GOING FISHING with Kotare

Press, Volume CXV, Issue 33816, 12 April 1975, Page 12

GOING FISHING with Kotare

One of the consequences of the completion of the Tongariro power scheme sends shudders down my spine.

I hope to make your spine shudder, too. If it supports a human bein'* concernred about Lake Taupo, fishing quality, weed production, preserving the environmental status quo, and handing these things on with a clear conscience to future generations, your spine will certainly shudder. The apathy brought about by the apparent immobility of geological time is an insidious disease. Nothing changes much in a generation, or even a lifetime. She 11 be right. Let things take their natural course. But the Tongariro power scheme is hardly a natural course. After Rangipo comes on stream within five years, the volume of waters flowing unnaturally into Lake Taupo will have increased water input by 26 per cent. Remember, 26 per cent. Mr C. S. Woods has been warmly congratulated over the past 10 years on his monumental report, Fisheries Aspects of the Tongariro Power Development Project, compiled for the Government in an attempt to survey and anticipate power-scheme consequences for this multi-million dollar asset of ours. Water quality Perhaps it was not surprising that water quality, an environmental consideration that in 1964 already worried men sick overseas, should not merit a mention. Wasn’t water quality in New Zealand, like Caesar’s wife, above suspicion? Only 10 years later, scientists slowly assembling water quality data

about New Zealand's priceless North Island lake and its catchment, have discovered some hair-raising facts — facts which, for instance, would seem to account directly for the current prolific weed growth in the tailrace area at Tokaanu.

That growth is just a symptom of a disease which may well cripple the whole of Lake Taupo quite soon, geologically speaking. Balanced growth Last year — and only as recently as that — scientists established for the first time that Lake Taupo is unique in the world because the chemicals suspended in its waters, include an astonishingly high proportion of phosphorus and a correspondingly low proportion of nitrogen, tne two nutrients largely responsible for all plant growth.

All other lakes from which water has been taken for analysis bear out the thesis that aquatic plant life of all sizes needs, for balanced growth, a water-supplied ratio of 15 parts of nitrogen to one part of phosphorus. Scientists at Lake Taupo estimate that 657,000 kg of nitrogen, and 116,000 kg of phosphorus, enter the lake each year; a ratio of 5.6 to 1.

In theory, to maintain balanced growth, algae and larger forms of plant life in the lake require some 43,500 kg of the available phosphorus to complement the 657,000 kg of nitrogen available to them. Some 720,000 kg of phosphorus is therefore not being converted into plant growth. It apparently sinks to the bed of the lake and enters the sediments lying there. Now, the implications of a nutrient imbalance

favouring sudden accelerated plant growth are well known the world over.

It Taupo suddenly received a much greater input of nitrogen, the potential weed growth would reach alarming proportions.

When the Tongariro power scheme is complete, scientsts expect an increase of 4 per cent more nitrogen — provided that all other w r aters newly flowing in to the lake yield a nitrogen content similar to that of the Tongariro rive itself. But that’s nothing. The crunch is yet to come.

Just suppose that a significant growth-accel-erating mineral, formerly present in minute quantity, were added in greater quantity. Just suppose that when combined with nitrogen and phosphorus, in much greater amount than before, it generated plant growth explosions previously undreamed of. You can cut out supposition. The inescapable fact is that such a mineral is now arriving in quantity suspended in waters fkowing into Lake Taupo via the Tokaanu powerhouse. More manganese This mineral is manganese. Scientists consider that it has always been present in Lake Roto Aira, a lake much closer to moderate eutrophication, or enrichment, than Lake Taupo (which is still marginally oligotrophic, though approaching the intermediate classification of enrichment known as mesotrophic).

True, the Poutu River has always, drained Lake Roto Aira, and this thus carried a certain amount of manganese into Lake Taupo via the Tongariro. After Rangipo is commissioned, Lake Taupo will be receiving 26 per

cent more water, remember, than it received before the power scheme was begun, and all via Lake Roto Aira. Possibly the manganese content of this sizeable increment of water will substantially increase the amount of available manganese present in Lake Taupo. Scientists don’t yet know the exact figure. What they do know, from trials conducted over four days just a few weeks ago is that the addition of manganese to a phosphate and nitrate supplement fed to algae predominant at that time resulted in a growth rate 1300 per cent greater than that of the control sample. Trial made A few days ago, scientists conducting a similar trial, but with the type of algae then predominant, recorded a growth increase of 360 per cent compared with the control. Are you receiving me? Already, only a matter of months after the new turbines at Tokaanu began to generate power, weed proliferates alarm-, ingly in the vicintity of the tailrace. The turbines themselves could hardly constitute a more efficient Roto Aira-weed mincing and distribution centre for the whole of Lake Taupo.

Before the power scheme, hydrologists calculated that the “average drop” of water at Turangi took 13 years to travel the length of the lake. After Rangipo, it will take 10| years.

One scientist believes that New Zealand is only that number of years behind the advanced stage of water-pollution evident in the United States.

Try driving the length of the lake. It takes only 45 minutes to cover 10? years. It’s much, much later than we think.