Updated model assessment of the effects of increased nutrient loads into Lake Benmore Prepared for Environment Canterbury August 2015 Prepared by : Bob Spigel (NIWA) David Plew (NIWA) David Hamilton (University of Waikato) Donna Sutherland (NIWA) Clive Howard-Williams (NIWA) For any information regarding this report please contact: Bob Spigel Scientist Hydrodynamics +64-3-343 8020 [email protected] National Institute of Water & Atmospheric Research Ltd PO Box 8602 Riccarton Christchurch 8011 Phone +64 3 348 8987 NIWA CLIENT REPORT No: CHC2015-089 Report date: August 2015 NIWA Project: ENC14506 Quality Assurance Statement Reviewed by: Sandy Elliot Approved for release John Quinn by: Ohau C Canal inflow entering Haldon Arm, Lake Benmore. [Donna Sutherland, NIWA] © All rights reserved. This publication may not be reproduced or copied in any form without the permission of the copyright owner(s). Such permission is only to be given in accordance with the terms of the client’s contract with NIWA. This copyright extends to all forms of copying and any storage of material in any kind of information retrieval system. Whilst NIWA has used all reasonable endeavours to ensure that the information contained in this document is accurate, NIWA does not give any express or implied warranty as to the completeness of the information contained herein, or that it will be suitable for any purpose(s) other than those specifically contemplated during the Project or agreed by NIWA and the Client. Contents Extended summary ............................................................................................................. 6 1 Introduction ............................................................................................................ 12 1.1 Background ............................................................................................................. 12 1.2 Scope of Work ......................................................................................................... 13 2 Methods .................................................................................................................. 17 2.1 The models .............................................................................................................. 17 2.2 Input data................................................................................................................ 19 2.3 Model configuration and calibration ...................................................................... 33 2.4 Scenario nutrient loads ........................................................................................... 34 2.5 Modelling strategy .................................................................................................. 43 3 Results .................................................................................................................... 45 3.1 Validation of model ................................................................................................ 45 3.2 Modelling TLI and its components – Chl-a, TN, TP ................................................. 53 3.3 Scenario testing ...................................................................................................... 56 4 Discussion ............................................................................................................... 59 4.1 Model uncertainty .................................................................................................. 59 4.2 Concern over matching with in-lake parameters ................................................... 59 4.3 Indications of nutrient limitation ............................................................................ 59 4.4 TLI simulations ........................................................................................................ 60 4.5 Land-use changes and nutrient input loads............................................................ 60 4.6 Final comments ....................................................................................................... 61 5 Acknowledgements ................................................................................................. 62 6 References ............................................................................................................... 63 7 Glossary of abbreviations and terms ........................................................................ 65 Appendix A Nutrient concentration vs stream discharge into Lake Benmore ........ 66 Appendix B Inflow sampling and laboratory validation of the Ahuriri Arm ........... 73 Introduction ...................................................................................................................... 73 Methods ............................................................................................................................ 73 Updated model assessment of the effects of increased nutrient loads into Lake Benmore Results ............................................................................................................................... 76 Discussion .......................................................................................................................... 77 Appendix C TLI values in 2008-09 (from Norton et al. 2009) ................................. 78 Appendix D CAEDYM derived nutrient limitation functions for diatoms ............... 80 Appendix E Remote sensing of vegetation adjacent to Ahuriri Arm ...................... 84 Introduction ...................................................................................................................... 84 Method .............................................................................................................................. 84 Study site ........................................................................................................................... 85 Results and Discussion ...................................................................................................... 85 Potential for further analyses ........................................................................................... 91 Reference .......................................................................................................................... 91 Tables Table 2-1: Nitrogen loads calculated for the model. 36 Table 2-2: Phosphorus loads calculated for the model. 38 Table 3-1: Lake Benmore - Baseline conditions. 56 Table 3-2: Lake Benmore modelled TLIs for Scenarios 2011-13. 57 Figures Figure 2-1: 400 m x 400 m model grid used for the Lake Benmore model application. 18 Figure 2-2: Inflows into the Haldon Arm from 2011 to 2013. 23 Figure 2-3: Gauged inflow into the Ahuriri Arm and ungauged inflow into the Ahuriri and Haldon Arms from 2011 to 2013. 24 Figure 2-4: Nitrogen species concentrations in the inflow and lake sites for the Ahuriri Arm from 2011-2013. 25 Figure 2-5: Nitrogen species concentrations in the inflow and lake sites for the Ahuriri Arm from 2011-2013. 26 Figure 2-6: Phosphorus species concentrations in the inflow and lake sites for the Ahuriri Arm from 2011-2013. 27 Figure 2-7: Phosphorus species concentrations in the inflow and lake sites for the Ahuriri Arm from 2011-2013. 28 Figure 2-8: Nitrogen species concentrations in the inflow and lake sites for the Haldon Arm from 2011-2013. 29 Figure 2-9: Nitrogen species concentrations in the inflow and lake sites for the Haldon Arm from 2011-2013. 30 Figure 2-10: Phosphorus species concentrations in the inflow and lake sites for the Haldon Arm from 2011-2013. 31 Figure 2-11: Phosphorus species concentrations in the inflow and lake sites for the Haldon Arm from 2011-2013. 32 Updated model assessment of the effects of increased nutrient loads into Lake Benmore Figure 2-12: Locations of water quality flows and loads used in the CLUES model (green circles). 35 Figure 3-1: Simulated (lines) and observed (dots) values for Chlorophyll-a, TN, NO 3-N, NH 4- N, TP and PO 4 at three depths in the Haldon Arm. 47 Figure 3-2: Simulated (lines) and observed (dots) values for POC, DOC, PON, DON, POP, DOP at three depths in the Haldon Arm. 48 Figure 3-3: Simulated (lines) and observed (dots) values for Chlorophyll-a, TN, NO 3-N, NH 4- N, TP and PO 4 at three depths in the Ahuriri Arm. 49 Figure 3-4: Simulated (lines) and observed (dots) values for POC, DOC, PON, DON, POP, DOP at three depths in the Ahuriri Arm. 50 Figure 3-5: Simulated (lines) and observed (dots) values for Chlorophyll-a, TN, NO 3-N, NH 4- N, TP and PO 4 at three depths in the Lower Benmore Basin. 51 Figure 3-6: Simulated (lines) and observed (dots) values for POC, DOC, PON, DON, POP, DOP at three depths in the Lower Benmore Basin. 52 Figure 3-8: Trophic Lake Index (Mean TLI) in 2011-13 calculated for summer in the Ahuriri Arm (Left Panel) and the Haldon Arm (Right Panel). 55 Figure 3-9: 3-D representations of the N and P loads in the different scenarios versus simulated TLI values. 58 fa Figure A-1: Correlation of phosphorus concentration and flow in the Ahuriri River. 66 Figure A-2: Nitrogen (N) and phosphorus (P) species concentrations versus flow in the Ahuriri River. 67 Figure A-3: Nitrogen (N) and phosphorus (P) species concentrations versus flow in the Ohau C Canal. 68 Figure A-4: Nitrogen (N) and phosphorus (P) species concentrations versus flow in the Tekapo-Pukaki River. 69 Figure A-5: Nitrogen (N) and phosphorus (P) species concentrations versus flow in the Tekapo-Pukaki River. 70 Figure A-6: Dissolved organic carbon (DOC) & particulate