Natural ocean acidification at Papagayo upwelling system (north Pacific Costa Rica): implications for reef development

摘要

Numerous experiments have shown that ocean acidification impedes coralcalcification, but knowledge about in situ reef ecosystem response to oceanacidification is still scarce. Bahía Culebra, situated at the northernPacific coast of Costa Rica, is a location naturally exposed to acidicconditions due to the Papagayo seasonal upwelling. We measured pH andpCO2 in situ during two non-upwelling seasons (June 2012,May–June 2013), with a high temporal resolution of every 15 and 30 min,respectively, using two Submersible Autonomous Moored Instruments (SAMI-pH,SAMI-CO2). These results were compared with published data from the 2009upwelling season. Findings revealed that the carbonate system in BahíaCulebra shows a high temporal variability. Incoming offshore waters driveintra- and interseasonal changes. Lowest pH (7.8) and highest pCO2(658.3 µatm) values measured during a cold-water intrusion event inthe non-upwelling season were similar to those minimum values reported fromupwelling season (pH  =  7.8, pCO2  =  643.5 µatm),unveiling that natural acidification also occurs sporadically in thenon-upwelling season. This affects the interaction of photosynthesis,respiration, calcification and carbonate dissolution and the resulting dielcycle of pH and pCO2 in the reefs of Bahía Culebra. Duringthe non-upwelling season, the aragonite saturation state (Ωa)rises to values of  >  3.3 and during the upwelling season falls below2.5. The Ωa threshold values for coral growth were derived fromthe correlation between measured Ωa and coral linearextension rates which were obtained from the literature and suggest thatfuture ocean acidification will threaten the continued growth of reefs inBahía Culebra. These data contribute to building a better understanding ofthe carbonate system dynamics and coral reefs' key response (e.g., coralgrowth) to natural low-pH conditions, in upwelling areas in the easterntropical Pacific and beyond.