Didymo is here to stay in the Waitaki River; while it may be controlled, it cannot be eradicated at present, the Environment Court was told yesterday.
If Meridian Energy Ltd builds a power scheme on the river, it is proposing ways to control it, although a group opposing the north bank tunnel concept project does not agree with some of its suggestions.
The court is hearing an appeal by the Lower Waitaki River Management Society against granting of water consents by Environment Canterbury for the scheme to take up to 260cumecs from the river between the Waitaki dam and Stonewall.
Yesterday's evidence focused on the effects of lower flows between the Waitaki dam and Stonewall if the scheme is built.
At present, the daily mean average is about 360cumecs.
Resource management consultant Ned Norton outlined effects on water quality and different flow regimes on periphytons (algae and other growths), including didymo, and macrophytes (aquatic plants).
Mr Norton said didymo could not be eradicated from the Waitaki River; but controlled easily by releasing high flows.
In January, flows of more than 900cumecs for two days removed more than 90% of didymo.
Chemical control would be "very impractical", because of the size of the river.
Conditions on Meridian's resource consents proposed a flow of 900cumecs be released when in-flows into storage lakes provided the water, on average every 1.9 years.
In addition, seven flushing flows of 450cumecs would be released a year.
Mr Norton was not certain flushing flows would deal with all the adverse effects, including didymo, to the same extent current flows did.
However, flushing flows would be monitored, to determine, among other things, how effective they were at removing didymo.
Under an adaptive management plan, flushing flows could be increased. The society will have its own environmental consultant, Bill Chisholm, give evidence on didymo later in the hearing.
Scientist Ian Jowett said lower flows would not be detrimental and should be beneficial to the trout fishery by providing a near optimal habitat for adult rainbow trout and their main food source.
The total habitat for invertebrates, native fish and most juvenile trout and salmon would be reduced with lower flows.
Freshwater scientist Donald Jellyman said the scheme was expected to have only minor implications for native fish.
Bullies and longfin eels would have an increase in habitat upstream of Kurow.
