River and Stream Water Quality and Ecology in the Wellington Region State and Trends
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River and stream water quality and ecology in the Wellington region State and trends River and stream water quality and ecology in the Wellington region State and trends A Perrie S Morar JR Milne S Greenfield Environmental Monitoring and Investigations Department For more information, contact Greater Wellington: Wellington Masterton GW/EMI-T-12/143 PO Box 11646 PO Box 41 ISBN: 978-1-927217-12-2 (print) ISBN: 978-1-927217-13-9 (online) T 04 384 5708 T 06 378 2484 F 04 385 6960 F 06 378 2146 June 2012 www.gw.govt.nz www.gw.govt.nz www.gw.govt.nz [email protected] DISCLAIMER This report has been prepared by Environmental Monitoring and Investigations staff of Greater Wellington Regional Council (Greater Wellington) and as such does not constitute Council policy. In preparing this report, the authors have used the best currently available data and have exercised all reasonable skill and care in presenting and interpreting these data. Nevertheless, Greater Wellington does not accept any liability, whether direct, indirect, or consequential, arising out of the provision of the data and associated information within this report. Furthermore, as Greater Wellington endeavours to continuously improve data quality, amendments to data included in, or used in the preparation of, this report may occur without notice at any time. Greater Wellington requests that if excerpts or inferences are drawn from this report for further use, due care should be taken to ensure the appropriate context is preserved and is accurately reflected and referenced in subsequent written or verbal communications. Any use of the data and information enclosed in this report, for example, by inclusion in a subsequent report or media release, should be accompanied by an acknowledgement of the source. The report may be cited as: Perrie A, Morar S, Milne JR and Greenfield S. 2012. River and stream water quality and ecology in the Wellington region: State and trends. Greater Wellington Regional Council, Publication No. GW/EMI-T-12/143, Wellington. Main cover photo: The Hutt River at Silverstream Executive summary Greater Wellington Regional Council (Greater Wellington) manages water quality in rivers and streams of the Wellington region for natural state, public water supply, recreation and amenity, trout habitat, and aquatic ecosystem health. Regular monitoring of physico-chemical and microbiological water quality, together with assessments of ecosystem health, are integral in managing rivers and streams for these purposes. This monitoring is also important for understanding the potential flow-on effects for the health of downstream receiving environments such as lakes and estuaries. This report provides a comprehensive assessment of state and trends in river and stream health across the Wellington region. The assessment focuses on the results of state of the environment (SoE) water quality and ecological monitoring undertaken at 55 river and stream sites since 2004 (although the analysis of current state only uses the most recent three years of data, July 2008 to June 2011). The report also includes an assessment of state and trends in native fish communities in the Wellington region, drawing largely on data from NIWA’s National Freshwater Fish Database collected between 1990 and 2010. Analysis of water quality, periphyton and macroinvertebrate data collected at 55 SoE sites (46 in the case of periphyton) over the period July 2008 to June 2011 found clear linkages between river and stream health and catchment land use. Sites classified as having ‘good’ or ‘excellent’ water quality (49% of sites) and ecosystem health (59% and 65% of sites for periphyton and macroinvertebrate indicators, respectively) tend to be located on the upper reaches of rivers and streams that drain the forested Tararua, Rimutaka and Aorangi ranges. 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. Similar patterns were observed with periphyton growth and macroinvertebrate health, although geological and climatic factors in part account for some sites (particularly those in the eastern Wairarapa hill country) being assigned to ‘fair’ or ‘poor’ classes for these indicators. Analysis of New Zealand Freshwater Fish Database records 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. Based on limited monitoring, fish community condition at some RSoE sites contrasts significantly with the water quality, periphyton and macroinvertebrate indicators measured at these sites. The majority of SoE sites exhibited relatively stable water quality and ecological health over the time periods