Potential Future Changes in Mangrove-Habitat in Auckland’s East-Coast Estuaries June TR 2009/079 Auckland Regional Council Technical Report No.079 June 2009 ISSN 1179-0504 (Print) ISSN 1179-0512 (Online) ISBN 978-1-877528-91-0 Reviewed by: Approved for ARC Publication by: Name: Megan Stewart Name: Grant Barnes Position: Group Manager, Monitoring and Position: Project Leader – Marine Research Organisation: Auckland Regional Council Organisation: Auckland Regional Council Date: 23rd March 2010 Date: 23rd March 2010 Recommended Citation: Swales, A.; Bell, R.G.; Gorman, R.; Oldman, J.W.; Altenberger, A. ; Hart, C.; Claydon, L.; Wadhwa, S.; Ovenden, R. (2008). Potential future changes in mangrove-habitat in Auckland’s east-coast estuaries. Prepared by NIWA for Auckland Regional Council. Auckland Regional Council Technical Publication Number TR 2009/079. © 2008 Auckland Regional Council This publication is provided strictly subject to Auckland Regional Council's (ARC) copyright and other intellectual property rights (if any) in the publication. Users of the publication may only access, reproduce and use the publication, in a secure digital medium or hard copy, for responsible genuine non-commercial purposes relating to personal, public service or educational purposes, provided that the publication is only ever accurately reproduced and proper attribution of its source, publication date and authorship is attached to any use or reproduction. This publication must not be used in any way for any commercial purpose without the prior written consent of ARC. ARC does not give any warranty whatsoever, including without limitation, as to the availability, accuracy, completeness, currency or reliability of the information or data (including third party data) made available via the publication and expressly disclaim (to the maximum extent permitted in law) all liability for any damage or loss resulting from your use of, or reliance on the publication or the information and data provided via the publication. The publication and information and data contained within it are provided on an "as is" basis. Potential future changes in mangrove-habitat in Auckland’s east-coast estuaries A. Swales R. G. Bell R. Gorman J. W. Oldman A. Altenberger C. Hart L. Claydon1 S. Wadhwa R. Ovenden (1) New Zealand Aerial Mapping, Hastings Prepared for Auckland Regional Council Environmental Research © All rights reserved. This publication may not be reproduced or copied in any form without the permission of the client. Such permission is to be given only 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. NIWA Client Report: HAM2008-030 November 2009 NIWA Project: ARC08216 National Institute of Water & Atmospheric Research Ltd Gate 10, Silverdale Road, Hamilton P O Box 11115, Hamilton, New Zealand Phone +64-7-856 7026, Fax +64-7-856 0151 www.niwa.co.nz Contents 1 Executive Summary 1 2 Glossary of terms 8 3 Introduction 13 3.1 Study objectives 13 3.2 Study estuaries 14 3.3 Background 16 3.3.1 Sedimentation rates in Auckland estuaries 17 3.3.2 Mangrove-habitat expansion 19 3.3.3 Sea level and vertical datums 20 3.3.4 Sea-level rise projections 22 4 Methods 27 4.1 Overview 27 4.2 Mapping the mean tide level (MTL) 2007 28 4.2.1 Aerial Photographic Survey 28 4.2.2 Tidal-flat Elevation Surveys 33 4.3 Recent estuary sedimentation 36 4.4 Map historical changes in mangrove habitat 39 4.5 Effect of tidal amplification on the MTL-2007 isobath 39 4.5.1 Present-day estuaries: model set-up 41 4.5.2 Past and Future estuaries: model set-up 42 4.6 Wave-exposure effect on mangrove-habitat extent 48 4.6.1 Empirical wave model 48 4.6.2 Sediment entrainment by waves 50 4.6.3 Influence of wave-exposure on the lower elevation limit of mangroves 50 4.7 Predicting changes in potential mangrove habitat area and distribution 51 4.7.1 GIS analysis and mapping 51 4.7.2 Hindcast mangrove habitat in c. 1950 A.D. 52 4.8 Potential for future changes in mangrove habitat 53 4.9 Sediment accommodation in