Revisited: The Dish on Sand Grain Size
While recently helping a friend set up a new 55 gallon SPS tank, we fell into a discussion about the correct sand grain sizes to use in his display tank and Deep Sand Bed (DSB). The discussion centered on water and organisms moving through the sand, and how this movement facilitates the removal of wastes. While the perception is that most of the water flows through the sand bed by passive diffusion (i.e. it seeps through on its own), the reality is that microorganisms and benthic infauna play very important roles in the process. Sand grain size, in turn, regulates the amounts and types of microorganisms and infauna that can develop in the bed. Thus, through the careful selection of the right sand for each application, excellent water quality is almost unavoidable!
Sand in Nature
When the word sand is used, it typically conjures up images of beaches and deserts, waves and dunes, and other summery things. Yet, have you ever really thought about what the word sand actually means, or why it comes to be there? What we think of as sand is really just an extension of a larger scale of grain size, called the Udden-Wentworth Scale, which progresses from the really tinysilt and clayto the really largecobbles and boulders. The names geologists assign to various grain sizes are functions of a precise set of ranges described by the scale. It is the size of grain which dictates a given particles destination. Large particles, like boulders, can only be moved by the most powerful of forces, such as volcanism, landslides, and raging torrents of water. On the other hand, truly fine particles, like silt and clay, require very little energy to move them. As such, they tend to settle in very low energy environments like the bottoms of deep oceans. Sand, in particular, is defined to have grain sizes ranging from 1/16 mm to a little over 2 mm, and tends to end up at the margins of land and sea, and in coastal shallows.
In some places, sand is the end result of erosive processes that carve larger rocks into tiny ones. In other cases, sand is derived from the mechanical breakdown of coral skeletons, either by wave action or resulting from predation on the corals themselves. Further breakdown of coral sand occurs when benthic infauna consume the sand grains in order feed on surface bacteria and microalgae. Each pass through the digestive track of an animal causes a little bit of the calcium-carbonate to dissolve, leaving the grain slightly smaller. Sand can also be created through the precipitation of carbonate salts directly from the water column itself. In general, it can be said that sand originates on land from geological processes, and at sea from biological means. Through the mechanical action of wind and waves, sand grains are distributed according to size and weight. This mechanical action typically makes sand form aggregates which are quite homogenous in composition. In essence, what happens is that the average water or wind velocities act to sort the sediment grains based on how much energy it takes to move them.
Sand in the Tank
Unlike the real world, the captive systems that are our aquaria provide us the opportunity to choose the size and composition of the sand bed. Though sand was not widely used in the beginning of the marine husbandry hobby, for over a decade now its implementation has dramatically improved our ability to maintain excellent water quality. There is a basic, three-fold explanation for why sand has made such a huge difference. First and foremost, the surface area available for microorganismal colonization is massive compared to bare glass, and significantly larger than that of live rock. Secondarily, the development of the oxygen concentration gradient that allows the needed bacterial diversity to mature is more predictable and reliable in sand. For a given grain size, there will be well-defined areas of aerobic, anaerobic, and anoxic activity if the sand is undisturbed. The third benefit of the sand is the habitat area it provides for benthic infauna, which help to ensure that waste products are recycled in a timely and safe manner.
Unfortunately, the needs of the infauna do not necessarily coincide with the maximization of surface area in the sand. What I mean is that the smallest grain sizes, in theory, provide the maximum surface area because the surface to volume ratio of any object increases as it gets smaller. The classic illustration of this principle is the cube. A cube with side lengths of two millimeters has a surface area of 24 millimeters squared, and a volume of eight millimeters cubed (a 3:1 ratio). Scale the cubes sides down to one millimeter, and the surface area drops to 6 millimeters squared, but the volume plummets to one millimeter cubed (a 6:1 ratio). However, as the sand grain size decreases, it becomes more difficult for infauna to move around in the interstitial (in-between) spaces. On the flip side of the coin, if the grains are too large, for example crushed coral and shells, surface area drops precipitously, the substrate offers little protection (and thus little habitat) to the infauna, and waste materials will begin to collect in the larger interstitial spaces.
