Demystifying Mud (Sediment Filtration)

Demystifying Mud (Sediment Filtration)

By Jonathan Lowrie

Without question, there has been more than a bit of curiosity regarding the use of mud in reef aquariums in the past few months.  Mud has been used in freshwater biotopes for decades in certain applications with good success, but where does mud fit in to a coral microcosm?  Perhaps some of the allure lies in the fact that many want a "miracle"...something new and exciting, without requiring any explanation.  Others tend to look at such "advances" with a skeptical eye, requiring information, and viewing such "discoveries" with a touch of skepticism.  The bottom line is that mud is not a new subject or a novelty.  The study of sediment ecology and biology is well rehearsed in great depth and has been for many decades.  While aspects of sediments and their populations are being discovered almost daily, the literature abounds with immense volumes of the nature of "mud."  In fact, to adequately cover the aspects of sediments in the marine environment would take a lifetime.  We would like to examine and clarify some aspects of mud, present some information concerning mud biotopes that are beneficial to reef aquariums, and finally present some experiences witht he use of such soft substrates.

What is Mud?

Mud, by definition, is wet soft earth or earthy matter.  

         Earth: dirt and soil, as distinguished from rock and sand. 

                  Soil is defined as the portion of the earth composed of                                  disintegrated rock and humus. 

                           Humus: the dark, organic matter in soil, produced                                 by the decomposition of vegetable or animal                               matter that results in the fertility of the                                     earth.

                  Dirt is defined as earth or soil, especially when loose.

Mud is not basically complicated, but certainly not magic, either.  However, mud does become quite complex when one looks at it from a geologist's point of view.  There are over 13 classifications, based on the Wentworth scale.  These classification schemes to desrcibe differences in sediment texture are based on the proportion of silt, clay, sand, and water.  Commonly used categories are shown below.

Wentworth Geometric Scale

The phi scale is based on the logarithmic transformation of a particle diamer (phi = logbase2 particle size in mm)

	Particle Type	Size (mm)	Phi units
Gravel	Boulder	>256	beyond -8.0
	Cobble	256-64	-8.0 to -6.0
	Pebble	64-4	-6.0 to -2.0
	Fine Gravel	4-2	-2.0 to -1.0
Sand	Very coarse sand	2-1	-1.0 to 0
	Coasrse sand	1-0.5	0 to 1.0
	Medium Sand	0.5 - 0.2	1.0 to 2.0
	Fine Sand	0.25 - 0.125	2.0 to 3.0
	vey fine sand	0.125 - 0.063	3.0 to 4.0
Silt	coarse silt	0.063 - 0.020	4.0 to 5.0
	medium silt	0.020 -0.005	5.0 to 7.0
	fine silt	0.004 -0.002	7.0 to 8.0
Clay	clay	<0.004	beyond 8

Meiofauna species might be expected to be more sensitive to alterations in sediment texture because of thier diminuative size.  McIntyre (1969) reviewed aspects of marine meiobenthos ecology which indicated that certain characteristic fauna occur in particular sand and mud deposits. Wieser (1960) determined that a certain Nematode only resided in muddy depostis, and Warwick and Buchannan (1970) showed that Nematode diversity decreased as particle size, increased or became more saturated with silts.  The proportion of silt and c;ay is of direct importance to many microorganisms and their distribution.  The porosity 9water content) and interstitial space are controleld directly by relative abunadanceof different sized particles.  Driscoll and Brandon (1973) observed that the distribution of Macoma tanta was directly related to the silt:clay ratio.  Also, sediment porosity, or ‘interstitial space’ is critical for small organisms living ewithin sediment.  Webb (1969) and Gray (1974) discussed the numerous types of marine sediments.  Many of the classificiatuions they developed were based on water movement through the sediment, which is dependant on particle shape and size.

Sediment water content (weight loss ofter dessication) or pore volumw (amount of water to achieve saturation) have been used to measure available space within sediments.  Frazer (1935) suggested that in systemically packed spheres:

p-kD2

Where permeability (P) varies directly with the square of the diameter of a spehere (D). 

Thewre are correlations of animal biomass and pre size within marine sediments (Parsons 1990)  Although total biomass of interstitial fauna was the same in all grades of sediment, the estimated volume of animals/voids was higher in samples which contained silts.

