SEBENS, Kenenth P.; University of Maryland: What Limits Coral Growth?: Location, Translocation, Allocation

Scleractinian corals use a wide variety of resources to grow and calcify in regions that are often low in potentially limiting resources. In shallow reef habitats, zooxanthellae translocate abundant photosynthate, but nutrients needed for tissue growth may be scarce. In deeper zones and other light limited reef locations, photosynthesis does not meet even energetic needs. Zooplankton and other particulate material can be major sources of nutrients, and probably energy, to reef corals, and dissolved nutrient uptake can also be significant. Experimental results show that plankton capture, for example, can control calcification and tissue growth rates in several coral species. The effect of feeding on growth and calcification is often greater than that of other proposed controls, such as available bicarbonate concentrations, calcium saturation state, and water flow. The growth response of corals to simultaneously modified available prey concentration and irradiance indicates that zooplankton capture can be a significant control for coral calcification even when irradiance is high. Comparisons of respiration and photosynthesis rates, along with chlorophyll and zooxanthellae density/size, illustrate changes occurring within the symbiosis with increased zooplankton capture. The differential allocation of energy and materials to tissue versus skeleton affects how corals grow, and may set limits on growth rate. Understanding interspecific differences in the utilization of this array of resources can help explain patterns of coral diversity, zonation, and abundance on reefs and may be useful in interpreting changes in growth rate with modifications of habitat quality and temperature.