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

River and stream water quality and ecology in the Wellington region: State and trends
Similar to the trends in periphyton growth, it is possible that declining trends in
macroinvertebrate metrics at some sites are a result of unusual flow conditions
affecting samples at the start of the trend analysis period. Many samples taken
during 2004 and 2005 were subject to unusually high river flows, short accrual
periods and in some cases low water temperatures due to samples being taken
later in the season. These conditions generally resulted in higher
macroinvertebrate metric scores in 2004 and 2005 than in later years, as
illustrated by the overall patterns in macroinvertebrate metrics at the 55 RSoE
sites across the trend analysis period (Figure 6.9).
MCI score
160
140
120
100
80
60
40
2004 2005 2006 2007 2008 2009 2010 2011
2004 2005 2006 2007 2008 2009 2010 2011
Figure 6.9: Box plots summarising the range of mean MCI (left) and QMCI (right)
scores recorded across all 55 RSoE monitoring sites for each year from 2004 to
2011
6.3 Synthesis
Based on mean MCI scores for the 2009 to 2011 period, 16 sites are classified
as having ‘excellent’ macroinvertebrate community health, 20 sites as ‘good’,
16 sites as ‘fair’, and 3 sites as ‘poor’ (Figure 6.10). All sites classed as
‘excellent’ have upstream catchments dominated by indigenous forest cover.
In contrast, the majority of the sites classed as ‘poor’ or as being within the
‘fuzzy boundary’ of the fair/poor threshold were soft-bottomed sites in
catchments dominated by pastoral or urban landcover. Although sites on the
mid and lower reaches of the larger rivers in the region such as the Otaki, Hutt,
Wainuiomata, Ruamahanga and Waiohine rivers tended to be classed as
‘good’, mean MCI scores at these sites were often considerably reduced
compared to upper catchment sites.
Mean MCI scores were strongly positively correlated with indigenous forest
cover and, for sites classed as ‘pastoral’, strongly negatively correlated with the
proportion of pastoral landcover. Although there were insufficient sites to
assess the relationship between MCI and urban landcover, RSoE sites classed
as ‘urban’ recorded the lowest mean MCI scores of all three major landcover
types. Further, comparison of macroinvertebrate data collected by Kingett
Mitchell (2005a) from 67 urban stream sites across Hutt, Wellington and
Porirua cities and the Kapiti Coast with the proportion of impervious cover in
the upstream catchment (Greater Wellington unpublished data) indicates that
stream health declines with increasing intensity of urban land use (Figure
6.11). These data suggest that as little as 10% impervious cover within a
catchment can reduce stream health.
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QMCI score
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