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

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Water quality (n=24) Periphyton (n=24) Macroinvertebrates (n=24) Water quality (n=23) Periphyton (n=17) Macroinvertebrates (n=23) Water quality (n=7) Periphyton (n=4) Macroinvertebrates (n=7) River and stream water quality and ecology in the Wellington region: State and trends Indigenous Forest Pasture 0 10 20 30 40 50 60 70 80 90 100 Percentage of sites (%) Excellent Good Fair Poor WGN_DOCS-#1100598-V3 PAGE 101 OF 160 Urban Figure 8.1: Breakdown of water quality, periphyton and macroinvertebrate health grades (classes) for 54 of the 55 RSoE sites, grouped according to their REC landcover class. Note that one site (RS44) belonging to the exotic forestry landcover class is not included in this summary forested Tararua, Rimutaka and Aorangi ranges. However, as the proportion of pastoral and/or urban landcover increases within a site’s upstream catchment, water quality and macroinvertebrate health tend to decline while nuisance periphyton and macrophyte growth increases. Of the 15 sites graded ‘poor’ for water quality, five are located in urban areas and ten drain predominantly pastoral catchments (of which most support at least some intensive agriculture, typically dairying). Most (12) of these sites are also located on relatively small (fourth order or less) river and streams, which suggests that these are more susceptible to degradation than larger rivers. Similar patterns were observed with periphyton growth and macroinvertebrate health, although geological and climatic influences mean that some sites assigned to ‘fair’ or ‘poor’ periphyton and/or macroinvertebrate classes (eg, those in the eastern Wairarapa hill country) are likely to naturally score lower for these indicators. Application of the Index of Biotic Integrity (IBI) to fish data held within the New Zealand Freshwater Fish Database (NZFDD) also showed that a significant relationship exists between fish community condition and upstream catchment landcover in the Wellington region. Fish community condition is significantly higher at sites located on rivers and streams draining predominantly indigenous forest catchments than at those draining pastoral or urban catchments. However, in some cases, fish community condition contrasts significantly with the water quality, periphyton and macroinvertebrate indicators. For example, some urban and pastoral sites (eg, Whareroa Stream at Queen Elizabeth Park) can have high fish community condition while some indigenous forest sites (including RSoE ‘reference’ sites, such as Beef Creek at Headwaters) can have poor community condition. Contrasting results between fish and other indicators are not unexpected; the Wellington region’s fish fauna are largely migratory and therefore respond to catchment scale influences as well as local habitat (cf. water quality, periphyton and macroinvertebrates which tend to respond to upstream or local influences), meaning that they can
River and stream water quality and ecology in the Wellington region: State and trends be significantly impacted by physical barriers (eg, culverts, weirs) that prevent upstream fish migration. 8.1.2 Temporal trends Temporal trend analysis performed on both raw and flow-adjusted physicochemical and microbiological water quality data found that water quality was typically stable at the majority of RSoE sites between July 2006 and June 2011. A total of 78 ‘meaningful’ trends (ie, statistically significant and a relative rate of change >1%/year) were identified across nine core variables (equating to 14.1% out of a total of 411 possible trends), with half of these trends associated with just three variables: total phosphorus (TP, 15 sites), nitrite-nitrate nitrogen (NNN, 13 sites) and total nitrogen (TN, 11 sites). In almost all cases, the trends in nutrient concentrations were decreasing. Such trends were present at a number of sites, including both nutrient-rich (eg, Mangaone Stream at Sims Road Bridge) and nutrient-poor (eg, Hutt River at Te Marua) sites. The reasons for the improvements are not known, but given that reference sites were well represented, these ‘improving’ trends may be related more to natural factors (eg, climate variability) than changes in land use/management. More importantly, while a number of ‘degraded’ sites showed improvements, these sites – as well as many sites that did not exhibit any meaningful trends – still recorded median nutrient concentrations well above ANZECC (2000) TVs. A few sites exhibited trends indicative of deteriorating water quality – such as an increase in DRP concentration (Mangaroa River at Te Marua and Whangaehu River 250 m from confluence) and a decline in visual clarity (eg, Horokiri Stream at Snodgrass and Whareroa Stream at Waterfall Road). In most cases, the causes for these trends are not known; as noted in Section 4.2.2 and discussed further in Section 8.3.2, declining water clarity in the Horokiri Stream may be associated with soil/stream bank erosion and sediment runoff from forestry activities. The results of trend analysis conducted on water quality data from NIWA’s monitoring sites located on the Ruamahanga (three sites) and Hutt (two sites) rivers for the same five-year period were not too dissimilar from the findings for RSoE sites located on these rivers. In fact, analysis over a longer time period (1989 to 2007) by Ballantine and Davies-Colley (2010) suggests that water quality in both of these rivers has remained relatively stable for some time. Where trends were identified, these were typically of a very small magnitude (eg, an increase in flow-adjusted NNN concentration of 0.006 mg/L/year was observed for the site Ruamahanga River at Wardells). Periphyton cover/biomass and macroinvertebrate community health were also stable at the majority of RSoE sites. However, and in contrast with water quality, where trends were present in one or more periphyton or macroinvertebrate metrics (19 and 15 sites, respectively, for the period 2004 to 2011), these were overwhelmingly indicative of deteriorating ecosystem health and tended to occur mainly at ‘impacted’ sites (ie, sites located in pastoral and urban catchments). The majority of the observed trends in these metrics did not coincide with one another, nor with any trends in water quality. It is likely that variation in climatic factors – in particular the frequency of flushing flows – PAGE 102 OF 160 WGN_DOCS-#1100598-V3
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