Attenuation coefficients of ultraviolet and photosynthetically active wavelengths in the waters of Heron Reef, Great Barrier Reef, Australia

Abstract

Variation in subsurface solar radiation has been identified as a second-order factor in coral bleaching. This study examines transmission of radiation at Heron Reef (southern Great Barrier Reef, Australia) testing the hypotheses that spectral transmission is independent of location, season and solar zenith angle. In June and December 2004, 132 underwater irradiance profiles were collected at six ultraviolet wavelengths (305, 313, 320, 340, 380, 395 nm) and photosynthetically-active radiation (PAR) to estimate spectral diffuse attenuation coefficients Kd(λ). Mean Kd(λ) decreased through the ultraviolet spectrum (0.435 m–1 at 305 nm to 0.115 m–1 at 395 nm) whereas mean Kd(PAR) was 0.129 m–1. Ultraviolet Kd(λ) values were highly correlated with each other, but less well correlated with Kd(PAR). Kd(λ) values were larger inside than outside Heron Lagoon, both across all wavelengths and at individual wavelengths. Similarly, Kd(λ) values were significantly larger in December than June. These results are consistent with greater resuspension and transport of particulates in Heron Lagoon at higher wind speeds. Kd(λ) values were not dependent on solar zenith angle. The implications are that location and season must be accounted for when setting Kd(λ) values to calculate spectral irradiances affecting corals at Heron Reef.