| Introduction
I plan for this article
to be the first of a series discussing marine reef aquaria
as ecosystems and how we should manage them using an ecosystem
or holistic approach, as opposed to the micromanagement philosophy
that seems inherent in most reef aquarium husbandry. I suppose
this more inclusive approach comes naturally to me; after
all, I am a professional ecologist and, to me, such an approach
simply seems natural. Nonetheless, to the average aquarist
whose familiarity with ecosystem management is fleeting at
best, such a management proposal must seem daunting.
Well, it shouldn't be. Actually, what this
kind of management means is that we let the organisms do most
of the dirty work, whilst we aquarists sit back and enjoy
the fruits of their labors. In other words, once we cut through
the buzzword bingo, this is the easy way to go, and in fact
many aquarists are well on their way to being ecosystem managers.
They just don't know it, yet.
Before I launch into the discussion of
aquaria as ecosystems, it is probably worth considering what
an ecosystem is. In this discussion, it is well worth considering
that the term "ecosystem" or, frankly, "eco-anything"
is severely overused in our hobby as a marketing tool and
by people who have a seriously deficient understanding of
the term. The term "ecosystem" has a standard definition
in ecological science as a community of organisms and its
associated physical environment. First off, most ecosystems
are not natural, in the sense of being a defined natural unit
with precise boundaries. Rather, as noted above, the term
"ecosystem" is a human label put onto an assemblage
of organisms within their environment. However, as it is generally
understood, the boundaries of that environment are well chosen
and reasonable. For example, a lake and all its inhabitants
may be considered to be a self-contained ecosystem with reasonably
precise boundaries. Other ecosystems have less precise boundaries
and, frankly, a coral reef ecosystem is one of the harder
ones to define and circumscribe. For example, I have been
in a research submersible descending on the outside of an
atoll, and it was very hard to decide when we passed out of
a coral reef environment into one that was simply a sea mount.
No matter how the boundaries are drawn,
ecosystems are generally considered to be stable over reasonably
long periods of time. Radical and unpredictable changes in
such systems do occur; for example, the massive fires which
burned much of the Yellowstone Park ecosystem in the late
1980s. However, in most cases the changes are generally predictable
and gradual. Once we know enough about the system, we may
be able to predict how the organisms will interact and how
the system will operate. In effect, if we know enough about
an ecosystem, we may choose to manage that ecosystem in a
conscientious and reasonable manner. This is the rationale
behind the management scheme for ecosystem-sized parks such
as Yellowstone National Park.
One of the truisms in the reef aquarium
hobby is that there are a lot of different approaches to solving
any problem. If I may be so presumptuous as to consider reef
aquarium keeping as a hobby "discipline," such as
horticulture, then the multiplicity of approaches we see in
the reef hobby is indicative of a discipline that is neither
mature nor successful. It highlights the fact that most aquarists,
and most aquarium authors, simply know very little about the
organisms they wish to keep or are writing about. Unfortunately,
as well, they know even less about how to keep them alive
and together for any length of time. Unfortunately, the "multiplicity
of approaches" also means that the organisms suffer while
various techniques of varying efficiency are used to determine
the care that the organisms require.
Consequently, it is worthwhile to ask the
following two questions. The first question is,"Why do
we want to consider yet one more approach to managing aquaria?"
The answer is that the approach of considering aquaria as
miniature captive ecosystems has been used by scientific researchers
for many years and it works - and works well. The second question
is, "Why should we want to consider and manage aquaria
as ecosystems?" The answer is simply that if aquaria
are some form of an ecosystem, then more than 50 years of
research about natural marine reef ecosystems may be used
to assist us in making decisions about our marine reef aquarium
systems. The closer we come to having an analogue of nature
in our systems, the more we may draw on the experiences of
researchers, divers, and natural historians to help explain
what happens in our tanks.
What Is An Ecosystem?
If we want to consider our boxes of salty
water, rock, colored sticks, and assorted funky critters as
ecosystems, we surely should be assured that we know the following:
first, what an ecosystem is, and second, how our aquaria compare
to this construct of nature. The thrust of this article will
be that comparison, and subsequent articles will discuss some
of the finer points of the management.
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Figure
1. The physical components of a coral reef are visible
in this image. There are shallow hard substrate rocky
areas (the dark blue submerged areas) in warm tropical
water. The white areas are unconsolidated or sandy sediments
where the corals do not grow.
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Three components comprise all ecosystems.
