Dr. Rob Toonen

Section of Evolution and Ecology

University of California, Davis

Davis, CA 95616

Part 1 previously printed in the January 2000 issue of FAMA.

In the last issue, I began this article by explaining that reefs naturally occur in close association with seagrass and mangrove habitats that encompass a very different environment and function than the reef itself. I tried to explain that the link between the sandy habitats found adjacent to natural reefs are both essential to and dependant upon the reefs themselves, and that this natural link between slow flow, fine sediment habitats and healthy reefs can be emulated in the aquarium. I provided a brief introduction to some of the more important physical and chemical factors that control nutrient use in marine sediments. I also promised to finish that introduction, continue to discuss the biological factors that play an important role in the function of sediment communities, and conclude with some recommendations for how best to take advantage of this natural association in our home aquarium design. In this issue, I hope to accomplish just that. I will first complete my discussion of the role of marine sediment communities in the decomposition of organic detritus. Next, I'll try to introduce some of the important organisms that play a role in the natural function of soft-sediment habitats, and I'll finish the article with some recommendations for incorporating these habitats into the design of reef tanks in the home.

Decomposition of organic detritus is a very complex subject, and I will only briefly discuss it here before moving on to a discussion of the biological factors I think are most important to the proper function of a sandbed. As I mentioned in the previous issue, decomposition can be either aerobic (taking place in an oxidizing environment) or anaerobic (taking place in a reducing environment). I have seen some authors try to make a distinction between anaerobic and anoxic environments, but in general the terms are used interchangeably. Technically, anaerobic means "unable to support aerobic respiration," while anoxic means "lacking oxygen." Thus, the zone in which oxygen is low enough that aerobic respiration is no longer efficient, but oxygen is still present is anaerobic, but not anoxic; in contrast, areas in which oxygen is not present are both anaerobic and anoxic. In most sediments, there is an area in which oxygen is detectable, but nitrate - NO3 - is the preferred electron receptor (in case you have forgotten, I explained respiration and the requirement for cells to have an electron receptor in the production of energy in the last issue). In discussions such as this, it is helpful to distinguish between zones in which there is an energetic advantage of using oxygen O2 (aerobic respiration), nitrate - NO3 - (typically by facultative anaerobes in the presence of very low oxygen levels) or sulfate - SO4 - (by obligate anaerobes in the absence of oxygen) as an electron receptor. This distinction is really concerned with the very thin layer around the redox discontinuity layer (RDL) where denitrification occurs (i.e., whether the redox potential is positive or negative around the RDL -- see Fig 1B), because of the long-standing fear of having anoxic zones in the aquarium. I think that this fear is over-stated and the presence of small areas of anoxic sediment deep in the sandbed of an aquarium can actually be beneficial to its function rather than harmful - I'll come back to this point and discuss it in more detail below. Just to make things more confusing, some authors try to make this distinction, and then arbitrarily define either term as "the one" that is specifically correct in discussing the this thin zone of positive redox just above the RDL. I checked a couple of biological dictionaries, and my sediment ecology reference text, and was unable to find a term applied specifically to the zone in which oxygen is present, but too low to support aerobic respiration. I can see the use of making such a distinction in discussions such as this, but in general, the definition of these terms varies by author and since I can find no consistent distinction between them, I (like most authors) use them interchangeably here.

The thin zone in which facultative anaerobic decomposition uses nitrate is generally the primary goal for most people who plan to include a sand bed (with or without a plenum) in their tanks. Respiratory processes of living organisms are essentially oxidation-reduction (redox) reactions involving a variety of oxidizing agents (electron receptors). These redox reactions occur by the transfer of an electron from one material to another to harvest energy, and much of the energy flow in marine sediments is regulated by the availability of suitable electron receptors (Day et al. 1989). Oxygen is obviously the most important and common electron acceptor for most organisms, but in marine sediments it is rapidly depleted, and other electron receptors, such as nitrate, sulfate, carbonate and methane become the primary avenues of energy transfer. Anaerobic reduction, primarily by facultative anaerobes which preferentially use NO3 as the electron receptor in the presence of low oxygen concentrations, is obviously the most desirable of the redox reactions occurring within the sediments, in terms of maintaining a low nutrient environment within a closed aquarium system. In order to achieve this goal, however, you need to develop an environment which is low enough in oxygen that NO3 is the preferred electron receptor, but not so low that SO4 becomes the receptor of choice.

So, how do we accomplish that? How do we take advantage of these natural populations of bacteria that both break down our detritus and perform nitrate reduction at the same time? Well, fortunately, there is a highly predictable gradient in the sequence of respiratory chemical processes with depth in marine sediments (Fig. 1B). At the water-sediment interface, oxygen is readily available, and aerobic respiration dominates the respiratory pathways. Depending on the organic load and grain of the sediments, oxygen can be very rapidly depleted. In most fine sediments, oxygen is depleted within the first 0-4cm (Day et al. 1989). As available oxygen decreases, nitrate (NO3) reduction becomes the most favorable respiratory pathway. This layer is generally very thin (usually on the order of 1-2 cm or less), and as the redox potential (Eh) decreases to zero (where available oxygen is undetectable), sulfate (SO4) and carbonate (mostly CO2) reduction becomes the dominant metabolic pathway over the next 10-50 cm (Fig. 1B) (see Martens 1978 for data and stoichiometric decomposition equations, if interested in more detail). In most well-flushed fine marine sediments, the redox potential (Eh) reaches 0 within the first 6 - 10 cm depth (Day et al. 1989), so by using sand of approximately the particle size and composition of natural sediments in estuaries, mangroves and seagrass beds, an aquarium sandbed on the order of about 10 cm deep should accomplish the goal of optimal nutrient recycling within the aquarium without the added complication of a plenum. However, as I mentioned under physical factors in the first portion of this article, the flow rate of water across the sediments can have a dramatic effect on the depth of oxygen penetration and rate of nutrient exchange in sediments. For example. in a slow-flowing sump anoxia can occur within 1 cm below the sediment surface, whereas in a well-flushed refugium, similarly-sized sediments can appear oxygenated to a depth of about 5-6 cm. Obviously there can be other factors (such as the rate of accumulation and decomposition of organic detritus in varying flow regimes) that may be important and interrelated to flow in this example, but the basic principle is that low flow decreases the rate of gas exchange and oxygen penetration into fine sediments.