Now that we have desribed the sediment, how about animals?  Animals and plants are also classified based on size.  Macrofauna include animals whose shortest dimension is greater than or equal to 0.5 mm.  Meiofauna are less than 0.5 mm, but greater than microfauna, who are less than 0.1 mm in size.

CHEMICAL COMPOSITION

Since benthic organisms affect and are themselves affetced by the chemical com[osition of bottom deposits, it is useful to brielfly consider chemical characteristics of marine sediments as they relates to biological processes within benthic communities. In all, but well flushed sediments, the concentrations of biologically important nutrients (silicates, nitrate, ammonia, phospahte) increase with depth to levels which are high relative to those in overlying water (Parsons, et all 1990)  Morse (1974) has develope a model for the transport process for exchange accross a sediment surface.  Its by simple diffusion.  Difussion accross a stagnant boundry layer (I cm think) may control flus into a layer where turbulent mixing ocurrs.  Waves also cause sediment/boundry layer mixing.  This action does release gases out of sediment in addition to the difussion at work.  Zeitzschel (1980) concluded that in shallow water (reef systems) up to 100% of the nutrient requirements for phytoplankton prioduction can be provided for.Becaue many nutrients are trapped within these sediments, they can be called on to provide for the reef community.  The action of the variosu detrivores within the ‘mud’ layers and thier interaction at the boundry layer will facillitate this mixing.

In an estuarrine habitat- a near shore community where an influx of seawater mixes with an outflow of freshwater, the transport of sedimentary compunds is regulkated by tidal action and estuarine flow.  These estuarine communites, commonly situated withing 300 yards of many carribean reef systems, are all associated with soft bottom, nmuddy bethic substartes.

However, not all muds are created by defintion.  In fact, a bag of top soil, one of the least humus enriched types of soil commonly available at nurseries, is composed of a perhaps quite surprising mixture.  If a bag of topsoil is added to a bucket of water, well over half of the volume will float to the surface as wood and plant debris.  Many of the fine "dirt components" cloud the water and will not easily settle out. Several washings later, the only thing that remains at the bottom of the bucket is sand and fine bits of rock of mixed types and origins. 

Of course, all this assumes that the mud is of some composition relative to its definition.  There is a special "mud" that is formed when certain bacteria decompose coral skeletons into a fine grayish mud-like carbonate based detrital sediment that has an organic content of 12%.  This was described as a "regenerative" sediment by DiSalvo in 1969.  Is this a magic mud?  No, but it is an interesting mud and does lead to the next topic.

When one begins to add mud to a reef aquarium, the addition is composed of various rock types which would not ordinarily be something added to a reef habitat, silicaceous sand (not aragonite, though limestone may be present to some degree), and a large volume of organic material.  Such organic material will continue to be decomposed by bacteria until it is finally totally released into the water column.  Most reef hobbyists spend great amounts of money and effort in order to maintain low organic content in their water, so purposely adding further organics would not seem prudent.  Furthermore, fine sedimentation of the clouded particulates comprises a stress that has many negative effects on marine life, including coral bleaching, fouling of filter feeding apparatus in invertebrates and multiple modalities in fish health.  Heavier organic loads also contribute to the eutrophication of reef communties, where algae outpace and overtake calcifying organisms.  Many of the decomposition products of the humus will also increase the nitrate and phosphate content of the water.  So why would it be advantageous at all?

Mud is Valuable

Most coral reefs are never found with muddy bottoms, though mixed calcareous and soft silted bottoms do occur in lagoons and nearby communties (covered later). Terrestrial based sand from multiple origin rock is rare, as is clay, which is mainly kaolinate.

