Biology of excessive weed growth in the hydro-electric lakes of the Waikato River, New Zealand

Abstract

In 1965 a massive growth of Ceratophyllum demersum caused the shut-down of one power station (Ohakuri) on the Waikato River. As a result a study was commenced of the biology of the weeds in the Waikato hydro-electric lakes. There are four major exotic weeds, Ceratophyllum, Lagarosiphon major, Egeria densa and Elodea canadensis, all of which form extensive beds. The first three have only appeared in the lakes since 1960 but Elodea was probably present earlier. Efforts to control the excessive growth by diquat have not proved successful and booms protecting the intake fail when a massive amount of weed is liberated after storms. Reasonable control can be obtained by means of lake lowering in mid-summer. Studies of Lagarosiphon, Egeria and Ceratophyllum have enabled the water temperature range, compensation points and certain nutrient requirements to be established under New Zealand conditions. Significant regions in the upper Waikato River are highly thermal and these waters are rich in arsenic. The weeds accumulate this arsenic and values of over 1000 ppm dry wt. have been recorded. This makes them very unsuitable for stock feed if mechanical harvesting were to be employed.

Ceratophyllum and Lagarosiphon both appear to be shade plants, the former existing in a coarse winter form and a finer summer form. Day length affects growth rate as also does light quality, dissolved bicarbonate and rate of water flow. Limnological data show a gradual nitrogen enrichment of the lakes as one passes downstream, with periodic local phosphate enrichment after pasture top-dressing activities. Soluble iron is present in the waters and this appears to be important for massive growth. In quieter waters massive Lagarosiphon growth can result in temporary oxygen depletion of the water as compared with open lake water.