A variety of factors influence the rate of decomposition of organic matter, including: temperature, particle size, original source, availability of a suitable electron acceptor, faunal abundance and season (e.g., Mitsch & Gosselink 1986). Although seasonality is unlikely to be a factor in a home aquarium (unless you program your lights and heater/chiller units for seasons), the remaining factors are as much an issue in the home aquarium as they are in nature. Of course, all these factors are somewhat inter-related, and the impact of one is often determined by the status of the others. For example, although water temperature strongly regulates the rate of organic decomposition, the increase in decomposition rates with temperature is much more rapid with smaller particle sizes than with large ones (e.g., Fenchel 1970).

The organic matter resulting from excretion, defecation and the deaths of organisms in our tanks is broken down into it's constituents primarily by the enzymatic activity of bacteria. The bacteria gain energy in the process and the elements composing the organic matter are released back into the tank in dissolved inorganic form once the decomposition process is completed (this process is called nutrient regeneration or recycling). A number of studies of natural nutrient regeneration levels during decomposition of organic detritus have shown that NH4, SiO2 and PO4 are released by decaying phytoplankton in the approximate proportions of 16:16:1 by atom (Redfield et al. 1963, D'Elia et al. 1983). Ideally, these nutrients are not released in excess, but are rather taken up by tank inhabitants in the same proportions (known as the Redfield ratio) as quickly as they are regenerated. It is the excess addition or selective removal of one nutrient or another (thereby changing the relative ratios of these nutrients) that tends to lead to problems in terms of nutrient loading.

The availability and source of nutrients in marine systems in general, and reef systems in particular are highly variable by location. For example, in some regions, detritus from terrestrial run off account for almost none of the total food source of sediment-dwelling organisms, but in others it may account for roughly 40% (Wolff 1977, Van Es 1977). The pathways through which a given nutrient passes may also differ among aerobic and anaerobic sediments. For example, aerobic bacterial activity typically converts NH4 to NO3 (nitrification) in oxygenated environments, but NO3 can itself be used as an electron receptor (denitrification, producing N2) in low oxygen and anaerobic conditions lower in the sediments (as I just discussed). Likewise, PO4 reacts relatively quickly with iron and manganese oxides and hydroxides to form insoluble precipitates in well-oxygenated environments. As sediments become reduced, and anaerobic conditions predominate, soluble forms of Fe(Mn)-PO43- are eventually produced, and phosphate begins to be released back into the system (Nixon 1981). These nutrients are typically taken up as quickly as they are released (hence the term "nutrient recycling") by sub-surface algae, bacteria and cyanobacteria. Although we are always trying to limt the introduction of nutrients such as iron and phosphate in our tanks, the fact remains that algae and even zooxanthellae require some trace amounts of the nutrients to thrive. I certainly do not advocate "supplementing" your system with such nutrients (or products containing them), but the addition of a sandbed will aid with efficient recycling of such nutrients within the system (much as a natural reef and its adjacent seagrass and mangrove lagoons function), and can ameliorate the effects of light nutrient loading in some cases. Having said that, however, the addition of a deep sandbed is not going to magically remove nutrients such as phosphate from your system. The rapid growth of the sandbed fauna will incorporate some phosphate, and phosphate can be directly adsorbed (stuck the surface) to marine sediments, but these benefits do not have an indefinite capacity, and just as a phosphate "sponge" can be exhausted, the capacity for sediments to aid in nutrient loading is limited. If such nutrients continue to be added to the aquarium, the rate of incorporation to organisms and adsorption to sediments is certain to be exceeded eventually and the aquarist will face the typical problems of eutrophication (nutrient overload typically leading to algal blooms) regardless of their system design.

A well-planned and properly functioning sand bed will have both the aerobic layer (with the multitude of infauna (living/burrowing within the sediments) and interstitial (living in the tiny spaces between individual sand grains that make up sediments) fauna that inhabit it, and an anaerobic layer in which NO3 reduction predominates. In order to ensure that you are taking full advantage of the zone in which nitrate reduction can occur, you should be aiming for some small sulfate reduction zones (yes, those ugly and "dangerous" black areas!) in your sandbed as well. Obviously a large and continuous production of hydrogen sulfide (H2S - the 'rotting egg' smell in salt marshes), methane or other anaerobic decomposition products will likely be detrimental to your tank inhabitants, but zones of anoxia deep in the sediments and the accompanying sulfate reduction or methanogenesis can perform a variety of useful functions, including binding toxic heavy metals, regenerating some nutrients, possibly even supporting populations of chemoautotrophic bacteria that can uptake compounds toxic to aerobes and convert them into a useful nutrient source for other bacteria and infauna in the aerobic layers above (some of which I'll discuss in the Biological Factors section below).

The combined area of estuaries, seagrass beds, lagoonal sediments and mangroves is far greater than that of the reefs associated with them (Lowrie & Borneman 1998). That observation is even more significant when we consider the bio-loading of our aquaria, and the relative size of the "reef" and "estuarine habitats" in our systems. Remember back at the beginning of this article, I mentioned in several places that natural communities of anaerobic bacteria are limited not by the availability of carbon sources (detritus) but rather the availability of suitable electron receptors (such as nitrate and sulfate). Again I would emphasize that point, because it means that these communities would be able to process more detritus in the presence of increased "nutrients" (primarily nitrate and nitrite) typical of aquaria. In order to obtain the maximal rates of nutrient recycling (which are required for our minuscule "estuarine habitats" to keep up with nutrient export from our "reef habitat" in our systems), we need to take advantage of both the aerobic and anaerobic pathways (see Fig. 1B). The addition of seagrasses to the system helps further, because the limiting nutrient for seagrasses, such as Thalassia, is phosphorous (Ogden 1988, cf Lowrie & Borneman 1998). Excess phosphate levels are one of the banes of successful reefkeeping, and the addition of export mechanisms such as seagrasses is an excellent tool in maintaining high water quality. In natural systems, seagrass beds and their associated lagoonal areas are capable of denitrifying and nitrogen fixing all of the accumulated organic material from adjacent to reefs (and in many studies significantly more than the total). These associated communities are "even dependent on both organic decay from within the community and from terrestrial runoff for sufficient organic input to sustain the floral and faunal constituency, making them a highly effective "filter" in the wild, and potentially in the aquarium" (Lowrie & Borneman 1998). Mangroves, on the other hand, are virtually self-sufficient, and do not contribute significantly to nutrient export of coral reefs (Ogden 1988, cf. Lowrie & Borneman 1998).

