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Executive Summary

Periphyton cover has been assessed at 73 of the 77 National River Water Quality Network (NRWQN) sites during monthly site visits since autumn 1989. Periphyton is evaluated visually as percentage cover of a standard wadeable area by mats or filamentous algae. The NRWQN sites include those classed as baseline (minimally impacted by human activities in their catchments) and impact, often with paired baseline and impact sites along single rivers.

The report addresses three questions in relation to the NRWQN sites for the period 1990-2006:

  1. Does periphyton exceed Ministry for the Environment (MfE) guidelines for nuisance effects on aesthetic and recreational values?
  2. Do human pressures influence the periphyton levels in and along the NRWQN rivers?
  3. Is the situation getting better or worse?

The mean monthly riverbed cover of periphyton between 1990 and 2006 across all 73 sites was 4.9% for mats, 5.6% for filamentous growth, and 10.6% for total (filamentous + mat) cover. The mean annual maximum periphyton covers were 19.6% (range 0 – 56.7%) for mats, 20.4% (range 0.1 to 68.6%) for filamentous growths, and 33.5% for total periphyton (range 0 – 80.4%). Periphyton cover was sufficiently high to impact on river recreation and aesthetic values (as indicated by exceedence of MfE guideline values) at about a quarter of the NRWQN sites at some time during an average year between 1990-2006. This suggests that nuisance periphyton may be a fairly widespread problem in New Zealand rivers, although direct extrapolation of the NRWQN findings to all New Zealand rivers is not possible because the NRWQN sites were not selected randomly and are biased towards larger than average rivers.

Both annual mean and annual maximum total covers were significantly higher at impact than baseline sites. Moreover, 28% of impact sites had mean annual maximum filamentous cover over the threshold for aesthetic nuisance effects, compared with 6% of baseline sites, and downstream sites typically had higher periphyton cover than paired upstream sites along the same river. Mean filamentous periphyton cover was also strongly positively correlated with percentage pastoral land cover in the catchment. Together these findings indicate that human activities in catchments have increased the occurrence of nuisance periphyton in the NRWQN rivers.

Analysis of trends in annual mean and annual maximum periphyton cover during 1990-2006 found more sites with decreasing cover than increasing cover. This encouraging finding was not expected given the increasing agricultural intensification and associated increase in nutrient inputs and instream concentrations over this period. Some of the trends of declining cover may be associated with improvements in point source effluent management (e.g., Mataura and Manawatu Rivers), but it appears that other factors may also be influencing periphyton cover at NRWQN sites.

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