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Notes 1769 salinity and temperature on results. Nevertheless, the clod cards proposed by Doty are easily prepared with standard ice-cube trays as molds and are suited to marine studies because they are large enough to be left out through a tide cycle without dissolving completely. It is a convenient and inexpensive method of measuring water motion in field studies. We have attempted to define the behavior of clod cards with respect to temperature and salinity and to develop calibration methods to enhance their usefulness. In his original description, Doty (197 1) recommended expressing field results as a “diffusion increase factor” (DF), defined as the ratio of weight loss of clod cards placed in the field to the weight loss of clod cards from the same batch held under specified still-water conditions in the laboratory. DF was meant to express the multiplication factor of diffusion in the field relative to diffusion in the absence of water motion. Unfortunately, his reported still-water values sometimes varied inexplicably among batches of clod cards that had been prepared and measured the same way. This variation introduced a large potential error into the calculation of DF, since this relatively small number is used as a divisor. Doty’s laboratory calibration method consisted of placing five clod cards on the bottom of a 15-cm-diameter container to which 2 liters of test solution were added. The water (motionless) was changed every day for 4 d. The initial and final dry weights of the clod cards were used in the DF calculations. Howerton and Boyd (1992) as well as Jokiel and Morrissey (1993) have demonstrated that the calibration container must have a minimum volume of 20 liters per clod card if it is to accurately represent weight loss due to diffusion in totally calm water. An additional problem is created by stratification in the calibration container, particu- larly when the clod cards are placed on the bottom. When a solid dissolves in quiescent water, material from the solid dissolves and diffuses away from the surface, producing a solution near the solid that is denser than the water itself. This solution flows downward, and the resulting flow increases the dissolution rate. However, the dense solution may “pool” on the bottom of the container, creating a layer of solution with a higher concentration of the dissolving material than in the bulk liquid. If the dissolving solid is placed on the bottom of the container, it will generate a layer of relatively concentrated liquid that will retard the dissolution rate compared to the solid in pure bulk solution. In unpublished experiments pointing the way to an improvement in methodology, Doty attached clod cards to a rotating arm in a tank of seawater and found that the dissolution rates of the cards were proportional to the rate at which they moved through the water. He called the apparatus a “water motion simulator” (WAMOSI) because it was the clod card, not the water, that was moving. The raw data tabulation of that study (Doty et al. 1986) shows that despite differences in apparent still-water dissolution rates among different batches of clod cards, their actual dissolution rates on the rotating arm were reproducible at a given velocity. Subsequently, Glenn and Doty (1992) used the rotating-arm calibration method in combination with field measurements to show that 85-98% of the variability of growth rates of eucheumatoid red seaweeds across seasons on a reef flat in Hawaii could be explained by water motion. These experiments indicated that calibrated clod cards would be valuable tools for correlating growth rates with water motion in the marine environment. Recently, Howerton and Boyd (1992) used clod cards to measure circulation patterns in aquaculture ponds, and Jokiel and Morrissey (1993) used clod cards to study water motion on coral reefs. In both studies, the investigators improved the still-water calibration method by increasing the volume of water into which the cards dissolve, and they measured rates of still-water dissolution of the cards as functions of duration of exposure, temperature, and salinity. Both studies used a rotating arm to estimate dissolution rates as a function of water velocity at a single temperature and salinity. They concluded that clod cards were valuable in measuring relative rates of water motion but, similar to earlier investigators, cited the need for equations relating card dissolution rates to fluid velocity as influenced by temperature and salinity before they could be used to measure absolute water velocities. Deriving and validating these equations was the purpose of our study. Clod cards were prepared essentially ac-
1770 Notes r=0.15 (typ.) ‘F q- El _ I /‘ ‘\, +45cm+ /F$2+., \ 3.3cm T=T Bottom surface sanded to adjust weight to 30 f 1.5 g Fig. 1. Dimensions of the clod cards. cording to the methods of Doty (197 1) and Howerton and Boyd (1992), except that neither the starting plaster of paris nor the finished cards were oven-dried before use. Our clod cards were made from reagent-grade plaster of paris (calcium sulfate hemihydrate, EM Science Co.) with 335 ml of water per 500 g. We slowly added plaster of paris to the water as we stirred it with a spoon; we poured the slurry into flexible plastic ice-cube trays and allowed it to harden for 20-30 min before removing the clod cards. We tapped the trays vigorously several times while the mixture was still liquid to dislodge air bubbles. Sometimes, a thin layer of liquid covered the surface of the hardening clod cards in the trays. This layer was decanted before the cards were removed. After removal, the cards were dried for 3 d or longer on a laboratory bench, and the bottoms were sanded until the cards had uniform weight within a batch (within 1.5 g); they were then glued to 5- x 7-cm sheets of waterproof plastic (Nalgene PolyPaper) with silicone cement. Weight loss due to water exposure was quantified by weighing dry clod cards before exposure, placing them in the test solution for 24 h, allowing them to redry for 3 d or longer on a laboratory bench, and then reweighing them. The ice-cube trays used in these experiments produced clod cards of shape and dimensions shown in Fig. 1. Internally, the dry cards had a foam structure with numerous interconnected minute air spaces within the calcium sulfate matrix. The cards had densities of 1.19-l .21 / at 25°C and weighed 28-32 g each. When placed in test solutions, the cards were fully saturated within 5 min, absorbing from 10 to 12 ml of solution. Water motion experiments were carried out inside a greenhouse in a shallow, wooden-sided, square tank (2.1 m long, 0.36 m deep) with a plastic liner (Fig. 2). The tank was filled with test solution to a depth of 25 cm (solution volume, 1.1 m3) and covered with sheets of Styrofoam to control temperature during each 24-h run. Test solutions were chilled by circulating the water through a 124-W refrigeration unit with external thermostat by means of a 30-W circulation pump. Solutions were heated with a thermostat-controlled, 150-W immersion heater. Clod cards were attached at various positions along a 2-m stainless steel arm (1 cm thick) which was rotated in the tank at 2.4 rpm by a 37-W electric gear motor secured over the middle of the tank by an external wooden framework. The cards were mounted on the arm by attaching them with rubber bands to rigid PVC-plastic plates (5 x 7.5 cm, 0.3 cm thick) screwed to the arm. Five cards were mounted along each of the two radii at distances from the center of 7, 27,47,67, and 87 cm. Thus, each position was represented by duplicate cards moving through the water at nominal velocities ranging from 1.8 to 2 1.8 cm s-l, which are within the range of water velocities encountered on tropical reef flats (Glenn and Doty 1992) but lower than extreme tidal bores, which are up to 80 cm s-l (Mathieson et al. 1977). Greater velocities can be achieved by selecting a gear motor with a higher rotation rate (Howerton and Boyd 1992) or by using a longer rotating arm (Jokiel and Morrissey 1993). Dissolution rates under free convection conditions were determined by suspending clod cards by wires 5 cm below the surface of 60 liters of solution contained in ice chests. The ice chests were kept under constant temperature in an undisturbed indoor location during each 4-6-d run. The cards were withdrawn and weighed wet each day, then replaced in the chests. Three solutions were tested in the water motion tank: tapwater, brackish water, and seawater. Tapwater was from the Tucson municipal supply, and seawater was collected from
Page 1: 1768 Notes coming tide over a broad
Page 5 and 6: 1772 Notes J 1.0 2 E 0.9 L 2 0.6 P
Page 7 and 8: 1774 Notes 0.6 A 25.5’ A 30.5. 0
Page 9 and 10: 1776 Notes 1000 500 < 200 p 100 7 f
Page 11 and 12: 1778 Notes Sample Points Along cent

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