Chair: Ulf Riebesell

Derek Manzello ^(1)*, Ian Enochs ^(1,2), Andrew Bruckner ⁽³⁾, Philip Renaud ⁽³⁾, Graham Kolodziej ⁽²⁾, David Budd ⁽⁴⁾, Renee Carlton ⁽²⁾, Peter Glynn ⁽⁵⁾

¹ Atlantic Oceanographic and Meteorological Laboratory, NOAA, Miami, FL, 33149, USA
² Cooperative Institute of Marine and Atmospheric Science, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, 33149, USA
³ Khaled bin Sultan Living Oceans Foundation, Annapolis, MD, 21403, USA
⁴ University of Colorado Boulder, Boulder, CO, 80309, USA
⁵ Department of Marine Biology and Ecology, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, 33149, USA

Background
Coral reefs are being severely impacted by warming and ocean acidification, yet we still have a relatively undeveloped understanding of how the combination of the two will impact coral reefs over this century. The coincidence of naturally high CO₂ conditions, periodic warming events caused by ENSO, and long-term (~40 yrs) monitoring make Galápagos coral reefs ideal for understanding the effects of combined warming and acidification that is not possible at other high CO₂ sites.

Methods
Seawater carbonate chemistry was measured with discrete samples and in situ instrumentation. Coral extension, density, calcification and skeletal P/Ca (a proxy for phosphate exposure) were determined from coral cores of massive Porites collected in June 2012.

Findings
Since the 1982-83 ENSO warming event, the persistence of reefs around the Galápagos Islands has differed across an acidification gradient. Reefs disappeared where pH < 8.0 and Ω_arag ≤ 3 and have not recovered, whereas one reef has persisted where pH > 8.0 and Ω_arag > 3. Where upwelling is greatest, calcification by massive Porites is greater than predicted by a published relationship with temperature despite high CO₂, possibly due to elevated nutrients. However, skeletal P/Ca, a proxy for phosphate exposure, negatively correlates with density (R = -0.822, p < 0.0001).

Conclusions
Coral calcification and reef structural persistence correlate with the regional trend in seawater pH in the Galápagos. Increased nutrients may stimulate coral growth with high CO₂, but the response of Galápagos reefs to warming suggests that elevated nutrients ultimately increase reef sensitivity to acidification by reducing skeletal density and further stimulating bioerosion already accelerated by low pH. The recent history of Galápagos coral reefs provides field evidence that reefs exposed to elevated nutrients may be the most affected and least resilient to changes in climate and ocean chemistry.