Award: OCE-1536547

Some tropical corals in the Atlantic have experienced slowing growth rates over the last several decades and the projections are for this to continue with the threats of ocean acidification and warming. Many climate change models also predict that vertical mixing—in which water rich in plankton and other organic matter are carried from the deep to the surface layers of the ocean—will be weakened. Therefore, a reduced food supply (heterotrophy) due to changing ocean circulation could be a third significant impact on coral growth and survival. Through a collaborative project with Woods Hole Oceanographic Institution, Princeton University and the Bermuda Institute of Ocean Sciences, we investigated the link between ocean nitrogen concentrations, coral feeding, and coral health. Specifically for this portion of the collaborative award in Bermuda, we conducted a laboratory-based coral culture experiment at the Bermuda Institute for Ocean Sciences designed to increase our understanding of the factors influencing nitrogen isotope incorporation into corals (the coral tissue and the algal symbionts, known as zooxanthellae). We were testing whether the coral?s nitrogen isotopic composition is affected by the consumption of different quantities of externally produced organic matter (i.e. heterotrophic feeding), without changes to the isotopic composition of the food source. We were also examining to what extent light and the impact of photosynthesis by the zooxanthellae, as well as the internal nitrogen cycling between the coral and zooxanthellae, affects this process. We used new coral recruits (Figure 1), i.e. juveniles that were settled from larvae, of the common mustard hill coral, so that our experimental corals spent their whole lives in the treatment conditions, allowing us to accurately sample the nitrogen isotope ratios. We constructed a biofilter that effectively removed all nitrogen from the intake water to the culture aquaria. This meant we were able to control all sources of nitrogen to the corals in culture. This research is ongoing at Princeton University but the analyses so far suggest that light does not have any significant affect on the coral nitrogen isotope ratios and increased feeding does not cause a breakdown of the coral-zooxanthellae internal nitrogen cycling. Frequent feeding increased growth rates by over 50% compared to low fed conditions, and increased zooxanthellae densities and chlorophyll a content. Light levels did not have a significant effect on growth rates. Food availability likely has a strong influence on the success of juvenile corals. In conjunction with these laboratory coral experiments we conducted a field study at several sites across the Bermuda platform to better constrain how the coral nitrogen isotope ratios are reflecting those of the fixed nitrogen in its environment. We measured the nitrogen isotope ratios for potential food sources (particulate inorganic matter and plankton net tows), comparator benthic heterotrophs (feather duster worms), comparator benthic autotrophs (macroalgae), and several species of coral. There was a gradient in food availability as well as in isotopic composition of the food sources across the Bermuda platform due to anthropogenic influences nearshore. Our initial results suggest that the gradients across the platform observed in the corals can be explained by gradients in the potential food sources. All gradients were similar, although absolute values varied according to trophic level effects, suggesting that the coral nitrogen isotopic composition is indeed tracking the food nitrogen isotopic composition, without effects from changing internal cycling dynamics, thereby supporting our laboratory experiment results. This research resulted in vital stand-alone data and will also be combined with results from our collaborators where field samples ("wild" corals) from Bermuda, Hawaii, and the northern south China Sea were examined to trace the different sources of anthropogenic N into the corals and evaluate the consequences for the coral communities. The overall aim for the collaborative research is to support the development and application of a new tool - nitrogen isotopes in the organic component of coral skeletons - to gain a better understanding of the relationship between ocean nitrogen concentrations and coral reef health. Our research will greatly aid in the predictions of coral reef health under global change. At BIOS, support was provided for one PI, and support and training provided for one graduate student, three undergraduate senior thesis students, one junior undergraduate research student over two years, and one Bermuda undergraduate student. Across the collaborative project overall, support was provided for the labs of three PI?s, two post-doctoral research projects, two PhD theses, one undergraduate senior thesis, three undergraduate interns and one Bermuda undergraduate student. To date, six scientific papers are published, under review or prepared for publication. Results of this work have also been shared at ten international conferences, workshops and seminars, including the European Coral Reef Symposium. At BIOS, the research has been highlighted on the BIOS newsletter (Currents), on weekly public tours, and has been incorporated into faculty presentations and undergraduate taught courses. Last Modified: 05/15/2018 Submitted by: Samantha De Putron