Abstract
To assess the viability of high latitude environments as coral refugia, we report measurements of seasonal changes in seawater parameters (temperature, light, and carbonate chemistry) together with calcification rates for two coral species, Acropora yongei and Pocillopora damicornis from the southernmost geographical limit of these species at Salmon Bay, Rottnest Island (32°S) in Western Australia. Changes in buoyant weight were normalised to colony surface areas as determined from both X-ray computed tomography and geometric estimation. Extension rates for A. yongei averaged 51 ± 4 mm y−1 and were comparable to rates reported for Acroporid coral at other tropical and high latitude locations. Mean rates of calcification for both A. yongei and P. damicornis in winter were comparable to both the preceding and following summers despite a mean seasonal temperature range of ∼6 °C (18.2°–24.3 °C) and more than two-fold changes in the intensity of downwelling light. Seasonal calcification rates for A. yongei (1.31–2.02 mg CaCO3 cm−2 d−1) and P. damicornis (0.34–0.90 mg CaCO3 cm−2 d−1) at Salmon Bay, Rottnest Island were comparable to rates from similar taxa in more tropical environments; however, they appeared to decline sharply once summer temperatures exceeded 23 °C. A coral bleaching event observed in December 2013 provided further evidence of how coral at Rottnest Island are still vulnerable to the deleterious effects of episodic warming despite its high latitude location. Thus, while corals at Rottnest Island can sustain robust year-round rates of coral growth, even over cool winter temperatures of 18°–19 °C, there may be limits on the extent that such environments can provide refuge against the longer term impacts of anthropogenic climate change.
Highlights
Declining rates of coral growth have stimulated debate over the future of coral reefs and the mechanisms or strategies that may be available for hermatypic corals to endure in the face of Earth’s rapidly changing climate
We show that calcification rates in A. yongei and P. damicornis exhibited minimal coherent seasonality, but that each of these species calcified at rates comparable to the same or similar species growing in more tropical habitats
We focused on two locations within the far eastern end of Salmon Bay given that this is the only known location at Rottnest Island where A. yongei coral are currently reported to occur (P. damicornis coral can be found at multiple sites)
Summary
Declining rates of coral growth have stimulated debate over the future of coral reefs and the mechanisms or strategies that may be available for hermatypic corals to endure in the face of Earth’s rapidly changing climate. Already there is evidence of various coral species actively expanding their distributions polewards (Yamano, Sugihara & Nomura, 2011; Baird, Sommer & Madin, 2012), to regions where there are fossilized reefs signifying the past suitability of those areas to support extensive coral growth during previous warmer geological periods (McCulloch & Esat, 2000; Greenstein & Pandolfi, 2008). The suitability of high latitude environments as future refugia for coral reefs, remains unclear at present (Pandolfi et al, 2011; Van Hooidonk et al, 2014) given the recent occurrence of high latitude coral bleaching events (Harrison, Dalton & Carroll, 2011; Smale & Wernberg, 2012). The capacity for corals to persist under future global climate change may largely depend on their capacity to cope with, or acclimatize to, changing environmental conditions (McCulloch et al, 2012; Rodolfo-Metalpa et al, 2014; Keshavmurthy et al, 2014)
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