First, there is the physical environment. Second, there is
the assemblage of living organisms, often called "the
biota." Finally, and perhaps more difficult to understand,
are the defined and stable pathways for the transfer of materials
and energy through the system. In a very real sense, the Earth's
biosphere may be thought of as a large and complex engine
that uses energy (primarily solar energy, but there are a
few other very minor energy sources) to turn raw materials
into various microbes (mostly bacteria), as well as plants
and animals. When any of these organisms die, their constituent
materials return to some inorganic reservoir and eventually
enter into the cycle again.
The biosphere is comprised of an interconnected
series of cycles of energy use and material transfer. Neither
the Earth's surface nor the availability of solar energy is
homogeneous, resulting in the myriad subdivisions of the biosphere,
ranging in size from minute to truly enormous. These subdivisions
each provide a basis for material and energy transfer in a
particular physical environment, and are termed "ecosystems."
As reef aquarists, the one that concerns us is the CORAL
REEF ECOSYSTEM.
CORAL
REEF ECOSYSTEM
Characteristics:
The Physical Environment
The CORAL REEF
ECOSYSTEM is discontinuous and delineated
by a rather limited set of physical constraints.
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First, coral reefs are found in areas of
hard substrata. Although unconsolidated or "soft
sediments" may be found adjacent to reefs, the reef
substrate itself is comprised primarily of hard rock and,
as far as is known, all reefs initially got started on
other bare rock substrates. |
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Second, the coral/dinoflagellate symbiosis requires
temperatures from about 68°F to about 92°F to
function and this symbiosis is necessary for the development
of modern coral reefs. Although a few reefs are outside
those ranges, the vast majority are found within those
limits. Such environments are basically found between
the Tropics of Cancer and Capricorn, although a few places
outside these latitudes support coral reefs. |
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Third, the physical milieu surrounding coral reefs is
relatively shallow water, generally water less than about
150 feet deep. This water typically has a salinity of
35ppt to 37ppt. |
A glance at a globe will quickly show that
while the total area of the tropics is immense, that portion
covered by shallow water is really very small, and of that
small proportion, the subset with hard substrate is smaller
yet. Nonetheless, if these areas have hard substrates, and
appropriate water and temperature conditions they generally
have coral reefs. Physical environments that fit all of these
characteristics get a great deal of relatively intense and
dependable solar radiation, and it is a characteristic of
modern coral reef organisms that they have evolved some ways
of utilizing this energy source.
The Biota
The biological assemblages of coral reef
environments are really beyond our complete evaluation, for
a couple of amazing reasons. First, these are amongst the
most biologically diverse environments on the planet, and
they are surprisingly unexplored and sampled. In contrast
to biologically rich terrestrial environments such as tropical
rain forests, the coral reefs are effectively terra incognita.
Explorers and naturalists have been carefully and scientifically
examining tropical rain forests for about 200 years. Coral
reefs have been looked at in some detail, really only in the
last 50 years or so. It has been said that science knows next
to nothing about what is found in a tropical rain forest.
If that is so, it is indeed daunting, as scientists know far
less about the biology of coral reefs than they do about tropical
rain forests.
A couple of factors contribute significantly
to our inability to adequately assess coral reefs. First,
the number of organisms per unit surface area or volume of
the reef substrate is staggeringly large. It is far larger
than you may expect. If you read Eric Borneman’s article
on bacterial feeding in reefs, in this issue of Reefkeeping,
you will find that bacteria cover, coat, and surround all
surfaces of a coral reef. In most cases, nobody has the foggiest
idea of what bacterial species are growing there, or in what
numbers, or how their populations change with time. Second,
many of the organisms found in a careful survey of a reef
will be unknown to science. This is particularly true of very
small animals, algae and bacteria, but may also be true for
the larger organisms that are encountered. And… if you
don’t know what it is, you can’t count it. And you
can’t figure out what it is doing there. Of course, you
can always sell it to an aquarist… Third, as with surveys
in any part of the marine realm, the length of time for surveys
is physically severely constrained by the limitations of underwater
field research using SCUBA or submersible vehicles.
These factors mean that scientific or natural
historic evaluations of coral reef ecosystems have been, and
will continue to be for quite some time, superficial, at best,
and ridiculous, at worst. Fortunately, for the purposes of
the reef aquarist, we really don’t need to enumerate
the biota.
So... I am off the hook, somewhat.
