River and stream water quality and ecology - Greater Wellington ...

River and stream water quality and ecology in the Wellington region: State and trends
8. Discussion
This section revisits the main findings from Sections 4 through 7. A regional
overview of state and trends in river and stream health is presented first, with
this information then considered in a national context. The principal issues
affecting the region’s rivers and streams are discussed and key management
issues identified, with brief consideration also given to downstream receiving
environments. Lastly, monitoring limitations and knowledge gaps are outlined.
8.1 Regional overview
8.1.1 State
Analysis of RSoE water quality, periphyton and macroinvertebrate data
collected at 55 sites (46 in the case of periphyton) over the period July 2008 to
June 2011 found that:
27 (49%) sites were assigned ‘good’ or ‘excellent’ water quality grades,
indicating that median values for at least five of the six key indicator
variables in Greater Wellington’s Water Quality Index (WQI) met
guideline values;
27 (59%) sites were classed as ‘good’ or ‘excellent’ in terms of
compliance with MfE (2000) periphyton cover/biomass guidelines,
indicating infrequent or nil occurrence of nuisance periphyton growth; and
36 (65%) sites were classed as having ‘good’ or ‘excellent’
macroinvertebrate community health, based on a comparison of mean MCI
scores against the thresholds in national guidance provided by Stark and
Maxted (2007).
The remaining sites (35–51% depending on the indicator) were classed as
having ‘fair’ or ‘poor’ water quality or ecosystem health. While this report has
not attempted to assess the suitability of rivers and streams for other, values 28
such as recreational use and trout fishery/spawning, these values are also likely
to be impaired at some RSoE sites. For example, the two Ruamahanga River
sites with a WQI grade of ‘fair’ (Gladstone and Pukio) are in close proximity to
recreational water quality monitoring sites with Suitability for Recreation
Grades of ‘poor’ (see Greenfield et al. 2012). Similarly, sites on a number of
river and stream reaches listed in Greater Wellington’s existing Regional
Freshwater Plan (WRC 1999) as supporting trout habitat (eg, Taueru River at
Gladstone, Kopuaranga River at Stewarts, Mangatarere Stream at SH 2 and
Huangarua River at Ponatahi Bridge), frequently exceeded MfE (2000)
periphyton guidelines for protection of trout habitat and angling values.
Water quality, periphyton cover/biomass and macroinvertebrate health all
showed clear linkages with upstream catchment landcover (Figure 8.1). Sites
classified as having ‘good’ or ‘excellent’ water quality and ecosystem health
tend to be located on the upper reaches of rivers and streams that drain the
28 The specific values that rivers and streams in the Wellington region are to be managed for are currently being considered as part of the
review of Greater Wellington’s existing Regional Freshwater Plan.
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