Time-series observations from the Atlantic and Pacific Oceans have revealed indisputable evidence of long-term acidification of open-ocean surface seawater due to uptake of anthropogenic carbon dioxide from the atmosphere. Concurrent evidence of negative effects of acidification on the production and preservation of calcium carbonate have fueled concern about the potential consequences to coral reefs. Although long-term acidification in coral reef environments in general has not been observed, seawater acidification rates on the Bermuda coral reef platform between 2007-2012 have been found to be three times faster than the long-term (1983-2012) acidification rate observed at a nearby offshore open-ocean time-series station. The investigators on this project believe that they now understand how this happens and have designed a study to confirm or refute their ideas. Specifically they believe that the observed changes in 2007-2012 are attributable to a recent shift in reef metabolic processes associated with an increase in net reef calcification and heterotrophy. The evidence they have in-hand suggests that these changes have been fueled by an increase in food supply to the reef as a result of increased offshore primary production seemingly linked to the state of the North Atlantic Oscillation (NAO), a periodic back-and-forth shifting of atmospheric pressure differences between the subpolar and the subtropical North Atlantic. In this project, by collecting an extensive set of physical, chemical, and biological data extending from the reef platform at Bermuda to the offshore open-water time-series station, they will explore this idea.
The primary scientific and societal broader impacts of this project will be its relevance to advancing current understanding of the effects of ocean acidification on coral reefs. Robust prediction of future effects on this ecosystem requires knowledge of the main drivers of reef biogeochemical processes and of local seawater acidification. Secondly, the project will support the education and research activities of both graduate and undergraduate students working as members of the research team, and foster community educational outreach through the Ocean Discovery Institute in San Diego and the Ocean Academy in Bermuda to engage students from underrepresented minorities.
The central hypothesis of this research is that during years of negative winter NAO, intensified mixing and increased nutrient supply enhance offshore production leading to coral reef calcification and reef heterotrophy, thus intensifying the local seawater acidification on the reef. To address this hypothesis, the team will measure and characterize inshore seawater biogeochemical properties (temperature, salinity, dissolved inorganic carbon, total alkalinity, calcium, partial pressure of carbon dioxide, inorganic nutrients, particulate organic carbon and nitrogen, total organic carbon and nitrogen, phytoplankton pigments, and bacterial abundance) on a monthly interval across the Bermuda coral reef. These data will be evaluated in concert with data collected as part of the Bermuda-Atlantic Time-Series and Hydrostation S programs and along inshore-offshore transects. It is expected that this approach will further the understanding of how seawater biogeochemical properties, including seawater carbonate chemistry, vary over time and space on the Bermuda coral reef, and identify the main drivers of these variations. It will specifically address the coupling between offshore and inshore biogeochemical processes and how they are linked to larger-scale oceanographic and climatic forcings, such as the NAO. It also addresses how reef biogeochemical processes may alleviate or exacerbate ocean acidification, and whether these changes are important to reef metabolism in the context of other forcings such as light, nutrients, and food availability.