Spatial Heterogeneity of Air–Sea Energy Fluxes over a Coral Reef—Heron Reef, Australia

Abstract

The thermal environment of a coral reef is moderated by complex interactions of air-sea heat and moisture fluxes, local to synoptic-scale weather and reef hydrodynamics. Measurements of air-sea energy fluxes over coral reefs are essential to understanding the reef-atmosphere processes that underpin coral reef environmental conditions such as water temperature, cloud, precipitation, and local winds (such as during coral bleaching events). Such measurements over coral reefs have been rare, however, and the spatial heterogeneity of surface-atmosphere energy exchanges due to the different geomorphic and biological zones on coral reefs has not been captured. Accordingly, the heterogeneity of coral reefs with regard to substrate, benthic communities, and hydrodynamic processes has not been considered in the characterization of the surface radiation budget and energy balance of coral reefs. Here, the first concurrent in situ eddy covariance measurements of the surface energy balance and radiation transfers over different geomorphic zones of a coral reef are presented. Results showed differences in radiation transfers and sensible and latent heat fluxes over the reef, with higher Bowen ratios over the shallow reef flat zone. The energy flux divergence between sites increased with wind speed and during unstable, southeasterly trade winds with the net flux of heat being positive and negative over different geomorphic zones. The surface drag coefficient at measurement height ranged from 1 3 10 23 to 2.5 3 10 23, with no significant difference between sites. Results confirm that spatial variation in radiation and air-reef-water surface heat and moisture fluxes occurs across a lagoonal platform reef in response to local meteorological conditions, hydrodynamics, and benthic-substrate cover.

Insights From a Decade of Research on Coral Reef—Atmosphere Energetics

“…Our research focused on Heron Reef, a typical lagoonal platform reef located on the southern Great Barrier Reef (GBR) 80 km northeast of Gladstone on the northeast coast of Australia (Figure ). As described in McGowan et al (), MacKellar, McGowan, and Phinn (), and MacKellar, McGowan, Phinn, and Soderholm (), Heron Reef covers ∼27 km 2 , having developed on an antecedent karst platform with episodes of growth corresponding with higher sea levels during Holocene sea level fluctuations (Hopley et al, ). The reef extends southeastward from Heron Island, which is located on the northwest margin of the reef.…”

Direct measurements of air‐sea CO2 exchange over a coral reef

“…Wind wave height on the reef is typically less than 0.5 m under a mean wind speed of 5 m s −1 although during high tide, oceanic waves may travel across the reef resulting in greater wave action [ Department of Environment and Resource Management (DERM) , ]. Mean 1500 eastern standard time (EST) air temperature varies from 28.3°C in January to 20.1°C in July, while rainfall is bimodal with an annual mean of 1050 mm, falling mostly during the austral summer [ MacKellar et al ., ].…”

“…Location map of Heron Reef showing the locations of the EC measurement sites with EC2 moored in the deep lagoon (insert) (modified after MacKellar et al . []).…”

“…Measurements of air-sea CO 2 exchange over Heron Reef were made using two pontoon-mounted eddy covariance (EC) systems (EC1 and EC2) [MacKellar et al, 2012[MacKellar et al, , 2013. Pontoon-mounted EC systems have been used successfully on lakes and reservoirs to provide continuous direct measurement of exchanges of CO 2 and latent and sensible heat between the water surface and the atmosphere under a wide range of meteorological conditions [Eugster et al, 2003;Vesala et al, 2006;Nordbo et al, 2011;McJannet et al, 2013].…”

“…This employs the Horst and Weil [1992] model for the crosswind component and functions for predicting the upwind dimension by Schmid [1994]. The method includes a stability correction function for the surface layer of air over the water and the influence of surface roughness [MacKellar et al, 2012].…”