In discussions with Eric Borneman, Jonathon Lowrie, and Morgan Lidster (of Inland Aquatics), all of them noticed cycles in the expansion and shrinkage of black reduction zones in their sand beds (as do I). I think the most logical explanation for this has to do with the accumulation of detritus in our sandbeds. As I mentioned (now several times) above, anaerobic respiration is most limited by appropriate electron receptors. As we load our systems, we continually add detritus (a carbon source), but our anaerobic bacteria are limited by electron receptors such nitrate and sulfate, not carbon, and the detritus starts to pile up. At the same time, the aerobic pathways continue to chug along, churning out NO3. Because the aerobic bacteria are carbon limited, the increased organic load allows them to increase both their population sizes and their rate of processing these organics. This speedup in aerobic metabolism means that oxygen begins to be used more quickly in the sediments, and the penetration of oxygen into the sediments is reduced. Reduced oxygen in the sediments leads to increased zones of anoxia, and the anaerobic reduction pathways start to roll. These can be remarkably efficient when electron receptors such as nitrate and sulfate are readily available, and soon you notice the spread of the sulfate reduction zones in your sediments. If all is well in your tank, this condition should lead to organics being broken down faster than they are introduced into the system (at least, this is ideally the goal of any closed system such as an aquarium - if such a balance between inputs and exports cannot be reached, the system will crash regardless of its design), and as those nutrients are recycled, the anaerobic zones begin to become limited by the availability of electron receptors again. Sulfate reduction zones ebb, and the cycle starts again. The "pseudoequilibrium" between nutrient input and recycling of nutrients by the sandbed should allow the system to run with minimal export mechanisms and modest regular water changes. This natural cycling of nutrients mimics the ebb and flow of nutrients on natural reefs and their associated sandy habitats, and is a much more simplistic and elegant method of aquarium design than the extreme chemical micro-management of reef aquaria that has become increasingly popular over time.

Biological regulatory factors:

Obviously these "regulatory factor" categories are not mutually exclusive, but rather were the simplest way for me to break this information into reasonable pieces. Although I discussed nitrate reduction under chemical factors, it could have as easily been discussed under biological factors, because that reduction is accomplished by bacteria, which are biological entities.

In closed cultures, microbial growth is often inhibited by the buildup of toxic end products of microbial metabolism. Free-flowing water generally prevents this, but in places where water movement is slow, microbial metabolism may produce these toxic end products faster than they can be removed. These toxic end products include fermentation end-products (such as fatty acids and alcohol), secondary metabolites and sulfides (Day et al. 1989). The buildup of these compounds is increased in the presence of organic detritus, both because microbial activity is increased, and because these compounds may also be produced by decaying organic material (Wilson et al. 1986). These toxic compounds can affect not only the microbial community, but can cascade through the sediment community and indirectly impact any organisms that are incapable of leaving.

Similar to the indirect effect described earlier in this article (toxic microbial by-products affecting other community members), direct interactions among organisms within the sediments can have a significant impact on species abundance and composition. Just as for the larger organisms with which we are all more familiar, there is constant competition for resources among sediment-dwelling species, there are predators and prey, and there are even many predatory microbes. The activities of some animals may strongly impact the survival of others. For example, burrowing thallasinid shrimps tend to be excluded from areas with dense populations of burrowing clams, or regions in which the sediments are stabilized by the growth of seagrasses (Rhoads & Young 1970). Although these examples are extreme, many benthic marine animals modify their environment in ways that may either enhance or reduce the ability of others to colonize and survive.

As an example, let's first consider the epifauna of the reef. The epifauna includes all those critters that live on or around structures on the surface of the sea floor. This community is typically dominated by corals, barnacles, hydroids, bivalves, tunicates, bryozoans, sponges and tubiculous (tube-building) polychaetes - or to put it simply, this is the stuff on our live rock that we buy to "decorate" our tanks. These sessile forms provide habitat for a variety of slow-moving organisms such as gastropods, errant (free-living) polychaetes, flatworms, and a variety of crustaceans and echinoderms. Together these animals form dense and (obviously to anyone who has a reef tank) beautiful communities. I won't try to cover any these animals in any detail because they are abundantly covered elsewhere (that's what most people discuss in "reefkeeping" articles, but the focus of this article is sandbeds).

The animals I do want to discuss in some very basic detail are the infauna (animals living beneath the sediment surface). I have already mentioned some of these players in passing along the way, but here I will try to cover a little more detail on the members of each group and the role they play in maintaining a healthy sediment community. Let's start with the macroinfauna (animals retained when sediments are strained through a 0.5 mm mesh). These are the worms, bivalves, echinoderms and other relatively large organisms that live beneath the sediment surface. In undisturbed sediments, these animals are frequently invisible except for the occasional fecal casting, burrow hole, or siphon tube. The apparently barren mudflats and seagrass beds in coastal waters conceal a diverse community of animals that are part of the most densely inhabited of aquatic environments in the world (see Table 2). Studies of these animals indicate that variability in species composition and abundance are the norm in marine sediments, and samples collected from as little as a few feet apart may give a very different impression of the community abundance and make-up.

