As part of the Ministry's national environmental reporting programme, this report summarises changes in nuisance periphyton (algal) growth at National River Water Quality Network sites between 1990 and 2006. Algal growth can be a nuisance for swimming, fishing and kayaking and it affects fish and insect life in rivers.
This is the first time this data has been analysed and reported on at a national level. The report adds important information to existing Ministry reporting on river water quality and further informs New Zealanders of the state of New Zealand's rivers and any changes over time.
Human activities have increased the occurrence of nuisance algal growth in rivers. The amount of periphyton was typically higher at sites impacted by human activities in their catchment than baseline sites.
There are strong relationships between filamentous algal cover in rivers and the percentage of catchment in pasture, with greater filamentous cover associated with sites with a greater proportion of pasture in their catchments. This is consistent with the report on water quality trends at National River Water Quality Network sites.
Around 30 per cent of rivers had algae cover above the Ministry for the Environment's recommended guidelines. Forty-five per cent of impact sites had average annual maximum periphyton cover above the recommended guideline compared with 13 per cent of baseline sites.
However, overall, between 1990 and 2006 more rivers had decreasing (ie, improving) periphyton cover than increasing cover. For example, average annual periphyton cover decreased at 16 sites and increased at seven sites. This highlights that periphyton cover is influenced by many factors, as well as nutrient levels, such as water temperature, shading and high flow events.