Responses of Hawaiian Scleractinian Corals to Different Levels of Terrestrial and Carbonate Sediment

Abstract

Goals of this investigation were: 1) Differentiate between the effects of turbidity and sedimentation on corals. 2) Differentiate the effects of carbonate and terrigenous sediments on coral growth and metabolism. 3) Develop a predictive model relating increased sediment loading to the photosynthetic rate, growth and survival of corals. 4) Evaluate the validity of the widely cited "10 mg cut-off point" and determine the levels of turbidity and sedimentation that are tolerated by corals. Earlier studies suggest that coral reefs decline when sediment loading levels exceed a sediment trapping rate of 10 mg cm-2 d-1 or a turbidity of 10 mg I-1. An integrated experimental design linked field and laboratory experiments. In the field, corals were transplanted to sites with varying levels of sedimentation. Turbidity, as linked to reduced light availability, had a significant affect on coral growth and metabolism in laboratory experiments but not coral growth or survival in field experiments. Turbidity in the field was highly variable and influenced greatly by wind velocity and rainfall. Terrigenous (or land-derived) sediments influenced light extinction more than carbonate (reef-derived) sediments. In laboratory experiments turbidity was more important than sediment trapping rate in affecting the photosynthetic ability and growth rates of corals. The predictive model stated that the lower the light level (as caused by increased turbidity) the lower the photosynthetic production of corals. The model was supported by results of laboratory experiments where conditions were precisely controlled and monitored. Under field conditions, the predictive model did not hold due to extreme natural variability in sediment loading and variation in a wide range of other environmental factors encountered at the experimental sites. Corals tolerated turbidity exceeding 120 mg I-1 and sediment trapping rates in excess of 11 mg cm2 d-1. Sediment loading is correlated with other environmental factors such as nutrient loading and reduced salinity. Composition of sediment varies widely between different locations. The "10 mg cutoff may prove to be useful in setting limits to vigorous coral reef development, although individual corals or coral assemblages may be able to survive and grow at levels exceeding this value.