Award: OCE-1829778

Coral reefs provide critical services for millions of people around the world, but are threatened by environmental perturbations due to human activities. In order to successfully maintain, protect, and restore coral reefs in the context of ongoing global environmental change, it is important that we have a sound understanding of the properties that allow corals and coral reef ecosystems to grow and accumulate reef structures made of calcium carbonate. In this project, we investigated changes in coral calcification rates and skeletal properties in cores extracted from three coral species (Diploria labyrinthiformis, Pseudodiploria strigosa, Orbicella franksi) from five different reef habitats on the Bermuda coral reef platform. Skeletal growth properties including linear extension, density, and calcification were analyzed for forty-two coral cores using computed tomography (CT) scanning techniques (Figure 1). For a subset of coral cores, geochemical analyses using mass spectrometry were applied to assess stable isotope variability (d13C and d18O) and trace metal ratios (e.g., Sr/Ca, B/Ca, Ba/Ca, Mg/Ca, Li/Mg, U/Ca). Uniquely, the coral growth, isotope, and trace metal records overlapped with a contemporary time series of surface seawater physical and chemical parameters from both inshore and offshore environments. These data provided an opportunity to compare variations in skeletal properties with seawater conditions. Specifically, we were interested to assess whether there were links between coral growth properties and the North Atlantic Oscillation index (NAO) as previous studies have proposed that certain modes of the NAO could lead to increased coral calcification rates in Bermuda due to increased food availability. The longest coral growth record recovered extended back to the 1930s but the majority of cores extended back to the 1970s and 1980s. No distinct long-term trend was identified in the mean growth record of each coral species, but substantial seasonal, inter-colony, and inter-species variability were observed (Figure 2). Statistical analyses showed correlation between coral calcification in D. labyrinthiformis and the NAO with higher rates of calcification coincident with negative winter NAO modes. Negative winter NAO modes are in general associated with deeper mixing, higher nutrient fluxes, and intensified spring blooms. The correlation was also stronger for corals living closer to the reef edge than for corals living closer to shore. Other coral species showed no correlation with the NAO. All of the D. labyrinthiformis cores also showed a reduction in calcification in 1988 coincident with a well-documented coral bleaching event in Bermuda, but this reduction was not apparent in the other coral species. For all species, seasonal variation in coral calcification closely tracked environmental variations in seawater temperature and pH (Figure 3). However, because of slow growth rates and limited resolution of isotope measurements, we were unable to elucidate seasonal variations and differences from these data. In contrast, trace metal ratios were measured at higher resolution and revealed distinct seasonal trends that tracked both growth rates and seawater properties, and were mostly consistent across all three coral species despite different skeletal morphology and growth strategies (Figure 3). Intellectual Merit: The results from this study highlight the importance of considering the range of responses within and between coral species as well as across inshore-offshore gradients that may arise from inter-annual climate modes (e.g., NAO), thermal stress events, and interactions between ocean warming and coral nutrition in developing predictive models aimed at projecting coral calcification under past, present, and future climate change scenarios. Furthermore, the resulting data with overlapping coral skeleton and trace metal properties with monthly seawater chemistry from inshore (~10 years) and offshore (~30 years) offer a unique opportunity for further studies aimed at refining the application of coral trace metal records as proxies for historical seawater reconstruction. Broader Impacts: This project directly supported the training and education of 2 PhD students, 1 MS student, 3 undergraduate students, and 1 postdoctoral scholar. The results have been presented at 7 national and international meetings, and shared with students and the general public at more than 20 outreach events. To date the project has generated or contributed to 5 peer-reviewed publications. Data generated from this project are publically available from the Biological and Chemical Oceanography Data Management Office (BCO-DMO) at http://www.bco-dmo.org/project/738326. Last Modified: 01/09/2025 Submitted by: AndreasAndersson