Resilience of coral calcification to extreme temperature variations in the Kimberley region, northwest Australia

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 lmol 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 B 0.04), as well as a significant effect of season on A. aspera and T. geoffroyi (p B 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.

The Future of Coral Reefs Subject to Rapid Climate Change: Lessons from Natural Extreme Environments

“…Corals on the reef flat of the Kimberley region have to cope with extended aerial exposure during spring low tides (up to several hours, Figure 2) and periods of stagnant water alternating with strong tidal currents (>10 knots). These physical conditions create extreme fluctuations in environmental parameters, such as temperature (e.g., the well-studied Shell Island where temperature can fluctuate by up to 7 • C daily, Dandan et al, 2015;Schoepf et al, 2015), pH (e.g., Tallon Island where pH can fluctuate from 7.6 to 8.8 units over a spring low tide, Pedersen et al, 2016), dissolved oxygen (Pedersen et al, 2016;Gruber et al, 2017), unusually turbid waters, and monthly SST exceeding 30 • C for several months a year. Despite these extreme conditions, highly diverse coral reefs exist throughout the Kimberley of up to 225 species (Rosser and Veron, 2011;Richards et al, 2015), which are comparable to inshore reefs in the central GBR ∼2 decades ago (Richards et al, 2015).…”

“…Furthermore, a seasonal calcification study revealed high resilience of coral calcification to both intertidal and subtidal temperature conditions as branching and massive corals calcified at rates that were comparable to those of similar species at a more typical coral reef 1,200 km to the southwest (Ningaloo Reef; Dandan et al, 2015). Given that rising sea levels are expected to reduce extreme temperature variation on macrotidal reef flats such as Tallon Island (Lowe et al, 2016), reef flat communities might have more time than other reef types to acclimatize and/or adapt to ongoing ocean warming.…”

“…Despite a lack in common coral taxa across extreme environments, critical thresholds in community structure do appear to exist as abiotic conditions become increasingly less optimal: (i) an initial reduction in species diversity (e.g., Craig et al, 2001;Fabricius et al, 2011;Enochs et al, 2015) with specialist species often able to survive and potentially even thrive within the new environmental niche (e.g., Kroeker et al, 2011;Fabricius et al, 2014;Dandan et al, 2015;Schoepf et al, 2015), followed by, (ii) a "lethal" threshold beyond which survival ceases (e.g., Inoue et al, 2013). For example, in the vent sites of Japan, hard corals dominate at 225 µatm, become limited at 831 µatm and are absent at the highest pCO 2 sites (1,465 µatm, Inoue et al, 2013).…”

“…For example, in the vent sites of Japan, hard corals dominate at 225 µatm, become limited at 831 µatm and are absent at the highest pCO 2 sites (1,465 µatm, Inoue et al, 2013). Similarly, in the Kimberley region, diverse coral communities dominated by branching Acropora exist in tide pools characterized by large diurnal temperature but not pH fluctuations (Dandan et al, 2015), whereas coral cover drops to <10% and Acropora ceases to exist on reef flats where extreme temperature, pH and oxygen fluctuations create a much more extreme environment (Gruber et al, 2017).…”

“…For example, corals across high temperature, low pH, and multistressor sites were able to maintain high calcification rates (e.g., Dandan et al, 2015;Wall et al, 2016), enabled via mechanisms such as unimpaired pH-homeostasis at the site of calcification (Wall et al, 2016) or increased photosynthesis rates to supply the required energy demand (Strahl et al, 2015a). Although this is sometimes associated with trade-offs such as lower skeletal density and increased bioerosion Crook et al, 2013), other corals are able to do so without indication of compromised energy budgets (Strahl et al, 2015a).…”