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11 February 2008

Ewens Ponds


ROBYN WILLIAMS: The ponds are like an antechamber. Underneath them are vast limestone caverns, whose water comes bubbling up through the sand-like bottom, like so many tiny volcanoes.

The tiny congolli fish lies in wait amongst this turbulence, preying on anything that might come its way.

The sand-like material is very fine limestone in the final stages of dissolution. By the next stage it will have disappeared altogether, absorbed into the clarity of the water that is the marvel of this place. Because it's so clear and sunlight penetrates so well, plants flourish here that normally need the open air. The effect is a submerged garden with flowers and grasses, even insects. Instead of birds, there are fish to provide the flashes of colour. These are galaxids, the native trout. And looking like some prehistoric grass-cutter, the spiny crayfish. It plays host to hundreds of tiny animals - the temnocephalid, a type of flatworm. They're not parasites - they don't live off the crayfish, but on it. They feed on the minute quantities of food left by the crayfish.

The outlet leads into a narrow shallow channel, or race, into the next pond, and the current takes the diver for a joy-ride. Enough water pours through here every day to supply Mount Gambier for three weeks, yet it all finishes up in the sea. The spacious grandeur of the ponds contrasts with the tiny delicacy of the creatures that live in them, like the hydroid. It looks like a flower, but it's an animal - a carnivorous one, at that. These are the larvae of the caddis fly, which is not really a fly but a hairy-winged moth. They carry around their larval case. Some are simply a piece of hollow reed, chewed off to length. Others are more elaborate structures of reed and bits of stone cemented together. The freshwater shrimp is one of the most common animals in the ponds. Gently peeling back a casing attached to the stem of a reed reveals a caterpillar. It will eventually become a moth and fly away to a brief life above the water. It gets its oxygen through tubes extending from inside its body - something like us putting our veins out through our skin and taking the oxygen direct from the air.

The water is rich in oxygen already when it enters the ponds and further enriched through photosynthesis, possible because the clarity of the water allows plenty of light to reach the submerged green plants. The ponds make visible one of nature's fundamental life-giving processes - plants using the sun's energy to transform carbon dioxide into food and oxygen. The bottom of the pond is busy with tiny scavenging mites. They're the garbage processors of the system, feeding on dead animals like this caddis fly larva, and helping keep the water clean in the process.