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The dissolution of gypsum or plaster of Paris has been widely used as an inexpensive integral measure of 'water motion' in the field and in laboratory tanks for studies of physical-biological interactions. Commonly, gypsumdissolution rates have been calibrated to steady flow speed or velocity in the laboratory and the calibrations have been applied to dissolution (i.e., mass-transfer) rates in the field or in tanks. We evaluated the gypsum-dissolution technique in a steady-flow, a fluctuating-flow, and a mixed-flow environment by comparing dissolution rate to direct flow measurements with an acoustic Doppler velocimeter. We found that dissolution rates were related to steady flow and to fluctuation intensity in the exclusively steady-flow and fluctuating-flow environments, respectively. The relationships were weak in the mixed-flow environment. Finally, dissolution and thus mass-transfer relationships were different in each flow environment, and the effects of steady flow and fluctuation intensity were not additive. Providing that it is rigorously checked and appropriately calibrated, the dissolution technique can be used to measure steady flow speed or fluctuation intensity in a steady-flow or fluctuating-flow environment, respectively. However, comparisons of dissolution rates between steady-flow, fluctuating-flow, and mixed-flow environments or within environments that change over time to determine water motion will be misleading. The gypsum-dissolution technique can be used as a good direct indicator of mass-transfer rates. However, mass-transfer rates are different in different flow environments. The gypsum-dissolution technique is not a universal integrator of 'water motion. ' The dissolution of gypsum or gypsum-based plaster of Paris from ''plaster balls'' or ''clod cards'' was first introduced by Muus (1968) and Doty (1971) as an inexpensive technique to measure water flow in the absence of expensive field instrumentation for direct measurements of water velocity. Effects of temperature, salinity, and water volume on dissolution rates were subsequently investigated by Jokiel and Morrissey (1993) and Thompson and Glenn (1994). Since its first introduction, the gypsum-dissolution technique has been widely used in many habitats and for many applications, especially for studies of physical-biological interactions (Table 1). The gypsum-dissolution technique has been thought to provide a comparative or absolute measure of water movement that ''incorporates any water motion due to tidal as well as wind-wave effects in the same integrated measure' ' (Wildish and Kristmanson 1997). Furthermore, the dissolution technique has expanded to include materials other than gypsum (e.g., Koehl and Alberte 1988;Bartol et al. 1999). AcknowledgmentsWe thank W. Boicourt for letting us participate in the field ''Tower'' experiment and for providing the temperature and salinity data to calibrate the field-dissolution results. We thank K. Sebens, D. Stoecker, V. Kennedy, M. Palmer, and three anonymous reviewers for ...

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