Download - ASLO

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-