Despite that variability, there are a number of generalizations that can be made concerning the composition of sediment communities in different areas. In general, polychaete worms and bivalve molluscs tend to dominate most soft-bottom habitats, both in terms of numbers and total biomass (Day et al. 1989). Bivalves have been mentioned a number of times throughout this article, but by bivalve molluscs, I do not mean the mussels and oysters you had for dinner last week. Sure those are bivalves, but the majority of sediment-dwelling bivalves are not quite like the ones that show up in the local Safeway. After years of borrowing a copy of Keen's classic book, Sea Shells of Tropical West America, I happened to find a copy in a small second-hand book store. They had no idea what a find it was, and sold it to me for $10 because it was missing the decorative jacket on the original hardcover. I relate this story because I have been paging through it in that smug "what a deal!" mood that I get when I'm really excited about something I just bought. Well, looking through that book has made me realize how few of the West Coast molluscs I have actually seen! Most of these animals are tiny clams (many on the order of a few mm in diameter), and I would say that the most common sizes (just flipping through the book) are around 3-7mm or 20-30 mm. When you start looking at the descriptions of the shells, however, the description "rare" is most often applied to the larger size-class and the description "common" to the smaller (Keen 1958). I would wager that few people pay much attention to even an abundant clam that small buried in marine muds. These are the type of critter that are most desirable in our sandbeds, however, and the ones most obviously missing from most "live sand" I typically see for sale in petshops. Based on his observations from hosting a workshop on the identification of live sand organisms at the Western Marine Conference in 1997, Ron Shimek has stated that low diversity appears a common feature among all tanks that he examined established using commercially available live sand (pers. comm.).

Among the other common groups of macroinfauna that can be significant players are burrowing gastropods, nemertean worms, flatworms, and a variety of echinoderms and micro-crustaceans (such as isopods, copepods and amphipods). Together these animals "rework" the sediments and are even more essential to the function of a healthy sand bed in a closed system than in natural systems where storms and other physical disturbances occasionally help with the task. What do I mean by "rework?" Reworking the sediments generally refers to the turning over of the sediments such that they remain only loosely packed, and generally well-oxygenated near the surface. Most animals that perform this function are actively consuming bacteria or bacterial products from the sediments and thereby maintaining an environment conducive to high bacterial growth. Some animals (e.g., the worm Arenicola) burrow through the sediments, consuming them as they go, and expel reduced deep sediments onto the surface around their burrow in the form of fecal castings, making the nutrients locked in these deep areas available for biological uptake on the surface.

Getting back to the more general topic of sanbed animals, despite abundant sampling and descriptions of the community composition of infauna in many regions, we really know very little about the functional roles of individual species in these habitats (such as their exact food requirements). Instead much of the research on benthic communities has focused on developing complex and elegant statistical tests to distinguish among different communities based on the similarities and dissimilarities in their species compositions. A general feature of most sediment communities in the wild is that they vary greatly in number, composition, and location over very small distances. There was at one time a serious argument over whether benthic communities were: 1) analogous to an organism, with its "body parts" comprised of the various dominant species of that community (e.g., a coral reef), or 2) a loose, or perhaps even random association of species that happen to share the same space (perhaps largely by chance), but have few (if any) obligate interactions (e.g., necessary for the survival or reproduction of the species). Benthic communities appear to have very few distinct boundaries, yet most differ slightly in their species compositions. Today most ecologists would agree that marine communities fall closer to the second view than the first.

In general, however, there are some common characteristics that tend to group infauna in various habitats. Suspension feeders (non-selectively trap & concentrate food from the water column) tend to predominate in sandier sediments, whereas deposit feeders (ingest sediments and digest the organic materials contained therein) tend to favor the more silty environments (Day et al. 1989). The suspension feeders collect food and eliminate wastes by pumping water through their burrows or by extending specialized feeding structures (such as siphons or modified gills) above the sediment surface into the water column. In contrast, the deposit feeders ingest the organic-rich sediments from which they derive their nutritional needs. In this manner the deposit-feeding macroinfauna generally rework the sediments, thereby aerating them and lowering the bulk density (most easily thought of as a measure of compaction or clumping - quicksand would have a very low bulk density, hard-packed clay a very high one) and stability of the sand bed (Rhoads & Young 1970). It is these latter animals (the deposit-feeding infauna) that are essential to the long-term stability of the sand bed in a closed system. Despite the tendency for people to treat a "cleanup crew" as if they don't exist, it should be obvious that it is important to feed the animals in your tank. For many of sandbed organisms, the organic detritus produced through the natural activity of their tank mates is sufficient, but additional feedings of zoo- and phytoplankton are uniformly beneficial, not only to the sand fauna but also to the corals in aquaria (Shimek 1997b, Bingham 1998). Although it is relatively easy to culture your own greenwater for feeding (Toonen 1998), the recent availability of both cryopreserved (e.g., Inland Seafarms) and live phytoplankton (e.g., DT's Phytoplankon) make it extremely simple to provide this essential food item to your reef inhabitants.

Note that this category does not include the large motile animals that are classified simply as benthic - those vertebrates and invertebrates that lead double lives and are equally as capable of burrowing into the sediments as they are scrambling across the surface or swimming into the water column. This group includes primarily the decapod crustaceans (mostly crabs and shrimps), but also includes many echinoderms (such as brittle stars, sea urchins and sea stars), gastropods, and some fishes. These animals are, for the most part, harmful to the longevity of a closed sand bed system (although there are some important exceptions including some species of brittle stars, and some sea cucumbers). Most are active predators whose activities tend to greatly disturb the sediments and reduce the populations of infaunal animals for which you are striving in your tank. Those "sand-stirring" sea stars and gobies from the local pet shop look great, and I often hear petshop employees stating with authority that they help maintain a live sandbed by "turning over the sand," and preventing the formation of dangerous "dead spots" in the sandbed. Well, hopefully, I've convinced you that those "dead spots" may not be so dangerous with my discussion of them earlier, but there is a much more important reason to avoid sifters - they eat stuff. Think about it - the animals are obviously eating something in the sand, otherwise they wouldn't be "stirring" it, and how quickly would those things in the sand have to reproduce to keep up with being eaten all the time? You just paid a lot to get "live" sand rather than going to Home Depot and buying a bag of the cheap stuff, so why would you want to then introduce something that will systematically remove everything that makes that expensive sand "live?" There are some obvious exceptions to this generalization, such as the sand-eating sea cucumbers (not filter-feeding species such as sea apples!), and some brittle stars (such as Ophioderma and Ophiocoma spp., but not the spiny Green Brittle Star Ophiarachna incrassata which is a voracious predator and can even catch fish) which are very useful for sediment reworking, but these animals are really the exception rather than the norm. If you really want to add some large creature to your tank to help clean the sediments, the best choice are the variious "turd" sea cucumbers. I know that doesn't sound flattering, but that really is the common name for many of the species (e.g., Donkey-dung sea cucumber, Holothuria mexicana). Recently some retailers have been coming up with more flattering names for their sea cucumbers, however, and things like "Tiger-tail sea cucumbers"are now becoming available on the market (although I have seen at least 3 species with this common name). These bottom-feeding sea cukes generally only survive in well established tanks with a reasonably well-established organic load in the sediments, however, so I wouldn't recommend adding one for at least the first 6-8 months.

