Over the last several years, so-called "sand beds," essentially layers of sand of various thicknesses and arrangements, have come to be common fixtures in marine reef aquaria. The use of these sand beds has been correlated with significantly increased survivability of many organisms in reef aquaria, particularly when compared to the bare bottom tank arrangements that were in the vogue about a decade ago. Nevertheless, few hobbyists seem to realize why sand beds should contribute to the success of their tanks, and fewer yet seem to understand how those beds work.

Over the last five years, I have made much of the point that our aquaria are artificial ecosystems, or microcosms, representative of the real reef environment. Of this there can be no doubt; our systems mimic relatively well many of the processes occurring in the natural world and, when populated by an appropriate group of organisms, many of the interactions occurring in natural habitats occur in aquarium systems. The approach of dealing with aquaria as artificial ecosystems has been criticized primarily on the basis that reef aquaria are patently artificial. However, such criticisms are both rather silly and quite wrong. Reef aquarium systems have to be quite good approximations of the real world, otherwise those animals that are kept in them would not be doing as well as they are. The organisms don't know they are not in the natural environment and any coral reef organism has evolved to deal with an environment that has as its limits those same limits as are found on real reefs.

The fact that we can deal with aquaria as good mimics of the real thing allows us to use "real world" or "scientific" data both to troubleshoot problems and to advance the techniques of animal husbandry. As we know that organisms must live within the ranges of their tolerances, we can recognize that problems will occur when something in an aquarium is far outside that range, such as the excessively high concentrations of poisonous heavy metals found in some artificial sea water mixes . Once that recognition is made, we can adjust for the problem and proceed. In this way, we may incrementally increase our understanding of the animals and what we need to do to maintain them.

If aquaria are artificial ecosystems, however, the component that is least artificial is the sand bed. This part of a reef aquarium, with little input from the aquarist, functions much as do the sandy areas near a real reef. That functionality is due to a rather complex interaction of physical and biological factors, but most of those interactions are unseen, and, I think, unappreciated by the average aquarist. Without those interactions our reef aquaria would simply fail. The fact that they don't fail is a tribute to the ease of constructing this one major functional analogue to an extremely critical coral reef community.

Sediments In Reefs

Sand beds are constructed physically of sands, and sand is defined as unconsolidated sediments made of particles between one sixteenth of a millimeter and two millimeters in diameter. Coarser sediments are referred to as gravel, finer ones as silts and clays (Holme and McIntyre, 1984). Of course, in the real world there is a continuum of sizes found in these environments, and the sediments actually found in any one spot reflect not only the geological and biologic history of the area, but also the hydrographic regime of the area. In other words, what is present is the result of what is available that hasn't been washed away by the waves.

Figure 1. A figure from Holme & McIntyre, 1984, showing possible ways of representing the variation among sediment types. All of these types of sediments will likely be found in various coral reef environments.

Sands surrounding natural coral reefs may be made of a number of substances. Around volcanic islands there are often regions of volcanic lava sand. Volcanic islands are the basis for most coral atolls and many fringing reefs, so lava sands are commonly found around reefs in nature. Coral reefs located near areas of river mouths or extensive areas of runoff often are surrounded by silica sands or fine sediments of other upland or inland sources. Of course, coral reefs may be surrounded by calcareous sands resulting from the breakdown of corals and other calcifying organisms. Calcareous sands may also be formed by the precipitation of particulate calcium carbonate in coral lagoons, one of the natural sources of oolitic sand. Calcareous sands may also be formed from the skeletal breakdown of other organisms, such as foraminiferans, bivalves, calcifying algae, or barnacles.

The chemical composition of the sands has a small effect on the organisms found in the sands, but that effect is minor compared to the effects due to differences in sediment particle size distribution. The sediment particles found in any given area are primarily due to the effects of sediment movements caused by wave action and water currents. Sediment density has some effect on what is present, but basically for any given sediment, finer particles will be found in areas with less water movement. Consequently, the pattern of sediments surrounding a coral islet that is one part of a coral atoll will be a complex tapestry of sediment sizes. In general, coarser sediments will be found in areas of higher current flow and wave action while finer sediments will predominate in areas of less kinetic energy. The absolute position of the sediments will often vary from season to season, particularly in intertidal and shallow subtidal areas. Tourists who always visit a given resort at a particular time of year are often quite amazed when they return to the resort six months out of sync with their usual pattern and find the sandy beach they expect to see has vanished, leaving a hard coral pavement instead. Movement of sediments is less in deeper waters but it still occurs. In fact, one major characteristic of natural sand beds is their mobility.

