by John Walsby
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.