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Our rivers: trends in algal cover

As part of its National Environmental Reporting Programme, the Ministry for the Environment reports on periphyton (algae) in rivers.

Data collected from 73 of the National River Water Quality Network sites between 1990 and 2006 was analysed to identify national trends.

This is the first time this data has been analysed and reported on at a national level. Algal growth can be a nuisance for swimming, fishing and kayaking and it affects fish and insect life in rivers. Some algae can also produce toxins that can be harmful to humans, stock and dogs.

Key findings

Occurrence of periphyton

  • Periphyton cover was typically higher at sites with a greater proportion of pasture in their catchment.
  • Around 30 per cent of rivers had algal cover that exceeded the New Zealand periphyton guideline.

Long-term trends

  • Between 1990 and 2006, a higher percentage of rivers had decreasing (improving) algal cover.
  • Some of these decreasing trends could be associated with improvements in point source discharges. However, algal cover is influenced by a number of factors – such as water temperature, shading and high flow events – in addition to nutrients levels.

Trends in periphyton cover, 1990-2006

Read table of data of this graph

Map showing trends in annual mean periphyton cover at National River Water Quality Network sites for 1990-2006.

Sites with a decreasing trend Sutton @ SH87
Clutha @ Balclutha
Mataura @ Seaward Down
Oreti @ Lumsden
Oreti @ Riverton HW Br.
Monowai below Gates
Motu @ Houpoto
Grey @ Waipuna
Haast @ Roaring Billy
Makaroro @ Burnt Br.
Tukituki @ Red Br.
Tarawera @ Lake outlet
Whirinaki @ Galatea
Opihi @ Waipopo
Hakatakamea above MH Br.
Wairua @ Purua
Sites with an increasing trend Waipa @ Whatawhata
Motueka @ Woodstock
Motueka @ Gorge
Wairau @ Tuamarina
Buller @ Longford
Whanganui @ Te Maire
Rangitikei @ Kakariki
No significant trend Clutha @ Luggate Br.
Kawarau @ Chards
Shotover @ Bowens Peak
Clutha @ Millers Flat
Hurunui @ Mandamus
Hurunui @ SH1 Br.
Waimakariri @ Gorge
Waimakariri above old HW Br.
Taieri @ Tiroiti
Taieri @ Outram
Mataura @ Parawa
Waiau @ Tuatapere
Waipoa @ Kanakanaia
Waikohu @ No. 1 Br.
Motu @ Waitangirua
Buller @ Te Kuha
Grey @ Dobson
Waipa @ Otewa
Waikato @ Hamilton Traffic Br.
Waikato @ Rangiriri
Waihou @ Te Aroha
Ohinemuri @ Karangahake
Ngaruroro @ Chesterhope
Ngaruroro @ Kuripapango
Mohaka @ Raupunga
Mohaka @ Glenfalls
Wairau @ Dip Flat
Rangitaiki @ Murapara
Rangitaiki @ Te Teko
Opihi @ Rockwood
Opuha @ Skipton Br.
Waitaki @ Kurow
Waitaki @ SH1 Br.
Tongariro @ Turangi
Waitara @ Bertrand Rd
Manganui @ SH3
Waingongoro @ SH45
Whanganui @ Paetawa
Rangitikei @ Mangaweka
Manawatu @ Weber Rd
Manawatu @ Teachers Coll.
Manawatu @ Opiki Br.
Waipapa @ Forest Ranger
Waitangi @ Wakelins
Mangakahia @ Titoki Br.
Hutt @ Boulcott
Hutt @ Kaitoke
Ruamahanga @ Waihenga
Ruamahanga @ Wardells
Ruramahanga @ SH2

This information supports the Government’s New Start for Fresh Water work programme and other freshwater policy initiatives.

For more information go to Trends in Nuisance Periphyton Cover at New Zealand National River Water Quality Network Sites 1990–2006.