Sediments in any one spot may be characterized by several discrete parameters. The first parameter is the average, or mean, sediment particle size. The second factor is the shape of the cumulative sediment particle size distribution; if a sample of the sediment is taken and the diameters of the sediment particles measured, the resulting graph will be a bell or "normal" curve centered around the average particle size. How that bell curve deviates from an ideal statistical bell curve reveals a lot about the sediments. For example, at the extremes, the curve may be low, broad, and flat or quite narrow and high. In the former case, it indicates a wide variety of sediment particles in each sample which, in turn, indicates lesser effects due to waves or currents. In the latter case, the sediments will be almost all of the same size, indicating a lot of movement of sediment particles by wave action and the resulting "sorting" of them by size. Well-sorted sediments are quite characteristic of areas with high currents or strong wave action, while poorly sorted sediments with a wide variety of particle sizes are characteristic of calmer waters. The third important parameter is the amount of organic material found in the sediments. I have worked in areas where the organic content was effectively zero. At the other extreme, I have sampled some areas where the organic content of non-polluted sediments was as high as about twenty percent by weight. In polluted areas the organic content may be even higher.

The size distribution of sediments in any marine soft sediment area is critical to the determination of the organisms living in those sediments. Organisms live on, and between sand grains, and the mixture of the sizes of the grains is critical. Sand grains of inappropriate sizes may be too big to move or, conversely, too small to be stable. Additionally, the mixture of the various sizes determines the ease with which water moves through the sediments.

Here are a couple of links showing bacteria on individual sediments:
Ocean Explorer
http://www.rnw.nl/science/assets/images/020903bacteria.jpg

There is obviously a complex interplay of factors then that determines the natural sediments found around a coral reef. Organic materials may come from the reef, adjacent areas, or from inland runoff. Tidal, wave, and storm patterns all influence the kinetic energy that will be transmitted to the sediments. Geological factors such as the presence of submerging or emerging coastlines may also contribute to the types of sediments present.

Figure 2. Image taken through the viewing port of a research submersible. The depth was 165 feet. Note the ripple marks and the relatively large sediment particle sizes (the ripples are about 6 feet away from the port and are over 2 feet high). This area receives occasional, but regular, winter storm waves in excess of 60 feet high. Shallow water habitats in this area have no sand whatsoever; the smallest particles are about 6 feet in diameter. Water action is the ultimate determinant of sediment particle size in natural environments.

Figure 3. Image taken from the inside of the Plexiglas sphere of the research submersible, Johnson Sea Link II. The flat featureless sandy substrate visible in search light glow is about 8 feet away (or about as far as the sand and gravel seen in the preceding figure). This picture was taken off the outer edge of the coral reef of the Bahamas in 1140 feet of water. Here there is mild gentle current, and the sediment is a well-sorted fine white sand.

In aquaria, the sediments are chosen by the aquarist and added to the system; however, that is only the beginning of the development of the sediments. As in the natural world, aquarium sediments are dynamic, albeit on quite a different scale than is seen in nature. Both the sediment particle size and physical distribution and amount of organic material in aquarium sediments will change through time.

The change in aquarium sediment particle distribution is most evident in fine calcareous sediments. The average particle size will tend to decrease in these sediments as particles are eroded or dissolved. In aquaria with poor carbonate buffering, there will be a tendency for finer particles on the surface to partially or wholly dissolve. Additionally, when deposit-feeding animals eat the sediments to digest the bacteria and algae off of them, some fraction of the sediment particles is also likely dissolved. Organic material will be added to the sediments and some of this will get incorporated into the sediments as fine particulate material. Often, this fine organic material is the site of organo-metal complexes forming an insoluble precipitate of toxic materials. Such precipitates are typically very small. The net result of all of these processes is that the average size of the sediment particles decreases in size over time.

