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Water Movement in the Reef Aquarium
By: Eric Borneman
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Perhaps only second to light in terms of its importance to the health of reef organisms is water movement. If one swims across and within a coral reef, various types and degrees of water motion become apparent. Near the shore, waves crash mightily against the beach or rocky shore on the windward side of islands, while delicate wavelets lap gently on sandy beaches on the leeward side. Storms churn even calm areas into frothy frenzies of turbulent water, obscuring water clarity to a mere few inches. Tides, depending on underwater topography and other factors, can be a gentle push and pull of rising water levels or they can form ripping currents that pull huge volumes of water through the reef at incredible velocities. Long shore currents can be wide gentle ocean rivers constantly moving along the face of a reef, or they can be mixing eddies of fast moving oceanic currents. Waves also create large mass movements of water: near the surface, coral reef denizens grip the substrate tenaciously as the continual pounding of oceanic waves breaks over their surface, frothing and surging through channels and breaks in the reef framework, while mid-depth waters experience a back and forth sway from the power of waves ebbing and flowing overhead. Deepwater habitats may experience the cool rush of water as variances in temperature cause an upwelling of plankton rich water. Finally, water careens from all the above listed forces amongst the various surfaces that compose a reef, creating internal waves and smaller eddies. Altogether, there is rarely a lack of rapid, turbulent or mass movement of water from reef areas, save perhaps for the calm tide pool at low tide, its surface rippled only by the wind.
| Typical flow rates from various reef environments, after Sebens, et. al. (1997, 1998) | | Reef Area | Typical Flow speed | | Reef crest, fast currents, wave surge | 7 - 34 cm/sec, up to 1m/sec | | Lagoon | 1 - 16 cm/sec | | Deep fore-reef (deeper than 25 m) | <5 cm/sec | | Mid- to deep fore-reef | 5 – 7 cm/sec, or less | | Shallow fore-reef | 9 – 16 cm/sec |
Why is the movement of water so important? Coral reefs are composed largely of sessile animals – they are incapable of significant movement. As such, these animals depend on water motion to bring them food and to whisk away their waste material. However, there is significantly more to the story. Water flow has been shown to increase food supply, increase gas exchange, improve enzyme action, increase metabolism, provide proper flushing of mucus, decrease disease, allelopathy and sedimentation damages, increase respiration rates, increase calcification rates, and increase photosynthesis rates. Can there be any doubt that providing proper water flow in captivity is highly desirable?
| Figure 1: | 
Figure 1: Waves that will not likely be replicated in the home aquarium. |
Reef aquarists are typically familiar with the fact that strong mixing currents should be provided in their aquaria, but not everyone has experienced the true nature of water motion in the wild. What most would consider being strong water flow in an aquarium is generally a calm deep lagoon in the wild. Even at sixty feet, gorgonians and low growing zoanthids can be bent almost flat in strong currents and surges. On the reef crest, it is nearly impossible for an equipped and experienced swimmer to avoid being dashed against the reef like a leaf in the wind, even on relatively calm days. Putting a couple powerheads in the corner of a 75 gallon tank is most certainly not what can be considered reef-type water movement, yet it is the norm for most aquarists. Before discussing the various options and solutions for providing currents in aquaria, it is necessary to examine clues in the animals themselves.
Which animals require strong flow and which ones don’t? One of the easiest ways to ascertain flow requirements is by examining the growth form and composition of the invertebrate. While there are always exceptions, the following generalities hold true for most taxa, including sponges, corals, algae, and others. Areas of strong water flow favor compressed and encrusting growth forms, or those with a low coefficient of hydrodynamic drag. For example, a rounded or “flat-to-the-pavement” coral will easily allow water to pass over it without exerting damage. Plate-like, branching, and vase shaped animals are found in lower water motion, where their growth form would otherwise be easily damaged by strong currents. In general, sessile invertebrates with smooth surfaces are from low water flow conditions, while those with very rough surfaces or which bear numerous vertical projections from their surface, tend to be from higher water flow; conditions where the surface texture functions to break up fast moving water into smaller surface eddies. Some animals will morph their appearance in response to long-term changes in water flow to maximize both food capture and reduce the likelihood of them being damaged from forceful water flow. As an example, soft corals may form folds in their surface to slow water flow, encrusting corals may send up vertical projections in response to lowered water flow, and branching corals may start encrusting in response to heightened water flow. In terms of some soft corals and gorgonians with planar branching patterns, they are normally oriented perpendicular to currents, and bifacial corals (polyps on both sides of a vertical plate-like or cylindrical skeleton) are frequently from areas which experience a back and forth wave motion.
| Figure 2: | 
Figure 2: Typical placement of a few powerheads in a reef aquarium. |
I know, I know – what about the branching “SPS” corals? They are a bit of an exception. Branching growth forms are primarily utilized by many reef crest corals for several reasons: first, this growth form maximizes their prey capture and surface area for exposure to light, given the circumstances. Furthermore, branching corals tend to depend heavily on asexual reproduction by branch fragmentation, so that the action of water breaking them is actually advantageous to these animals and ensures their continued presence in an area. Even so, the higher the water flow, the more compressed their form and the more tightly spaced are their branches. Growth forms of some of these branching corals can vary so much from high water flow to low water flow areas as to make them almost unrecognizable as the same species.
