Introduction

Coral reefs such as the Great Barrier Reef (GBR) are the net result of the productive efforts of marine animals and plants that build structures able to withstand waves and currents. These structures are predominantly composed of skeletal material produced by a myriad of simple organisms that can achieve high productivity and nutrient-cycling efficiency in seas that are generally nutrient poor. Scleractinian corals and coralline algae are the major contributors to reef growth on most coral reefs. Both utilize solar energy via photosynthesis to realize calcification rates that are significantly higher than those possible if they were solely heterotrophic (Chalker and Dunlap, 1983; Barnes and Chalker, 1990). Calcification rates, and thus the accumulation of calcium carbonate products, or reef growth, proceed most rapidly in the euphotic zone, where photosynthetically active radiation (PAR) can be accessed and utilized. In tropical seas, where the majority of corals live, this may extend to depths exceeding 100 m under exceptional circumstances; for example, corals are known to grow at depths below 140 m in the Red Sea, and to reach similar depths on outer reefs of the GBR such as Myrmidon Reef off Townsville (Hopley, 1994; see Fig. 9.12). Generally, however, PAR rapidly diminishes at depths below 30 m, or even shallower in turbid settings (Anthony and Fabricius, 2000). As a result, reefs grow best within 30 m of the sea surface, with reef accretion rates usually highest above 20 m (Hopley, 1982; Carter and Johnson, 1986).