Ocean acidification due to anthropogenic CO2 emissions is a dominant driver of long-term changes in pH in the open ocean, raising concern for the future of calcifying organisms, many of which are present in coastal habitats. However, changes in pH in coastal ecosystems result from a multitude of drivers, including impacts from watershed processes, nutrient inputs, and changes in ecosystem structure and metabolism. Interaction between ocean acidification due to anthropogenic CO2 emissions and the dynamic regional to local drivers of coastal ecosystems have resulted in complex regulation of pH in coastal waters. Changes in the watershed can, for example, lead to changes in alkalinity and CO2 fluxes that, together with metabolic processes and oceanic dynamics, yield high-magnitude decadal changes of up to 0.5 units in coastal pH. Metabolism results in strong diel to seasonal fluctuations in pH, with characteristic ranges of 0.3 pH units, with metabolically intense habitats exceeding this range on a daily basis. The intense variability and multiple, complex controls on pH implies that the concept of ocean acidification due to anthropogenic CO2 emissions cannot be transposed to coastal ecosystems directly. Furthermore, in coastal ecosystems, the detection of trends towards acidification is not trivial and the attribution of these changes to anthropogenic CO2 emissions is even more problematic. Coastal ecosystems may show acidification or basification, depending on the balance between the invasion of coastal waters by anthropogenic CO2, watershed export of alkalinity, organic matter and CO2, and changes in the balance between primary production, respiration and calcification rates in response to changes in nutrient inputs and losses of ecosystem components. Hence, we contend that ocean acidification from anthropogenic CO2 is largely an open-ocean syndrome and that a concept of anthropogenic impacts on marine pH, which is applicable across the entire ocean, from coastal to open-ocean environments, provides a superior framework to consider the multiple components of the anthropogenic perturbation of marine pH trajectories. The concept of anthropogenic impacts on seawater pH acknowledges that a regional focus is necessary to predict future trajectories in the pH of coastal waters and points at opportunities to manage these trajectories locally to conserve coastal organisms vulnerable to ocean acidification.
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Acknowledgments This document results from contributions from MEDEICG projects funded by the Spanish Ministry of Economy and Competitiveness (CTM2009-07013), MedSEA of the FP7 of the EU (contract no. FP7-ENV-2010-265103), TEMSPATH Marie Curie Intra European Fellowship of the FP7 of the EU (254297: FP7-PEOPLE-2009-IEF), the CSIRO Marine and Coastal Carbon Biogeochemistry Flagship Cluster, and the Baltic Nest Institute. We thank Jon Sharp and Peter Raymond for sharing alkalinity data on the Delaware River and Mississippi River, respectively, Wei Jun Cai and an anonymous reviewer for useful comments, and Iris C. Anderson and Wayne S. Gardner, co-editors in chief of Estuaries and Coasts, for useful advice and the invitation to write this piece.
- Anthropogenic impacts
- Ocean acidification
- Watershed changes
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics
- Aquatic Science