The physiological response of hermatypic corals to nutrient enrichment
Nutrient enrichment of tropical waters constitutes an increasing threat to the health and biodiversity of coral reefs. In order to manage these ecosystems effectively, the onset of nutrient pollution has to be closely monitored. This thesis examined the possibility of using some physiological responses of hermatypic corals as an early-warning bio-assay, to detect nutrient enrichment before reef deterioration has taken place. To this aim, the physiology of the common branching coral Porites porites and the massive coral Montastrea annularis was studied both in the laboratory and on the reef under different nutrient conditions. By measuring the organic and inorganic productivity of corals and by constructing carbon budgets, it was hoped to relate differences in the fixation, allocation and utilisation of carbon to differences in nutrient regimes. Nubbins of Porites porites and explants of Montastrea annularis were chosen as the experimental units. Nubbins were obtained by cutting coral tips (approx. 20 mm), grounding their cut surface flat, and gluing them onto a perspex tile with cyanoacrylate glue. To obtain explants, a coral head was cored under a drill press fitted with a hole saw. Cores were then cut to fit, and sealed into polyethylene cups with underwater epoxy putty. A new culturing system was developed to grow corals successfully in the laboratory under completely controlled and repeatable conditions. This system (the 'photostat') consisted of glass aquaria (30x21x18 cm) placed in a constant temperature water-bath under metal halide lamps. The aquaria were fitted with specially designed air lines and coral trays to maintain a strong water motion around the corals, independent of the rate of water-flow. A peristaltic pump ensured a daily water turn-over. A new improved carbon budget methodology was developed by comparing the well established methods of Davies (1984) and Muscatine et al (1984) on Porites porites. These methodologies differed in the measurement of zooxanthellae respiration rate (Rz) and zoozanthellae growth rate (). Rz,DAVIES was found to be twice as small as Rz,MUSCATINE (RZ, MUSCATINE (RZ, DAVIES = 18.1 gC cm-2d-1 vs. Rz,MUSCATINE = 33.1 gC cm-2d-1), but this accounted for a difference of only 3% when Rz was expressed as a percentage of the total daily carbon input. By comparison, a 25-fold difference between methods occurred in the component of carbon required for the daily growth of the zooxanthellae. Davies' method measured the net rate of zooxanthellae growth (NET) from the increase in surface area, assuming a constant zooxanthellae population density. In this case NET was only 1.65 gC cm-2d-1.