Some authors also recommend "cleaning" or otherwise disturbing the sediments (or some small portion of them) as part of standard aquarium maintenance. This advice is reasonable only if the sediments lack all of the critters on which I will focus for the rest of the article. There is a well documented relationship between disturbance and species abundances - without any disturbance, competitive interaction among animals dominate, and the species diversity of the system is decreased. Conversely, with a lot of disturbance, the constant removal of individuals and resources leads to the extinction of some groups, and again, species diversity is decreased. It is only at a low level of constant disturbance that species diversity is enhanced by opening new habitats/freeing resources and thereby preventing competition among groups from leading to the exclusion of some species. But the removal of individuals and resources must remain low enough that resources do not become limiting, and the chance removal of individuals of a rare species does not impact their survival. Cleaning the sediments by vacuuming or otherwise stirring, siphoning or removing detritus is definitely a form of disturbance. Given that, you may find yourself swayed by the argument that some sediments should be cleaned regularly as part of your maintenance schedule, but this ignores the disturbance caused by the animals themselves that live within the live sand. Animals that live in the sediments burrow though them, move them, aerate them, eat them (and the critters in or on them sometimes), and generally cause a fair bit of disturbance without any help from the aquarist. Ron Shimek (1998a) provides sediment disturbance rates for single organisms burrowing and feeding in benthic sediments ranging from 100 to around 10,000,000 mm³ per day. At densities approaching 40,000 worms per m² in my tanks, that is a LOT of disturbance already! Aside from the detrimental rearrangement of the sediments and the disruption of the chemical gradient formed by the sediments, a small bit of siphoning or cleaning is certain to tip the balance against some animals in the tank, and you will almost certainly have a less diverse and efficient sandbed than if the bed were left untouched except for the action of the organisms living in the sediments.

Those animals that I am talking about above are primarily the meiofauna (pass through a 0.5 mm screen but are retained on a 0.05 mm mesh screen), which are unfortunately rather poorly studied and even less well-understood. Several early works from the 60's and 70's still provide the majority of available information regarding the importance and function of meiofauna in benthic marine habitats (Day et al. 1989). The meiofauna include a taxonomically diverse group of critters, including rotifers, gastrotrichs, kinorhynchs, nematodes, tradigrades, copepods, ostracods, many turbellarians and oligochaetes, some polychaetes, and a few specialized hydrozoans, nemerteans, bryozoans, gastropods, aplacophorans, holothurians and tunicates. Whew, that's a mouthful, but I think you get the idea - there are lots of 'em (again, if you want more information on some of these groups, check out Ron Shimek's monthly "Without a Backbone" column in Aquarium Frontiers and my "Reefkeepers Guide to Invertebrate Zoology" column in Aquarium.Net). This is one case in which the general rule of thumb that finer sediments support more diverse communities than sandier ones is reversed. For the meiofauna, sandy beaches (in which the meiofauna can live in the interstices between sand grains) tend to support more diverse communities than do very fine sediments (through which the animals must burrow by displacing particles as they move). This is due in part to the high energy and coarse grained nature: there is rapid exchange of water, and with it renewed oxygen and organic matter, through the subsurface sediments. This exchange, in turn, allows meiofauna to frequently exist deeply (on many beaches up to a meter or more) into the sediments (Fenchel & Riedl 1970). In finer sediments, however, most meiofauna are restricted to upper few centimeters which remain well oxygenated, although some can tolerate anoxic conditions for extended periods, which allows them to penetrate the reduced sediments. Often these forays into reduced sediments are foraging trips, and the meiofauna provide an important and major pathway for nutrients to re-enter the aerobic zone. This explains why people often observe animals or animal tracks/burrows in anoxic regions of their sandbeds. For example, anaerobic sulfur bacteria may account for nearly 1/3 of the diet of ciliates (Fenchel 1969), and Montagna (1984) subsequently found that grazing rates by infauna averaged 3% of the total bacteria and 1% of the total diatoms in the sediments per hour. The diet of meiofauna examined consisted primarily of diatoms, flagellates, sulfur bacteria and "other" bacteria (including cyanobacteria), which together accounted for over 90% of the diet. This argues that meiofaunal grazing probably represents an important stimulatory effect on the benthic microbial community, and is also essential to the long-term health and stability of sandbeds in closed systems. Again, these animals are woefully lacking in most commercially available live sand I have seen, and those Ron examined at the live sand workshop.

Many studies have examined the vertical distribution of meiofauna in relation to sediment depth, and the general pattern is that the animals are most abundant in the upper 2-3 cm of the sediments, with the abundance falling off rapidly with depth until about 6-7 cm (Fenchel & Riedl 1970), where most sediment infauna become rare or absent.

Temperature is again an important factor in controlling the abundance of meiofauna, but is once again sort of counterintuitive for tropical meiofauna. In temperate zones, the peak abundances of meiofauna occur in the mid-summer months when temperature is at its maximum, but in tropical and sub- tropical climes, peak meiofaunal abundances occur in the winter months when temperatures at their lowest, and these annual fluctuations in the abundance of the maeiofauna can vary by a factor of 5 or more (Day et al. 1989). This is perhaps the best argument of which I am aware for maintaining an aquarium at temperatures below the mean summer values for the tropics, which average in the mid- to high 80s (F). Despite the fact that these animals are much smaller than the obvious macroinfauna, their density and metabolic rate is proportionally much higher, such that they end up being approximately equal to or even exceeding the larger infauna in their annual impact on the sediments (Warwick et al. 1979).

