Abstract
We report seasonal changes in coral calcification within the highly dynamic intertidal and subtidal zones of Cygnet Bay (16.5°S, 123.0°E) in the Kimberley region of northwest Australia, where the tidal range can reach nearly 8 m and the temperature of nearshore waters ranges seasonally by ~9 °C from a minimum monthly mean of ~22 °C to a maximum of over 31 °C. Corals growing within the more isolated intertidal sites experienced maximum temperatures of up to ~35 °C during spring low tides in addition to being routinely subjected to high levels of irradiance (>1500 µmol m−2 s−1) under near stagnant conditions. Mixed model analysis revealed a significant effect of tidal exposure on the growth of Acropora aspera, Dipsastraea favus, and Trachyphyllia geoffroyi (p ≤ 0.04), as well as a significant effect of season on A. aspera and T. geoffroyi (p ≤ 0.01, no effect on D. favus); however, the growth of both D. favus and T. geoffroyi appeared to be better suited to the warm summer conditions of the intertidal compared to A. aspera. Through an additional comparative study, we found that Acropora from Cygnet Bay calcified at a rate 69 % faster than a species from the same genus living in a backreef environment of a more typical tropical reef located 1200 km southwest of Cygnet Bay (0.59 ± 0.02 vs. 0.34 ± 0.02 g cm−2 yr−1 for A. muricata from Coral Bay, Ningaloo Reef; p < 0.001, df = 28.9). The opposite behaviour was found for D. favus from the same environments, with colonies from Cygnet Bay calcifying at rates that were 33 % slower than the same species from Ningaloo Reef (0.29 ± 0.02 vs. 0.44 ± 0.03 g cm−2 yr−1, p < 0.001, df = 37.9). Our findings suggest that adaption and/or acclimatization of coral to the more thermally extreme environments at Cygnet Bay is strongly taxon dependent.
Highlights
Rising ocean temperatures accompanied by the increased frequency and severity of marine heat waves are testing the physiological ability of coral to withstand extreme thermal stress (Hoegh-Guldberg 1999; Hoegh-Guldberg et al 2007; Donner et al 2009; Frieler et al 2012)
We report seasonal changes in coral calcification within the highly dynamic intertidal and subtidal zones of Cygnet Bay (16.5°S, 123.0°E) in the Kimberley region of northwest Australia, where the tidal range can reach nearly 8 m and the temperature of nearshore waters ranges seasonally by *9 °C from a minimum monthly mean of *22 °C to a maximum of over 31 °C
3 The Western Australian Marine Science Institution, Perth, WA, Australia comparative study, we found that Acropora from Cygnet Bay calcified at a rate 69 % faster than a species from the same genus living in a backreef environment of a more typical tropical reef located 1200 km southwest of Cygnet Bay (0.59 ± 0.02 vs. 0.34 ± 0.02 g cm-2 yr-1 for A. muricata from Coral Bay, Ningaloo Reef; p \ 0.001, df = 28.9)
Summary
Rising ocean temperatures accompanied by the increased frequency and severity of marine heat waves are testing the physiological ability of coral to withstand extreme thermal stress (Hoegh-Guldberg 1999; Hoegh-Guldberg et al 2007; Donner et al 2009; Frieler et al 2012). Corals in the backreef and shallow pools bounding the island of Ofu in American Samoa can occasionally experience daily fluctuations up to *6 °C, with a 4 °C seasonal variation superimposed on that, while the average daily temperature of Ofu lagoon is *29 °C in summer (Piniak and Brown 2009; Oliver and Palumbi 2011) These reef communities support reasonable levels of coral cover (25–26 %) and high diversity (Craig et al 2001) with reasonably robust rates of coral growth (*1.4 g cm yr-1; Smith et al 2007). In the Red Sea, Pineda et al (2013) found higher rates of bleaching in corals living on the more exposed, seaward sides of nearshore reefs than corals living on the more protected, shoreward sides, despite much higher temperature elevations and variations within the more protected sites, suggesting some form of environmentally dependent resilience related to prior history of thermal variability and exposure (Carilli et al 2012; Castillo et al 2012)
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