Species-Specific Coral Calcification Responses to the Extreme Environment of the Southern Persian Gulf

Emily J Howells,Dain Mcparland,John A Burt,Morgan S Pratchett,Andrew G Bauman,Scott F Heron,Glenn Dunshea,Grace O Vaughan

doi:10.3389/fmars.2018.00056

Abstract

Sustained accretion of calcium carbonate (mostly by scleractinian corals) is fundamental for maintaining the structure and function of coral reef ecosystems, but may be greatly constrained by extreme and rapidly changing environmental conditions. Corals in the southern Persian Gulf already experience extreme temperature ranges (34C), chronic hypersalinity (>43 psu) and frequent light limitation (<100 μmol photons m-2 s-1). We compared annual rates of calcification for two of the most common massive coral species in the region (Platygyra daedalea and Cyphastrea microphthalma) along marked gradients in environmental conditions in the southern Persian Gulf and into the Oman Sea. Overall calcification rates were 32% higher in P. daedalea colonies (x = 1.103 g cm-2 y-1, n = 46) than in C. microphthalma (x = 0.835 g cm-2 y-1, n = 37), probably reflecting inter-specific differences in energy allocation and skeletal density. There was also considerable variation in calcification rates among individual colonies from the same locations that was unrelated to depth or photosymbiont type. However, most interestingly, P. daedalea and C. microphthalma exhibited contrasting trends in mean annual calcification rates across locations. For P. daedalea, calcification rates were lowest at Delma, where the minimum temperatures were lowest and salinity was highest, and increased across the southern Persian Gulf with increases in minimum temperatures and decreases in salinity. These data suggest that calcification rates of P. daedalea are most constrained by minimum temperatures, which is consistent with the strong relationship between annual calcification rates and minimum local temperatures recorded across the Indo-Pacific. Conversely, linear extension and calcification of C. microphthalma in the southern Persian Gulf was lowest at Ras Ghanada, where there was lowest light and highest maximum temperatures. These data reveal striking taxonomic differences in the specific environmental constraints on coral calcification, which will further reinforce changes in the structure of coral assemblages with ongoing global climate change.

Highlights

Scleractinian corals are one of the foremost contributors to carbonate accretion (Vecsei, 2004), which is fundamental for the formation and maintenance of coral reef frameworks (Perry et al, 2012)
Growth rates from the Gulf were compared to growth estimates for both P. daedalea and C. microphthalma sampled on shallow (4–6 m) fringing reef in the Oman Sea, as well as published values for Platygyra and Cyphastrea spp. from tropical and subtropical Indo-Pacific reefs (Weber and White, 1974; Romano, 1990; Babcock, 1991; Harriott, 1999; Roberts and Harriott, 2003)
There were generally faster extension rates on colony nodules and slower extension between nodules for C. microphthalma (Figures 2c,d), whereas the more even surface of P. daedalea colonies tended to result in more uniform intra-colony extension and less intra-colony variation in extension measurements (Figures 2a,b; Table S1)

Summary

Introduction

Scleractinian corals are one of the foremost contributors to carbonate accretion (Vecsei, 2004), which is fundamental for the formation and maintenance of coral reef frameworks (Perry et al, 2012). Extreme and rapidly changing environmental conditions are expected to increasingly constrain calcification rates of corals (De’ath et al, 2009; Cantin et al, 2010; Pratchett et al, 2015), if not decimate coral populations (e.g., Hughes et al, 2017), and drastically alter the composition of coral assemblages (Loya et al, 2001; Hughes et al, 2012). Most notably, sustained increases in global ocean temperatures, unequivocally linked to anthropogenic climate change, are challenging the thermal limits of most coral species, causing increased incidence and/or severity of mass coral bleaching (Heron et al, 2016b). Calcification rates may be further constrained as corals are exposed to increasing concentrations of dissolved CO2, thereby decreasing seawater pH and aragonite saturation (Schoepf et al, 2013)

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