by John Walsby
The animals
that live in ponds, lakes and rivers are relatives of marine animals
- but they have to cope with quite different problems. Freshwater
is a difficult medium to live in. In the process we call osmosis
water always passes across biological membranes from the dilute
side to the more concentrated side until the salt concentrations
on both sides are equal. To prevent their bodies swelling up,
and to keep their body fluids at the right strength, freshwater
animals must constantly pump water out.
All seashores can be
reached by the continuous expanses of sea that surround and connect
land masses. But lakes, streams and rivers are isolated bodies
of water; moving from one stream or lake to another is very difficult.
Except near their openings to the sea, where there are tidal flows,
rivers and streams flow in only one direction - downhill. If mobile
animals are not to be washed out to sea, they must swim strongly
against the current. Sessile animals are not troubled by moving
water while they are firmly attached but their larvae must have
ways of moving upstream or the species will eventually be washed
out of the river system.
Lake and river water
levels vary with rainfall. In summer some may dry out but during
winter storms those same spots may become raging torrents, bursting
their banks, scouring out their beds and tumbling along logs and
boulders that crush creatures that they crash against. Summer
and winter conditions can therefore threaten the survival of both
mobile and attached freshwater animals.
The term "freshwater"
makes us think of crystal clear water, cool and refreshing to
drink; but not all freshwater is like this. The term really means
that the water is not salty, but though inland waters are low
in common salt, they may contain other chemicals, fertilisers
and pesticides washed from farmland.
If the water catchment
area has been cleared of the bush that otherwise delays the runoff
of heavy rain, then the topsoil will frequently be eroded and
the streams will become brown with a heavy sediment load.
Various other pollutants
also end up in our streams and rivers, some accidentally and others
poured in deliberately or through negligence. These pollutants
include cowshed wastes, factory wastes and spillages and effluent
flows from municipal sewerage works. Incredibly there are still
instances of virtually raw human sewerage being allowed to flow
into our fresh waterways, preventing us even from swimming there
let alone taking food from the waters.
New Zealand's lakes
and rivers are respected worldwide for their scenic beauty, clear
water and magnificent fishing. The large brown and rainbow trout
and the many different species of freshwater animals that make
up their food, all depend on a healthy water system.
How healthy is your
local pond or stream ? Have a look into it - without taking any
foolish risks - and see what animals you can find.