Chart:

165 m depth, reef shelf edge slope, soft compacted sediment, medium to fine sand

98 m depth, shelf edge, medium to coarse compacted sediments, 

71 m depth, outer shelf, coarse loose sediments, mainly Halimeda

63 m depth, inter-reef, mixed sediment sizes

69 m depth, inter reef, soft loose fine sand

46 m depth, leeward reef talus, well worn coarse sediments

40 m depth, lagoon, near reef, coarse unsorted sand

                  , lagoon, away from reef, medium to fine sediments with                 much macrolife

(Scoffin, et. al.,1985)

Nonetheless, the calcareous and non-humus containing sand bottoms that surround coral reefs have been part of the reason for their success.  Such nitrogen and phosphorous enriched sediments would quickly cause fleshy and microalgaes to overwhelm coral growth (Delgado, 1994, and others). Although the previous section would lead one to believe that mud would certainly not be beneficial to a marine aquarium, this is not the case.   In fact, the microbial (bacterial) fauna present in organically rich muds such as those of estuarine systems can be one of the most productive regions on earth in terms of their decomposition abilities and their primary productivity.  However, mud and other habitat specific sediments cultivate their own flora and fauna, composing a community uniquely adapted to that environment. 

What happens in the sediments?

The sediments that surround and lie adjavent to coral reefs, as mentioned, are not muddy.  However lagoonal sediments are quite high in organic matter.  Mud, as with other carbonate sediments, can play an integral role in denitrification and nutrient processing.  The highest rates of denitrification on reef found in dead coral heads (live rock), Thalassia sea grass beds and lagoon sediments (Seitzinger and D'Elia, 1984) Most aquarists using live sand beds in natural nitrate reduction (NNR), believe that the top aerobic (oxic) layers overlay the anoxic layers where denitrification takes place.  However, denitrification can also take place in oxic areas, and some of the highest rates of denitrification have been found in the top 1 cm of sediments where nitrate and oxygen levels are highest (Oren and Blackburn, 1979).  Nonetheless, anoxia commonly develops in the top 1/2" to 1" (5mm - 10 mm) of reef sediments, though this depth varies according to the grain size and composition of the substrate.  It can occur from the top millimeter down to 10-15 cm or more, such as the sediment areas near the Bermuda shelf. Areas without bioturbation may become anoxic within millimeters of the (carbonate) mud surface of shallow water sediments (Matson, 1985).

Methanogenesis can also occur often within centimeters of the surface of lagoonal sediments(Matson 1985).

The amount of bacterial populations present depend to a large degree on sediment particle size ( Rublee, 1982, etc.)

They are the highest in very fine sand year round and in very coarse sand  sediments during the winter (Johnstone, 1990 and Matson, 1985).  Sediments have been found to be generally oxidized in winter, and reduced in summer since higher temperatures favor higher anearobic activity.  Coarse sand has higher photosynthesis rates of algae within the sediments and in overall respiration of the community (Johnstone, 1990). Even such coarse grained muds. have a rate of anoxic catabolism that equals oxygen reduction .(Matson, 1985)  Bacterial populations in sediments may even be nutrient limited (Hansen, 1987) by phosphorous or nitrate; in other words, they are so effective that they could theoretically process more organic material than that to which they are exposed.

Anoxic decomposition, via reduction, is the most completely regenerative method of disposing of excess nutrients, and could account for the decomposition of all deposited organic matter to the lagoon (Matson, 1985). The energy web of most sediments in and around coral reefs revolves around detritus.

        

detritus and DOM-->bacteria and fungi--->mixed detrital consumers (omnivores/herbivores)--->lower carnivores---> higher carnivores (Ogden, 1988)  -include diagram of nutrient cycling, D'Elia

Biogenic sorting ocurrs as well withing a mudbed.  Burrowing organisms often generate a strong verticla inhomogeniciyty (maybe a too technical word?) in the sedimentert column.  Tyically, a sediment ingesting organisms consume preferentially small particles and transfer them to the surface or boundry layer.  The Atlantic Polychate, Clymenella torquatta, resides in its tude head down.  Particles less than 1 mm are ingested and defacated at the sediment surface.  This allows for a mixing and transport of nutrients accorss layers.