examined (2006 to 2011 and 2004 to 2011, respectively). Generally speaking, the majority of statistically significant trends tended to be indicative of improving water quality (predominantly declining nutrient concentrations) but deteriorating ecological condition (increasing periphyton cover/biomass and declining macroinvertebrate community health). In most cases the reasons for the observed trends were unclear. The presence of improving nutrient concentrations across a wide spectrum of sites, including several reference (pristine) sites, suggests that the improvements are more likely related to natural factors such as climate variability than changes in land use or land management practices. Similarly, variation in river and stream flow probably influenced many of the trends identified in periphyton and macroinvertebrate metrics. Overall, while the absence of wide scale deteriorating trends in water quality and ecological health is positive, many of the RSoE sites are considered ‘degraded’, with some very degraded when considered in the national context. For example, most urban sites and several lowland pastoral sites recorded nutrient concentrations well above their respective national median values for similar urban and rural streams. The RSoE sites in poorest condition – in particular those with small catchments dominated by urban or intensive agricultural land uses – share in common one or more of the following ‘stressors’: nutrient enrichment, poor water clarity, nitrate or heavy metal toxicity, microbiological contamination and instream habitat degradation. Management of these stressors requires a whole of catchment approach that addresses municipal wastewater discharges to water (in the Wairarapa Valley in particular), nutrient loss (from both overland runoff and leaching via shallow groundwater) in intensively farmed rural catchments, sediment runoff associated with erosion-prone farmland, exotic forestry and urban development, sewer infrastructure leaks/faults, urban stormwater discharges, water abstraction, and direct stock access to streams and riparian margins. Contents Executive summary i 1. Introduction 1 1.1 Report purpose 1 1.2 Report scope 1 1.3 Report outline 2 1.4 Terms and definitions 2 2. Overview of river and stream health monitoring in the Wellington region 4 2.1 Background 4 2.2 Monitoring objectives 4 2.3 Monitoring sites 4 2.4 Monitoring variables 6 2.4.1 Water quality variables 6 2.4.2 Biological variables 7 2.4.3 Supporting variables 7 3. Rivers and streams in the Wellington region 9 3.1 Permanently flowing rivers and streams 9 3.2 Intermittently flowing rivers and streams 11 3.3 Regionally significant rivers and streams 12 3.4 Key anthropogenic pressures on river and stream health 12 3.4.1 Landcover and land use 12 3.4.2 Significant consented activities 16 4. Physico-chemical and microbiological water quality – state and trends 20 4.1 Current state 20 4.1.1 Approach to analysis 20 4.1.2 Summary statistics and compliance with guideline values – overview 22 4.1.3 Water temperature 25 4.1.4 Dissolved oxygen (% saturation) 27 4.1.5 pH 29 4.1.6 Visual clarity, turbidity and total suspended solids 29 4.1.7 Conductivity 31 4.1.8 Total organic carbon 31 4.1.9 Nitrogen 32 4.1.10 Phosphorus 33 4.1.11 E. coli 35 4.1.12 Heavy metals 36 4.1.13 Water Quality Index 38 4.2 Temporal trends 43 4.2.1 Approach to analysis 43 4.2.2 Results 45 4.3 Synthesis 54 5. Periphyton – state and trends 57 5.1 State 57 5.1.1 Approach to analysis 57 5.1.2 Periphyton cover 58 5.1.3 Periphyton biomass 60 5.1.4 Periphyton classes and relationship with landcover 60 5.1.5 Relationships between periphyton growth and environmental variables 62 5.1.6 Macrophytes 65 5.2 Temporal trends 66 5.2.1 Approach to analysis 66 5.2.2 Results 67 5.3 Synthesis 73 6. Macroinvertebrates – state and trends 75 6.1 State 75 6.1.1 Approach to analysis 75 6.1.2 Macroinvertebrate metrics 77 6.1.3 Relationships between MCI, landcover and environmental variables 79 6.2 Temporal trends 81 6.2.1 Approach to analysis 81 6.2.2 Results 82 6.3 Synthesis 86 7. Fish – state and trends 89 7.1 State 89 7.1.1 Approach to analysis 89 7.1.2 Results 91 7.2 Temporal trends 98 7.2.1 Approach to analysis 98 7.2.2 Results 98 7.3 Synthesis 98 8. Discussion 100 8.1 Regional overview 100 8.1.1 State 100 8.1.2 Temporal trends 102 8.2 National context 103 8.2.1 Water quality 103 8.2.2 Ecological health 105 8.3 Key issues affecting river and stream health 105 8.3.1 Nutrient enrichment 105 8.3.2 Poor clarity and sedimentation 107 8.3.3 Toxicity 109 8.3.4 Microbiological contamination 111 8.3.5 Habitat degradation 112 8.4 Downstream receiving environments 114 8.5 Monitoring limitations and knowledge gaps 115 9.