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The planktonic algal communities found in natural lakes and wetlands are expected to be quite different to those of nutrient-enriched ponds (such as sewage treatment ponds). Similarly, we expect differences between clean, unpolluted streams and those receiving significant discharges from farmland, industry, or treatment plants.

Note that many algal genera contain numerous species, and that the different species within a genus may occur in quite different habitats. Biological monitoring involving identification to genus level - as in this guide - therefore should be based on community assemblages rather than individual taxa.

If you have a stream sample containing ten or more algal taxa, all of which are listed in the “Good quality streams” column, then the chances are that this is a normal, healthy algal community. If future monitoring in the same stream shows that the algae community has become dominated by taxa more typical of wastewater treatment ponds, then the first management action might be to identify a source of nutrients and algae, and to assess whether there is an unauthorised pollution issue.

However, there are instances in which the detection of particular individual taxa warrants a call to the authorities. These include the detection of sewage fungus,Didymosphenia, or large blooms of cyanobacteria that are potentially toxic.

As well as the composition of algal communities, the quantity of algae at a site is often an important issue. Measures of algal quantity are outside the scope of this guide, but may be found in the Periphyton Monitoring Manual produced by NIWA. If growths of algae are large enough to be objectionable, possibilities for control should be considered.