A microbiota adapted to the anoxic zone below the RPD (Redox potential discontinuity) environment can decompose organic material through fermentation, where some organics are used as hydrogen acceptors for the oxidation of other compiunds, yileding end oroiducts, such as fatty acids, or dissolved sulphates, nitrates, carbonates, and water can be used as hydrogen acceptors by different bacteria, yileding compounds liek H2S, NH3, CH4 ,H2.  This is NOT what we are looking for, yet our typical fauna in a live rock system, thrives on these compounds.  The mineralization of organic matter, although dependant on anaerobic oprocess, can be significant.  In an experiemnt using Zostrea detrituds and living plants, over half the oxidation and reduction of organic matter couldbe atributed to sulfate and nitrate reducing bacteria (Jorgenson and Fenchel, 1974)

 

This organic detritus (mostly algae matter and coral mucus) is decomposed primarily by microbial action. Up to 80% of dissolved organic compounds (DOC) pass through and are absorbed by the lagoon communtiy, and most of particluate organic sompounds (POC) settle on the lagoon sediments (Ogden, 1988).  Sandy lagoons also account for more than 70% of the nitrogen fixation in the reef (Shasar, 1994).  A slow downward flux of O2 appears to be at least partly responsible for sedimentary anoxia (Matson, 1985), lending further credence to the use of a plenum in sand beds.  The end products of anoxic deomposition are returned to near the sediment surface where they feed a diverse microflora involved, once again, in primary productivity.

What are the fates of nitrate?  There are many, but among the most prominent are assimilation by algae and bacteria and dissimilation by bacteria. The upper oxic layers of bacteria oxidize organics to CO2 while the anaerobic fermeners and denitrifiers oxidize organics to CO2 and convert nitrate to ammonia and nitrogen gas (N2).

Terrestrial and estuarine muds have higher rates of dissimilatory nitrate reduction back to ammonia and not nitrogen gas, thereby conserving nitrogen in the system for use by photosythesizers within the sediments.  In the reduction of nitrate to nitrogen gas, nitrogen is simply removed from the system by release into the environment, and these products can then be used by sulfate reducers and methanogenic bacteria. There is a low pH in muds, and therefore carnon dioxide (CO2) and organic acids (humic and fulvic) produced by the N2 community may then be shunted to sulfide (SO4) reduction and methanogenesis only if anoxic conditons exist. In fact, these sulfide and methanogenesi goups do exist, with redox levels as low as -450 mV.  In general, redox levels lower than 200 indicates these processes are taking place.   Sulfate reducers occur primarily in enriched lagoon sediments and are also associated with cyanobacterial mats in the reef flats (Kinsey, 1985).  The end product of their decomposition is carbon dioxide which contributes greatly to the CO2 content of the water. This carbon source can be used by algae or corals for calcification and/or respiration (Skyring, 1985). (...good or bad?  good for community, but is it good for closed systems?)

microbes: viruses, bacteria, fungii, actinomycetes, molds, yeasts,

         algae **very important

meiofauna : protozoans, crustaceans, polychaetes, annelids,

Furthermore, there are many specific areas of sediments in and around coral reefs that all support a unique benthic fauna and flora.  In the most simple of terms, these adjacent communities all play a role in the entire macrocosm of coral reefs, and in their nutrient regulation and recycling. 

Adjacent Communties

The description of nutrient flow (flux) over a coral reef is complex and not entirely known.  However, a brief description is necessary.  Basically, upwellings and currents bring plankton rich water across a coral reef.  There, the incredible array of life strips the water of its "food."  Much of the energy from this food is recycled and conserved within the reef habitat though the food chain within the community. Primary production of food by sunlight creating plants and algae which are in turn eaten by progressively higher consumers is not considered here. Bottom sediments and their accompanying flora and fauna are among the most important ways of recycling organic reef material. (Sorokin, 1981) The coral reef and its adjacent communties are very effective in absorbing nutrients and recycling them within the community, preventing loss of such energy sources back to the ocean, and therefore allowing the vast complex web of species to exist (Crossland and Barnes, 1983).  They are largely dependent upon each other. Kinsey states that, "Gross production and calcification in coral reefs are, nevertheless, clearlydominated by benthic processes..."

As waves and currents wash over the reef, waste, mucus, sediment, and particulate organic matter (detritus) is carried across the reef and deposited into near shore communties.  These communities depend to some degree on the organic input of the coral reef community to fuel their own growth and productivity.  To some degree, like the reef, they are self sufficient.  Nonetheless, the flow of nutrients does foster and influence these adjacent communties (Hansen, et.al 1987, Johsnstone, et.al. 1990).  To illustrate their importance, Ogden (1988) states, "Mangrove and seagrass systems are sinks, trapping and accumulating organic and inorganic material and permitting the growth of coral reefs offshore (while) coral reefs buffer the physical influence of the ocean and permit the development...of lagoon and sedimentary environments suitable for mangroves and seagrasses." 

