious threat by ocean acidification calls for our better understanding of its status and progress. There is an urgent need for spatially and temporally resolved measurements of biogeochemistry and physics in order to optimize modeling for future projections and adequate actions for protection and adaptation [Newton et al., 2012].
pH measurements provide essential information on all of these processes and they are key to understanding subsequent effects on ecosystems that result from the changing pH and calcium carbonate saturation state [Bednaršek et al., 2012]. pH reductions will lead to the entire water column in some ocean areas becoming undersaturated with respect to the mineral aragonite in just a few decades [Gruber et al., 2012]). This could lead to a variety of ecosystem changes that are now only poorly monitored. For example, a decrease in aragonite production may lead to a weakening of the biological pump ([Riebesell et al., 2009]; [Hofmann and Schellnhuber, 2009]). Until recently, the observing network for ocean pH was comprised of ship-based repeat hydrographic surveys, VOS pCO2 tracks for near-surface observations, and high-frequency time-series stations by ships or moorings. Accordingly, the array of Biogeochemical-Argo floats with pH and other biogeochemical sensors could resolve the spatial and vertical patterns and short-term variations such as seasonal and interannual dynamics in essential variables of CO2 chemistry, including fluctuations and trends of aragonite saturation depth, net community production as well as the acidity of seawater globally.

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