One particular characteristic of the coral
reef biota, however, sets it apart from that found in all
other ecosystems. The characteristicat is that coral reefs
are largely dominated by organisms that are neither simple
plants nor animals but instead are stable algal-animal symbioses.
In these symbioses, the animal “host” has living
within its tissues or sometimes even within its cells, algal
or microbial cells. The term “alga” (plural = algae)
is a vernacular term with no real scientific meaning, resulting
in a large, diverse and unrelated variety of organisms getting
lumped together under that name. About the only thing most
algae have in common is that they are photosynthetic organisms
similar in some biochemical regards to plants. One example
of the diversity should be illuminating. The algal symbionts
found in coral reef animals are often, but not always, from
an algal group called the Dinoflagellata, or as they are commonly
called, dinoflagellates. Recent work on the structure of the
dinoflagellate genome indicates that dinoflagellates are unique
among all living organisms and are no more closely related
to other algae than they are to animals or fungi. It has been
seriously proposed that this group of strange organisms be
given their own biological kingdom equal in the taxonomic
hierarchy to the animal or plant kingdoms. When these bizarre
little single-celled algae reside in animals we call them
zooxanthellae, refer to them as algae, and pretend we know
something about them. Right…
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Figure
2. Some zooxanthellae squeezed (with pleasure) out
of an Aiptasia tentacle. The greenish spots are
chloroplasts. These dinoflagellates, unlike most other
members of the group, lack flagella once they take up
residence in their host, and are immobile. These are
only a few micrometers across.
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One other significant characteristic of
this coral reef biota is that it, like the tropical rain forest,
is dominated by organisms that can significantly increase
the structural three-dimensional heterogeneity of the environment.
However, in coral dominated areas the three-dimensionality
of the environment is produced by the symbiotic organisms
called corals, rather than by trees or other plants. The resultant
structurally heterogeneous and topographically complex structures
are called “reefs.” In a few other areas of the
world, different kinds of biogenic reefs are formed, but they
are neither as large nor as structurally complex as coral
reefs. The ability of reef corals to increase the spatial
heterogeneity of their area is a key to the diversity of life
found on them, as this increase provides many more microhabitats
for small organisms to live.
Energy and Material Pathways
The basis for life is the absorption of
solar electromagnetic energy and its subsequent utilization
as chemical energy. When life first evolved on the Earth about
3.8 billion years ago, this was likely an inefficient and
wasteful process, but in the intervening years, it has been
fine-tuned significantly. In a very real sense, any ecosystem
is a body of organisms devoted to squeezing the last tasty
drop of light-juice energy out of any given area on the Earth’s
surface. The basis of life in most of the Earth’s ecosystems
are organisms, such as bacteria, algae or plants, that use
the energy they can collect from sunlight to bind carbon dioxide
and water together to form a basic sugar along with the waste
product of oxygen. This sugar formation is called photosynthesis,
and it is truly the basis of life.
On coral reefs, many of the organisms such
as algae, true plants, and photosynthetic bacteria have photosynthetic
capabilities. Others, including corals, tridacnid clams, some
sponges, and even some snails, act as partial photosynthetic
organisms through the action of their algal symbionts. All
of their photosynthesis results in the same thing; it forms
sugars.
So why is this sugar production so important?
The primary reason for that importance is that each sugar
molecule took a lot of sunlight energy to make, and that solar
energy can be recovered in an organism, by breaking down the
sugar back down to its constituent carbon dioxide and water
molecules. This process provides all living things with a
source of useable energy. The energy used to make sugar by
photosynthesis can be recovered from it by combining it with
oxygen, or burning it. If you take some sugar and burn it
completely, you will get light, heat, carbon dioxide, and
water. However, in the water inside of cells, the sugar may
be wetly burned in a controlled series of reactions, yielding
energy that can be utilized by the cell, as well as carbon
dioxide and water. In such reactions, the production of heat,
or waste energy, is minimized. These reactions, occurring
within cells, are basically the reverse of photosynthesis.
and are called respiration, and the energy liberated during
the process is used for all metabolic functions.
Corals and other animals with zooxanthellae
or other photosynthetic symbionts are really maximizing their
energy gathering capabilities. Most algae (and bacteria) are
"leaky," and a lot of the sugar they produce oozes
out of the algal cells as a dilute syrup. As the algae are
living in and between the cells of their host, this sugar
solution goes directly into the host cells and is utilized
as food. In some cases, the host organism produces signals
that trigger the release of more "syrup" than would
ordinarily leak out. Respiratory energy is used by all organisms
to power their cellular reactions; or put another way, it
is absolutely necessary for life. Some other chemicals are
necessary for growth since sugars cannot form the constituents
of tissues. These other chemicals, primarily proteins and
skeletal materials, must be obtained by other sources such
as feeding or the absorption of materials through the surface
of the organisms. Nevertheless, for these other materials
to be used, the energy derived from sugar is necessary.
