Water motion is something that aquarists often overlook, but it is one of the most critical physical parameters that determines the health of the aquarium as an environment and the inhabitants as individuals. When we first started keeping these reef tanks we were so concerned about filtration, and the "buzzword" was
"turnover". "How many times do I turn over the volume of water j through a filter?" was the primary question about water movement. More important is the water motion within the tank; its velocity and variability.
Three main types of water movement characterize the reef environment: surge, turbulence and laminar flow. Surge is the back and forth movement caused by ocean swells and wind-driven waves. As the waves move into shallow water their circular motion components become compressed and flattened, causing the water to move back and forth. This is best demonstrated by the to and fro motion of large sea fans in shallow reef areas. Surge is an important factor in the biology of many reef organisms, especially flexible ones. The back and forth movement exposes more of the surface area to light, increases the feeding efficiency of polyps and greatly helps in the exchange of metabolites and gases with the water. Turbulence is a random swirling of water in all directions, often caused by opposite currents clashing or water swirling around objects. Laminar flow is water movement in one direction and is the type of flow commonly generated by aquarium pumps. It is typically encountered on deeper portions of the reef. Although small scale turbulence can be generated in aquariums by laminar flowr, the effect is usually isolated, not tank wide (Sprung, 1988).
When it comes to keeping reef organisms we must take into account the area from which they were collected, since this
determines which forms of water movement they have adapted to. The shape of the organism is often a clue, but it isn't always, and fortunately most reef creatures have the ability to modify their orientation as they grow. Portions of a colony may die off while others grow and give it a new shape.
There are many organisms that are adapted to zones where the water velocity is very slow most of the time, and they expand during these periods of calm, and retract in stormy or more turbulent times. These include mushroom anemones and some stony corals such as Cynarina lac ry ma lis, Nemenzopbyllia turbida, and Plerogyra sinuosa. These should be placed somewhere in the tank where the currents are very slight.
The vast majority of reef organisms like a good turbulent flow, or surge, especially soft corals such as gorgonians, leather corals, Anthelia, and Xenia, and the branching and encrusting reef-building stony corals. Because they are sessile organisms, corals, sponges and other invertebrates orient their growth to the water flow, so that their ultimate shape is a product of the flow regime where they occur on the reef, in addition to their orientation toward or away from the light. As with lighting, if you can vary the energy, providing it in a pulsed fashion, so much the better, because that's what happens in the natural setting. Water motion does change. The tides change, waves come with storms, and the depth change associated with the tides alters the flow of currents. In an aquarium you almost have to be some kind of engineer to figure out the best arrangement of currents to provide both turbulent and quiet zones to accommodate different organisms, based on the structure of the reef that you've built, where you have positioned the corals, and the means that you provide to vary the flow of water.
What's really important about the flow of water is what it achieves. Water brings food to the specimens and flushes away fouling organisms. It allows the exchange of oxygen and carbon dioxide with the environment. If there isn't sufficient flow, an organism becomes isolated from the environment because of dampening of the water flow adjacent to it. This creates a thin film of motionless water surrounding the organism, hindering photosynthesis and respiration (Dennison and Barnes, 1988). Without sufficient water flow, the organism can consume all of the oxygen from this layer of water, and literally suffocate in its own waste. Water motion is
also essential for calcification to proceed at optimum rates.
Reduced water flow can lower dark calcification rates by up to 60% and light calcification by 25% (Dennison and Barnes, 1988). Strong water velocity not only strips away the thin layer of motionless water surrounding an organism, it also moves soft-bodied or flexible species to and fro, achieving the same end. Furthermore, strong water motion serves the same function that a heart does, in that it helps circulate the gelatinous internal fluids in corals through its impact on the corals surface, aiding the exchange of respiratory gasses, dissolved food, and waste across their surface. The motion of the water literally pumps the organism. This is why pumping Xenia, Pulse Coral, pumps when the water is relatively still. The action circulates the internal fluid. During the day this helps Xenia release excess oxygen accumulated from photosynthesis. At night the pumping allows release of CO ? and uptake of oxygen. Water motion achieves these functions for other corals that do not pulse, and the state of expansion of the coral tissue and polyps assists the exchange of respiratory gasses, food, and waste by increasing or decreasing surface area.
The pumping action also serves a less obvious purpose, the pulsed illumination and shading of the organism. In many flexible species, surge wrater motion achieves this effect, preventing the organism from over-illumination or constant shade to one side because it moves to and fro. On the reef, water motion and light are o intimately related. Even for stony corals the water motion alters the light. The ripples or waves on the water surface create so-called "glitter lines ', which pulse the light like a strobe, so that it is briefly brighter than ambient light and briefly less bright.
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