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

River and stream water quality and ecology in the Wellington region: State and trends
Table 5.2: Criteria and MfE (2000) thresholds used to assign each RSoE site to
one of four periphyton classes. Note that a site only had to exceed one of the
streambed cover or biomass thresholds to be categorised into a particular class
Quality
class
Excellent
Good
Fair
Poor
Explanation
Minimal exceedance of
streambed cover guidelines
and full compliance with
biomass guidelines
Infrequent non-compliance
with streambed cover and/or
biomass guidelines
Intermittent non-compliance
with streambed cover and/or
biomass guidelines
Frequent non-compliance
with streambed cover and/or
biomass guidelines
Guidelines/thresholds used to determine class
Streambed cover
(%n exceedances)
Mat Filamentous
(60%)
(30%)
Chlorophyll a
(n exceedances)
50 mg/m 2 120 mg/m 2
20 3 ≥ 2
quality and shade (one-off measures, see Appendix 8) substrate size (mean of
three assessments), and estimates of mean annual maximum accrual derived by
Thompson and Gordon (2010b).
(a) Cautionary notes
The number of observations of periphyton streambed cover made at each of the
46 RSoE sites varied significantly over the reporting period. On average,
observations could not be made on seven sampling occasions at each site
(range 0 to 20), due to either high turbidity, high flows or because the site
could not be accessed (eg, road closure). As a result, several sites had less than
60% of the potential number of streambed cover assessments (max n=36).
The MfE (2000) cover and biomass thresholds for trout habitat and angling in
Table 5.1 do not apply to all RSoE sites as not all rivers and streams in the
Wellington region are valued for trout fishing or spawning. In addition, the
periphyton biomass thresholds for the protection of aquatic biodiversity do not
take into account natural variation in periphyton biomass that can occur in
response to differences in catchment altitude, geology and other geophysical
characteristics.
5.1.2 Periphyton cover
Twenty six RSoE sites exceeded MfE (2000) guidelines for periphyton
streambed cover at least once (and typically more than once) during the reporting
period (Table 5.3). The guideline for filamentous streambed cover was more
frequently exceeded than the mat cover guideline (25 sites and 6 sites,
respectively). Sites that exceeded the guideline for mat periphyton cover
typically also exceeded the filamentous periphyton guideline at some time during
the reporting period except at one site, Hutt River opposite Manor Park Golf Club.
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