Sediments and Water

In either the real reef or in aquaria there are some aspects of the sediments and water that are important. First, it is important to realize that passive water movement through the sediments is essentially impossible. The channels between the sand grains are so small that the resistance to passive water movement is, for all practical purposes, absolute. Unless the water is pumped through the sediments, it simply doesn't move. Contrary to a lot of reef aquarium mythology, water does not "diffuse" through the sediments. Materials dissolved in the water may diffuse within the water medium, but that movement is very slow and generally inconsequential. As we will see, unless the aquarist arranges for some sort of active pumping, all water movement in aquarium sediments is mediated by organisms.

Water flow over the sediments may be either turbulent, such as caused by a power head, or laminar, such as uniform bulk water flow. Turbulent flow will move some water through the upper few fractions of an inch of sediments, laminar flow generally will not. However, in either case, there will be little real interchange of water from the sediment interstices into the water column and vice versa. Even in aquaria with strong surge devices, as long as the sediment is not physically moved, there will be little mixing between the water in the sediments and that in the water mass above the sediments.

This division of the water in an aquarium into two discrete bodies of water, the water mass above the sediments and the water mass in the sediments is very important for the functionality of the sand beds and aquaria. In the presence of bacteria, it results in the formation of relatively discrete layers in the sediments based on the diffusion of gases through the sediment water mass. These layers are generally characterized by the concentration of oxygen in the water, and they are classified as aerobic, anaerobic, and anoxic. Aerobic layers have oxygen concentrations near or at the level found in the free flowing water above the sediments. Anaerobic layers have some oxygen present, but the concentration is reduced from that found in the overlying waters. Anoxic layers have no free dissolved oxygen, and may be also referred to as reducing, as opposed to oxidizing, layers.

If there was no life in the sediments, there would be no layering. The layers are caused by the action of bacteria, micro-organisms, and animals which live on the sediment particle surfaces, and between the sediment grains. As these organisms metabolize, they use up the available dissolved oxygen. All of the oxygen in the sediments is consumed relatively rapidly, resulting in anoxic layers, wherein the only life is bacterial. Oxygen diffuses into the sediments from the water above the sediments, but such diffusion is very slow. In the absence of animals in the sediments, the aerobic and anaerobic layers would each be a few hundredths of an inch in thickness, and the anoxic layers would effectively extend to the surface. Such layering is found in highly organically enriched areas or in areas with toxic materials in the sediments. In these areas, animal life is absent from the sediments. These areas generally are polluted areas, but they don't have to be; there are naturally occurring areas that mimic mankind's best (worst?) efforts at pollution.

Here are some images of bacterial mats on natural anoxic sediment surfaces: http://www.geomar.de/projekte/komex/gallery3.html

Organisms and Sediments

Because organisms live on and between sediment particles, the interactions between those various organisms are what makes sand beds so important in reef aquaria. Bacteria, some microalgae, protozoans, and a few animals are small enough to live upon sand grains. To these organisms, the sediment bed, as such, does not exist; rather their whole world is quite literally a grain of sand. On this super small scale, the food web starts with the bacteria and microalgae, and on this scale, the microalgae are predominantly cyanobacteria and diatoms. These organisms live by absorbing dissolved materials in the water around them and by metabolizing those nutrients, creating more bacteria, cyanobacteria, and diatoms. Both the bacteria and cyanobacteria will also actively secrete enzymes into the surrounding environment, and these enzymes will breakdown organic particulate material so that it may be absorbed. There is sufficient light in all aquarium sediments for some photosynthesis to occur; light sufficient for photosynthesis generally can penetrate several inches or more into these sediments. The relative proportions and types of bacteria and algae in the sediments will depend upon the depth in the sediments, and the relative amount of dissolved oxygen available. In the upper layers of the sediments, diatoms and aerobic bacteria predominate on the sediment particles. In the anoxic lower regions of a deep sand bed, anaerobic bacteria predominate. In between, there is a transitional mix of several types of organisms, depending on the amount of nutrient, sediment disturbance and water movement.