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Figure
3. Even though they have zooxanthellae, corals need
to feed on some source of nitrogen to be able to build
tissues. The sources might be, either animals, as apparent
in these eating zooplankton, plants, or bacteria, or
dissolved organic material rich in nitrogen. Obviously,
in these situations, the corals are secondary consumers.
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All of this food enters an ecosystem and
contains useable energy in the form of sugars, and structural
materials in the form of proteins, minerals, and other materials.
Energy and matter are moved through natural ecosystems by
being passed from one ecosystem component to another. For
example, plants form sugars which get passed to animals by
the action of animals eating the plants. When those animals
are eaten, the materials and energy get moved through the
system even more. In most natural ecosystems, typically about
three or four transfers occur before all the useable energy
and matter are either fixed within a group of organisms, or
lost to the system as gas or waste heat.
Natural ecosystems have what are called
trophic (or feeding) levels. The first trophic level produces
the sugars or chemical energy. These organisms are referred
to as "primary producers" and are photosynthetic
organisms such as the zooxanthellae or algae. The next trophic
level is made of those organisms called "the primary
consumers;" these are the first organisms that eat the
product of producers, either by eating the primary producer
itself or its byproducts. Corals are primary consumers when
they consume the byproducts of their zooxanthellae, or in
those rare cases where they directly consume phytoplankton.
The third trophic level is made of those animals that are
"secondary consumers." These are carnivores, and
are organisms that eat only animals or other non-photosynthetic
organisms. Corals are secondary consumers when they eat planktonic
animals. There may be several levels or links in various food
chains comprised of secondary consumers; however the absolute
number of these levels is limited by the efficiencies of feeding.
An examination of a hypothetical food web
from an open ocean environment will show the limitation. Generally,
each organism can assimilate about ten percent of the food
that it eats. So, to produce one pound of tuna, a top carnivore,
takes ten pounds of some small fish such as herring. Those
ten pounds of herring are produced by 100 pounds of herring
food such as large carnivorous zooplankton such as euphausids
or krill. To produce 100 pounds of krill takes 1000 pounds
of small zooplankton such as carnivorous calanoid copepods.
Those 1000 pounds of copepods are the result of the consumption
of 10,000 pounds of herbivorous microzooplankton such as ciliates
or rotifers. Those 10,000 pounds of microzooplankton resulted
from the consumption of 100,000 pounds of unicellular algae
or cyanobacteria, the primary producers in the system. This
food web would have six levels, and would imply that to produce
1 pound of tuna took about 50 tons of algal biomass. It is
easy to see that there really could not be a specific oceanic
predator living only on tuna. Each pound of that predator
would need the production of about 1,000,000 pounds of algae,
and that amount of production would cover so much ocean area
that such a predator could not get enough food to stay alive
and get big enough to eat a tuna. Most oceanic food webs have
actually five or fewer levels.
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Figure
4. This rotifer is a primary consumer living in
the plankton. The green area seen in the gut are algal
cells undergoing digestion.
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If the organisms at each level only assimilate
about ten percent of the food they eat, this means a lot of
unassimilated food is pooped out of the animals of each level.
This unassimilated, but still nutrient-rich, food goes to
power the decomposers. These are the organisms, primarily
bacteria and detritivores, that break down the wastes, debris,
and detritus back to the basic building blocks so that other
organisms can again utilize those materials. In doing this
final breakdown, the decomposers also harvest the last useable
energy out of the materials.
For the ecosystem to remain functional,
all of these levels must be present and functional. Otherwise
material will accumulate at one level or another and either
demand will outstrip supply for food or there will an be accumulation
of partially utilized food resulting in pollution and fouling
of the environment.
The Aquarium Comparison
I think that most aquarists can see that
our aquaria are functionally organized similarly to natural
ecosystems. In fact, the same trophic levels have to be found
in our aquaria for them to remain healthy and, really, the
only way our organisms may behave in a normal manner is if
they are given a reasonable analog of the normal amount and
kind of food. The organisms have no way of knowing they are
not in a normal ecosystem, and so will react as naturally
as they can to situations as they occur.
