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Introduction:
Biological filtration, or the conversion
of waste and excess nutrients into some innocuous form by
means of organism metabolism, is one of those concepts that
everybody seems to understand. If there is a problem with
the term, it is that everybody understands it differently.
In its broadest sense, it seems to be a term used to mean
the conversion of biologically produced noxious wastes into
something benign. In a stricter sense, the term is frequently
used to mean, specifically, the conversion of ammonia and
nitrate ions into nitrogen gas. Whatever its usage, it is
a given that aquaria, in general, and reef aquaria, in particular,
must have some sort of efficient biological filtration.
How, where, by what means, and how efficiently
various substrates or methods provide biological filtration
are argued by aquarists with the conviction of zealots arguing
that theirs is "The One True Faith." Unfortunately,
many of these arguments are backed by just as much hard scientific
evidence as are the tenets of many cult religions. As aquarists,
we ALL "believe," and in most cases, what we believe
in is a matter of faith, rather than any sort of scientific
evidence. Arguments, pro and con, are often met by thinly-veiled
cries to burn the heretics. So… always being fond of
a bit of human pyrotechnics, I thought maybe I could sprinkle
a little gasoline on the fire.
One of the tried and true methods of providing
biological filtration is by the use of "live rock."
This method has been suggested by virtually every author,
including myself, who has written about the aquarium hobby
for the last 15 years. The advice to use live rock as a biological
filter has been accepted for so long that it has become dogma.
I think it is always useful to question dogma. It is easy,
and comfortable, to be dogmatic, but progress comes only with
periodic and critical re-evaluation of dearly-held ideas and
practices. With this thought in mind, I believe it is time
to critically re-examine the uncriticized use of live rock
in reef aquarium systems.
Some Background:
My first introduction to the concept of
"live rock" occurred in the 1980s when I initially
considered setting up a coral reef aquarium. At that time
I lived in a large city and was only vaguely aware of the
concept of ordering livestock by mail. The internet was in
its infancy and online vendors were non-existent. Consequently,
like everybody else, I got my live rock from my local aquarium
store. Each piece was critically and lovingly examined for
its animals and algae. There was even a competition of sorts
in my aquarium club to see who could get the best rock for
their aquarium. What constituted "good" were the
color and variety of algae and animals on the rock.
Most of this rock originated in Floridian
waters, although some seemed to trickle in from other exotic
places. This rock was lush with life and often obviously riddled
with holes or pores. Consequently, when somebody started discussing
how the rock acted as a biological filter, it was pretty easy
to accept. The porous nature of the live rock's interior was
a ready-made substrate for denitrification bacteria. It seemed
like a logical idea at the time, and we all pretty much accepted
it. And we accepted it without very much thought. I don't
recall anybody asking the really critical question, "How,
exactly, do bacteria growing inside of rocks 'filter' aquarium
water?" Or, even better, "How, exactly, does water
get into and out of live rock?" If anything was said
at all, it was something along the lines of, "Bacteria
growing on the surfaces of the interior of the rock did the
denitrification, and the water flow through the rock was slow,
so that the appropriate conditions of reduced oxygen were
met to facilitate the whole process." That almost sounds
like it makes sense, but does it? I would like to examine
the assumptions underlying that statement, one by one.
These assumptions are:
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Live rock is porous.
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The interior of the rock contains denitrifying
bacteria.
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Water moves through the rock, at just
the right velocity to facilitate denitrification.
Assumption 1: The Porosity Of Live Rock
One thing that should be evident is that
for live rock to function as a filter medium it must be porous.
Additionally, that porosity has to be sufficiently small so
that the rock contains many pores and cavities, for only if
there is a lot of pore space is there going to be sufficient
filter area to accommodate the bacterial populations that
produce the actual chemical changes. Finally, the interior
of the rock has to provide the appropriate physical environment,
primarily slow water flow and low oxygen concentrations, to
facilitate the appropriate bacterial processes.
Live rock is composed of numerous materials
having differing characteristics with regard to porosity.
However, many studies have documented the porous nature of
coral reef rock. Our so-called "live rock" is generally
coral reef rubble collected and sold to aquarists. This rubble
is comprised primarily of coral skeletons, or a mixture of
coral skeletons cemented together by calcareous algae. Upon
examination of the material of a coral reef it becomes apparent
that corals are only one component of the life on such a reef,
and although they appear large and evident, their contribution
to the actual amount of living material on the reef is relatively
small. In their pioneering study of the reef at Enewetak atoll,
the Odums in 1955 showed that the majority of non-bacterial
biomass on a "coral" reef was actually in the form
of algae. They found that various types of algae were everywhere.
There were algae growing in the coral tissues, of course,
as zooxanthellae, but additionally there were algae growing
freely and widely across the coral reef. The algae on the
surface of the reef were diverse in form, and belonged to
many groups, from large green algae such as Codium,
to coralline red algae, to coralline green algae, to diatoms
and dinoflagellates. In point of fact, they found enough algae
on the reef to consider it far more reasonable to call such
reefs algal reefs instead of coral reefs. I wonder how many
aquarists would be hobbyists today if these biogenic structures
were named after their most abundant life forms and called
"algal reefs" rather than coral reefs.
