Page 31 - BKKS National Show Programme 2018 highres_Neat

pond, which can be done every time we feed our koi, will be Measuring dissolved oxygen
sufficient.
Oxygen can be measured using manual kits based on a
Oxygen and ammonia process called the Winkler Principle but these and some
electronic meters can show false readings if chemical
The optimum level of ammonia in a pond is as near zero as treatments are in the water.
is possible but this value sometimes cannot be achieved.
Typical examples are the elevated ammonia levels that Photosynthesis
occur when a new biofilter is started and before it matures
or the raised ammonia levels in spring in an unheated pond Plants in the water exposed to sunlight convert carbon
when the fish begin eating but before the biofilter rematures dioxide to oxygen by photosynthesis (see figure 2). At night,
after its winter dormancy. Temporary housing in tanks the reverse occurs: the plants become oxygen consumers
without proper biological filtration or at koi shows without (figure 3) along with the fish and aerobic bacterial action.
biofilters will also cause raised ammonia levels. Provided Ponds with a heavy plant load can have significant
these ammonia levels are short term and are kept below 0.5 fluctuations in dissolved oxygen content. At dawn, in
mg/L they will not cause long term harm if the pond, tank or ponds without supplementary aeration, the dissolved
show vat is very well aerated. oxygen level will be low because none has been added
Oxygen and nitrite poisoning from photosynthesis for several hours. Due to oxygen
from photosynthesis, dissolved oxygen levels will peak in
(methaemoglobinemia) mid-afternoon.
Oxygen is transported around a koi’s bloodstream by the
haemoglobin in red blood cells as show in figure 1. If the
nitrite level in a pond exceeds 0.2 mg/L, it will enter the
gill and combine with the haemoglobin changing it into Science panel - methaemoglobin
something called methaemoglobin (pronounced met-
haemoglobin) which can no longer carry oxygen. Small quantities of methaemoglobin form
Very low levels of methaemoglobin occur naturally in fish spontaneously in fish blood even when nitrite isn’t
but, if there is excessive nitrite in a pond, it enters the present in their environment and the amount may
bloodstream through the gills and increases the amount even get as high as 10%.
of red blood cells that can no longer carry oxygen. There is an enzyme, called a reductase, that
Methaemoglobin is brown which causes the gills, which are removes the nitrite but the process is slow.
normally ruby red due to the blood flowing through them, to
look brown instead and so the common name for this effect If parameters are being regularly tested and a
is “brown blood disease”. nitrite problem in a pond is spotted early so that the
reductase can reduce it to negligible proportions
If the level of nitrite isn’t high enough to cause suffocation before too much harm is done then, any red blood
but persists for a considerable time it can cause nerve cells containing methaemoglobin could be restored to
damage, liver damage and suppress the immune system functioning as normal.
making the fish likely to succumb to disease or parasite
attack. As with ammonia, the ideal value should be near However, if a high level of nitrite goes unnoticed,
zero and should never exceed 0.2 mg/L. and the amount of haemoglobin that is changed to
methaemoglobin reaches a level of 70% to 80% it is
likely that death due to acute hypoxia will occur.

Figure 1. A simplified block diagram showing how oxygen is transported from the gills to the tissues that need it and how the waste
carbon dioxide from those tissues is transported to the gills and removed.
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