How water movement affects coral reef organisms: Aquarius mission 9918, Conch Reef, Florida Keys National Marine Sanctuary.
Principal Investigator: Dr. David Wethey, University of South Carolina.

The condition of corals and other bottom dwelling reef organisms depends ultimately on materials dissolved in the water that flows past them.  In fact, most reef organisms, because they live permanently attached to the bottom, and because they can only absorb, capture, or release materials at the surface of their bodies, depend entirely on the water that flows over and around them for everything they need to survive and reproduce.  The rate at which they can take up substances from (or release them to) the water is directly controlled by the amount of mixing in the water.  More mixing (turbulence) increases the ability of these organisms to exchange nutrients, wastes, reproductive products, or pollutants with the surrounding water.  This Aquarius mission is designed to 1) measure the water movement within centimeters of coral surfaces and 2) to measure the rates at which chemicals disperse to and from those surfaces.  By doing so, we will gain the ability to better predict how chemicals in the water (naturally occurring or introduced by humans) affect the reef environment. 

Though focussed on a small spatial scale, the results of this work will provide important information to better understand large scale problems facing coral reefs, like coral bleaching and nutrient pollution.  Coral bleaching is most severe during warm summer months, which are often characterized by high water temperatures and calm ocean conditions.  Calm conditions reduce turbulent flow on the reef, and the reduced flow can increase water temperature at specific "pockets" on the reef.  We will conduct experiments to understand how corals with different shapes and at different locations (high up on the surface of the reef compared to down low protected by spurs and grooves) interact with water movement to affect temperature where it matters most – immediately around corals.  On coral reefs, where nutrients are naturally found at low concentration, it's important to understand how flow dynamics can control the ability of corals to acquire the nutrients they need to grow and reproduce.  Nutrient pollution occurs on a coral reef when damaging amounts of nitrogen and phosphorus are introduced.  The amount of pollution available to harm an organism is a product of the pollutant's concentration, its input flow rate, and the amount of turbulence delivering the chemicals to the organisms' surfaces.  With relatively low levels of turbulence, organisms quickly deplete the chemicals from the water immediately around them, leading to lower uptake rates.  Conversely, high levels of turbulence continually refresh the concentrations of chemicals near the organisms, enhancing uptake rates.  Recent work suggests that reefs as a whole take up more nutrients than would be predicted by standard engineering models of flow over typical rough surfaces.  The implication is that the complex structure of the reef environment generates high levels of turbulence, and hence increased uptake rates.  This mission will make the measurements that are needed to understand that turbulence. 

Summary of Techniques (Technical discussion)

The target of this study is to measure flow characteristics and exchange rates on the coral surface at a spatial scale relevant to individual coral colonies.  We will use a diversity of mutually reinforcing approaches to make these measurements. 

"Smart" Front End

Since our work site is about 30 meters from the Aquarius underwater laboratory, we wanted to avoid running low-voltage sensor signals over that distance.  Therefore, we are placing a Campbell CR10X data logger as the "smart" front end data processor.  It will have various sensor packages plugged into it to make a variety of measurements.  The data logger is encased in a waterproof housing constructed from PVC pipe with a 3/4" Plexiglas faceplate.  Into the faceplate are mounted Seacon underwater connectors that permit sensor cables to be attached and detached in the water.  An RS-232 serial connection allows the aquanauts in the habitat to program the logger, monitor its operation, and collect data from it.  Power for the logger is provided by 12 volt DC in the same underwater cable as the RS-232 serial wires.

Flow Speed Measurement

Our main water flow speed measuring devices are based on thermistors.  "Hot bead thermistor" flow probes consist of a thermistor bead a few millimeters in diameter mounted at the end of a foot-long probe holder.  The electronics associated with the probe maintain the thermistor bead at a temperature several degrees above ambient water temperature.  Faster flow speed removes heat from the bead more quickly, requiring more electrical current through the bead to maintain its temperature.  The amount of current required is used (with appropriate calibration) as a measure of flow speed at the probe tip.  Using these probes we will be able to make flow speed measurements at 10 Hz (10 times per second) at multiple places simultaneously, allowing us to map the vertical distribution of flow speeds above coral heads.  That information informs us directly about the turbulence present in the water.  We will also be using these probes in conjunction with the other observations described below.The thermistor probes were fabricated in our laboratory from simple components (bare-wire thermistors, standard multi-conductor signal wire, and Plexiglas tubing for the probe supports).  The tips were soldered together then "potted" in an epoxy-like solution to waterproof all the exposed wire within the probes.  The probes are controlled and measured from small electronic assemblies, also fabricated in the laboratory.  The electronics are encased in underwater housings constructed from PVC pipe parts with a 3/4" Plexiglas faceplate.  Power comes from lantern batteries in the housings.  Each PVC housing hosts the electronics to run three flow probes simultaneously.  The signals from those electronic assemblies are carried over short (1 meter) underwater cables to the underwater housing containing the Campbell CR10X data logger.  The logger digitizes the signals from the flow sensor electronics and sends them back to the habitat as part of its serial data stream.

