Acidification of the Coastal Ocean: Are Deep Waters of the Gulf of Maine already Corrosive to Pteropods?

Zhaohui Aleck Wang, Marine Chemistry & Geochemistry
Gareth Lawson, Biology



As a result of increases in atmospheric carbon dioxide (CO2), the ocean is taking up extra CO2 and becoming more acidic, in a process referred to as ocean acidification (OA). Certain coastal regions, such as the upwelling system along the U.S. West Coast, are more susceptible to the effects of ocean acidification than others, because their waters are naturally low in pH and saturation of aragonite (a calcium carbonate mineral), but higher CO2 concentration, at least at some times of year. In such OA ‘hot-spots’, continued anthropogenic perturbations to the carbonate chemistry will quickly push the system towards a more corrosive (aragonite under-saturated, ΩA < 1) environment that many calcium carbonate shell-forming organisms may not tolerate. Coastal acidification in the Gulf of Maine (GoME) has generally not been considered to be a pressing concern, but new data collected by our group and collaborators suggest that in the deep waters of the GoME low seawater pH may cause aragonite saturation states (ΩA) to be close to a chemical and ecological threshold (i.e. ΩA = 1). Currently, there are no year-round CO2 system measurements to assess conclusively whether the deep waters in the GoME are already experiencing seasonal ΩA under-saturation. If seasonal under-saturation is present, however, this may have detrimental consequences to thecosome pteropods, a group of aragonite shell-forming zooplankton that are important members of the pelagic food web and key contributors to biogeochemical cycles.

We propose an interdisciplinary project aiming to assess seasonal variations of the CO2 system in the deep GoME and the associated impacts on thecosome pteropods. The main objectives of this project are to: (1) investigate if deep waters of the GoME are already seasonally under-saturated with respect to aragonite saturation state, and if these waters are more susceptible to acidification pressures; (2) quantify seasonal patterns in the abundance of the most common thecosome pteropod in the GoME, Limacina retroversa, and examine the impacts of potential under-saturation of aragonite on its vertical distribution; (3) investigate the physiological response of the animal to its chemical environment. Demonstration that the deep waters of the GoME are already seasonally under-saturated with respect to aragonite would be an important development. If the GoME does indeed qualify as a coastal acidification ‘hot spot,’ the proposed study would undoubtedly have significant implications for future funding of coastal acidification research in the GoME.