Patterns and rates of recent sedimentation and intertidal vegetation changes in the Kaipara Harbour Prepared for Auckland Council & Northland Regional Council November 2011 Authors/Contributors: A. Swales M. Gibbs R. Ovenden K. Costley N. Hermanspahn R. Budd D. Rendle C. Hart S. Wadhwa For any information regarding this report please contact: A Swales Scientist - Group Manager Coastal & Estuarine Processes +64-7-856 7026 [email protected] National Institute of Water & Atmospheric Research Ltd Gate 10, Silverdale Road Hillcrest, Hamilton 3216 PO Box 11115, Hillcrest Hamilton 3251 New Zealand Phone +64-7-856 7026 Fax +64-7-856 0151 NIWA Client Report No: HAM2011-040 Report date: November 2011 NIWA Project: ARC10224 © 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 1 Executive summary ..................................................................................................... 9 2 Background ............................................................................................................... 15 2.1 Study objectives ................................................................................................. 15 2.2 Estuary sedimentation ........................................................................................ 16 2.3 Study area .......................................................................................................... 16 3 Methods ..................................................................................................................... 27 3.1 Sediment-core sampling ..................................................................................... 27 3.2 Sediment bulk density ........................................................................................ 29 3.3 X-radiographs .................................................................................................... 29 3.4 Particle size ........................................................................................................ 30 3.5 Radioisotope dating ........................................................................................... 30 3.5.1 Sediment accumulation rates (SAR) ..................................................... 31 3.5.2 Surface mixed layer - sediment residence time .................................... 32 3.6 Recent changes in the spatial extent of vegetated intertidal habitats .................. 32 3.6.1 Analysis of aerial photographs .............................................................. 33 4 Results ....................................................................................................................... 35 4.1 Sediment core profiles ........................................................................................ 35 4.1.1 Site KAI-1 (southern Kaipara) ............................................................... 35 4.1.2 Site KAI-2 (southern Kaipara) ............................................................... 38 4.1.3 Site KAI-3 (southern Kaipara) ............................................................... 41 4.1.4 KAI-4 (southern Kaipara) ...................................................................... 43 4.1.5 Site KAI-5 (southern Kaipara) ............................................................... 45 4.1.6 Sites KAI-6 and 6A (southern Kaipara) ................................................. 48 4.1.7 Site KAI-7 (southern Kaipara) ............................................................... 51 4.1.8 Site KAI-9 (Omokoiti Flats) ................................................................... 54 4.1.9 Site KAI-10 (Omokoiti Flats) ................................................................. 56 4.1.10 Site KAI-14 (Kakarai Flats, Hoteo) ........................................................ 58 4.1.11 Site KAI-16 (Hoteo River mouth) .......................................................... 61 4.1.12 Site KAI-17 (Tauhoa River mouth) ........................................................ 64 4.1.13 Site KAI-20 (Arapaoa River) ................................................................. 67 4.1.14 Site KAI-21 (Arapaoa River) ................................................................. 69 4.1.15 Site KAI-22 (Arapaoa River) ................................................................. 71 4.1.16 Site KAI-23 (Arapaoa River) ................................................................. 75 4.1.17 Site KAI-24 (Arapaoa River) ................................................................. 77 4.1.18 Site KAI-25 (Intertidal flats, Wairoa River mouth) .................................. 80 4.2 Recent changes in vegetated intertidal habitats ................................................. 81 Patterns and rates of recent sedimentation and intertidal vegetation changes in the Kaipara Harbour 4.2.1 Summary .............................................................................................. 81 4.2.2 North Kaipara ....................................................................................... 83 4.2.3 Whakatu mangrove forest ..................................................................... 84 4.2.4 North Head ........................................................................................... 85 4.2.5 Arapaoa River ...................................................................................... 86 4.2.6 Otamatea River .................................................................................... 87 4.2.7 Oruawharo River .................................................................................. 89 4.2.8 Whakaki River ...................................................................................... 91 4.2.9 Central Kaipara .................................................................................... 92 4.2.10 Tauhoa – Hoteo .................................................................................... 95 4.2.11 Kaipara Flats ........................................................................................ 97 4.2.12 South Kaipara ....................................................................................... 99 4.2.13 Omokoiti Flats .................................................................................... 101 4.2.14 South Head ........................................................................................ 103 5 Synthesis ................................................................................................................. 105 5.1 Recent sedimentation in the Kaipara Harbour .................................................. 105 5.1.1 Reliability of the sediment dating ........................................................ 105 5.1.2 Comparison of 210Pb and 137Cs SAR estimates ................................... 106 5.1.3 Comparison of sedimentation rates with other North Island estuaries ............................................................................................. 107 5.1.4 Fine-sediment fate in the Kaipara Harbour ......................................... 110 5.2 Recent changes in intertidal vegetated habitats ............................................... 120 6 Acknowledgements ................................................................................................. 125 7 References ............................................................................................................... 126 8 Appendices .............................................................................................................. 131 8.1 Location of sampled core sites ......................................................................... 131 8.2 Appendix: Dating of estuarine sediments ......................................................... 131 8.2.1 137Cs dating ........................................................................................ 132 8.2.2 210Pb dating ........................................................................................ 133 8.2.3 Sediment accumulation rates (SAR) ................................................... 134 8.2.4 Sediment Mixing ................................................................................. 135 Tables Table 4-1: Changes in the area (hectares) of major intertidal vegetation habitats in the Kaipara Harbour: 82 Table 4-2: North Kaipara: changes in the area (hectares) of mangrove and salt-marsh habitats (1977–2002). 83 Table 4-3: Whakatu mangrove forest: changes in the area (hectares) of mangrove and salt-marsh habitats (1977–2002). 84 Patterns and rates of recent sedimentation and intertidal vegetation changes in the Kaipara Harbour Table 4-4: Arapaoa River: changes in the area (hectares) of mangrove and salt-marsh habitats (1977–2002). 86 Table 4-5: Otamatea River: changes in the area (hectares) of mangrove and salt- marsh habitats (1977–2002). 88 Table 4-6: Oruawharo River: changes in the area (hectares) of mangrove and