Figure 4. Shallow water sediment surface from a Caribbean coral reef. The sponges were attached to shell fragments under the sediment surface. Note the brittle star arm, it is about an inch long, but otherwise similar to the small brittle stars found in reef aquaria. It moves food from the water column into the sediments where further processing of the food occurs. Note as well the diversity of shapes and sizes of particles seen on the surface. This sediment was near shore and very poorly sorted.

Although these minute organisms are simultaneously the ultimate consumers of dissolved nutrients, the aquarium's biological filter and the source of food for other organisms, they are but one part of complex web of interdependent sediment organisms. This web is dependent upon the shallow-sediment-dwelling animals for its existence and functionality. These most important animals are the various sediment worms, snails, and crustaceans that many aquarists refer to as "the clean-up crew."

It is important to realize that the diversity of this group of organisms is really the cause of its utility. Very few marine animals are omnivores, or eaters of everything. Rather, they all tend to specialize on one kind of food or another. Consequently, to make certain that all kinds of excess foods are "disposed of," aquarists need to ensure a rich and diverse sediment fauna.

Figure 5. A small scale worm, about one half inch long, crawling on the surface of a temperate sand bed. Note the anemone burrow. Burrowing anemones are common in both temperate and tropical sand beds, and pump much water into and out of the bed as they expand and contract. In reef aquaria with sand beds, other animals generally serve the same purpose as these small anemones are seldom sold in the hobby.

At any level in a food web or any link in a food chain, most of the food that is eaten is not assimilated into the tissue of the animal doing the eating. Generally, as an ecological rule of thumb, only about ten percent of the food eaten by any animal stays in that animal as part of its tissue. Some of the remaining ninety percent of the eaten food is burnt as fuel in respiration to provide energy for the organism. Burnt fuel exits the animal as water and carbon dioxide and this eventually leaves the aquarium. A lot of food is "spent" this way; so much food is converted into carbon dioxide in every aquarium every night that the carbonic acid produced by this exhalation will significantly lower the pH of the system. Additionally, some of the food is used in other metabolic functions, and the byproduct of this is the waste ammonia and phosphates excreted by the animal through its urine, or simply across its body surface. However, that is still only a small part of the food. The majority of the unassimilated food is passed out of the digestive tract as feces. Fecal matter in marine ecosystems is simply indigestible or undigested foods mixed with some digestive enzymes and intestinal bacteria. As unappetizing as this stuff may sound, it is a major food source for much of the fauna of a coral reef, including corals, and fishes such as clown fishes (See Hamner, et al, 1988 for a discussion of just how much "coprophagy" (or eating of feces) is a part of the reef).

Aquarists feed their systems to keep their decorative animals in good health. The amount of food necessary to maintain a large well-stocked aquarium is quite significant. However, most of that food is not used by the organisms that it is meant for, it either is either converted into dissolved nutrients or it is converted into feces. Both of these materials must be removed from the aquarium or converted into some harmless product. That conversion is almost entirely the done in the sediments, and it is done by cycling food over and over through various animals and microbes until there is either no nutritional value left in it or it has been totally converted to soluble gases that leave the system.

This process begins in the uppermost sediments where carrion-feeding animals such as the small fireworms, Linopherus, and snails, such as Nassarius, eat excess meaty foods such as dead brine shrimp or flake food rich in meat byproducts. Other animals in the uppermost sediments eat "vegetable" material. In natural systems, this vegetable material would be primarily algal remnants or the remains of sea grasses. In aquaria algal remnants are present, but so are vegetable byproducts in flake foods. In most aquaria the animals that eat this material are amphipods, some semi-omnivorous snails such as the temperate Illynassa obsoleta, and surface grazers such as the conchs, Strombus species, and mopping sea cucumbers.

One aspect of all of these animals living and feeding in the sediments at the surface is that a lot of dissolved nutrients are excreted by these animals into the sediments.  These nutrients will, in their turn, go to fuel algal growth on the sediments.  In aquaria, these algae are predominantly diatoms and the photosynthetic bacteria called cyanobacteria.  An interesting small sub-cycle of nutrient utilization occurs where algae are:

  • eaten by the various grazers, including at this level, the small harpacticoid copepods, and the small seed shrimps or ostracods, which scrape microalgae off of the individual sand grains, 

  • processed through their metabolism resulting in,

  • part of the algal mass being assimilated by the grazers, 

  • part being respired, and

  • part being excreted as dissolved nutrient to fuel more algal growth.

