Poisonous chlorine compounds
by John Walsby
Since the Second World War there have been three common sources
of chlorine based poisons that have polluted New Zealand streams
and rivers. These are pesticides like DDT and Dieldrin, bleaches
Paper making mills like those at Kawerau on the Tarawera River
continue to bleach paper with chlorine and some runs out with
the waste into waterways. Today, there is greater control of waste
discharges, a gradual change to non-chlorine based bleaches and
a growing demand from the public for unbleached paper.
As a disinfectant, simple chlorine based chemicals have been routinely
used for both domestic and industrial purposes. Most are correctly
discharged into drains that lead to water treatment stations but
some are run to waste into storm water drains and end up in streams
and rivers. The heavily chlorinated flushings and overflows from
swimming pools are also a source of toxic discharge in some areas.
If commercial chemicals supplied to keep swimming pools clean
are strong enough to stop any life growing in the water, they
will also poison both microscopic and larger organisms in nearby
creeks if they are drained into them. Some people fail to take
this into account when draining or cleaning pools.
When insecticides are poured on to farm animals and sprayed on
pasture, horticultural blocks and home gardens, growers aim for
a good kill of troublesome insects and to often use excessive
amounts of spray. If the insecticides are effective on the land,
they often continue to be a hazard to insect life elsewhere when
washed off by rainfall and and any animals that eat those dead
insects can also suffer.
Toxic residues of old organo-chlorine insecticides are still present
in many areas but fortunately most modern insecticides breakdown
quite quickly in the soil. However, they must not be used near
In lakes, food networks start with algal production and continue
through algal consumption by small filterers and surface scrapers.
These consumers fall prey to larger animals that include fish.
The natural life of streams varies from that in lakes and ponds
in that most of the organic matter in them is washed in from the
surrounding land after rainfall. The living organisms in streams
are therefore highly sensitive to nearby land use.
The organic matter naturally consists of fallen leaves, branches,
flowers, pollen, and fruit, along with the faeces of both large
and small animals and the bodies of those animals after they die.
The dead animals naturally include millions of insects many of
which have only short lives as flying adults.
Most dead insects are buoyant and easily washed down into streams
where, floating on the water surface as "insect drift",
they are a regular and important part of the natural diet of many
fish. If the dead insects flushed off the land are crop pests
that have been killed by pesticides then they will pass on the
poisons that killed them to fish in the next step of the food
Organo-chlorine compounds tend to accumulate in the fatty tissues
of fish and as these are concentrated around fish ovaries they
are thought to be partly responsible for the reduced fertility
of some species.
Another important source of food for river fish are the creatures
that cling to the bottom or hide in crevices to avoid being swept
down stream. Some of these filter fine particles from the water
and some scrape off algal films growing on river stones. Others
process the large amounts of plant and animal debris that falls
or is washed into waterways and sinks to the bottom as it becomes
Many of these creatures are the aquatic larvae of insects such
as mayflies, stoneflies caddisflies, lacewings, craneflies, midges
and certain beetles, in all of which the larval stage of the life
cycle is by far the longest with adults living just long enough
to mate and lay eggs; frequently only a few hours to a day.
When populations of these larval insects are knocked back by insecticides
and chlorine based pollutants in the water, both native and introduced
fish are deprived of food and the whole ecology of the waterway
can be upset.