Chair: Ulf Riebesell

Ian C. Enochs ^(1,20), Derek P. Manzello ⁽²⁾, Lyza Johnston ⁽³⁾, Emily M. Donham ⁽⁴⁾, Graham Kolodziej ^(1,2), Lauren Valentino ^(1,2), Charles P. Young ⁽⁵⁾, Nichole N. Price ⁽⁶⁾

¹ Cooperative Institute for Marine and Atmospheric Studies, University of Miami, Miami, FL 33149, USA
² Atlantic Oceanographic and Meteorological Laboratory, NOAA, Miami, FL 33145, USA
³ CNMI Bureau of Environmental and Coastal Quality, Division of Coastal Resources Management, Saipan, Northern Mariana Islands 96950, USA
⁴ Moss Landing Marine Laboratories, Moss Landing, CA 95039, USA
⁵ Joint Institute for Marine and Atmospheric Research, NOAA/University of Hawaii, Honolulu, Hawaii 96822, USA
⁶ Bigelow Laboratory for Ocean Sciences, East Boothbay, ME 04544, USA

Background
Coral reef ecosystems are expected to be strongly impacted by ocean acidification (OA) in the coming century. The influences of OA on coral reefs will be numerous, involving diverse species with different degrees of susceptibility. Naturally acidified systems provide a way to study these individual responses and a means to investigate how myriad alterations manifest at the ecosystem-scale over long periods of time.

Methods
We characterized the unique coral reef ecosystems surrounding the volcanic CO ₂ vent at Maug Island in the Commonwealth of the Northern Mariana Islands (CNMI). We present data on the spatial extent and temporal dynamics of carbonate chemistry alteration. We analyse trace-gas concentrations and characterize fluctuations in light, current, tides, and temperature. We tie these environmental data to high-resolution photo mosaics and in situ biodiversity surveys to examine changes in reef community structure correlated with this natural OA gradient. Finally, we investigate the influence of vent proximity on biological processes affecting reef habitat growth and erosion. Using computed tomography (CT) and analysis of coral growth bands, we analyse changes in the calcification of massive Porites coral. Using crystalline calcium carbonate blocks, we quantify the settlement and erosion rate of microboring algae.

Findings
Venting at Maug contributed to localized acidification conditions similar to those predicted to occur worldwide by the end of the century. This OA gradient was correlated with a spatial shift from healthy coral reefs to an ecosystem dominated by macroalgae. Reduced calcification and higher settlement of microboring flora were also correlated with CO ₂.

Conclusions
Together these measurements underscore the strong influence that OA will have on reef persistence and highlight the value of the Maug site for future OA research.