The final group to mention is the microinfauna (animals that pass through a 0.05mm mesh screen) - these animals include the bacteria, flagellates, diatoms, cyanobacteria and ciliates. I don't see much point in going into any further detail than I have done by mentioning this group in a variety of other places throughout this article. Like the meiofauna, these organisms can be extremely important to the healthy functioning of a sand bed, but are relatively poorly understood and understudied. The function these organisms provide, however, are exceedingly important to the issues of nutrient recycling I discussed earlier.

Summary and Recommendations:

"My basic assumption is that the natural way is best. Emulating nature facilitates predictability and prevent organism stress. I claim that our reef aquaria systems are artificial ecosystems and function as do natural ones." (Ron Shimek, 1998a).

I completely agree with Ron's statement above, and have tried my best to give you a brief explanation of how natural sediment systems function and why. If you are willing to accept the basic assumption that emulating natural conditions is the ultimate goal of setting up and maintaining a reef aquarium (or at least an effective way to do so), then the inclusion of a deep sandbed in your aquarium is a leap in the right direction.

This is probably more detail than most people ever wanted to know about sandbeds and sediment ecology, but I thought that the recent level of interest in sand-beds warranted some more detailed information about them. After reading through all this, it should be apparent that I have not merely said "add so much sand of this size to some depth and swirl at 80F for 3 weeks." Well, I'm sorry to say that you won't find any such "recipes for success" in this article. That is, in part, because I do not believe in such cookbook techniques (there are almost always multiple ways to achieve the same end and any such over generalizations are bound to fail in some cases), but also in part it is because there is no simple answer to the question "How should I set up my aquarium or sandbed?" The answer starts with another question: "What are you trying to accomplish?" I think that the answer to the simple question of a goal is all too often missing in the design and implication of a tank, but provides the foundation of most successful hobbyists, professional aquarists and researchers alike - if one has a good basic knowledge of the system, and that knowledge is used to design a setup that will accomplish your specific goals, the results will certainly be more successful than anyone who follows any "Fool-proof, Easy-Reef" recipe (but I'm sure there are lots of folks who will be happy to mail you one for only $19.95 plus $4.95 S&H).

Many biologists have an envy of fields such as chemistry and physics in which there are "laws" that uniformly apply to the interactions being studied. Some ecologists continue to search for some unifying "law of biology" that will allow us to understand how complex communities (such as a coral reef) become established and are maintained, but if there is any unifying "law" of biology that occurs to me, it is that there is always an exception to the rule. Organisms are remarkably diverse and adaptive in their interactions with one another and the environment, and attempts to generalize about habitats or ecosystems always require caveats and squishy terms like "in general." It is often difficult for people to accept that there are no black-and-white answers to most questions, and approaches based on simple "reef recipes" are therefore highly attractive. Another common path for both beginners and advanced hobbyists is to gravitate towards the high-tech chemical micro-management of the reef environment, which seems much more authoritative and reliable for some reason than emulating nature (despite the fact that nature is much better at supporting these animals than we are). I know that both reef recipes and micro-management can work (check the web for some pictures of tanks based on these approaches, and its easy to see that some people are very successful with them), but I urge people not to discount more natural methods without considering what they have to offer and what you, as an aquarist, are trying to accomplish. I think too many people default to these approaches simply because they "feel right" - especially when compared to an approach that advocates virtually no technology and always asks return questions rather than giving simple answers to queries. I think this misplaced need for a simple answer (I include technologically complicated mechanisms such as electro-chemical denitrifiers into this category, because the answer to the question "How do I decrease my nitrate levels?" is a simple one: "Spend another $250 on this whiz-bang gadget!") is the biggest hurdle to be overcome in the general acceptance of emulating nature in our aquaria; nature is not simple, and there are generally many pathways to reach the same goal.

I strongly believe that the bottom line of what is best for your aquarium depends on the specific conditions, critters and goals you have for your tank. In my opinion, a lack of understanding of the basic properties of how habitats and organisms interact and make a living is the primary cause for otherwise smart and motivated people to fail at this hobby - there is no magic here, but it is a complex and dynamic system, and the more you know about how it functions and why, the more likely you will be able to maintain it successfully. Those who are not willing to learn about the needs and natural conditions under which their organisms live may still be able to successfully grow them in a fancy aquarium setup, but I see many people who have equivalent (or more commonly better) success with the same organisms in a well thought out aquarium with few gadgets and a little knowledge. I urge beginners to speak to as many people as they can find, examine their tanks and find out about their techniques. Only after having some idea of the variety of methods available to set up and maintain a reef tank can you make an informed decision about how to set up your own.

OK, having said that, I need to get off my soap-box and at least give some general guide lines for making your own sand bed. The first question you have to ask yourself is what type of reef habitat are you trying to emulate? For example, based on a recent study from the Great Barrier Reef in Australia, Ron Shimek (1998a) calculated water flow across the reef crest was equivalent to 834 gallons per minute in a 100 gallon tank. Obviously it will be hard to even generate that flow rate in an aquaium, but in any tank with a very high flow rate, the presence of fine grained sediments is likely to cause problems. That does not mean that such tanks preclude the ability to take advantage of natural sediment systems, it simply means that you need to be more imaginative in your incorporation of sediments such as I have described here. There are a variety of ways to include a "remote" sandbed as an attached sump system, possibly also functioning as a refugium (a separated system in which microcrustaceans, worms and other typical prey items are allowed to grow here but the fishes and other predators cannot reach them - the benefit of such a system is that the reproduction of animals in the refugium provide a constant source of live food for the animals in the main aquarium, and those that are missed serve to constantly reintroduce potential prey organisms into the main tank). Refugia can range from commercially offered in-tank models (e.g., Inland Aquatics sells an in-tank refugium that can be easily incorporated into tanks for which a sump-style refugium is impossible) to fancy multi-level sumps mimicking different habitats associated with coral reefs in the wild (e.g, Eric Borneman's system in which water is circulated between a series of connected tanks that sequentially simulate the intertidal, reef zone, seagrass beds and mangrove swamps). It is a simple matter to include a sandbed within the refugium or sump container instead of within the main display tank itself for high flow-rate systems.