Of course, with each pass through the cycle, the amount of nutrient available for algal growth would decrease.  Or, it would if no more was being added by feeding.  But, of course, more food always has to enter the system.  Nevertheless, this algae-grazer nutrient cycling process does go a long way to remove a lot of excess food from the system.

However, not all of the food eaten by the surface grazers remains on the surface; many of the surface grazers will dive under the sediment surface as soon as they have eaten, and digestion will occur in the relative safety of the substrate. Excretion of the various wastes occurs at varying depths below the surface. Additionally, other animals such as the tube-dwelling, suspension-feeding Phyllochaetopterus worms, or the suspension-feeding small brittle stars, add both dissolved wastes and feces below the sediment surface. These worms eat small particulate material in the water, and should therefore also be considered to be a part of the clean-up crew. Basically, these suspension-feeding animals are living mechanical filters. Other subsurface worms that may feed upon surface particulate material, such as the cirratulid hair worms and the tube dwelling spaghetti worms, do much the same thing. In effect, all of these worms move material from the surface and deposit it some distance down into the sediments.

Figure 6. Small surface deposit feeding tube worms called "oweniids." These worms are common in some tropical habitats, such as sea grass beds, and feed by "daubing" the surface for food. As they move up and down in their tubes, in a manner similar to tube worms in aquaria, they pump water in and out of the sediments.

Such material still is food and, of course, there are yet more worms and other animals that process it. One thing that is often overlooked, in discussions of food transfers such as this, is an interesting reversal of a trend seen above the sediment surface. Each of these food transfers corresponds to going one more link or level up a food chain or web. When this occurs in the water column or on land, the animal that is that link gets eaten by yet another larger animal. The final animal in the food chain is generally the biggest critter around. In these sediment-based systems, the animals of each succeeding level are generally smaller than those of the preceding layer. Although there are subsurface predators in the sediments, they are limited in size by the sediment properties and prey sizes. The largest wholly infaunal predatory animals are worms and are generally no more than a foot or so long, and they are almost never found in aquaria.

Nonetheless, the sediments in the lower part of the aerobic layer and upper anaerobic areas are a busy place. In addition to the surface feeding animals discussed above, there are animals here that are only found under the sediment surfaces. These include some of the nematodes or round worms. This is a diverse group containing both herbivores and carnivores; nonetheless, except for a few species, their natural history and aquarium biology is effectively unknown. Some of them will undoubtedly be eating small particulate organic material, either worm feces, algal or bacterial clumps or some other material. Others may eat small polychaete worms or protozoans.

The subsurface community of organisms also includes a rich array of protozoans. These include highly mobile ciliates, some which look quite like flatworms, and shelled but effectively immobile foraminiferans. All of these are predators that graze upon bacteria, bacterial aggregates, or algae. In turn, these algae and bacteria thrive in this area because of the action of the surface feeders that pump food and nutrients into this zone.

Flatworms are found throughout the upper sediment layers, but are most commonly found within the sediments near the lower boundary to the aerobic layers. Many of these are predatory and eat copepods and small worms; others eat the abundant microalgae in this area.

Some of the larger and most impressive animals found wholly within the sediments of this level are polychaete worms, such as the syllids. These worms will reach lengths of an inch or more. The ones I have seen appear to subsist, depending upon the species, on other polychaete worms, or bacterial aggregates.

One other byproduct of the animals living in the sediments needs to be addressed, as it is very important to aquaria. This particular product is produced in sand beds where the animals are doing well, and that product is the spawn from the animals in the bed. Once the bed animals are thriving, they reproduce regularly, and this reproduction is in the form of eggs, sperm, and larvae liberated at the sediment surface into the overlying water. This material is, of course, recycled food added to the aquarium some time before hand, and it is now in the form where it is eminently good food for many suspension-feeding animals in the system. So, here again, nutrient has been moved back up out of the sediments and into the water for corals and other animals to eat.

