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

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River and stream water quality and ecology in the Wellington region: State and trends summer and can support high periphyton growth despite relatively low nutrient concentrations. While reduced flows in these rivers largely reflects the less frequent summer rainfall and naturally lower groundwater inputs associated with the eastern Wairarapa hill country, in some other streams, water abstraction may be a contributing factor. As noted in Section 3.4.2, there has been a considerable increase in consented water abstraction in the Wellington region in recent years, for both urban water supply and irrigation in rural areas (principally for dairying in the Wairarapa). Despite a cap being in place on new surface water takes from some small streams in the central Wairarapa Valley, such as the Parkvale and Mangatarere streams, Hughes and Gyopari (2011) demonstrated that the flow in these streams is depleted by the abstraction of hydraulically connected groundwater. Further much of this groundwater has not to date been considered as part of the consented surface water allocation. A new framework for conjunctive management of surface and groundwater in the Wairarapa Valley has recently been recommended to address this issue (see Keenan et al. 2012). The benefits of riparian vegetation for stream health by way of increasing stream shade and reducing water temperatures and periphyton growth are well known (eg, Parkyn 2004) and have been demonstrated to be particularly effective in small rivers and streams. For example, Perrie (2008) reported a reduction in maximum water temperatures of several degrees for the Enaki Stream in the Mangatarere catchment near Carterton (Figure 8.5), despite the presence of relatively young riparian plantings. Monitoring at RSoE sites has shown that high levels of shade can limit periphyton growth even at sites with elevated nutrient concentrations (eg, Parkvale Stream tributary at Lowes Reserve and Whareroa Stream at Waterfall Road), thereby potentially mitigating some of the effects of nutrient enrichment associated with upstream land use activities. There are also a number of other potential benefits from riparian vegetation, including interception and attenuation of nutrients, sediment and pathogens, improved bank stability, and improved habitat diversity and complexity through leaf litter and woody debris inputs (eg, Parkyn 2004). Water temperature (º C) 25 20 15 Upstream Downstream 0 16/02/2006 18/02/2006 20/02/2006 22/02/2006 (Source: After Perrie 2008) Figure 8.5: Daily variation in water temperatures upstream and downstream of a planted reach of the Enaki Stream (near Carterton) for a week during February 2006 WGN_DOCS-#1100598-V3 PAGE 113 OF 160
River and stream water quality and ecology in the Wellington region: State and trends While riparian vegetation can provide a variety of benefits to river and stream habitat (and water quality), Perrie (2008) noted that in some cases the benefits of riparian vegetation may be limited due to the overriding influence of land use in the upstream catchment. For example, as noted in Section 8.3.3, riparian rehabilitation along a reach of the Karori Stream in the vicinity of the RSoE site at Makara Peak that is subjected to significant urban stormwater contamination (and soluble nutrient inputs) resulted in no improvement to aquatic ecosystem health. Similarly, despite improvements in some variables such as water temperature and bank stability, the lack of any significant improvement in periphyton or macroinvertebrate metrics associated with riparian rehabilitation undertaken on reaches of the Enaki Stream was in part attributed to poor upstream farming practices (including stock access to stream beds and unauthorised dairy effluent run-off). This highlights that although the benefits of riparian rehabilitation are well established, it may not be sufficient to mitigate the detrimental effects of other factors such as urban stormwater discharges and poor land management practices. To improve indigenous fish community condition, the removal of physical barriers that prevent or impede fish migration is also required. Limited monitoring to date indicates that fish communities at some RSoE sites at least are affected by instream barriers (eg, Beef Creek at Headwaters has poorer than expected fish community condition). Further, in an assessment of 28 barriers across 13 river/streams in the Wellington region, Atkinson (2008) concluded that they were all, to some degree, negatively impacting on the ability of native fish to migrate upstream. This indicates that barriers to fish passage are a potentially widespread problem. Greater Wellington has recognised this and developed a specific strategy to address native fish passage issues in the Wellington region (GWRC 2011). 8.4 Downstream receiving environments Although beyond the scope of this report, it is important to recognise that the state of some of the region’s rivers and streams can have flow-on effects for downstream receiving environments. For example, Greenfield et al. (2012) identified that poor microbiological water quality in the lower reaches of the Mangaone Stream at times negatively impacts on water quality at Te Horo Beach. Similarly, Oliver and Milne (2012) identified sedimentation, nutrient enrichment, and toxicant and microbiological contamination as some of the main issues affecting estuaries in the Wellington region. All of these issues are particularly pertinent for Porirua Harbour, with sedimentation and nutrient enrichment the main concerns for Lake Onoke. Nutrient enrichment is also a key issue for Lake Wairarapa (Perrie & Milne 2012). With further land use change expected in their upstream catchments (forestry clearance, and residential and roading development in the case of Porirua Harbour, and rural land use intensification in the cases of Lakes Wairarapa and Onoke), it is important that measures introduced to manage the effects of this change on river and stream health are implemented with consideration of the need to also manage potential ‘downstream’ effects. PAGE 114 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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4.2 Temporal trends River and strea
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Page 72 and 73: TN (mg/L) Temperature ( 0 c) Shade
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Page 80 and 81: Chlorophyll a (mg/m 2 ) 90 80 70 60
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Page 106 and 107: IBI Score 60 50 40 30 20 10 0 River
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