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 ...

To test the effect of sediment resuspension on the nutrient and ecosystem dynamics we performed a 4 wk experiment in three 1000 l shear-turbulence-resuspension-mesocosm (STURM) resuspension (R) tanks and three 1000 l non-resuspension (NR) tanks. All tanks contained defaunated muddy sediment and brackish estuarine water and had similar water-column turbulence intensities (~1 cm s-1), energy dissipation rates (~0.08 cm 2 s-3), and tidal cycles (4 h mixing-on and 2 h mixing-off). However, while bottom shear velocity (stress) was low in the NR tanks, high instantaneous bottom shear produced resuspension in the R tanks during the mixing-on cycles. Tidal resuspension in the R tanks resulted in concentrations of 120 to 220 mg l-1 total suspended solids when mixing was on, decreasing to between 10 and 20 mg l-1 when mixing was off. Particulate nitrogen, phosphorus, and carbon concentrations, as well as dissolved inorganic nitrogen, nitrate + nitrite, and phosphate levels were higher in the R tanks. Phytoplankton biomass was also higher in the R tanks, though light was limiting. Tidal resuspension affected water-column algal and zooplankton community composition and induced significantly higher concentrations of brown tide Aureococcus anophagefferens. Microphytobenthos biomass was significantly higher in the NR tanks. Dissolved inorganic nitrogen sediment effluxes were similar in both tanks; however, polychaetes and amphipods developed in the NR, but not in the R tank sediments. Tidal resuspension shifted processes from the benthos to the water column. Regular tidal resuspension profoundly affected ecosystem structure and function, often through indirect pathways and linkages. KEY WORDS: Tidal resuspension · Benthic-pelagic coupling · Shear stress · Shear velocity · STURM· Ecosystem Resale or republication not permitted without written consent of the publisher OPEN PEN ACCESS CCESS J J J J J 0 J NR J J J J J 0 J J J J J J J J J J J J J J J 0 0.05 0.1 0.15 0.2 0.25 J J J J J Editorial responsibility: Graham Savidge,

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Three shear turbulence resuspension mesocosms (STORM tanks) were used to examine the release of polychlorinated biphenyls (PCBs) from resuspended Hudson River sediment. Twenty-two percent of the resuspended PCBs desorbed after 2 h, and 35% +/- 8% of PCBs were in the dissolved phase after apparent steady state was reached in 2 days. After the first resuspension event, the solids were allowed to settle and the quiescent time was varied to determine whether the labile pool of PCBs is recharged during sediment consolidation. The steady-state log Koc values for the third subsequent resuspension were higher than for the first event due to lower dissolved PCB concentrations; the particulate PCB concentrations were constant between events. With 1 day of consolidation between resuspension events, the dissolved concentration of all congeners decreased an average of 8% +/- 5% between subsequent resuspension events. With 4 days between events, only the dissolved pentachlorinated PCBs decreased significantly (p = 0.002), suggesting that the easily desorbable PCBs recharge when there is sufficient time between resuspension events.