The functionality of these sediment layers, in the context of either the aquarium or natural ecosystem, is dependent upon the diversity and richness of organisms in the sediments, and this is directly related to the sediment particle distributions that were mentioned previously. Well sorted sediments with a narrow particle size range, are generally quite optimal for a few organisms, primarily those adapted to that particle size range. For everybody else, well… they don't work so well. In aquaria, where the maximum diversity and richness is required, the aquarist needs to ensure that the sediment particle size range is fairly large. Of course, "fairly large" is a matter of opinion. Marine benthic ecologists and other folks that study sediments categorize sediments in a series of sizes based on the negative logarithm to the base 2 of the size. Sounds pretty complicated, but really isn't. What this means is that starting with the upper sand limit of four millimeters in diameter, the sand size categories are: 2 mm to 1 mm, 1 mm to 0.5 mm, 0.5 mm to 0.25 mm, 0.25 mm to 0.125 mm, and 0.125 mm to 0.063 mm.

Figure 7. Silty sediments, where the sediment particles are typically smaller than one-sixteenth millimeter in diameter are common in lagoonal backwater areas protected from wave action. These sediment areas are at the opposite extreme of the sediment areas illustrated in Figure 1. Here, the environment is very stable and there are multitudes of animal burrows. These sediment areas are the "power houses" of nutrient processing because of the high density of animals found in them.

For a sand bed to contain the most animals of the most species, it really should have a distribution where sediment sizes span from about 2 mm to 0.063 mm (2 mm to 1/16th mm), and where most of the particles are in the 0.250 mm to 0.125 mm range. This will make a sediment that is acceptable, if not perfect, for most animals.

The animal life in the subsurface layers is, of course, only a single component of the rich array of organisms found in this level of sediments in our tanks. This area is rich with various algae and bacterial species. Most aquarists tend to think that most algae are something deleterious and their presence in reef tanks is considered a problem. However, most of the non-bacterial life on a "coral" reef is algae. In fact, the biomass of algae on such a reef is often on the order of five to ten times the biomass of corals. The reality of the situation is that these reefs are algal reefs, with a thin frosting of corals and other animals. (Odum and Odum, 1955).

Since both the corals and the algae thrive under the same environmental conditions, and since it is therefore impossible to keep algae out of reef tanks, it is best to manage the tanks so that the algae are beneficial. The sand bed subsurface algae definitely are beneficial. They utilize dissolved nutrients and, in turn, provide nutrition for many of the small animals found in these layers. A similar situation is seen with bacteria, they also utilize nutrients and are food for deposit-feeding animals. Animals that feed on sediments are not feeding on the mineral grains of the sediments but rather are consuming the bacteria and algae adhering to those sediment particles.

By now, the basic properties of all of these cycles should be obvious. Dissolved nutrient is utilized by the algae and bacteria to produce more bacteria and algae. In doing so, it is removed from solution, and some of it is respired away as dissolved gas. The gas eventually leaves the aquarium. In turn, the algae and bacteria are eaten by some animal, and some more of the once dissolved nutrient is respired away. Some of the once dissolved nutrient is incorporated into the predator and some is once more released as dissolved nutrient in the predator's urine. Generally, if a piece of food drops to the bottom of an aquarium (or the ocean), enough energy (in the form of sugars or carbohydrates) is in it to provide fuel for five to six cycles through decomposers and detritivores.

Each of these cycles progressively removes some of the useful energy and materials from each bit of food, until all that remains are materials that are insoluble or materials locked up in some organism. Such a continual cyclic process can remove an amazing amount of material from an aquarium, but it can't remove all of it, by itself. The single most critical factor in all of these processes is the transfer of materials from one state to the next, whether it is from one organism to another by feeding, or by passing from nutrient to organism. Each time such a change occurs, energy is used up and materials are respired away.

The key to the success of such a sand bed community is water movement between the sediment grains. I mentioned above that it is essentially impossible for waves or water currents to move water in sediments. However, there is an exceptionally useful method of generating slow and even water movement through sediments. This water movement is caused by the motion of the animals in the upper inch or so of sand, particularly in those vertically-oriented tube worms such as Phyllochaetopterus, but also by all other animals moving in the upper sediment layers. The amount of water moved by one worm is quite small, on the order of a few fractions of a milliliter per day to a couple of milliliters per hour, but the cumulative total of all the water moved by all the animals in the sand bed is quite considerable. It is enough to push water into and through the sediments.

