Refining reef-coral refugia

One of the most important contemporary issues for coral reefs, apart from the need to reduce greenhouse gas emissions, is to understand the vulnerability of reef corals to climate change and to identify climate-change refugia. We recently used global species distribution models with two predictive variables, photosynthetically available radiation and sea-surface temperature, to identify 12 localities in the Indian and Pacific oceans that may serve as reef-coral refugia from climate change (Cacciapaglia & van Woesik, 2015). The two environmental variables were used as our main predictors because: (i) corals extract most of their metabolic resources from their endosymbionts that photosynthesize, and (ii) symbiotic dysfunction, observed as coral bleaching and subsequent coral mortality, occurs through the interaction between high light (photoinhibition) and high-temperature anomalies. Keppel & Kavousi (2015) suggested that we examined only ‘one global stressor (ocean warming)’ when in fact we examined two stressors – temperature and irradiance – and the interaction between these variables. They also suggested that we ignored ‘ocean acidification’, which we did purposefully. We also ignored many other secondary factors, including flow rates and nutrients, all of which have not been the primary biogeographic drivers of coral species across the Indian and Pacific oceans over the last several millennia, but may locally influence future coral distributions. A declining ocean pH, or ocean acidification, is a consequence of the rise in atmospheric carbon dioxide (CO2) that dissolves into the ocean and increases the concentration of dissolved CO2. The dissolved CO2 combines with water to form carbonic acid (H2CO3), which mostly dissociates into bicarbonate (HCO 3 ) and hydrogen ions (H), and a smaller proportion dissociates into carbonate ions (CO 3 ). The increase in H + ions lowers the pH of the ocean. Climate models predict that by 2100 (IPCC, 2013), the ocean pH will decrease potentially to 7.8, under the most severe Representative Carbon Pathway of 8.5 Wm . Although the pH in the open ocean averages around 8.1, and such a decrease in pH will influence calcareous life in the open ocean (Fabry et al., 2008), contemporary shallow coral reefs can experience diurnal fluctuations in pH from 8.4 at midday to 7.8 in early morning (Ohde & van Woesik, 1999). Recent incubation experiments show little sensitivity of corals to reduced pH (Comeau et al., 2013; Ohki et al., 2013), and field measurements in Palau showed high diversity and high coral cover in nearshore systems that experience on average a pH of 7.85 (Shamberger et al., 2014). Moreover, corals can upregulate their internal pH through hydrogen pumping (McCulloch et al., 2012) and therefore have the potential to tolerate decreased ocean pH. Together, these contemporary results show that the effect of ocean acidification on live reef corals, through to 2100, may be more subtle than previously suspected (Mumby & van Woesik, 2014), although the problems involved in carbonate framework dissolution under ocean acidification will remain (van Woesik et al., 2013). By contrast, temperature stress is anything but subtle (Hoegh-Guldberg et al., 2007; Baker et al., 2008). It is difficult for corals to adjust to high temperature and irradiance stressors, and thermal anomalies that elevate sea-surface temperature of 2–3°C have caused severe coral mortality in many localities around the globe and have changed the composition of corals on reefs (Loya et al., 2001; van Woesik et al., 2011). Even in some of the world’s warmest waters, for example in the northern Persian Gulf, coral bleaching occurs when the water temperature is above 33.5°C (Kavousi et al., 2014). The temperatures of the oceans will continue to rise rapidly (IPCC, 2013), potentially increasing on average 3°C by 2100, and therefore, dealing with increasing temperatures is the most immediate and critical stress that marine organisms must face. We share the appreciation of the need to search for microrefugia from climate change at <1 km (Mosblech et al., 2011), particularly on coral reefs, as suggested by Keppel & Kavousi (2015). Although such an analysis is desirable, Keppel & Kavousi (2015) correctly state that ‘relevant environmental data to facilitate downscaling oceanic environment models to sufficiently fine scales (≤1 km) is currently not available for marine environments’. We would encourage analyses using such highresolution data, if they were available, but it is unreasonable to urge analysis of data that does not yet exist. Correspondence: Robert van Woesik, tel. +1 321 674 7475, fax +1 321 674 7238, e-mail: rvw@fit.edu