Abstract
Hydrographic measurements were collected on nine offshore reef platforms in the eastern Red Sea shelf region, north of Jeddah, Saudi Arabia. The data were analyzed for spatial and temporal patterns of temperature variation, and a simple heat budget analysis was performed with the goal of advancing our understanding of the physical processes that control temperature variability on the reef. In 2009 and 2010, temperature variability on Red Sea reef platforms was dominated by diurnal variability. The daily temperature range on the reefs, at times, exceeded 5°C—as large as the annual range of water temperature on the shelf. Additionally, our observations reveal the proximity of distinct thermal microclimates within the bounds of one reef platform. Circulation on the reef flat is largely wave driven. The greatest diurnal variation in water temperature occurs in the center of larger reef flats and on reefs protected from direct wave forcing, while smaller knolls or sites on the edges of the reef flat tend to experience less diurnal temperature variability. We found that both the temporal and spatial variability in water temperature on the reef platforms is well predicted by a heat budget model that includes the transfer of heat at the air–water interface and the advection of heat by currents flowing over the reef. Using this simple model, we predicted the temperature across three different reefs to within 0.4°C on the outer shelf using only information about bathymetry, surface heat flux, and offshore wave conditions.
Highlights
In 2009 and 2010, temperature variability on Red Sea reef platforms was dominated by diurnal variability
We found that both the temporal and spatial variability in water temperature on the reef platforms is well predicted by a heat budget model that includes the transfer of heat at the air–water interface and the advection of heat by currents flowing over the reef
The topographic complexity of the reefs leads to a high degree of temperature variability at small spatial scales, with cooler temperatures on the forereef or wave-exposed side of the reef and warmer temperatures in the reef interior and on the wave-protected side of the reef
Summary
Elevated temperature is the primary cause of mass coral bleaching events (Glynn 1993), but recent work suggests that organismal response to temperature variation is complex; it can depend on other biological and physiochemical factors such as the history of thermal exposure, ability to adapt or acclimate to thermal changes, short-term temperature variability, water flow, heterotrophic feeding, and light (Nakamura and van Woesik 2001; Berkelmans 2002; Lesser et al 2004; McClanahan et al 2005; Sammarco et al 2006; Palardy et al 2008; Weller et al 2008) These factors contribute to the high degree of spatial variability in coral bleaching observed at global, regional, and even individual reef scales (Riegl and Piller 2003; McClanahan et al 2005). The authors found that mean horizontal advection largely controls SST in the region for most of the year, but that eddy-induced heat flux becomes important during extreme SST anomalies
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