Sea Grass Beds

Sea Grass Beds receive large amounts of detritus from nearby coral reefs and are thus the site of large microbial and microalgal populations.  The seagrasses, commonly known as turtle grass (Thallasia sp.), mangrove grass (syringodium sp.),  and eel grass (Zostera sp.), are not algae, but true grasses (rooted plants gaining nutrients from the sediments (Ogden and Zieman) that grow underwater.  They may be exposed to air during low tides, and play a key role in both contributing to and stablilizing the sediments in which they live.  They are also relatively free of predation.  Reef sediments in sea grass beds are predominantly calcium carbonate debris from (in order) foraminiferans, Halimeda algae, mollusks, and corals. The sea grass sediments are mostly anoxic, and are primarily carbonate reef sand with small amounts of clay and silt.    Bacterial production an populations are the highest near the sea grass roots and are significantly higher than "normal" reef sediments.  Furthermore, bacterial production in the water column is very high in sea grass beds.  Considering that corals and sponges filter bacteria from the water column at up to 95% efficiency (Morairty, 1985, Sorokin, 1978, Reiswig, 1971, Wilkinson, 1978), the loss of this microbial community from the water column could be excessive, especially in closed aquaria with high coral coverage.  Thus, sediments become even more critical.  Sulfate reduction is also at a high level, occurring at its greatest rate in the top 1 cm of  sediment (Skyring, 1985), and is dominant as the final step in decomposition of material. (Moriarty, et. al. 1985) The sediments are finer than those around the reef, and mostly oxidative (Williams, 1985), though Matson found reduction rates to also be greatest in the fine particled sand of Thallasia beds. Therefore, it appears that all types of decomposition, buth oxidative and reductive are high in sea grass beds. 

Though the sea grasses and bacteria may compete for some of the same nutrients, it is the unique sediment and species composition that accounts for the productivity and their ability to manage the surplus effluent of the reef community.  Phosphorus seems to be the limiting nutrient in Thallasia beds (Ogden, 1988), no small benefit for the often excessive phosphate levels in reef aquaria.  Seagrass beds and lagoonal areas with their associated infauna have up to ten times the area of the reef and are (by most references, conservatively) capable of denitrifying and nitrogen fixing all of the accumulated organic material from the reef (Seitzinger and D'Elia 1983).  They are even dependent on organic decay from within the community and from terrestrial runoff, making tham a highly effective "filter" in the wild, and potentially in the aquarium.

Mangroves.

There has recently been an increase in the interest of maintaining mangrove trees as an interesting and functional addition to reef aquariums.  Not only are they quite beautiful, but their roots are quite adept at removing nutrients from the sand and water.  Therefore, the nutrients which can stunt coral growth are used to feed the growth of the mangrove instead.  Mangroves are unique habitats where many fish come to spawn in the protected waters.  Unique flora and fauna abound in these rich habitats, including many species of gastropods and mollusks.  Within the sediments of a mangrove, algal mass is low, because the mangrove forest shades the soft bottom and prevents sunlight from reaching their chloroplasts.  Terrestrial runoff and fallen branches and leaves provide a rich organic sediment that is the cause of very high bacterial productivity, and they can compose over 90% of the biomass (Alongi, 1988). These bacteria act as a sink for nutrients, and can thus be very important in aquarium nutrient control.  Mangroves, except for the occasional tidal inputs, are surprisingly self-sufficient, and do not appear to be significant in terms of export of coral reef nutrients

(Ogden, 1988).  Still, given an environment free from terrestrial, supplemental, organic inputs, mangroves would certainly be capable of utilizing and exporting reef material. 

