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Koi ponds and organic wastes
If we ignore the fate of metabolic ammonia for a moment and
concentrate on the organic matter, it will be easier to understand how
these pollutants can affect the long term health of our koi if we have a
clear idea of what happens to them in the pond. To do this we have to
understand a little about fungi and bacteria, Nature's rubbish
disposers. We tend to think of microorganisms such as fungi and bacteria
as being disease-causing agents but, in truth, relatively few of them
are pathogenic (disease-causing). Indeed, without the continued
decomposing actions of these microorganisms, the planet Earth would by
now be covered with a layer of sewage several miles deep!
Heterotrophs & autotrophs
We can divide microorganisms into two basic types. First are those
that need a supply of ready-made organic carbon to provide them with
energy and the building blocks for other molecules such as amino acids.
Organisms that require organic carbon, which includes humans, are called
heterotrophs. Some microorganisms
and all plants can extract carbon from inorganic carbon dioxide. These
are called autotrophs. This
distinction between the modes of nutrition of various micro-organisms is
important because an environment that encourages one type of
micro-organism -for example, autotrophic nitrifying bacteria - may be
unsuitable for heterotrophs such as Aeromonas bacteria and
vice versa.
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There are two
distinct types of bacteria involved with pond filtration.
heterotrophs - feed on organic
matter such as fish waste and mulm
autotrophs feed on
inorganic compounds such as ammonia & nitrite |
Mineralisation
Solid organic matter, such as fish waste, is broken down in a series
of steps by heterotrophs into progressively simpler compounds. Most of
the initial decomposition is done by fungi, which use enzymes to break
down the larger, complex organic molecules into simpler, soluble
nutrients that the fungi can reabsorb. This process of using enzymes to
break organic matter into smaller molecules is carried out by all
microorganisms, each time producing a different organic compound,
until finally the original matter is converted into basic non-organic
components such as nitrogen, potassium and phosphorous. This whole
process of converting organic materials into non-organic matter is
called mineralisation.
The ultimate fate of these inorganic elements is to be converted back
into organic matter, usually by plants and other autotrophic organisms,
and the whole cycle starts all over again. While the biology of
decomposition may be mildly interesting, the important point is that it
is carried out in many stages, requiring large numbers of different
species of microorganisms that produce a wide range of different organic
compounds in the process.
Ideally there would be little in the way of 'free'
organic compounds
In an ideal situation, the rate of mineralisation would be matched by
the production of organic materials and there would be little in the way
of free organic carbon compounds (or other pollutants) in the
surrounding water. But, unless our filtration and pond husbandry is
designed to remove these organics at the same rate as they are produced,
there will be a small but often significant level of free organics. As a
direct consequence of this mild form of pollution a rising DOC level -
the following problems may be encountered:
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excessive algal growth leading to either green water from
free-floating algae or dreaded blanketweed. These aquatic plants
(or weeds, depending on your point of view) will thrive on the
non-organic products of mineralisation, such as nitrate and
phosphates
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high levels of heterotrophic bacteria, which is as good a time
as any to point out that many of the common pathogenic bacteria
(such as Aeromonas and Pseudomonas) are
actually opportunistic heterotrophs. This means they are usually
present in the pond feeding as decomposer organisms but can, if their numbers are high enough or conditions
are right, turn their attention to the fish. The
classic example is when fish are stressed and the normal defence
systems are weakened. While it would be impossible and undesirable
to eliminate these opportunists entirely, I think that most people
would agree there is no point in encouraging them either!
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high levels of ectoparasites such as flukes and protozoa. These
parasites thrive in waters with a high organic load and, because
some of them feed on bacteria, the presence of high levels of
bacteria will encourage an increase in parasites
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there is some evidence that water with a high organic load can
depress the immune system. However, this may be a result of the
increase in parasite and bacteria levels, or it could be that some
organic compounds produced during mineralisation are stressful to
fish.
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organic matter consumes a lot of oxygen while it is being
oxidised or decomposed which could, under certain conditions, be
detrimental to the well-being of the fish
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high organic loads are also implicated in environmental gill
disease, a serious and relatively common koi health problem
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raised levels of organic compounds can make the water look
mucky, often resulting in foam being produced at water returns, e.g.
filter outlets and waterfalls.
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While high levels of
dissolved organic and inorganic compounds are not directly
dangerous, they do encourage disease and affect overall water
quality |
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