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.
