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

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TN (mg/L) Temperature ( 0 c) Shade score Mean annual maximum accrual (days) 17 16 15 14 13 12 11 10 9 0.1 1 10 100 1000 10 1 0.1 Chlorophyll a (mg/m 2 ) 0.01 0.1 1 10 100 1000 25 20 15 10 5 0 200 180 160 140 120 100 80 60 40 Chlorophyll a (mg/m 2 ) 0.1 1 10 100 1000 Chlorophyll a (mg/m 2 ) 20 0.1 1 10 100 1000 Chlorophyll a (mg/m 2 ) River and stream water quality and ecology in the Wellington region: State and trends 0.0 0.1 1 10 100 1000 Chlorophyll a (mg/m 2 ) WGN_DOCS-#1100598-V3 PAGE 63 OF 160 TP (mg/L) Water clarity (m) Habitat score Substrate index 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.1 0.01 0.001 0.1 1 10 100 1000 160 140 120 100 80 60 Chlorophyll a (mg/m 2 ) 40 0.1 1 10 100 1000 6.2 6.0 5.8 5.6 5.4 5.2 5.0 4.8 4.6 4.4 4.2 Chlorophyll a (mg/m 2 ) 4.0 0.1 1 10 100 1000 Chlorophyll a (mg/m 2 ) Figure 5.3: Scatter plots showing the relationships between mean periphyton biomass (as chlorophyll a) and selected environmental variables, based on data collected at 46 RSoE sites between July 2008 and June 2011. All environmental variables are medians apart from shade and habitat scores (one-off measures), substrate (mean of three assessments) and mean annual maximum accrual (determined by Thompson & Gordon (2010b)). Note the log (and different) scales on some axes
River and stream water quality and ecology in the Wellington region: State and trends Mean and maximum mat periphyton cover demonstrated few statistically significant correlations apart from moderate to weak correlations with water temperature, shade, dissolved oxygen and dissolved inorganic nitrogen (Table 5.4). It is important to note that a strong correlation between periphyton growth and a particular environmental variable does not necessarily mean that there is a cause and effect relationship between the two. Although variables such as water temperature, shade and nutrient concentrations have been shown to directly affect periphyton growth (MfE 2000), other variables are unlikely to directly influence periphyton growth. For example, the moderate correlation between filamentous periphyton cover and pH is likely to be related to the four RSoE sites with the highest mean filamentous periphyton cover being located in eastern Wairarapa. These sites typically have high pH (median pH of 7.6– 8.0) due to their catchment geology (see Section 4.1.5) but proliferation of filamentous periphyton is more likely to be a result of low summer-time flows and infrequent freshes that are also typical of these sites. The strong negative correlation between periphyton biomass/filamentous cover and visual (water) clarity was unexpected because all other factors being equal, higher water clarity and the associated increase in light availability to the streambed should result in increased periphyton growth. It is likely that the strong negative correlation is due to high water clarity being associated with oligotrophic conditions present at some RSoE sites. The relatively poor correlation between periphyton growth and average accrual period (rs=0.38–0.02) was also surprising given that the frequency of flushing flows is widely recognised as one of the key factors determining periphyton growth (eg, MfE 2000). The poor correlation is likely to be related to the overriding effect of land use related factors such as nutrient concentration and water temperature at some sites. For example, although sites such as Motuwaireka Stream at Headwaters and Waikanae River at Mangaone Walkway are characterised by long accrual periods (average annual maximum accrual of 110 and 97 days, respectively) they have low periphyton biomass as they are located in the headwaters of forested streams. Conversely, although Ruamahanga River at Gladstone Bridge has the second shortest average annual maximum accrual period (39 days) it has moderate periphyton growth – most likely due to moderate nutrient enrichment found at this site. Most RSoE sites assigned to the ‘excellent’ periphyton class (refer Table 5.3) were characterised by very low median concentrations of TN and/or TP (100), reflecting their location in catchments dominated by indigenous forest (refer Figure 5.1). Two notable exceptions were Whareroa Stream at Waterfall Road (Figure 5.4) and Parkvale Stream at Lowes Reserve; these two sites recorded moderate to high median nutrient concentrations but are also characterised by a high degree of shade (shade scores of 19 and 18, respectively) which is likely to be the key factor controlling periphyton growth at these sites. PAGE 64 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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