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

River and stream water quality and ecology in the Wellington region: State and trends
It should also be noted that MCI and QMCI thresholds suggested by Stark and
Maxted (2007) do not take into account the natural variation in
macroinvertebrate community composition that can occur in response to
differences in catchment altitude, geology and other geophysical characteristics.
6.1.2 Macroinvertebrate metrics
Summary statistics for the four metrics calculated (MCI, QMCI, %EPT taxa and
%EPT individuals) calculated for each of the 55 RSoE sites are presented in
Table 6.2. Mean MCI scores across the 55 RSoE sites ranged from a minimum
of 67 (Waiwhetu Stream at Wainuiomata Hill Bridge) to a maximum of 146
(Ruamahanga River at McLays). As outlined in Section 2.3, the Waiwhetu
Stream site is known to be occasionally influenced by tidal backflow and taxa
indicative of estuarine conditions have been recorded at this site. Because the
MCI was not designed to take into account estuarine influence its application at
this site may not be appropriate. The next lowest mean MCI score recorded
across the 55 sites was 75.9 (Parkvale Stream at Weir).
Based on mean MCI scores, 16 sites were classified as ‘excellent’, 20 sites as
‘good’, 16 sites as ‘fair’ and 3 sites as ‘poor’. All sites classed as ‘excellent’
drain upstream catchments dominated by indigenous forest. The seven sites
classed as ‘poor’ or within the ‘fuzzy boundary’ of the fair/poor threshold were
found in catchments dominated by pastoral or urban landcover (Figure 6.1). All
but one of these sites have substrate dominated by soft sediment – the
exception being Parkvale Stream at Weir which has a relatively small
catchment dominated by intensive agricultural land use.
Number of sites
25
20
15
10
5
0
Indigenous forest Pasture Urban Exotic forest
Excellent
Good
Fair
Poor
Figure 6.1: Macroinvertebrate quality classes assigned to each of the 55 RSoE
sites, grouped according to REC landcover. Quality classes are based on the
application of Stark and Maxted (2007) thresholds to mean MCI scores from
annual sampling in 2009, 2010 and 2011. Hatched boxes represent sites within
the ±5 MCI unit ‘fuzzy boundary’ between each class (note the result from the
single exotic forestry site falls on the boundary of the fair/good threshold)
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