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

25th percentile summer time flow m 3 /s
25th percentile summer time flow m 3 /s
14
12
10
8
6
4
2
35
30
25
20
15
10
5
2003/04 2004/05 2005/06 2006/07 2007/08 2008/09 2009/10 2010/11
River and stream water quality and ecology in the Wellington region: State and trends
Otaki River at Pukehinau
Hutt River at Birchville
Hutt River at Taita Gorge
Waiohine River at Gorge
Ruamahanga River at Waihenga
2003/04 2004/05 2005/06 2006/07 2007/08 2008/09 2009/10 2010/11
2003/04 2004/05 2005/06 2006/07 2007/08 2008/09 2009/10 2010/11
WGN_DOCS-#1100598-V3 PAGE 73 OF 160
25th percentile summer time flow m 3 /s
25th percentile summer time flow m 3 /s
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0.12
0.10
0.08
0.06
0.04
0.02
0.00
Pauatahanui Stream at Gorge
Porirua Stream at Town Centre
Waiwhetu Stream at Whites Line East
Whangaehu River at Waihi
Ruakokoputuna River at Iraia
2003/04 2004/05 2005/06 2006/07 2007/08 2008/09 2009/10 2010/11
Figure 5.10: 25 th percentile summer-time (November–April) flows between 2003/04
and 2010/11 at ten flow monitoring sites across the Wellington region. Data from
the Whangaehu and Ruakokoputuna river sites provided courtesy of NIWA
5.3 Synthesis
Both periphyton cover and biomass were lowest at RSoE sites with catchments
dominated by forested areas of the Tararua, Rimutaka and Aorangi ranges.
These sites rarely exceeded the MfE (2000) guidelines for protection of
instream values and, along with two highly shaded sites on streams with
pastoral catchments, were assigned an overall periphyton class of ‘excellent’
(Figure 5.11).
Periphyton growth was greatest at urban sites in Wellington and Porirua, as
well as at pastoral sites in central and eastern Wairarapa (Figure 5.11). These
sites frequently exceeded guidelines for protection of recreational, aquatic
biodiversity and trout fishery/spawning values and were classed as ‘poor’.
Several of these sites, including Taueru River at Gladstone, Kopuaranga River
at Stewarts, Mangatarere Stream at SH 2 and Huangarua River at Ponatahi
Bridge, are listed in Greater Wellington’s existing Regional Freshwater Plan
(WRC 1999) as supporting important trout habitat.
Environmental variables that exhibited the strongest correlations with
periphyton biomass and/or cover were nutrient concentrations and water
temperature. Many RSoE sites classified as having ‘poor’ compliance with the
MfE (2000) periphyton guidelines recorded very high median concentrations of
TN and/or TP, and high water temperatures. Some sites, such as those in
eastern Wairarapa, recorded only low to moderate nutrient concentrations but
high water temperatures. Rivers and streams in these areas are subject to
periods of extended low flow during summer and can support high periphyton
biomass despite relatively low nutrient concentrations. Conversely, at two sites