In addition to the algae growing visibly
on the surface of the rocks, the Odums were surprised to find
that algae were growing INSIDE of all the substrates on a
reef. Algae, primarily filamentous green algae, lived inside
of coral heads, inside of dead coral skeleton, and inside
of all coral rock and rubble. In fact, on an old coral atoll
such as Enewetak where all evidence of the volcano that gave
the reef its start has vanished with subsidence (the volcanic
basis for the reef at Enewetak is found under some 5,000 feet
(1515 m) of coral reef deposited over several million of years
coral growth), virtually all of the rocks are riddled with
algae and contain a lot of algal growth and biomass.
The Odums found that in the average coral
head, in the region of the polyps, the density of the algal
component was about 0.004 grams/cm3
and the animal component was about 0.021 g/cm3,
while among the bases of the polyps the filamentous algae
had a density of about 0.022 g/cm3.
Below the polyp zone of the coral head the algae had a density
of 0.037 g/cm3. In other
words, in a coral head with living coral tissue on it, the
animal component accounted for about one-fourth of the total,
0.021 g/cm3, while the various
algal components amounted for 0.063 g/cm3.
Interestingly, as well, the filamentous algal component of
a coral head had a much greater biomass (about 16 times greater)
than did the zooxanthellae in the coral.
The algae in the coral heads do not die
when the coral animal does, and the amount of the coral algae
in various rock components of the reef is shown in Figure
1. This figure, modified from the Odums' 1955 paper, shows
the relative biomass of several rocky areas on the reef. I
have colored the algal biomass amounts green, and the biomass
inside rock that could be collected as live rock in yellow.
The amount of algae living inside the various components of
coral rubble and rock is quite significant, and those algae
are quite important to our discussion of live rock porosity.
If corals are grown in environments free of the algae that
colonize their skeletons, those skeletons are typically quite
porous. However, the algae growing within the rock add to
the porosity by dissolving fine holes for their filaments.
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Figure 1. Dried biomass from several different
coral reef sites on Enewetak.
Modified from Odum and Odum, 1955.
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Assumption 2: The Interior Of The Rock Contains
Denitrifying Bacteria
This is probably the easiest of the assumptions
to validate. Most authorities (see Capone, et al., 1992) consider
that such bacteria are ubiquitous. They are likely found in
virtually all habitats at least in small numbers, but thrive
in almost all areas where the conditions are to their liking.
The inside of the live rock would be a good place for them,
and it appears that they are probably there (Risk and Muller,
1983). It would seem that the assumption that live rock contains
the appropriate denitrifying bacteria is therefore valid.
Assumption 3: Water Moves Through The Rock At
Just The Right Velocity To Facilitate Denitrification
This assumption is probably the hardest
to evaluate. The major question we have to ask ourselves is,
"How does water move through the rock?" One occasionally
hears the statement from aquarists that water "diffuses"
through live rock, just as one occasionally hears the statement
that water "diffuses" through sand. Neither statement
is correct. Water doesn't diffuse through either of these
substrates. Materials dissolved in water may diffuse from
regions of higher concentration to regions of lower concentration,
but the water itself doesn't move in these situations. The
only time that water moves through diffusion is in the special
case of diffusion called osmosis, and in that situation a
membrane has to separate the two regions of differing concentrations
of solutes in the water. Such conditions are not met in live
rock, and there is no net movement of water into or out of
it by diffusion or osmosis.
Interestingly enough, it is possible that
dissolved materials such as the various chemicals constituting
the denitrification cycle do diffuse into and out of live
rock; however, it is unlikely that such diffusion moves any
significant amount of materials. Calculating diffusion rates
into and out of the volume of a rock is complicated and includes
such variables as the water flow over the rock, the dynamic
viscosity of water, the rock's size, and the diffusion coefficient
of the material in question through water. When all of the
various parameters are factored in, for the various gases
or ions in question, the passive diffusion rates are probably
on the order of 1 x 10-4
m2/sec. Assuming a constant
concentration gradient, and a uniform porosity such a rate
means that gases would diffuse in through the volume of the
rock at the rate of roughly a micrometer per second, so that
in an hour the gases would diffuse about 3.6 mm, or about
an eighth of an inch. If a volume is 10 cm in diameter, gases
would diffuse to its center from its outside edge in about
14 hours. However, the gases would move considerably more
slowly through the live rock than they would through an empty
volume. The small diameter of the passages or pores in the
rock would restrict the flow significantly. It is unlikely
that the flow rate would approach the estimated value, and
even if it did the relative volume of the gases exchanged
would be minimal.
