Heat budget and thermal microenvironment of shallow‐water corals: Do massive corals get warmer than branching corals?

Abstract

Coral surface temperature was investigated with multiple temperature sensors mounted on hemispherical and branching corals under (a) artificial lighting and controlled flow; (b) natural sunlight and controlled flow; and (c) in situ conditions in a shallow lagoon, under naturally fluctuating irradiance, water flow, and temperature. Under high irradiance and low flow conditions, hemispherical corals were 0.6uC warmer than the surrounding water. Hemispherical corals reached higher temperatures than branching corals, by a measure of 0.2uC to 0.4uC. Microsensor temperature measurements showed the presence of a thermal boundary layer (TBL). The TBL thickness was flow dependent, and under low flow conditions, a TBL up to 3 mm thick limited heat transfer to the ambient water. Combined microsensor measurements of temperature and oxygen showed that the TBL was approximately four times thicker than the diffusive boundary layer, as predicted from heat and mass transfer theory. A simple conceptual model describes coral surface temperature as a function of heat fluxes between coral tissue, skeleton, and surroundings. The slope of the predicted linear relationship between coral temperature and solar irradiance is fixed by the efficiencies of light absorption and the heat losses to the skeleton and the water. Although spectral absorptivity may play a significant role in coral warming, shape-related differences in thermal properties can cause hemispherical corals to reach higher temperatures than branching corals. Shape-related differences in thermal histories may thus help explain differences in susceptibility to coral bleaching between branching and hemispherical coral species.The increasing occurrence of coral bleaching over the last two decades has focused attention on temperature fluctuations on corals reefs (Brown 1997;Berkelmans and Willis 1999;Hoegh-Guldberg 1999). Under mass coral bleaching conditions, small excursions in the ambient water temperature on a coral reef (of just a few degrees Celsius above the normal average temperature maximum) induce the expulsion of the endosymbiotic dinoflagellates (zooxanthellae) and/or the loss of pigments from a wide variety of corals (Glynn 1996;Hoegh-Guldberg 1999). If such high temperature anomalies last for a week or more mass mortality of corals can occur (Glynn 1996; HoeghGuldberg 1999;Coles and Brown 2003).In most of the literature on coral bleaching, temperature of the ambient water is always assumed to be the same as the coral temperature. However, as a result of the shallow nature of many coral reef lagoons, radiant energy reaching the coral surface can increase its temperature relative to the surrounding water. Few studies have considered the 1 Corresponding author (Peter.Ralph@uts.edu.au). AcknowledgmentsWe thank N. Ralph for constructing the flow-through chamber and R. Bilger and J. Kent for valuable discussion of heat transfer theory. We thank M. Ball for discussions of heat transfer in corals at an early stage of this investigation and two anonymous reviewers ...

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

“…They are also subject to diurnal tidal amplitudes of up to 11 m (Kowalik 2004) that can expose corals to potentially stressful and damaging levels of temperature and light ). Furthermore, water motion can become stagnant during such low tide (Lowe et al 2015), decreasing rates of oxygen export and increasing oxidative stress (Lesser and Farrell 2004;Anthony and Kerswell 2007;Mass et al 2010), as well as increasing the temperature of coral tissue above already elevated ambient levels (Fabricius 2006;Jimenez et al 2008). Thus, intertidal and nearshore environments along the Kimberley coast provide a challenging thermal environment to which corals have adapted, yet to date we have found no record of extensive coral bleaching on a regional scale along the Kimberley coast.…”

“…This makes the nearshore intertidal of Cygnet Bay comparable to other high and variable temperature environments where scleractinian coral is abundantly found, such as backreef habitats in American Samoa (Craig et al 2001;Oliver and Palumbi 2009) and outer reef environments of the Red Sea, albeit not quite as extreme as in the Persian Gulf (Riegl et al 2011;Bauman et al 2013) or nearshore environments of the Red Sea (Pineda et al 2013). Furthermore, the temperature of coral tissue within the shallower and more isolated pools in Cygnet Bay (B0.2 m deep) could be reaching in situ temperatures of up to *36°C at midday in summer due to the stagnant water motion being limited to flows on the order of *1 cm s -1 or less at low tide (Fabricius 2006;Jimenez et al 2008). At the same time, both daytime elevations and night-time depressions in water temperature within the intertidal will generally last just 2-4 h before these environments are renewed by rising tides and temperatures return to offshore levels, thus providing some respite to transient thermal stress (Mayfield et al 2013).…”

The physiological response of reef corals to diel fluctuations in seawater temperature

“…Daily fluctuations as large as 10°C also affect the near-shore, shallow reefs of Oman and Taiwan (Coles, 1997;Lee et al, 1999), and daily fluctuations of~6°C are common in the lagoon and over fringing reefs (1-10 m depth) in American Samoa and other Pacific islands (Brown, 1997b;Craig et al, 2001). SST records can also miss more subtle characteristics of the thermal environment affecting corals, notably the variation in temperature of individual colonies due to their color and the ways in which seawater flows around them (Fabricius, 2006;Jimenez et al, 2008).…”

Thermal effects of tissue optics in symbiont‐bearing reef‐building corals

“…This can result in coral bleaching, i.e., the expulsion of the zooxanthellae and/or loss of photosynthetic pigments from the coral host tissue (Lesser 1997;Coles and Brown 2003). A newly recognized additional effect of the elevated light absorbing capacity of corals is the risk of increased coral surface temperature, particularly under conditions of high irradiance and low water flow in shallow environments (Fabricius 2006;Jimenez et al 2008Jimenez et al , 2012. This additional source of heat has hitherto largely been ignored in coral stress physiological studies but could have implications for our understanding of the thermal exposure and tolerance of individual corals before and during a bleaching event.…”