Award: OCE-1040952

The goal of this project was to use the low pH submarine springs (ojos) in Mexico for in-situ studies of the impacts of low aragonite saturation (?arag) on coral calcification. Specifically we set out to determine if effects of natural ?arag gradients can be captured with relatively little impact from other potentially confounding parameters (e.g. salinity, temperature and light). We used pH, temperature and salinity sensors to collect hourly physicochemical measurements to characterize the range and variability (temporal and spatial) of these parameters around some ojos (Fig 1). After high resolution "mapping" of ?arag gradients around the ojos, we conducted ecological surveys along the ?arag gradients (Fig. 2A). The numbers of coral species present (Fig. 2B) and the sizes of coral colonies (Fig. 2C) declined as ?arag dropped, but three species of hard corals persisted at the lowest ?arag (Fig. 2D). The settlement, survival and growth of some corals at very low ?arag highlight the need to investigate mechanisms enabling calcification under such extreme conditions. We then collected 14 coral cores across the ?arag gradients to investigate impacts on calcification and bioerosion rates in resident corals (Fig 3). We compared our results to data obtained in controlled laboratory experiments and show that the sensitivity of calcification to natural variation in aragonite saturation (?arag) in the coral Porites astreoides is identical at the ojos and in laboratory experiments where ?arag is manipulated under constant temperature and salinity (Fig. 4). If salinity and/or temperature were affecting calcification negatively, the decline in calcification along ?arag gradients should be larger than in the laboratory experiments. The close agreement between laboratory and field data strongly suggest that ?arag is the dominant factor driving declining coral calcification at our field sites. Using the same species in the field and aquaria incubations where OA is induced by increasing pCO2 under controlled conditions (constant alkalinity, salinity, and temperatures) enables disentangling of the responses to the multiple potential drivers in the field. These results illustrate the potential for obtaining interesting and unique information from the ojos that complements data from controlled laboratory experiments. Knowledge from natural settings is essential for understanding how natural heterogeneity is reflected in the diversity of biological responses to OA. Our field site is ideal for studying coral calcification at low ?arag, particularly when considering factors operating over longer time scales (i.e., allowing acclimation). Overall our results indicate that some corals may be able to acclimate and adapt to the changing conditions more than others although this may be associated with lower calcification and less robust reef structures. Areas where corals are already exposed to low pH conditions should be protected as natural refiugia. Last Modified: 09/30/2015 Submitted by: Adina Paytan