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

River and stream water quality and ecology in the Wellington region: State and trends
The ecological significance of these trends is unclear. However, at some sites
where a deteriorating trend in MCI was identified (eg, Whareroa Stream at
Waterfall Road and Waiwhetu Stream at Wainuiomata Hill Road), the
magnitude of the deteriorating trend resulted in the site moving into a lower
macroinvertebrate class during the reporting period (Figure 6.6). Application
of two thresholds used in other regions to identify ecologically significant
trends – a relative rate of change greater than 1% per year and an overall
change greater than 20% (Stark 2010; Collier & Hamer 2012) – resulted in all
23 and 16 trends, respectively, being classed as ecologically significant.
However, these thresholds are considered arbitrary and so should be used with
caution. For future reporting of macroinvertebrate metric trends it is
recommended that alternative methods for assessing ecological significance be
explored, including the method used by Collier and Hamer (2012); this method
bases the threshold for ecological significance on the degree of ‘natural
variation’ at reference sites over time.
MCI score
140
120
100
80
60
RS11: Whareroa S - Waterfall Rd
RS27: Waiwhetu S - Wainui Hill Br
RS33: Ruamhanga R - Gladstone
RS38: Kopuaranga R - Stewarts
RS41: Waingawa R - South Rd
Excellent
Good
40
2003 2004 2005 2006 2007 2008 2009 2010 2011 2012
Figure 6.6: Mean Macroinvertebrate Community Index (MCI) scores for samples
collected annually between 2004 and 2011 from five RSoE sites where significant
declining trends were detected. Red lines and the grey bands represent Stark and
Maxted (2007) thresholds and corresponding fuzzy boundaries, respectively
In general, no clear reasons could be found for the observed trends in
macroinvertebrate metrics. However, corresponding trends in water quality or
periphyton variables may have had a bearing on results at some sites. For
example:
The decline in %EPT taxa identified at Whangaehu River 250 m from
confluence coincided with a deterioration in visual clarity and an increase
in turbidity, dissolved reactive phosphorus and total phosphorus (based on
non-flow adjusted data) (Section 4.2.2).
PAGE 84 OF 160 WGN_DOCS-#1100598-V3
Fair
Poor