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

River and stream water quality and ecology in the Wellington region: State and trends
downstream of the Carterton and Masterton WWTP discharges, respectively.
This is well established for the Carterton WWTP discharge which Milne et al.
(2010) estimated contributed as much as 90% of the total DRP load to the
Mangatarere Stream during low flow conditions. The influence of the Carterton
WWTP discharge appears to extend well beyond the Mangatarere Stream; the
Waiohine River at Bicknells (located approximately 4 km downstream of the
Mangatarere Stream confluence), also recorded a median DRP concentration
above the ANZECC (2000) lowland TV. Ausseil (2011) noted that DRP
concentrations at this site remain elevated under low flow conditions,
confirming that the WWTP discharge is the likely reason for this.
In an assessment of nutrient ratios (DIN to DRP) across the 55 RSoE sites,
which provide a general indication of which nutrient may limit periphyton
growth, Ausseil (2011) estimated that 41% of RSoE sites are nitrogen-limited
or co-limited (where the limiting nutrient switches regularly from nitrogen to
phosphorus). Of the remaining sites, 38% are phosphorus-limited (principally
pastoral sites as a result of nitrogen inputs) and 21% have concentrations of
both nitrogen and phosphorus that are high enough for neither nutrient to be
limiting periphyton growth. This suggests that management of point and nonpoint
source nutrient inputs to rivers and streams across the region should focus
on control of both nitrogen and phosphorus; a similar conclusion to that
reached by Wilcock et al. (2007) when considering nutrient management in the
Manawatu-Wanganui region.
8.3.2 Poor clarity and sedimentation
Median visual clarity values at all but one of the 15 RSoE sites graded ‘poor’
and seven of the nine sites graded ‘fair’ for water quality failed to meet the
MfE (1994) minimum of 1.6 m. In fact, failure to meet the 1.6 m threshold was
also the principal difference separating ‘good’ and ‘excellent’ sites. Reduced
water clarity can affect the sight range for humans and aquatic animals as well
as the availability of light for aquatic plant growth (Davies-Colley et al. 2003),
resulting in degradation of recreational and ecological values.
The occurrence of poor water clarity and high concentrations of total
suspended solids (TSS) at many sites indicates the potential for increased
sediment deposition on the streambed. Increased sedimentation is recognised
as a key impact of land use change which results in degradation of aquatic
habitat for macroinvertebrates and fish in particular (Clapcott et al. 2011).
Although streambed sedimentation is not currently assessed under the RSoE
programme, the occurrence of soft substrate at a number of sites gives a clear
indication of where sediment deposition rates are highest.
All but one of the nine RSoE sites with substrates dominated by soft sediment
are located on small lowland streams along the Kapiti Coast or in the eastern
Wairarapa hill country (the exception being Waiwhetu Stream at Wainuiomata
Hill Road). Although some of these sites may be naturally inclined toward
being soft sedimentary due to catchment geology (eg, Taueru River at
Castlehill), others are likely to be hard-bottomed sites which have been
inundated with fine sediment (eg, Waiwhetu Stream at Wainuiomata Hill
Road) as a result of upstream land use. Assessment of macroinvertebrate
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