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

River and stream water quality and ecology in the Wellington region: State and trends
6. Macroinvertebrates – state and trends
This section assesses state and trends in macroinvertebrate community health
in river and streams across the Wellington region, based on the results of
monitoring conducted annually at 55 RSoE sites in late summer/early autumn.
The current state of macroinvertebrate health is presented first, using annual
data from 2009, 2010 and 2011. Temporal trends are then examined for the
period 2004 to 2011.
6.1 State
6.1.1 Approach to analysis
Macroinvertebrate samples collected annually in 2009, 2010 and 2011 were
used to determine the current state of macroinvertebrate health at each of the
55 RSoE sites. Due to changes in the number of samples collected over the
reporting period (from 2004 to 2009 three replicate samples were collected
from each site, whereas in 2010 and 2011 just one sample was collected) and
some inconsistencies in the resolution of taxonomic identification between
years, an initial data screening and ‘clean up’ process was undertaken prior to
data analysis. 22 As a result of this exercise, some of the metric scores presented
here may differ slightly to those presented in earlier reports (eg, Perrie &
Cockeram 2010).
Consistent with the SoE reporting recommendations in Stark and Maxted
(2007), the current ‘state’ of macroinvertebrate community health was
determined using the Macroinvertebrate Community Index (MCI). However,
additional metrics – Quantitative MCI (QMCI), %Ephemeroptera-Plicoptera-
Trichoptera (EPT) taxa 23 and %EPT individuals 23 – were also calculated and
are discussed where relevant (see Appendix 3 for more information on the
various macroinvertebrate metrics). Mean metric scores were calculated for
each site across the three years and compared against the quality class
thresholds in Stark and Maxted (2007) (Table 6.1). Stark and Maxted (2007)
note that these thresholds should be interpreted as having ‘fuzzy boundaries’,
with a margin of ±5 or ±0.2 units applied to the MCI or QMCI thresholds,
respectively.
Table 6.1: Interpretation of MCI-type and QMCI scores (from Stark & Maxted 2007)
Quality class MCI and MCIsb QMCI
Excellent ≥ 120 ≥ 6.00
Good 100―119 5.00―5.99
Fair 80―99 4.00―4.99
Poor