When balancing the needs of infauna and microorganisms, it is also prudent to consider the ability of water to move throughout the sand bed. Fortunately, this is easier than it sounds. The prevailing thought on the subject, amongst aquarists, has historically been that water passively diffuses through the sand bed to the bacteria it houses. Indeed, it is not unfounded to believe that water could do such a thing; it is well-established that water can pass through subterranean sediments, as it does in aquifers and river-mouth sand bars. However, according to Ron Shimek (2006), virtually all of the water passage through the sand bed is biologically mediated. Through the activity of infauna and the rapid metabolisms of microorganisms, dirty water is literally sucked into the sand bed and spit out again when it is clean. At the surface of the sand bed, nutrients are taken up by bacteria and algae (which can photosynthesize down to several inches deep in the sand). The sand grains housing the colonies are then eaten, along with particulate wastes, by benthic infauna. These infauna then retreat to burrows in the sand, where they rest and digest their meals. As the animals move up and down through the substrate they bring water down to greater depths and jostle individuals grains, opening space for further water penetration. They also urinate out ammonia, phosphate, and other nutrients while at depth, ensuring that deeper colonies of microorganisms can feed as well. Through this miniature food web, nutrients are eliminated from tank water without the need for much water to pass through the sand itself. In fact, the requisite anaerobic and anoxic regions need for denitrification would not occur if tank water were allowed to move through the sand bed as a whole.
The Final Recommendation
I like to think of the benthic food chain as a kind of bucket brigade, with each participant handing a certain volume of water, with just the right chemical composition, down to the next organism in line. The organisms act to strain out the nutrients from the water column, only passing on what is necessary for the next deeper level to survive. In order to create an effective food web, it is necessary to select a sand grain that will maximize the potential for a diverse benthic community. For a display tank, with a sand layer 2-4 inches thick, medium grain sand works very well. Medium grains range from mm to mm in length, and provide a good compromise between surface area and space for benthic infauna. In my opinion, the display tank should house a significant population of infauna, since most of the wasted food and expelled feces will end up here. Therefore, the sand bed should be one tailored to those animals needs. Additionally, free-living corals such a solitary mussids and plates, prefer this coarseness. At the same time, fine grain sand, or sugar sand, is superior for DSBs housed in refugia or sumps. Not only does the smaller grain size (1/8 mm to mm) provide greater surface area, macroalgae will have an easier time taking root and flourishing within this type of substrate. Fine sand that is mixed with a moderate proportion of silt has an exceptionally high surface area, and can house immense colonies of bacteria. The smaller interstitial spaces will also help to preserve an adequate concentration gradient of oxygen. Another good idea is to use an inch or two of medium grain sand on top of the sugar sand, to give infauna a good habitat in the sump as well. Though some aquarists may fear the development of poisonous hydrogen sulfide in anoxic regions of the bed, unless disturbed, there is little likelihood that any of this foul-smelling chemical will ever actually end up in the system. Moreover, the pungent rotten-egg smell will become overpowering long before the hydrogen sulfide concentration reaches deadly levels.
You might never have considered it as such, but the sand bed in a saltwater tank is a lively place. There is, quite literally, a lot of moving and shaking going on down there. But the best part is, all you really have to do is put in the right size sand, seed it with a little bacteria and infauna, and feed everyone else in your tank. In a short time, your sand will have a thriving population of cleaners that runs itself. And your corals and fish will love you for it (or they would if they could).
Works Cited:
Calfo, A. An Introduction to Deep Sand Beds for the Natural Marine Aquarium, Part 1Only Part. Wet Web Media Online. 2003. URL: < http://www.wetwebmedia.com/deepsandbeds.htm > Garratt, S. The Deep Sand Bed. Reef Eden Online. 2007. URL: < http://www.wetwebmedia.com/deepsandbeds.htm >
Poppe, L.J., et al. U.S. Geological Survey Open-File Report 00-358; Chapter 1: Grain-Size Analysis Of Marine Sediments: Methodology And Data Processing. U.S. Department of the Interior. 2000. URL: < http://pubs.usgs.gov/of/2000/of00-358/text/chapte... >
Shimek, R.L. Dearest Mudder.... The Importance of Deep Sand. Aquarium Fish Magazine, Mar. 2001. Revised 2006. URL: < http://www.ronshimek.com/deep_sand_beds.html >
Shimek, R.L. How Sand Beds Really Work. Reefkeeping Online Magazine, June 2003. URL: < http://reefkeeping.com/issues/2003-06/rs/feature/... >
Tullock, J.H. Water Chemistry for the Marine Aquarium. Barrons: Hauppauge. 2002.
Thurman, H. V. and A. P. Trujillo. Introductory Oceanography, Tenth Edition. Upper Saddle River; Pearson Prentice Hall. 2004.