For a significant amount of gas exchange
to occur there has to be continual movement of the water into
and out of the rock. Given the minuscule pore sizes in these
rocks, the water movement cannot be generated by water currents
outside the rock. The resistance to movement of water in small
tubes, such as the pores in live rock, is considerable. The
only motive force sufficient to move enough water through
the rock, so that it may act as an efficient denitrating site,
is the force generated by the animals, mostly the worms, living
in their burrows. These worms move back and forth in their
burrows and in doing so they move the water in the burrows
in a pulsating fashion. Many of the burrows and pores are
interconnected, either intentionally or by happenstance, and
these interconnections result in water movement into and out
of the rock. Additionally, many of the worms and other animals
in the rock pump water over themselves in their burrows. They
do this to facilitate gas exchange over their gills, but the
net result is a significant, constant, and moderate current
through the rock. Such a current, coupled with oxygen utilization
of animals in the rock, could result in the interior of the
rock becoming the efficient denitrification site that it has
been thought to be.
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Figure 2. Individuals of this species of small
worm, possibly a species of Polydora, about 1/25th
of an inch in diameter, live in calcareous substrates
where they chemically excavate burrows. Motion of such
small worms up and down in their tubes helps to pump
water slowly, but regularly, through "live"
rock.
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Problem Rock:
There is only one problem related to the
use of live rock as an effective source of biological filtration.
For the rock to be the site of efficient biological filtration,
water has to be passed slowly and steadily through the rock.
The most likely way that will happen is by the activities
of the myriad of animals that live in the rock. Of course,
for this to happen there must be animals living in the rock,
and lots of them. Therein lies the problem with using live
rock as a biological filter. Live rock comes from many sources
in today's hobby, and the products that these vendors provide
are by no means uniform in their capability to provide biological
filtration.
The live rock may be collected and shipped
"as is," or it may be "treated" or "cured"
in different ways to remove various components of, primarily,
the animal fauna living on and in the rock. Some collectors
and vendors go to great lengths to ensure that their rock
is free of as much of the material as possible that can potentially
rot and foul a system. These vendors provide rock that often
is covered with a large amount of coralline algae, and very
little else. This rock is free of much of the material that
can die in transit and rot in the destination tank. It is
also free of most animal life. This rock can provide a beautiful
backdrop or substrate in a tank, but, unfortunately, it simply
can't provide much in the way of biological filtration. The
small animals that moved water through the rock are not only
dead and gone, but there is likely no fauna available to colonize
the live rock and replace them in the destination tank. This
rock is full of dead space and algae. Once in the destination
tank, such rock will become populated with algae, much as
it was in nature. However, there will be no water pumped through
the small channels and pores in the rock, and such pores will
begin to fill in, primarily by the growth of algae. Such rock
has quite a potential for the internal buildup of noxious
compounds. If a significant amount of algal and worm biomass
was killed by the collection and curing process, this material
will mostly remain in the rock, where it will rot. Instead
of functioning as a biological filter, such rock would contribute
to the system's organic load as these rotting materials slowly
diffuse out of the rock over a period of several months.
Growth of coralline algae over the rock's
surface will close off most of the small channels and passageways,
largely contributing to the decline of the rock's biological
filtration capacity. Hobbyists may further contribute to this
degradation of filtering capacity by gluing coral fragments
to the rock's surface, thus sealing off more of the pores.
This may make the inside of the rock anoxic and, if there
is much organic material inside the rock, it will start to
rot. If the interior of the rock is subsequently exposed to
the tank environment, such material could be deleterious.
Some of the larger openings into the rock
will remain. In some cases, these will become occupied by
larger worms. The movement of these larger worms can help
facilitate biological filtration in the rock, but they cannot
maintain it anywhere nearly as efficiently as could the large
populations of small worms that had been previously living
in the rock.
Good Rock:
One of the characteristics one should look
for in live rock that would contribute to its biological filtration
capacity in a tank would be a good and diverse growth of animals
on its surface. If these are present, it is likely that the
necessary smaller animals that live in the rock are present.
Rock that is naturally porous and relatively light weight
for its size would likely have more highly perforated internal
regions and would function better in this regard. Aquacultured
rock with a good growth of animals on its surface should be
as good as natural rock when it comes to biological filtration.
A good growth of animals on the surface implies a good recruitment
of smaller burrowing forms in the rock.
It is possible that some of the so-called
live rock available for the aquarium hobby can provide significant
biological filtration; however, that rock has to be carefully
chosen for its array of animal life present. Rock without
animals in it will not be effective at being a filtration
medium as there is no way for the interior porosity and presumptive
bacterial beds to be functional without a way of moving water
through the rock, and the only way that movement may be accomplished
is by animal action.
A Request for Data:
Over the next several months, Eric Borneman
and I will be examining several types of live rock, and we
will determine just how much life is found living within these
rocks and how effectively they could act as biological filters.
These results will be published in Reefkeeping Magazine.
I would like ask that if any aquarists have 1) test kits for
sulfide or 2) dissolved oxygen, and a syringe with a long
narrow hypodermic needle, that they attempt to make measurements
of these two variables from the centers of large pieces of
live rock. The hypodermic needle would need to be placed deeply,
and carefully, in the rock through a small opening or pore.
The water would have to be carefully and very slowly withdrawn
from the rock and then tested. Such data should be sent to
me for incorporation into the master data. At the present
time, the data about the internal environment of the rock
are ambiguous; but they are also very sparse, and more data
are needed.
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