Measurements of Mass Transport Using Quantitative Video

We will use video imaging to measure the dispersal of dissolved chemicals away from the reef surface.  (This also gives us complementary information about transport of chemicals to the surface.)  Plaster models of basic coral shapes (hemispheres, cylinders, and plates) impregnated with a brilliantly colored non-toxic dye (fluorescein) will be placed in locations of interest.  As turbulent water movement dissolves dye away from the surface, it will be lit using a lighting system based on blue LEDs (light emitting diodes).  That light is at an appropriate wavelength to excite fluorescence in the dye.  Using a yellow filter over our video camera lens, we can image the light from fluorescence while excluding the light from illumination and reflection.  By lighting the model with a slab-shaped plane of light and measuring the brightness of the dye fluorescence around it, we will be able to produce quantitative maps of the concentration distribution around the coral shapes.  These experiments will be conducted at night to avoid ambient light from the sun.  The camera is a commercial monochrome CCD camera mounted in a waterproof housing made of PVC with a 3/4" Plexiglas faceplate.  The video signal is returned directly to the Aquarius habitat on waterproof coaxial cable.  Using the same camera and lighting system, we will also measure the downstream plume of dye dispersing away from the surface of a flat plate.  Dye will be introduced through porous tubing a few millimeters in diameter embedded in the plate.  The dye is introduced at a controlled rate using a diver-operated syringe pump.  This measurement will give us a direct measure of the rate at which chemicals are transported away from (and conversely, toward) the reef surface. 

Heat Analogues for Mass Transport Measurements

In exactly the same way that increased turbulence causes increased transfer of chemicals at an object's surface, it also increases the transfer of heat at that surface.  Hence, by heating (or cooling) models and measuring the rate with which they return to ambient temperature, we will get an indirect measurement of mass transport rate at the surface.  To make those measurements we have brass models of standardized coral shapes (hemispheres, cylinders, and plates).  Brass was chosen for its excellent thermal conductivity and resistance to saltwater corrosion.  Each model is drilled and tapped to contain a brass screw with a thermocouple at its tip (hence putting the thermocouple close to the center of the model).  The thermocouple measures temperature based on the fact that placing certain dissimilar metals in specific configurations across a temperature gradient will generate small voltage potentials along the length of the wire.  Using one thermocouple junction in the brass model and another in the ambient seawater, we can measure the rate with which a model reaches the ambient temperature.  The models will be cooled using ice water (or heated using hot water), then immediately exposed to the ambient water at that site of interest.  Measuring the models' rate of temperature adjustment and comparing that to the rate in still water will tell us how much turbulence in the environment elevates the rate of exchange at the models' surfaces.  Like the thermistor flow probes, the thermocouple wires will be plugged into the Campbell CR10X front end processor.  The CR10X will digitize the voltages, convert the measurements to a temperature difference in degrees Centigrade, then send those measurements back to the habitat. 

Direct Measurement of Mass Transport

The final technique we will use is to measure the rate of mass transport at objects' surfaces directly by observing dissolution of plaster models.  We have prepared Plaster of Paris models of standard coral shapes (hemispheres, cylinders, and plates) that will be placed in numerous locations on the reef.  The difference in their dry weights before and after a day of exposure to the water movement gives a direct measurement of the rate of loss of mass at their surface.  Numerous replicates in a variety of places will give us a range of integrated measures of mass transport across the reef site.  An appealing feature of this technique is that, unlike many of our other experiments, it is not dependent on circuitry, electronics, computers, cables, connectors, cameras, lights, or any other technological feature that might fail (other than the habitat itself).