Effects of irradiance, flow, and colony pigmentation on the temperature microenvironment around corals: Implications for coral bleaching?

Experiments were conducted to determine the effects of colony pigmentation, irradiance, and flow on the temperature microenvironment that corals experience in shallow water. The warming of colony surfaces increased with increasing colony pigmentation (darker surfaces) and at high irradiance but was alleviated by higher water flow. Dark colonies were up to 1.58C warmer than ambient seawater at high irradiance and slow flow. In contrast, very light colonies were similar in temperature to ambient water at all levels of flow and irradiance. The darkness of corals progressively increased along a gradient of decreasing water clarity from oligotrophic offshore reefs toward turbid high-nutrient reefs near the coast. The surface temperature of these darkly pigmented turbid-water corals was significantly greater than that of the paler corals in the clear-water environments at comparable seawater temperatures, light, and current conditions. The surface warming of darkly pigmented colonies in coastal environments is sufficiently high to exceed their bleaching threshold during warm, calm, and clear seawater conditions. The term coral bleaching describes the loss or expulsion of endosymbiotic dinoflagellates from the coral host, often resulting in colony death or reduced growth and fecundity of the surviving colonies (Glynn 1993; Brown 1997; Podesta and Glynn 2001). Warmer-than-normal temperatures are widely accepted as the most important external trigger for mass coral bleaching (Glynn 1996; Brown 1997; HoeghGuldberg 1999). Corals live close to their upper thermal tolerance limit and start to bleach when local average summer temperature maxima are exceeded by 1–28C for a number of days (Berkelmans and Willis 1999). Additionally, high irradiance, low flow, and low water turbidity have been identified as factors that can be responsible for coral bleaching, especially in combination with high water temperatures (Lesser et al. 1990; Glynn 1996; Nakamura and Van Woesik 2001). Typically, the relationship between bleaching and temperature has been assessed on the basis of data obtained on seawater or sea surface temperatures. However, it is the temperature of the colony surface and of the boundary layer directly above it that determines physiological processes, which can deviate substantially from those of the larger body of surrounding seawater. This deviation of surface temperatures from ambient seawater temperatures is a function of several factors. Two main factors are the short-wave solar radiation incident on the surface and the proportion of the incident radiation absorbed by the surface (short-wave absorptivity); the latter factor itself is a function of the reflec1 Corresponding author ([email protected]).