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

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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
River and stream water quality and ecology in the Wellington region: State and trends Figure 5.11: Periphyton quality classes for each of the 46 RSoE sites with hard substrate suitable for periphyton growth. Classes were assigned to each site according to compliance with MfE (2000) guidelines, based on both monthly assessments of periphyton cover and annual assessments of periphyton biomass made between July 2008 and June 2011 a high degree of stream shade resulted in low periphyton growth despite exposure to moderate to high nutrient concentrations. Although formal monitoring of macrophyte cover was not undertaken during the reporting period at the nine RSoE sites dominated by soft substrate, observations recorded during other monitoring suggest that nuisance macrophyte growth regularly occurs at four of these sites. Nuisance macrophyte growth also occurs at a number of hard-bottomed sites characterised by small substrate and/or long accrual periods. Significant increases in periphyton cover and/or biomass were detected at 19 RSoE sites while a significant decreasing trend was detected at one site. The magnitude of increasing trends was considerable at some sites – including many sites classified as ‘poor’ – and in some cases may have resulted in significant degradation of river and stream values. The reasons for these increasing trends are unclear. At many sites, the increasing trends have likely been influenced by unusually high river flows and short accrual periods at the start of the analysis period (summers of 2004 and 2005) which resulted in very low periphyton growth in these years. Further, at some sites, decreasing summer-time river flows over the duration of the trend period may also have influenced the observed trends. PAGE 74 OF 160 WGN_DOCS-#1100598-V3
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River and stream water quality and
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DISCLAIMER This report has been pre
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observed trends were unclear. The p
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5. Periphyton - state and trends 57
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Figure 2.1: RSoE monitoring sites a
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2.4.2 Biological variables River an
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Appendix 1: Monitoring site details
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River and stream water quality and ecology - Greater Wellington ...

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