Another aspect of sessile invertebrates in various water flow conditions lies in their composition. In general, those with the most support can endure the strongest water flows. Stony corals are often more adept at surviving pounding waves than a large soft bodied anemone. On a perhaps more familiar note, this is one of the reasons we tend to find corals with smaller polyps in high energy environments, and corals with larger polyps dwelling in deeper water. Similarly, soft corals and sponges which are very soft and easily torn tend to inhabit somewhat more protected areas than those with lots of internal support in the form of silicaceous or calcareous spicules.
| Characterization of flow rates: | | Low Flow | 1 - 5 cm/sec | | Medium Flow | 6 - 20 cm/sec | | High Flow | 21-50 cm/sec | | Very High Flow | >50 cm/sec | | Flow rates can be estimated at feeding time. A ruler and a stopwatch can be used to measure the time it takes for a bit of food to travel a certain distance. Simple calculations will then give the flow rate in meters/second to ascertain the various flow regimes in different areas of an aquarium. The use of food dispersal can also be used to analyze the various eddies present, allowing the aquarist to see where mixing, laminar, and tumultuous flow exists in the tank. |
Understanding water flow and the characteristics of animals adapted to a given flow regime is fine, but what about the aquarium? If we have acquired an organism and have successfully determined the type of flow in which it is best suited, how do we provide that flow? First, it is very difficult to provide very many types of flow within a given aquarium. Once again, it is best to emulate a given area of the reef, as providing more accurate approximations of local conditions is more feasible. The use of powerheads is probably the most common method of providing currents in a reef aquarium, and the positioning of these small submersible pumps in various locations throughout the tank can be used to create some reasonable facsimile of water motion. The poor laminar flows of such devices can be careened off tank glass and other powerheads flow to create a more beneficial mixing-type current. With some experimentation in position and in various aquascaping techniques (caves, channels, etc.), areas of lower and higher flow can be created within a single aquarium. Larger external pumps can be used, as well, and they often are utilized as returns from sump areas. In such cases, the large unidirectional water flow is best broken up through the use of multiple outlets, “funny pipe” fittings, or even spray bars. I find spray bars to be generally too diffuse, however.
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Figure 3: Obtaining proper water flow becomes even more challenging as the density on the reef increases. |
In becoming somewhat more innovative, the use of various water flow devices can also be employed. Dump buckets are tilt-able pans of water that, upon filling to a certain height on a pivoted base, tilt and release the contents of the pan in a single dump. The result is a standing wave within the tank that causes a very natural oscillation of sessile invertebrates and algae. These devices are often incorporated as part of Algae Turf Scrubbers, although they can be used as stand alone water flow devices. Surge tanks use a separate container above the main tank where water is pumped in and released when the water reaches a certain height, either triggering a float device or exceeding the head pressure needed to start a mechanical siphon. The results of surge devices are quite dramatic, with rapid movement of diffuse volumes of water rushing periodically into the aquarium. These devices were described by Carlson (SeaScope) and Borneman (FAMA). Public aquariums have also made use of surge devices (Cabrillo, Los Angeles and Waikiki, Hawaii). Some public displays have also made use of hydraulic or mechanical plates that use a forward push to drive water in a wave throughout the tank, much like a wave generator at water parks. A similar device can be utilized by affixing a thin piece of acrylic to a motor assembly located outside the tank, perhaps acting as a type of “windshield wiper” across the back glass. Other water flow devices include billows pumps and air lift tubes, although these are even more infrequently available or utilized. The use of flumes, or raceways, can even be employed for animals which live in areas exposed to constant unidirectional mass flow, such as deepwater corals and sponges. In such cases, water would flow from one end of a tank, out the other, returning all from the same direction. Often, the innovation of the aquarist is the only limitation as to the number and types of methods that can be employed in creating proper water movement.
In summary, it becomes of critical importance to the success and health of a reef aquarium to provide adequate water motion. I feel that it is an area which should receive a great deal of planning and attention, yet is often treated in a somewhat cursory fashion. There is definite room for improvement in the actual devices which are currently available to create water flow, and I hope to see some of the more innovative aquarist designs brought into the aquarium marketplace in the future. Non-traumatic plankton friendly pumps would be an extremely beneficial product to see become readily available. In the meantime, reef aquarists should use their skills and creativity to accurately replicate current velocities, total water movement, and types of flow to profoundly affect the health of their aquaria. The results can be quite dramatic.
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