Partitioning of underwater direct and diffuse ultraviolet irradiance in a shallow water coral reef
Cameron J. Veal A C D , Kelvin J. Michael B and Manuel Nunez AA School of Geography and Environmental Studies, University of Tasmania, Hobart, Tas. 7001, Australia.
B Institute of Antarctic and Southern Ocean Studies, University of Tasmania, Hobart, Tas. 7001, Australia.
C Present address: Centre for Marine Studies, The University of Queensland, Brisbane, Qld 4072, Australia.
D Corresponding author. Email: c.veal@uq.edu.au
Marine and Freshwater Research 60(12) 1244-1253 https://doi.org/10.1071/MF08318
Submitted: 19 November 2008 Accepted: 21 April 2009 Published: 17 December 2009
Abstract
In optically shallow waters of coral reefs, the contribution of both direct and diffuse ultraviolet irradiance is poorly documented yet its involvement in biological processes is highly significant. Here we present a new approach of partitioning the ultraviolet attenuation coefficient into direct and diffuse components in the top 5 m of waters surrounding Heron Reef, Queensland, Australia (23°27′S, 151°55′E), during the austral summer solstice from 11 to 21 December 2005. A commercial ultraviolet radiometer, which operated in selected spectral bands centred at 305, 313, 320, 340, 380 and 395 nm, was fitted with an aquatic shadow-band that allowed the measurement of both global and diffuse irradiances. Direct irradiance was estimated by subtracting diffuse irradiance from global irradiance. Results revealed that direct irradiance was attenuated more quickly than diffuse irradiance, with stronger attenuation in the shorter wavelengths. Attenuation coefficients for the global, diffuse and direct components of irradiance were significantly different, justifying their separate treatment through partitioning. A significant dependence on turbidity explained more than 70% of the variation in attenuation of ultraviolet irradiance at the shorter wavelengths. The direct ultraviolet irradiance can exceed 50% of the global ultraviolet irradiance in shallow waters (0–5 m), a significant result that is generally overlooked in optical and biological studies.
Additional keywords: Great Barrier Reef, Heron Island.
Acknowledgements
This work has been funded by the Australian Research Council Discovery Grant DP0453361, led by Professor Ove Hoegh-Guldberg at the Centre for Marine Studies, The University of Queensland. The authors acknowledge technical assistance from: Mr Peter Dove, Mr Paul Waller, Mr Vagn Jensen and Dr Ohm Masiri as well as the input of two anonymous reviewers that greatly improved the manuscript. Thanks must finally go to the staff at Heron Island Research Station as well as Miss Bonnie Lewis, Miss Rebecca Shaw and Mr Graeme Veal for volunteering in the field.
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