Collaborative Research: Mass Transfer Due to Oscillatory Flow Over Coral Reefs

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The relationship between new production, organic carbon cycling, and nutrientuptake into coral reef systems is not well understood. Nutrient uptake into experimentalcoral reef communities has been shown to be mass-transfer limited, or limited by the rate at which nutrients can be physically delivered to the active surfaces of reef autotrophs. A set of simple, mass transfer relationships have been developed to predict uptake of dissolved inorganic nutrients. These relationships were derived from experiments which were conducted in 10-20 m long flumes and under steady, non-oscillatory flow conditions. Drs. Atkinson and Monismith reason that these mass transfer equations are inappropriate for understanding nutrient uptake by shallow-water reef communities in natural reef environments, where wave-drive water motion dominates the surrounding flow field. They have argued with both theoretical and empirical evidence that there should be some enhancement of mass transfer due to oscillatory flow. They emphasize that there are no mass transfer studies with both waves and naturally rough surfaces and that this is a problem unique to the study of benthic ecosystems. The project has three general goals: 1) To understand how waves influence rates of mass transfer to naturally roughsurfaces; 2) To determine how waves influence nutrient uptake into reef communitiesadapted to wave-driven flow environments; 3) To actually demonstrate that waves enhance in situ rates of nutrient uptakeby natural reef communities.To achieve these goals, these scientists propose an interlocking set of two lab studies and one field study. The first of these, a laboratory experiment on nutrient uptake by a coral community under both steady and oscillating flow conditions, will be conducted at the HIMB lab on Coconut Island. The second, a detailed study of the fluid mechanics ofstress and mass transfer over corals will be carried out at the Environmental FluidMechanics Laboratory at Stanford. Finally, to test the application of the lab results, aseries of field experiments will be carried out to determine whether rates of nutrientuptake by natural reef communities under wave-driven flow conditions are positivelycorrelated with incident wave energy, and, to determine whether these rates scale withestimates of bottom shear stress according to mass transfer correlations derived fromlaboratory experiments. Results of these experiments will give us the quantitative basis to determine thespatial variability in rates of nutrient uptake into reef communities, and to promote anunderstanding of how the dynamic physical environment typical of reef communitiesmay be defining their metabolism.