An added benefit of including a refugium is that the animals living within it (e.g., copepods, mysid shrimps, gammarid amphipods, isopods, polychaetes, etc.) are able to provide a consistent and nutritious food source for the remainder of the aquarium (see Shimek 1998b for a description of the plankton harvested from such a system). This is particularly important when one considers the calculation Ron made regarding the amount of live food that reaches a reef on a daily basis: 1 oz. wet weight of live food / 10 gallons / day. That's a lot of food, and because I am pretty confident that few, if any, of us feed at that level, anything that helps corals meet their energetic requirements by supplementing intentional feedings is a good thing.

The first thing to consider is sediment size and depth. From Tables 1 & 2 in the last issue, you can see that smaller particles allow for increased bacterial growth, but also remember that the pattern is the opposite for meiofauna. In order to maintain a diverse and effective sediment community, you need to have a mixed range of sediment particle sizes. I can't stress this point enough - simply dumping in a deep layer of uniformly sized sediments is unlikely to give you the results for which you are looking. You need a mixture of sediments from various places and of various sizes to get the maximum benefit from including a deep sandbed community. Ideally the mix should be at least 60% fine sand (something around 0.5 to 0.05 mm in diameter) mixed with some coarser and some even finer particles. Anything larger than about 2 mm diameter is, in my opinion, more-or-less just wasting space in a sandbed -- there are relatively few organisms found in such sediments in natural communities (see Table 1).

In my tanks, I run about 10% coarse sand (2mm - 0.5 mm), something around about 30% or so of "sugar sand" (0.3-0.5 mm), and the remainder of the bed is comprised of very fine sands (on the order of 0.05 to 0.1 mm). Coarse sand is basically fine crushed coral gravel - often the only size that is offered as "live sand" substrate in petshops (buying live sand from a petshop can be both the "seed culture" and make up the approximately 10% of coarse sand incorporated into the sandbed). Sugar sands can be purchased from vendors such as CaribSea. Very fine sands can be difficult to locate, but are becoming more and more common from vendors such as CaribSea as demand increases, and can also be found in most bags of crushed coral (that "dust" you rinse off). I actually bought my fine sand as "founders sand" (at least that's what the guy at Home Depot called it - he said it was calcareous sand used by sand blasters), but I have not been able to locate the same stuff since that original purchase. In any case, the idea is to get as close as possible to the size ranges typical of estuarine and lagoonal sediments to take advantage of the infaunal community found in those habitats.

After you have the particle size down, you have to decide how deep to make the bed. In general, at "normal flow rates" if you follow the particle size recommendation I make above, you need roughly 7 - 10 cm of sand to accommodate the full range of bacterial communities outlined in Figure 1. The deeper the bed, the more sure you can be to include the full range of reducing communities in your bed, but the more water column space you lose to stock with the animals in which you are really interested as well. If you have large sediment particles, you will need a deeper sediment layer to accomplish the same goal, but the relationship between sediment particle size and depth is not a simple linear one. In other words, if you double the sizes of each particle size category I suggested above, you couldn't simply double the depth of the bed and expect it to work exactly the same way. When establishing a new sandbed in a tank in which vertical space is an issue, I would start out with about 6 - 7 cm of mixed grains sands as outlined above, and continue to add 1-2 cm of depth every couple of weeks until anoxic regions (evidenced by dense black sulfide reduction zones) begin to appear at the bottom of the bed. Once these zones appear, the bed is deep enough to support both oxidative and reduction pathways.

Warnings against the danger of hydrogen sulfide (H2S) production by deep sandbeds have appeared many times in the popular literature, often accompanied by dire stories along the lines of "the black smoker that killed everything in the tank of my best friend's other best friend." In fact, under normal circumstances, the production of hydrogen sulfide in a deep sandbed is likely to be a benefit rather than a dangerous and unwanted consequence of anoxia - the end products of sulfide reduction (CO2 and organic acids) are used by other organisms and function to increase the diversity and stability of the sandbed. Organic acids do have the unfortunate consequence of coloring the water if allowed to concentrate in an aquarium. However, periodic use of activated carbon remove organic acids, and "excessive levels of humic acids do not seem to occur to any great degree in long term established sand systems" (Lowrie & Borneman 1998). This is likely due to the fact that the decomposition of organic acids by microbes, although slow, is complete and the end products are available for direct uptake by a variety of other organisms (as discussed above under Chemical Regulatory Factors). For example, Morgan Lidster of Inland Aquatics in Terre Haute, IN runs sandbeds that exceed 20" (yes, you read that right -- 20 inches!) with great success in his ATS-based culturing facility, and has seen none of the widely broadcasted pitfalls of deep sandbeds (Lidster, pers. comm.). Jonathan Lowrie and Eric Borneman (1998) present a similar view in their article, as do the writings of Ron Shimek and I. As I have stated in several places already, in a properly balanced aquarium a type of equilibrium is reached in which sulfide zones expand and shrink with nutrient loading. If the system is continually overloaded, the aquarium is bound to collapse and the eruption of "sulfide volcano" in the aquarium is one potential outcome - of course, given continued nutrient overloading in a system, there is no other method that could prevent a similar outcome...
Starting with good quality live sand is also essential, because (to quote Jonathan Lowrie 1998), "Live sand will not form from dead sand. Period." No matter what kind of live rock you bought, or how good you think it is, it won't include the soft-sediment organisms that are necessary for the proper functioning of a healthy live sand bed as outlined above (even if a few of the animals crawl out of the rock and into the sand). Worse still, sand is very rough, and those slimy, mushy, soft-bodied critters I've been talking about don't handle being ground to bits by the sand during shipping very well. Imagine the trauma of a worm, sea cucumber or thin-shelled mollusc being smashed, ground and pureed by the movement of the sand grains as it is collected and then tossed around by baggage handlers two or more times before it arrives in your hands (just consider how your suitcase fares on most trips!!). How, then does one obtain good quality live sand? Is it even possible?