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Figure
5. The large white object is a foraminiferan, an
amoeba-like protozoan that secretes a calcareous shell
around itself. It is a member of the decomposer/detritrivore
guild, and feeds on bacteria it gathers from between
sand grains by the use of specialized pseudopods, called
rhizopods. This organism was collected from one of my
aquaria.
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To allow the organisms to live as naturally
as possible in our marine reef aquaria means we must provide
an analogue of the producer level. Although the producers
in marine reef aquaria simply cannot produce enough biomass
to feed the system, they have other important functions. These
photosynthetic organisms use dissolved nutrients from the
tank water and convert them into useable food, and in the
process lower these levels toward the levels that are found
in nature. Of course, nature finds a way, and if we don’t
provide the appropriate producers, there will be some in our
tanks, such as microalgae and cyanobacteria, that will take
advantage of the situation and thrive. Additionally, we must
provide at least one, and more likely several, good and functional
consumer levels, to eat excess food and thrive and, in the
process, to pass that food to the decomposer level to recycle
the nutrients, and to liberate all the stored ecological energy.
With all of these levels in place, a coral
reef aquarium is neither more nor less than a small captive
analogue of an ecosystem. It meets all the necessary requirements
of a small ecosystem, and there are numerous natural ecosystems
just as small. Is it natural? It most decidedly is not! Can
it be a functional ecosystem? Absolutely! When this functional
ecosystem is well set up, it is an efficient and easy way
to manage and keep our coral reef animals in the peak of health.
The producer organisms in such an artificial
ecosystem are the various algae, microbes (photosynthetic
protests and bacteria), corals, and zooxanthellate organisms.
The algal production converts soluble nutrients into exportable
materials, such as macroalgal tissue which may be harvested
periodically. The corals’ zooxanthellae produce sugars
and facilitate coral growth. The consumer organisms are the
fish, but also the corals, and other decorative animals such
as feather duster worms, small bugs and scavenging snails.
These animals consume animal foods for the energy and structural
materials present within it. Left-over foods, dead microscopic
or tiny animals, detritus, and animal feces are utilized by
decomposers, primarily microscopic animals, protozoa, fungi,
and bacteria. These organisms remove the last of the usable
energy from foods and cycle the nutrients back to the consumers.
Virtually all of the decomposers are found living in the sediments
or within tiny spaces on the rocks. The most important of
all of the trophic levels for the health of the system is
the decomposer level, a guild of numerous kinds of organisms
that work out of sight in the sediments. Without a functional
decomposer assemblage or some other mode of export, nitrogenous
wastes accumulate, nutrient levels skyrocket, and the system
becomes polluted, will eventually crash, and likely kill or
severely impact all other life in the system.
So, for the best maintenance of the animals
we like to watch (the corals, fishes, and other attractive
organisms), we must work to maintain the health of many animals
we cannot often see. Even if we could see them, they would
probably be considered unattractive. Fortunately, being the
ecosystem manager of a small marine reef aquarium artificial
ecosystem is just a tad bit easier than being the manager
of, say, Yellowstone National Park. It simply requires learning
about and maintaining the animals under normal, natural conditions,
and providing them with sufficient food to keep going. Increasing
species diversity, particularly of the decomposer pathway,
also facilitates stability by allowing the development of
alternative ways for decomposition to occur. Export of excess
materials is facilitated by the use of harvestable algae,
and together with reasonable filtration by foam fractionation
or other means will help keep the system stable and healthy
with a minimum of maintenance. Scientists and researchers
call these small ecosystem analogues "microcosms,"
and they have been used in marine research since the 1960s.
They are a tried and tested type of system that promotes animal
health. A lot of researchers use them for two major reasons:
they keep the animals healthy, and they require a minimum
of effort and maintenance. Of course, you don't get to spend
a lot of money buying a lot of strange additives with miraculous
claims, and generally odd pieces of expensive equipment are
also considered excessive, but hey, you can't have everything.
Conclusion
The concept of the aquarium as an artificial
ecosystem is an easy and simple methodology for maintaining
coral reef animals at the best of health, with a minimum of
effort and expense by following the examples apparent and
inherent in nature. This series will continue, with future
articles concerned with discussing some of various examples
of behavior, animal morphology, or system dynamics that allow
us to maintain our small "slices of life." Over
the next several months, I hope to go into more detail discussing
the various aspects of setting up a functional aquarium ecosystem
"in a box."
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