WEED POLLUTION THREAT TO S.I. LAKES AND RIVERS

Press, Volume CXI, Issue 32637, 19 June 1971, Page 9

WEED POLLUTION THREAT TO S.I. LAKES AND RIVERS

(By a staff writer)

Propellers of power-boats fouled with weeds from North Island lakes and rivers and home fish tanks are an immediate and serious pollution threat to South Island lakes and rivers.

The establishment of I certain lake-weeds from] the North Island would be disastrous for fishermen, boat-lovers, swimmers, and hydro-electric schemes.

The weeds need neither seeds nor roots to propagate. A few leaves wrapped around a propeller could introduce the variety, as could a person who cleans his home aquarium, containing exotic water weeds, at a river or lake. If boatowners bringing boats from the North Island make sure that their propellers are clean, and if home acquaria owners keep the weeds in their tanks away from rivers and lakes, the threat may be averted. In the North Island it is too late; the talk is no longer of eradication but hopefully, though not always confidently, of control. Four weeds There are four of the weeds: Canadian pondweed ] (Elodea canadensis), oxygen weed (Lagarosiphon major), egeria (Egeria densa) and hornwort (Ceratophyllum demersum). Canadian pondweed already occurs in Canterbury lakes and rivers (the Christchurch City Council has a constant task in cleaning out the Avon River) but so far oxygen weed has not taken hold. There is every reason to believe that if it were introduced it would thrive and become an immense problem. There is some oxygen weed already in the Nelson area. For those not familiar with the weeds in Lake Rotorua or the lakes of the : Waikato basin, some idea of the size of the problem may be gauged from the fact that various hydro-electric stations have had to be shut down for some days because the weeds have blocked the

p I screens to such an extent I that the screen cleaners were J i unable to cope. Rafts of weed 1 200 ft to 300 ft long and up to ■ 15ft thick have been re- . ported. One measure adopted , to avoid a screen becoming • clogged was to extend a boom carrying netting 12ft down into the lake. One mat ’ of weed at Atiamuri, slightly deeper than 12ft, clogged up the netting and fresh weed rolled underneath. The weed accumulated in the boom faster than it could be removed with a grab arrangement in a boat. Control diffcult The adage that prevention is better than cure could hardly be more aptly illustrated than from lake-weeds. Various solutions have been attempted, none of them wholly effective, and most of j them having undesirable < other effects. < There have been two sorts , of mechanical methods. One i has been the use of a weedcleaning machine. The first ■ type left the weeds in the i lake and the lake-dwellers i knew that this amounted to a sowing campaign and the Rotorua "Daily Post” re- ■ ported: “Their enraged howls echoed all the way to Wellington and the weed machine was summarily recalled from its voyaging.” Another mechanical method is the lowering of the lakes. Sometimes this is not possible, but, even when it is there are problems. Lakeweed can be cleared from lake edges when the lakes are lowered, but those who remove it cannot help but remove sources of food for trout If they don’t remove the food the sun will. Whatever mechanical means are used there is a very large amount of weed left to deal with, and a further problem is created. One method of disposing of the weed has been to convert it. to feed for sheep. A major difficulty here is that sheep

do not like it. Besides that, the weed consists of between 80 per cent and 97 per cent water. So the Electricity Department, in whose interests it is to remove the weed, finds itself in the ironical position of removing its own source of power from lakes and rivers. There have been the chemical solutions. One chemical was an arsenic compound. The arsenic did prove effective to some extent, but since people drank water from the lake and ate fish from the lake, no-one seemed sure what else the arsenic would eradicate. A chemical called diaquat is the one presently favoured. Again, the other effects are not at all clear.

One major problem with . the chemical means of control is eutrophication. Eutrophication is the process of enrichment of water, the consequent promotion of growth and the effects of decaying vegetation. The whole problem of lake- , weed is compounded by the : nutrients supplied by the run- . down from fertilised land, or ' from aerial top-dressing, but the killing of the weed creates a special problem. The dead weed sinks in the lake and rots, the rotting process using oxygen. Given enough rotting matter, the oxygen in the water becomes seriously depleted, enough sometimes to cause the death of plant and fish life. The lake would become dead. Fish trial Biological control is being investigated. An early experiment was with an exotic snail, but it did not eat enough weed and ate smaller snails. Experiments are being conducted with the Chinese grass carp, a fish that can eat its own weight in weed a day. One problem with it is that once in the lakes it might find something .it likes eating better than the weed. Another is that if it finds the

weed to its liking and eats all. the weed in its area what will it eat after that?

The Electricity Department is seriously concerned about the lake-weeds. In Auckland it is supporting research done under the supervision of Professor V. J. Chapman (whose D.S.I.R. bulletin, “A History of the Lake-weed Infestation of the Rotorua Lakes and the Lakes of the Waikato Hydroelectric System,” was extremely useful in the preparation of this article) and in Christchurch it is supporting research done under the supervision of Dr Vida Stout, senior lecturer in the zoology department of the University of Canterbury (whose South Island knowledge of lakes and whose general advice also aided in this article). The positive aspect of this threat of pollution is that it remains a threat and may be averted. Propellers of boats do need to be cleaned if they are -fouled with North Island lake-weeds and aquaria should never be emptied on lake or river shores. These are acts over which humans do have some control; apparently even sheep are relucted to clean up after this particular human mess.