An Effective Sediment

From the preceeding information, it should be obvious that an effective sediment in terms of decomposing and denitrifying abilities is one which is high in organic material to support copious microbial populations.  However, such rich benthos also support communties of meiofaunal and flora, and macrofauna and flora.  Other organisms, like the seagreasses, mangrove trees and macroalgaes will not be the only competition for the desirable by products of bacterial metabolism.  Other infauna occurs as well.  Primary deposit feeding macroinfauna of lagoonal systems include the sea cucumbers (Holothuria), gastropods (Tellina, Rhinoclavis),  mollusks, echinoderms, and certain fish such as the tommyfish (Limnichthys) and gobies (Amblyeleotris) (Ogden, 1988).  One particular animal which has been found repeatedly to dramatically influnce the productivity of lagoonal sediments are the thalasinid shrimp (Callianassa).  These shrimp, which burow into the sand and create small mounds of substrate around their burrows, are both prolific and efficient.  Thallasinids are very effective "substrate sifters," and they significantly reduce the micro and mieofuanla populations.  "(Callianassa) play a major role in the restructuring and functioning of lower trophic groups in lagoonal sediments." (Hansen, et. al. 1987, Johnstone, 1990).

The meiofaunal consumers such as protozoans, ciliates, nematodes, copepods, turbellarias, polychaetes also scavenge the sediments for detritus, algal remains, and may even forage on bacteria directly.

Many macroalgaes may be present that vie for the rcih organic content of lagoonal sediments.  The most competitive are member sof the genera Microdictyon and Caulerpa.  Caulerpa may significantly uptake ammonia produced from microbial action via their rhizoids  (Williams , 1985). 

In general, bioturbation and competition negatively affects microbial populations.  therefore, the overall effectiveness of a sediment area is reduced over what would be present throught he actions of microbes alone.  It is interesting that many proponenets of "live sand beds" still recommend the use of "substrate sifting" organisms such as sea cucumbers, sleeper gobies (Valencienna sp.) and other burrowing animals.  Such bioturbation does mix the upper layers of the sand and, in effect, clean it of excess organic matter.  However, it also removes substrate for microbes, changes the oxygen composition of the sand, and therefore alters resident bacterial populations.  The normal populations of meiofauna, coupled with perhaps a few lightly bioturbasive animals should be all that is required for a well functioning substrate.  Keeping the sand "clean" as has been assumed in the past, should not be a priority. 

Reasons to Use Mud?

The recently publicized "Ecosytstem" method has been received with great and great skepticism.  In fact, the principles behind it are not as "novel" as they may seem. From descriptions in the trade, the Caulerpa present in this method would  uptake ammonia from sedimentary breakdown and be theoretically used in nutrient export, provided it is harvested.  Certainly the use of algae for effective filtration has been used (and with more effective species than Caulerpa) for many years successfully.  Algae turf scrubbers, despite certain negative reviews in the popular literature, are highly effective filtration devices capable, in our expereince, of sustaining all manner of coral reef aquaria. Caulerpa aside, what are the reasons to use mud?

Sediments high in organic matter are capable of a greater diversity and level of microbial growth.  Fine silty particles also increase the amount of sulfide reduction within their depths.  Anoxia is, arguably, the most important condition of effective decomposition and denitrification.  As will be discussed in the next section, conditions favorable to sulfide reduction are not necessarily deleterious to the auqatic environement.  The production of hydrogen sulfide (H2S) has appeared many times in the popular literature to be a dangerous and unwanted consequence of those using "live sand" beds.  The production of hydrogen sulfide is not, in fact, likely to be a great risk, and the end products of sulfide reduction, carbond dioxide and organic acids, will be used by other animals and algae both within adn exterior to the sediments, servincg to increase biodiversity and stability of a system.  The organic acids (humic and others) have also been described in a negative light in the literature as being harmful through their light absorptive qualities, etc.  Excess humic acids do not seem to occur to any great degree in long term established sand systems.  The periodic use of activated carbon would remove accumulated organic acids from the water column, should they occur.

A new sense of sediements may be initiating at this time.  However, one caveat exists in the use of any organically enriched "mud."  The organic and mineral material present, which supports the microbial biota, should theoretically, in time, be exhausted.  It is doubtful that sedimentary deposit of detritus and reef "wastes" would be of a similar composition to sustain the specialized community.  If it were, lagoonal and reefal sediments would resemble estuarine or terrestrial infl coastal communities.  theyd o not.  furthermore, the initial populations of specialzied communtities would not be present merely by adding, for example, a soil to the substrate.  An incoculum of flora and fauna would need to be introduced to the sediments.  therefore, we question whether or not peridodic replacement of some of the original mineral and organic content of any mud would be required.  It seems likely that this woudl be the case.  Perhaps the most important role of "mud" would be in its ability to establish sufficient levels of anoxia, and to support a diverse and possibly more unique population of meiofaunal and meiofloral components.