Additionally, it has been estimated that each small animal over the course of a day disturbs around a hundred cubic millimeters of sediment. A hundred cubic millimeters is not very much, but when multiplied by the number of animals in a sand bed… well, the bed positively vibrates. In my 45 gallon lagoonal reef, by doing sediment samples and counting the number of animals in the sands, I estimated that there were between 90,000 and 150,000 animals in the sand bed with a foot print of about three feet long and one foot wide. Such a population density translates into about 300,000 to 450,000 animals per square meter, a value quite consistent with values found in rich sand or sandy mud ecosystems in nature. There was enough activity in the tank to move virtually all the sand in the tank every few days. Of course, it doesn't really occur that way; most of the motion is limited to the upper layer where it facilitates water movement. Proper functionality of the lower parts of the sand bed require no disturbance except the gentle, and slow, movement of water through them.

In other words, in the sand bed of a normal reef tank, there is the capability of having a sediment community with a population comparable to natural systems. Such a bed functions like a natural system as well. It metabolizes and uses organic materials moving excess materials through food webs and chains, and allowing their export from the system.

Not everything will leave the system, however, and what remains also follows a pattern seen in natural systems. Only a few gaseous materials will exit the system as respiration byproducts. Other soluble materials will accumulate in the system's water and will have to be removed by skimming. Still others, particularly the toxic materials, will tend to get concentrated in animals or precipitated as insoluble minerals by bacteria. The slow water movement caused by the action of the upper sand surface worms pumps water slowly through the lower anoxic regions of the sand bed. Here, bacteria and chemistry combine to produce conditions that result in the precipitation of many toxic heavy metals such as sulphide and iron hydroxide minerals. (Pincher, et al., 1999, 2000) Such materials accumulate in the tank with time, but as long as these sediments remain anoxic, those poisons are locked there and can be considered "safe."

Conclusion:

By simply setting up a deep sand bed, and then maintaining that bed with the proper diversity and mix of animals, reef aquarists can facilitate the utilization of the necessary excess nutrient resulting from normal feeding. Such beds also efficiently, but slowly, detoxify toxic trace metals. The large populations of sediment animals also transfer nutrients from the sand bed back to organisms such as corals and soft corals by their action of moving sediments and water which generates bacterial particulates in the tank water mass. Finally, as these small animals reproduce they also transfer excess nutrients back from the sediments into the water mass in the form of larvae and reproductive products.



If you have any questions about this article, please visit my author forum on Reef Central.

Cited References:

A Listing of a Couple of Hundred Other References Detailing Sediments and Sediment-Organism Interactions Will Be Found by Following This Link:
http://www.rshimek.com/reef/sediment_ref.htm

Hamner, W. M., M. S. Jones, J. H. Carleton, I. R. Hauri, and D. McB. Williams. 1988.
Zooplankton, planktivorous fish, and water currents on a windward reef face, Great Barrier Reef, Australia. Bulletin of Marine Science. 42: 459-479.

Holme, N.A. and A.D. McIntyre, eds. 1984. Methods for the study of marine benthos. IBP Handbook no. 16, 2nd. ed. Blackwell Scientific Publications. Oxford. 387 pp.

Odum, H. P. and E. P. Odum. 1955. Tropic structure and productivity of a windward coral reef community on Eniwetok Atoll. Ecological Monographs. 25:291-320.

Pichler, T., J. Veizer and G. E. M. Hall. 1999. Natural input of arsenic into a coral-reef ecosystem by hydrothermal fluids and its removal by Fe(III) oxyhydroxides. Environmental Science and Technology. 33:1373-1378.

Pichler, T., J. M. Heikoop, M. J. Risk, J. Veizer and I. L. Campbell. 2000. Hydrothermal effects on isotope and trace element records in modern reef corals: A study of Porites lobata from Tutum Bay, Ambitle Island, Papua New Guinea. Palaios. 15:225-234.




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How Sandbeds REALLY Work by Ronald L. Shimek, Ph.D. - Reefkeeping.com