Abstract
Abstract. Numerous experiments have shown that ocean acidification impedes coral calcification, but knowledge about in situ reef ecosystem response to ocean acidification is still scarce. Bahía Culebra, situated at the northern Pacific coast of Costa Rica, is a location naturally exposed to acidic conditions due to the Papagayo seasonal upwelling. We measured pH and pCO2 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 2009 upwelling season. Findings revealed that the carbonate system in Bahía Culebra shows a high temporal variability. Incoming offshore waters drive intra- and interseasonal changes. Lowest pH (7.8) and highest pCO2 (658.3 µatm) values measured during a cold-water intrusion event in the non-upwelling season were similar to those minimum values reported from upwelling season (pH = 7.8, pCO2 = 643.5 µatm), unveiling that natural acidification also occurs sporadically in the non-upwelling season. This affects the interaction of photosynthesis, respiration, calcification and carbonate dissolution and the resulting diel cycle of pH and pCO2 in the reefs of Bahía Culebra. During the non-upwelling season, the aragonite saturation state (Ωa) rises to values of > 3.3 and during the upwelling season falls below 2.5. The Ωa threshold values for coral growth were derived from the correlation between measured Ωa and coral linear extension rates which were obtained from the literature and suggest that future ocean acidification will threaten the continued growth of reefs in Bahía Culebra. These data contribute to building a better understanding of the carbonate system dynamics and coral reefs' key response (e.g., coral growth) to natural low-pH conditions, in upwelling areas in the eastern tropical Pacific and beyond.
Highlights
Ocean acidification (OA) caused by human-induced increase of atmospheric CO2 (Sabine et al, 2004; Feely et al, 2009) is considered one of the major threats to marine calcifying organisms and ecosystems (Fabry et al, 2008; Hofmannn et al, 2010; Doney et al, 2012; Gattuso et al, 2015)
The present study provides data from in situ measurements from a system that is naturally exposed to low-pH conditions and seeks to characterize the carbonate chemistry within a bay (Bahía Culebra) and its potential impact on the reefs
Our results indicate that physical processes, such as the coastal upwelling and the exchange of water between the bay and the open ocean, influence the carbonate chemistry on timescales of weeks to months, where metabolic processes influence the diurnal cycle
Summary
Ocean acidification (OA) caused by human-induced increase of atmospheric CO2 (Sabine et al, 2004; Feely et al, 2009) is considered one of the major threats to marine calcifying organisms and ecosystems (Fabry et al, 2008; Hofmannn et al, 2010; Doney et al, 2012; Gattuso et al, 2015). C. Sánchez-Noguera et al.: Natural ocean acidification at Papagayo upwelling system face waters will become undersaturated with respect to aragonite in the decades (Gattuso et al, 2015), emphasizing the need to study the response of natural ecosystems to OA. Aragonite undersaturated surface waters occur naturally in some parts of the ocean, as a consequence of underwater volcanic seeps (Hall-Spencer et al, 2008; Fabricius et al, 2011, 2015; Enochs et al, 2015) or upwelling that drags corrosive deep water into the surface mixed layer (Feely et al, 2008; Hauri et al, 2009; Fassbender et al, 2011; Harris et al, 2013)
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