All multicellular organisms are dependent on the movement of fluid (air or water) for their survival. Some animals exert energy to move fluid; for example, we humans move air in and out of our lungs using muscles, and circulate blood through our bodies using our hearts. Many organisms, however, must rely on the movement of water outside of their bodies to deliver materials such as oxygen. Anemones, seaweeds and corals all exchange oxygen and carbon dioxide directly with the surrounding water moving over their surfaces. Understanding patterns of water motion over them is key to understanding the physiology and ecology of these benthic organisms.
Physical oceanographers can learn a great deal about ocean circulation patterns using an array of equipment that is either operated from a ship or deployed in the water column. Scientists may use a Conductivity, Temperature, and Depth recorder (CTD) to measure the salinity, temperature and pressure throughout the water column, and an Acoustic Doppler Current Profiler (ADCP) to record the current velocity. These tools provide a broad picture of the physical conditions present in the water column.
However, water flows at the bottom of the ocean are often difficult to measure. Whenever fluid such as water moves over a solid surface, like the ocean bottom, that fluid has a tendency to stick to the bottom. That “stuck layer” in turn sticks to the water moving above it, slowing down its momentum. The result is a gradient in velocity over the bottom of the ocean, often several meters thick, which we call a boundary layer. The thicker the boundary layer, the farther a substance (such as oxygen or nutrients) must travel from the water column to the surface of an organism living on the bottom.
Suspension feeders, such as the crinoid and corals shown here, rely on the movement of water for the delivery of food and oxygen. Water flow patterns play an important role in driving the distribution patterns of suspension feeding invertebrates. Click image for larger view.
In order to measure rates of exchange between organisms and the overlying water, we can measure levels of oxygen, nitrogen, etc. at varying distances above the bottom. Alternatively, we can measure the rate at which materials are removed from the bottom. One common technique used by benthic ecologists is to deploy cards or balls made of plaster of Paris (calcium sulfate hemi-hydrate) or alabaster (calcium sulfate dihydrate). These “clod cards” dissolve at a rate related to the “shear” applied to them. The faster the flow and the greater the rate of “scrubbing” by the moving water, the faster the clod cards will dissolve. The higher the rate of mass loss, the higher the rate of exchange at that location.
Using a submersible or ROV, researchers can deploy the pre-weighed clod cards at predetermined sites. After a few days, the clod cards are retrieved. After retrieval, they are dried and reweighed. The post-deployment weights are then compared to the pre-deployment weights. To help quantify the actual current velocity, dissolution rates of clod cards are calibrated in a controlled flow tank under salinity/temperature conditions similar to those present in the area of study. The values for dissolution obtained in the field are compared to the values obtained in the lab. This information indicates the relative amount of exchange of nutrients between the benthic community and the water column, as well as the physical forces the attached epifauna encounter in different regions. Scientists then compare these measurements to patterns of species distribution, feeding type, size and morphology, in order to learn how flow might control where organisms live.
Clod cards can be made of both alabaster and plaster of Paris. The rate at which the two materials dissolve in seawater is directly proportional to the strength of the flow, but plaster of Paris dissolves more rapidly than alabaster. Five sides of each flat, rectangular clod are painted with waterproof boat paint, so that only the top surface is exposed to the flow of water. The clods are then mounted on sheets of Plexiglas. Each Plexiglas card holds one alabaster clod and one plaster of paris clod attached to it. Lead weights are bolted to the bottom of each card to anchor it in position. Finally, a knotted rope handle is attached to ease the deployment and retrieval of the cards by a submersible's manipulator arm.
Although they are not as precise as current meters, clod cards are simple in design and inexpensive to make. There are many potential problems that could make it impossible to retrieve the clod cards intact or even at all. If scientists fail to recover some clod cards it may be unfortunate, because they sacrifice labor, data and a small amount of money. However, most instruments used by physical oceanographers to measure currents and turbulence cost thousands of dollars and it would not be feasible to deploy them in areas where the chances of recovery are small. Clod cards are therefore an economical means of characterizing the near-bottom flow patterns in little-known ocean environments.
The Web team gratefully acknowledges this contribution by Dr. Brian Helmuth an assistant professor from the Department of Biological Sciences and Marine Science Program at the University of South Carolina and Dara Dawn Hooker from the Department of Geological Sciences and Marine Science Program at the University of South Carolina.