Experiences with Mud, Adjacent Communites, and Other Sediments

Finding the commercial allure of the complexities of mud, along with their basic neccesity and influence over coral reef growth, somewhat objectionable, we would like to offer our own past and current experiences with different sediments.  We do this in hopes that a more complete uynderstanding occurs, and a basis on which to evaluate the use of calcareous or organic sediments in an aquarium. 

In a recent internet sequence, Dr. Ron Shimek proposed that the use of live sand in aquariums probably fulfills the samd function as the use of more recent arrivals in "the sediment scene."  Indeed, the most active sediments of the lagoonal and adjacent reef communites are, in essence, an almost completely calcareous "live sand" enriched with large amounts of detritus and other organic matter.  They have been shown to be capable of complete recycling and decomposition of organic matter oin the wild, and our own experience with unskimmed "Jaubert" style reef aquariums would indicate similar functioning of a live sand bed in the captive environment.  We have found that methanogenesis and sulfide reduction are occurring within the sand bed.  After dismantling one sand bed, in particular, deeper layers were noticeably warmer (approximately 100 degrees F) than upper layers.  Subsurface stratification of productive algal mats and cyanobacterial layers establish their critical function much the same as they do in nature. The results of our experiences with such sand beds in maintaing water quality can equal or exceed the use of more "traditional" methods employing heavy foam fractionation.  Inland Aquatics in Terre Haute,  currently have systems with sand beds depths in excess of twenty inches without the much vaunted deleterious effects of "deep sand beds."  In fact, the populations and reductive aspects of such depths can make them even more effective.  The use of "remote" sand beds can also be a very effective way to utilize benthic microbial "filtration," since such relatively undisturbed areas will be free of significant bioturbation and competition allowing full development of microbial populations.  In summary of live sand, there is no doubt that the use of "live sand" is a capable and important component of the total captive reef environment.  But, can it get better?  We feel it can.

The recent use of refugiums to provide a culture of zooplankton and food for the aquarium is a wonderful example of how a functional separate communtiy can be established in connection with a main reef display.  Given the nature of the adjacent communties of sea grass beds and mangroves, establishing a separate, but connected microhabitat is of great benefit.  Not only are these sub-communties an interesting and attractive display in their own right, but when coupled with a detrital producing reef in need of nutrient export, they become even more valuable.  While we are incapable of duplicating nature, we feel that the understanding of nutrient flow in nature has provided us with a unique way of natural, non-mechanical nutrient regulation.  If designed so that the flow of water from a reef display enters a seagrasss or mangrove community, the native populations are capable of complete "denitrification" deemed so valuable to reef aquarists.  The removal of foam fractionation devices and other mechanical filtration will further allow for headier populations of planktonic organisms.  Indeed, the seagrass and mangrove communties are natural spawning habitats for many vertbrates and invertebrates.  Over time, it is likely that such areas will be exploited by reef organisms for that purpose, lending a hopefully better opportunity for breeding marine organisms, as well as increasing water column plankton.  We hope to cover the establishment and care of such communties in a future article.

In summary, the use of adjacent communtites and organically rich sediments can become an exciting area for reef aquarists.  The composition of the sediments, whether they are calcareous, silty muds, or combinations, can be used in different functional manners.  The procurement of non-traditional sediments should be weighed carefully.  While finding a non-polluted natural source for estuarine mud might be ideal, the compositon of such sediments may be of a nature where unwanted toxins, chemical compounds and mineral makeups create potentially great problems.  Furthermore, for any sedimentary community to be fully effective, the complment of niche organisms, both indigenous and habitat attrracted, must be present.  To merely add an organic sediment to an established or new system without understanding its nature or function could easily be as harmful to the aquatic environment as it could be beneficial.  However, with proper use, organic rich sediments can be exploited to increase biodiversity and total function of a natural reef aquarium.