State and Trends of River and Lake Water Quality in the Otago Region 2000-2020 Otago Regional Council Private Bag 1954, Dunedin 9054 70 Stafford Street, Dunedin 9016 Phone 03 474 0827 www.orc.govt.nz © Copyright for this publication is held by the Otago Regional Council. This publication may be reproduced in whole or in part, provided the source is fully and clearly acknowledged. ISBN Report writer: Rachel Ozanne, Freshwater Science Team, Otago Regional Council Data and statistical analyses performed by: Landwaterpeople (LWP Ltd) Reviewed by: LWP Ltd Published 2 Otago Water Quality: Rivers and Lakes 2000-2020 3 Otago Water Quality: Rivers and Lakes 2000-2020 Contents Executive Summary ............................................................................................................................... 14 Introduction ................................................................................................................................. 15 Otago Region ................................................................................................................................ 16 2.1 Region Description ................................................................................................................ 16 2.2 Freshwater management units ............................................................................................. 16 2.3 Upper Lakes Rohe (Clutha Matu/Au FMU) ........................................................................... 18 2.4 Dunstan Rohe (Clutha Matu/Au FMU) .................................................................................. 18 2.5 Manuherekia Rohe (Clutha Matu/Au FMU) .......................................................................... 18 2.6 Roxburgh Rohe (Clutha Matu/Au FMU) ................................................................................ 19 2.7 Lower Clutha Rohe (Clutha Matu/Au FMU) .......................................................................... 19 2.8 Taieri FMU ............................................................................................................................. 19 2.9 Dunedin Coast FMU .............................................................................................................. 20 2.10 North Otago FMU.................................................................................................................. 20 2.11 Catlins FMU ........................................................................................................................... 21 ORC monitoring programme ........................................................................................................ 22 3.1 Water Quality Sites ............................................................................................................... 22 3.2 Water quality variables ......................................................................................................... 22 3.2.1 Phytoplankton, Periphyton and Nutrients .................................................................... 22 3.2.2 Toxicants ....................................................................................................................... 23 3.2.3 Suspended sediment ..................................................................................................... 23 3.2.4 Aquatic Life ................................................................................................................... 24 3.2.5 Escherichia coli (E. coli) ................................................................................................. 25 Methods ....................................................................................................................................... 26 4.1 Water Quality State Analysis ................................................................................................ 26 4.1.1 Grading of monitoring sites .......................................................................................... 26 4.1.2 Handling censored values ............................................................................................. 28 4.1.3 Time-period for assessments ........................................................................................ 28 4.1.4 Calculation of water clarity ........................................................................................... 29 4.1.5 pH Adjustment of Ammonia ......................................................................................... 29 4.1.6 Evaluation of compliance statistics ............................................................................... 29 4.2 Water Quality Trend Analysis ............................................................................................... 30 4.2.1 River water quality data ................................................................................................ 30 4.2.2 Lake quality data ........................................................................................................... 30 4.2.3 Flow data ....................................................................................................................... 30 4.2.4 Sampling dates, seasons, and time-periods for analyses ............................................. 30 4 Otago Water Quality: Rivers and Lakes 2000-2020 4.2.5 Handling censored values ............................................................................................. 32 4.2.6 Flow adjustment ........................................................................................................... 33 4.2.7 Seasonality assessment................................................................................................. 34 4.2.8 Analysis of trends .......................................................................................................... 34 4.2.9 Trend direction assessment .......................................................................................... 34 4.2.10 Assessment of trend rate .............................................................................................. 35 4.2.11 Evaluating changes in discontinuous data .................................................................... 36 4.2.12 Interpretation of trends ................................................................................................ 37 4.2.13 River data availability .................................................................................................... 38 Results Overview .......................................................................................................................... 39 Upper Lakes Rohe (Clutha Matu/Au FMU) .................................................................................. 40 6.1 State Analysis Results ............................................................................................................ 41 6.1.1 Periphyton and Nutrients ............................................................................................. 44 6.1.2 Toxicants ....................................................................................................................... 44 6.1.3 Suspended fine sediment (Rivers) ................................................................................ 44 6.1.4 Aquatic Life (Rivers) ...................................................................................................... 44 6.1.5 Human health for recreation ........................................................................................ 45 6.2 Trend Analysis ....................................................................................................................... 46 6.3 Water quality summary and discussion: Upper lakes Rohe ................................................. 47 Dunstan Rohe (Clutha Matu/Au FMU) ......................................................................................... 50 7.1 State Analysis Results ............................................................................................................ 51 7.1.1 Periphyton and Nutrients ............................................................................................. 54 7.1.2 Toxicants ....................................................................................................................... 54 7.1.3 Suspended fine sediment (Rivers) ................................................................................ 55 7.1.4 Aquatic Life (Rivers) ...................................................................................................... 55 7.1.5 Human health for recreation (Rivers) ........................................................................... 55 7.2 Trend Analysis ....................................................................................................................... 56 7.3 Water quality summary and discussion: Dunstan Rohe ....................................................... 57 Manuherekia Rohe (Clutha Matu/Au FMU) ................................................................................. 61 8.1 State Analysis Results ............................................................................................................ 62 8.1.1 Periphyton and Nutrients ............................................................................................. 64 8.1.2 Toxicants ....................................................................................................................... 64 8.1.3 Suspended fine sediment (Rivers) ...............................................................................