Meso and Micro-scale light fields within Coral Reef Environments

Light fields in shallow water control the development of coral reefs. In fact, the morphology of reef-building corals defines the three-dimensional surface where photosynthesis occurs. However, little detailed information is available about the optical properties of waters over coral reefs and the consequent penetration of solar radiation. An understanding of these physical processes is particularly important in assessing how coral reefs are likely to respond to global climate change. This thesis pursues this understanding through a combination of experimental and field measurements as well as modelling and related computational approaches to integrate many separate fields of research to define the shallow water light field around coral reefs. The principal research objective of this thesis was to develop a greater understanding of the behaviour of light within shallow coastal waters by acquiring the physical information needed to model solar irradiance fields in a realistic coral reef environment using high-resolution, fine-scale methodologies. The second objective was to develop a better understanding of the contribution of light to the stress phenomenon of mass coral bleaching with a view to informing a better design of satellite-based sensors and analytical algorithms. In order to address these overarching objectives, the thesis addressed 3 key questions: (1) What are the optical properties of waters surrounding coral reef regions of the world and how accurately can they be measured from remote sensing platforms? (2) What are the most important optical and photophysiological processes in shallow water coral reefs? (3) How are light fields incident on coral colonies controlled by shape and morphology, and how can these shapes be better captured at the scale of centimetres to millimetres to increase the understanding of photon interactions? The optical water quality of a region was found to strongly affect the spectral and absolute irradiance field incident on a coral reef, with the waters and coral reefs of the Mexican Meso-American Barrier Reef System being significantly different to those of the Australian Great Barrier Reef and the Israeli Gulf of Aqaba. These differences were driven by: distance of the coral reefs from land, the extent of human disturbance of associated coastal areas, and the subsequent amount of optically-sensitive dissolved components and suspended sediment in the water column. The significant focussing of light energy by small waves in shallow water coral reefs, despite producing irradiance intensities in excess of 2.5 times surface irradiance conditions at depth on the most light-exposed coral reef in the world (Northern Red Sea), did not result in altered or depressed photosynthetic productivity under either wave lensing or static illumination conditions. The light field experienced by shallow water coral reefs is primarily linked to the skeletal shape and complexity with most coral physiological studies normalising parameters to the surface area of the primary surface of the coral skeleton. The accuracy of surface area detection is critical to this normalisation process, which revealed that, compared to surface area detection using the highly accurate X-Ray Computer Tomography technique, single wax dipping will permit highly accurate surface area estimates of most coral skeletons. If surface area models or three-dimensional shapes are required, instead of just surface area, the X-Ray CT provides the most realistic coral surface. Such a realistic coral morphology was subsequently used to investigate light fields over a shallow water coral reef with a three-dimensional radiative transfer light model. The preliminary results of the model indicated that skeletal morphology, photosynthetic tissue albedo and location and substrate albedo contributed to the total photon yield at any point on a coral surface. The results and analysis associated with this thesis illustrate the highly complex nature of shallow water light fields in coral reefs and indicate the need to increase the accuracy and resolution of light monitoring currently being undertaken in coastal waters globally. Focusing on this issue is critically important to improving the predictive products that are available for coral reefs. Given the central role that light plays in the biology of corals and coral reefs, understanding the complexities of solar radiation, optical properties and tropical coastal environments is vital, especially given the importance of coral reef ecosystems to millions of people worldwide.