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Organic
Pollution
a common koi pond pollutant
Written by: Frank Prince-Iles
FishDoc
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:
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
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!
- 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
- 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.
- 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
- high organic loads are also implicated in environmental gill disease, a
serious and relatively common koi health problem
- 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 |
Read further in Part 3
Article placed here with permission from the author,
Frank Prince-Iles
FishDoc
http://www.fishdoc.co.uk
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