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An algal group to watch out for is the introduced nuisance diatom Didymosphenia. This invasive alga has appeared in many South Island streams and rivers, particularly those frequently visited by anglers (who have been accidentally spreading the stuff around).

Didymosphenia has large sarcophagus-shaped cells that are mounted on long mucilaginous stalks. Masses of this alga (commonly referred to as "didymo" or “rock snot”), primarily consisting of the stalks, can smother streambeds, and pile up along stream and river margins. Dried-up wads of this alga look like paper pulp dumped on the stream/river banks.

Note that there are native species of Gomphoneis and Gomphonema that also grow on long mucilaginous stalks, and may have a similar cell shape from certain angles, but their cells are much smaller than those of Didymosphenia (see images below).

Any sightings of Didymosphenia in North Island waters should be reported to MPI or your regional council. If you think you’ve encountered this alga, be sure you do not spread it to any other site – any gear that entered the infected site should be washed and dried thoroughly before further use.