Award: OCE-1416518

Oceanic uptake of industrially produced CO2 from the atmosphere results in acidification of surface seawater, that is, ocean acidification. As a result, it has been hypothesized that calcifying organisms and ecosystems, including coral reefs, could be negatively affected by these changes in seawater chemistry. In fact, coral reefs could transition from a state of net accretion to net erosion, which could have far reaching consequences for millions of people dependent on coral reefs and the services they provide including financial revenue from tourism and fisheries, food and nutrition, and protection from storms and waves. Most of our current understanding of the effects of ocean acidification on coral reefs have been derived from small-scale and short-term laboratory experiments. However, to fully understand and accurately predict the potential effects of future ocean acidification on coral reefs, it is necessary to first understand the natural environmental drivers and biogeochemical function of coral reefs. Consequently, the goal of this project was to investigate the natural temporal and spatial variations of biogeochemical properties and processes on the Bermuda coral reef, and ultimately, identify the main drivers of this variability. Bermuda was selected because of its relatively high latitude location in the North Atlantic, which makes it a marginal reef system and potentially either more resistant or more vulnerable to ocean acidification than reefs at lower latitudes. Knowledge gained from this location may be instrumental in understanding how reefs in general may respond to ocean acidification. Our research approach took advantage of autonomous instruments measuring physical and biogeochemical properties on the coral reef as well as monthly cruises inshore and offshore in the Sargasso Sea making additional measurements (Figure 1). Furthermore, this project built on a previous NSF sponsored project, making the resulting dataset one of the world?s longest records of biogeochemical parameters measured on a coral reef. The resulting time-series data showed that the Bermuda coral reef undergoes predictable seasonal variation in physical and biogeochemical parameters, but with both sub-seasonal and interannual variability linked to a range of forcings including mesoscale eddies and larger climate modes such as the North Atlantic Oscillation (NAO) (Figure 2). During summer, surface seawater temperature (SST) were high while dissolved inorganic carbon (DIC), total alkalinity (TA) and pH were low. During winter, the opposite trends were observed. These trends in DIC and TA mainly reflected the influence from net reef metabolism, oscillating between net organic carbon production (autotrophy) during spring/summer, and net respiration (heterotrophy) during fall/winter, while maintaining high rates of calcium carbonate (CaCO3) production during summer/fall, and lower rates during winter/spring (Figure 3). Overall, the Bermuda coral reef was net calcifying with only occasional observations of net dissolution during winter. During the summers of 2010 to 2013, maximum rates of reef scale calcification were higher compared to other years. Except for 2012, these years coincided with a negative winter phase of the NAO, which is associated with stronger winds, intensified mixing, and increased injection of nutrients and nutrition to the Bermuda reef. We believe this additional supply of energy fueled the higher rates of reef scale calcification during these years. In contrast, the anomalously high rates of calcification observed in 2012 may have been related to a cyclonic eddy that transported large amounts of Sargassum on to the reef that subsequently fueled high rates of reef scale calcification. Regardless of the exact mechanism, the results offer strong evidence that Bermuda reef metabolism and coral reefs in general are strongly influenced by offshore biogeochemical processes. Consequently, the oceanic regime is an important determinant to consider in the context of coral reef resistance and resilience in response to ocean acidification. Furthermore, numerical model analysis of the relative importance of environmental parameters on reef scale and coral colony calcification rates in Bermuda revealed that SST was by far the strongest driver of the observed variability in calcification rates. In fact, the positive effect of SST was so dominant it masked any potential negative effects resulting from low pH and high pCO2 conditions. However, if SST were to exceed the mean summer SST by one degree or more it would most likely cause coral bleaching and potential coral mortality. Notably though, our model results revealed that if SST warming during the 21st century were constrained to gradual warming predicted under a minimum CO2 emissions pathway (Representative Concentration Pathway 2.6), coral calcification and CaCO3 production in Bermuda could actually increase and benefit from the anticipated moderate warming. These findings highlight the potential benefits of rapid reductions in global anthropogenic CO2 emissions for coral reefs and the ecosystem services they provide. Last Modified: 10/06/2019 Submitted by: Andreas Andersson