One trick is to buy the top inch or so of live sand from a variety of petshops the day after they get in their new shipment of live sand. Why would you want to do this? Well, because those shipments usually get dumped into a relatively small tank, and the oxygen content starts to drop deeper in the sand as the animals and bacteria use it up. As the oxygen level drops, the animals migrate towards the surface, and if you time it just right, you can get most of the animals that survived the shipping by purchasing the upper layers of sand from the tank (I'm sure your retailer is going to love this bit of advice!). The rest of the sand is likely devoid of much life other than bacteria.
Although that method does work, the best answer is to get the animals shipped separately from the sand itself. You can still buy all the sand "live" you want (there are usually some animals that miraculously survive the shipping and the sand grains will be coated with bacteria as well), but if you really want to get a really good "live sandbed" you'll need to contact facilities like Inland Aquatics (http://www.inlandaquatics.com/) or IndoPacific Sea Farms (http://www.ipsf.com) about purchasing cultured infaunal critters to populate your sandbed. That's right, many of these animals are being cultured now, and I strongly encourage everyone to buy cultured animals whenever possible to minimize the impact of aquarium collection on natural reefs. We should be encouraging the growth of these captive breeding facilities, because especially given the current political climate, captive raised animals may soon be the only source of aquarium animals in the US. Breeding marine invertebrates in aquaria actually predates anything that could be called a reef tank (Toonen 1997), and it doesn't take any fancy equipment to succeed with breeding reef animals in your own home (Toonen 1996-97, Toonen 1998). For people interested in breeding marine organisms, the Breeders Registry is a great non-profit organization that aims to share information among breeders and provides a database of reports for all known marine species bred thus far (http//www.breeders-registry.gen.ca.us). There are many people breeding a variety of marine animals, and I have spoken to several people who are trying to start sandbed starter kit businesses by breeding some of these animals in their homes.

Ideally, you should buy a sandbed starter kit from as many different locations as you can, introduce these critters and give them an opportunity to establish and reproduce before adding any potential predators (such as fishes, shrimp or hermit crabs) to the tank. Again, I emphasize that these animals need to be fed, and regular additions of foods such rotifers (available from suppliers such as Wasatch Aquacultre, Aquatic Eco-Systems Inc. and Florida AquaFarms), copepods, enriched baby brine shrimp, and phytoplankton (aka greenwater -- available from suppliers such as Inland SeaFarms and DT's Phytoplankton Farms) will greatly improve the nutritional health of both your sandbed and corals. A reasonable estimate of the time for the sandbed to become properly established to support potential predators would be on the order of six months. Once the sandbed community is well established, it should begin to reproduce at a rate sufficient not only to maintain itself in the presence of a few predators, but also provide supplemental feedings for the filter-feeders introduced to the aquarium. If you decide to go the route of including a refugium rather than incorporating the sediments into the tank directly, there is no need to delay the addition of other animals, because tank inhabitants are excluded from the refugium, and the sandbed community can reproduce without predation and eventually establish themselves in the main aquarium (or sump) as well. This last option is my preferred method because it provides the most flexibility in terms of design for the main aquarium, although I currently have tanks of each design. Regardless of the design you chose, it is also a good idea to "recharge" your sandbed periodically (I try once every year or so) with a new "starter culture" to make up for the accidental loss of any animals from the system, and to hopefully increase the diversity of animals introduced to the sandbed.

Well, I hope that I have convinced you that a well-balanced sand bed can be a significant contribution to a healthy reef tank. Having said that, however, I must emphasize that the design of the tank, and the components that one decides to include in its design must be suited to the needs of the animals to be kept. While a sand bed seems to be a great addition to many tanks, they are obviously not well-suited to inclusion in a tank designed to mimic the conditions best suited for an Acropora dominated reef crest (discussed above).

I hope that I have also convinced you that the benefits attributed to the addition of a plenum are basically the same as those of a deep sand bed. In my opinion, the addition of a plenum is an additional level of complexity that is neither necessary nor beneficial over the inclusion of the deep sediments themselves. I certainly do not mean to say that plenumed systems are in any way less successful or inferior to systems based on a deep sandbed - I'm sure that there are many readers who have perfectly successful tanks with plenums. What I do mean to say is that I have yet to locate any data showing that the addition of a plenum provides any additional benefit that cannot be attributed to the presence of the deep substrate itself, and given this introduction to sediment ecology, I hope the reader can understand why I doubt such a result will ever be found. Therefore, I would argue that incorporating a plenum into aquaria constitutes an unnecessary complication which has no clear benefits to the long-term success of marine reef aquaria over the inclusion of a deep bed of fine sediments. In fact, given my discussion of the potential benefits of including anoxic reduction zones deep in the sandbed, I would argue that a deep sandbed should provide some excess benefit beyond a system based on a plenum. Lowrie & Borneman (1998) conclude that using a deep sandbed to maintain water quality can equal or even exceed the results from using more "traditional" methods employing heavy foam fractionation (protein skimming), and provides the basis for removing skimmers from well-established aquaria (which is another subject for future debate).

To come back to the original question posed by the article that prompted this reply, "are plenums obsolete?" I would have to say that the word obsolete does not accurately reflect my feelings on this matter. I would rephrase that question to "are plenums necessary?" and answer with a confident "no."

In conclusion, I am not trying to convince anyone that the addition of a plenum is likely to hurt the functioning of a healthy sandbed, but in my experience the addition of a plenum didn't seem to really help either. I must admit that I obviously fall into the anti-plenum camp, and personally consider them an unnecessary hassle (so I don't include one in any of my tanks), but if you're interested in a much more positive discussion of plenums and their function, I'd suggest the writings of Sam Gamble in the archives of Aquarium.Net (http://www.aquarium.net/misc/b_issues.shtml) or those of Bob Goemans within the pages of FAMA.

Personally, I don't think that the addition or omission of a plenum will have nearly as much effect on the ultimate success or failure of a hobbyist as a good basic knowledge of the structure and function of coral reef, mangrove and seagrass ecosystems, the biology of the organisms inhabiting each habitat, and how they interact in nature. Given that it is possible get the benefits of a plenum (primarily denitrification) in addition to others not provided in a plenumed system (e.g., the carbonate reduction pathways, sulfide reduction and methanogenesis) by simply adding about 10 cm or so of mixed fine sands and silts along with as much of the associated infauna possible onto the bottom of the aquarium, I am left wondering why anyone would want to go to all the extra trouble and expense to set one up...

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