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

River and stream water quality and ecology in the Wellington region: State and trends
software (version 3.20). While all trend classes are summarised, only
‘probable’ and ‘clear’ trends are presented and discussed. For sites where
trends were present, a linear regression line was fitted and the slope of this line
used to provide some context for the rate of change or the magnitude of the
trend at each site. The relative rate of change was also calculated by dividing
the slope by the mean macroinvertebrate metric value.
While useful indicators of macroinvertebrate community health, trends in
abundance-based metrics (eg, QMCI and %EPT individuals) should be
interpreted with caution as these metrics are more likely to be influenced by
changes in sampling intensity, micro-scale habitat variables and fluctuations in
the numbers of tolerant taxa than other presence/absence based metrics (Collier
et al. 1998; Scarsbrook et al. 2000).
6.2.2 Results
The trend analysis results are summarised in Table 6.3. Across all four of the
metrics assessed, the majority of RSoE sites were typically stable over the
seven-year reporting period (78% to 87% of sites depending on the metric).
More trends were apparent in MCI than in the other metrics, although half of
these were deemed ‘possible’ trends. The majority (83%) of all trends
(possible, probable and clear) were indicative of deteriorating invertebrate
community health. Only ‘clear’ and ‘probable’ trends (summarised in Table
6.4) are discussed further. The complete set of trend analysis results can be
found in Appendix 7.
Table 6.3: Number of RSoE sites assigned into different trend categories for each
of four macroinvertebrate metrics, determined by performing Spearman Rank
correlations on macroinvertebrate results from annual sampling at 55 sites
between 2004 and 2011
Metric
Deteriorating trend
Possible Probable Clear
decline decline decline
Total
No
trend
Improving trend
Possible
increase
Probable Clear
increase increase Total
MCI 5 1 4 10 43 1 1 0 2
QMCI 2 3 3 8 47 0 0 0 0
%EPT taxa 2 4 1 7 48 0 0 0 0
%EPT
individuals
2 1 2 5 46 1 2 1 4
Sixteen sites (29%) demonstrated at least one ‘clear’ or ‘probable’ trend in one
of the four metrics assessed (Table 6.4), with 23 trends detected in total. Three
sites exhibited trends in two metrics (Mangapouri Stream at Bennetts Road,
Waiwhetu Stream at Wainuiomata Hill Bridge and Waingawa River at South
Road) and two sites exhibited trends in three metrics (Whareroa Stream at
Waterfall Road and Kopuaranga River at Stewarts). Of the sites with trends in
two or more metrics all but one of the trends detected were indicative of
deterioration. The exception was Mangapouri Stream at Bennetts Road, where
an improving trend in MCI coincided with a deteriorating trend in QMCI. This
site, along with three other sites (Waitohu Stream at Forest Park, Ruamahanga
River at Pukio and Waiohine River at Gorge) also exhibited an improving trend
in %EPT individuals.
PAGE 82 OF 160 WGN_DOCS-#1100598-V3