Long Term Management of an Invasive Alga in a Controlled River

Didymosphenia geminata (didymo) is a colonial diatom that can form extensive mats on the beds of rivers, transforming the appearance and ecosystem function of the affected rivers.  Didymo is native to the northern hemisphere and was accidentally introduced into the South Island of New Zealand. It was discovered there in 2004.  Didymo mats grow to at least 50mm thick and cover large areas of the beds of rivers.  Rivers with clear water, low nutrients, long periods of low flows, and boulder/cobble substrates provide the most suitable habitat for didymo. Lake outlet rivers and those controlled for hydropower generation are the most affected. A programme to identify, and provide, increased flows to remove the didymo biomass to levels that do not adversely affect angling opportunities and fish biomass has been developed with Meridian Energy, holders of consents to dam and divert river flows to generate electricity in the Waiau River Catchment, South Island, New Zealand.  Flows at least 10 times the median at least 4 times per year, with increased frequency in the summer months, are required to reduce didymo biomass to acceptable levels on a continuing basis.