Water motion on coral reefs: evaluation of the 'clod card' technique

Assumptions underlying the technique of using dissolution rates of plaster blocks as a dimensionless index of water motion were tested. 'Diffusion factor' (DF) was originally defined by The DFconcept must be interpreted with caution because measurement of the rate of loss in the control block is influenced by many factors. The volume of standing water containing the control block must be relatively large in order to avoid saturation of the water with solute from the card. Temperature and salinity also influence rate of weight loss. Relationship between current velocity and weight loss was linear over the range of current velocities tested (0 to 47 cm S-'). Weight loss remained relatively constant until the block dissolved to less than 30 % of its original weight. The DF measurement is analogous to various dimensionless indices of mass transfer currently being applied in coral reef hydrodynamics. Weight loss due to turbulence (i.e. wave-induced orbital motion) and weight loss due to unidirectional flow (i.e. current) apparently are additive, but in a complex fashion. Exemplary data are presented to illustrate the utility as well as the limitations of thls method on coral reefs.