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

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Chlorophyll a (mg/m 2 ) 90 80 70 60 50 40 30 20 10 (5) (21) River and stream water quality and ecology in the Wellington region: State and trends (40) (55) 0 2004 2005 2006 2007 2008 2009 2010 2011 Figure 5.8: Chlorophyll a concentrations at Ruamahanga River at Gladstone, based on annual periphyton biomass sampling between 2004 and 2011. The numbers in brackets indicate the accrual period (days) prior to sampling and the red horizontal line indicates the MfE (2000) 50 mg/m 2 threshold The reasons behind the increasing periphyton cover and/or biomass trends at such a large number of sites across a wide range of landcover types are unclear. At two sites changes in sampling location may be behind the apparent increase in periphyton growth. For example, the increase in both annual mean and annual maximum filamentous periphyton cover at Pauatahanui Stream at Elmwood Bridge is the likely result of shifting the site 40 m downstream in 2008 to enable easier access to the stream. Habitat at this downstream site is more suitable (gravel/cobble) for periphyton growth compared to the former silt-dominated upstream site. Similarly, trends in periphyton growth at Hutt River at Boulcott may be related to the sampling site being shifted to a shallower site 100 m upstream in 2009 when the previous sampling site became too deep to access safely. There were no clear links between trends in periphyton growth and trends in water quality variables such as nutrient concentrations and water temperature that are known to be key drivers of periphyton growth. At most sites where clear increases in periphyton growth and/or biomass were detected there were no significant increases in nutrient concentrations; in fact, the temporal trend analysis presented in Section 4.2 indicates that significant decreases in TN and/or TP concentrations were recorded at some of these sites (although water quality trends were assessed over a shorter period). The exception was Mangaroa River at Te Marua which exhibited a significant increase in TP concentrations. Variation in river flow over the 2003/04 to 2010/11 period may be a key factor behind the observed trends in periphyton growth. Due to the occurrence of frequent freshes during the summer months of 2003/04 and 2004/05, periphyton biomass samples taken from many RSoE sites during these years followed only a short accrual period and, in some cases, coincided with lower WGN_DOCS-#1100598-V3 PAGE 71 OF 160 (32) (22) (31) (51)
River and stream water quality and ecology in the Wellington region: State and trends water temperatures due to samples being taken later in the season. These conditions are reflected in both periphyton biomass and periphyton cover measurements being at their lowest across all 46 sites in the first two years of the trend period (Figure 5.9). The low biomass values recorded in 2003/04 and 2004/05 are likely to have influenced the increasing trends detected at several sites (eg, Figure 5.8). Mean filamentous streambed cover (%) Chlorophyll a (mg/m 2 ) 80 70 60 50 40 30 20 10 0 300 250 200 150 100 50 0 2004 2005 2006 2007 2008 2009 2010 2011 2004 2005 2006 2007 2008 2009 2010 2011 Figure 5.9: Box plots summarising the range of annual measurements of mean filamentous periphyton cover (top) and periphyton biomass (as represented by chlorophyll a, bottom), based on data collected from 46 RSoE sites with hard substrates between September 2003 and June 2011. Note that some outliers have been excluded from the periphyton biomass plots for clarity Assessment of summer-time 25 th percentile flows 21 at flow recorder sites at or near RSoE sites that exhibited increasing periphyton trends showed that across the region, flows during the 2003/04 and 2004/05 summers were considerably higher than the rest of the reporting period (Figure 5.10). At some rivers or streams (eg, the Ruakokoputuna River in eastern Wairarapa which is considered to have similar flow patterns to the Awhea River) there was a notable decline in 25 th percentile summer-time flows across the period while at others (eg, Porirua Stream) summer-time low flows fluctuated following a peak in 2004 and 2005. In general, Keenan et al. (2012) noted that since 2005 there have been lower annual 7-day low flows and more low flow days per year compared to average. 21 25 th percentile flow was used to indicate lowest summer-time flows when periphyton growth tends to be greatest. Other statistics such as median and minimum flow showed similar patterns. PAGE 72 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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