mangrove habitats 55 4.9.1 Catchment sediment loads 56 5 Results 57 5.1 Catchment sediment loads 57 5.2 Sediment accommodation space in existing mangrove habitats 59 5.3 Estuary infilling as a function of system properties 60 5.4 Present-day mangrove habitat extent in estuaries 63 5.4.1 Lower elevation limit of mangroves in estuaries 64 5.5 Hydrodynamic effects on mean tide level 68 5.6 Hindcast & projected future changes in tidal-flat habitat 79 5.7 Historical changes in mangrove habitat 80 5.7.1 Hindcast changes in potential mangrove habitat 80 5.7.2 Observed changes in mangrove habitat 81 5.7.3 Validation of the hindcast MTL-1950 isobath 98 5.8 Future changes in potential mangrove habitat 99 5.8.1 Whangateau Estuary (Omaha) 103 5.8.2 Matakana Estuary 107 5.8.3 North Cove and Bon Accord (Kawau Island) 110 5.8.4 Mahurangi Harbour 113 5.8.5 Puhoi Estuary 116 5.8.6 Waiwera Estuary 118 5.8.7 Orewa Estuary 120 5.8.8 Weiti Estuary 122 5.8.9 Okura Estuary 124 5.8.10 Upper Waitemata Harbour 126 5.8.11 Central Waitemata Harbour 128 5.8.12 Shoal Bay 130 5.8.13 Hobson Bay 134 5.8.14 Tamaki Estuary 136 5.8.15 Whitford Bay 142 5.8.16 Wairoa Estuary (Clevedon) 145 5.8.17 Te Matuku Bay (Waiheke) 148 5.8.18 Awaawaroa Bay (Waiheke) 151 5.8.19 Putiki Bay (Waiheke) 154 6 Synthesis 157 6.1 Influence of estuary infilling on mangrove-habitat expansion 157 6.2 Controls on mangrove-habitat expansion 158 6.2.1 Propagule dispersal 158 6.2.2 Tidal-flat elevation 159 6.2.3 Wave climate 159 6.3 Historical trends in mangrove-habitat expansion 159 6.4 Potential for future mangrove habitat expansion 161 6.4.1 Oceanographic and hydrodynamic effects 167 6.5 Sources of uncertainty 168 7 Acknowledgements 172 8 References 173 9 Appendices 181 9.1 Appendix 1: Ground control points for aerial photography 181 9.2 Appendix 2: Comparison of elevation survey methods 183 9.3 Appendix 3: Details of RTK-GPS surveys in the study estuaries 184 9.4 Appendix 4: Aerial photographic survey – dates and times 186 9.5 Appendix 5: Present-day estuaries – calibration of hydrodynamic models 190 9.6 Appendix 6: Past and future bathymetry for modeled estuaries 200 9.6.1 Mahurangi Estuary bathymetry hindcasts and forecasts 200 9.6.2 Okura Estuary bathymetry hindcasts and forecasts 202 9.6.3 Waitemata Harbour bathymetry hindcasts and forecasts 203 9.6.4 Tamaki Estuary bathymetry hindcasts and forecasts 205 9.6.5 Whitford Bay bathymetry hindcasts and forecasts 207 9.6.6 Appendix 7: Present-day mangrove-habitat extent and mean tide level 209 ___________________________________________________________________________________ Reviewed by: Approved for release by: Doug Ramsay David Roper Formatting checked 1 Executive Summary Mangrove-habitat expansion has been documented in many of Auckland’s east-coast estuaries over the last several decades. Historical accounts also indicate that substantial loss of mangrove-habitat occurred locally during the first half of the 20th century. This was associated with reclamations for farmland, landfills, roads and ports, as well as construction of causeways that restricted tidal flows. Some of this loss would have been offset by mangrove-habitat expansion at unaffected sites. However, it is difficult to quantify the net effect of these natural and human-induced changes in mangrove habitat in Auckland’s estuaries that predate the systematic aerial photographic surveys that began in NZ in the 1940s (Morrisey et al., 2007). Increases in mangrove habitat since that time have coincided with estuary infilling due to accumulation of sediments eroded from the land. Although sedimentation is a natural process, increased soil erosion due to catchment deforestation (from the mid-1800s), conversion to pasture and rapid urban development over the last 50 years or so has accelerated sedimentation in Auckland’s estuaries. These eroded sediments have built intertidal flats that provide potential habitat for mangroves. In the Auckland Region, changes in mangrove distribution have been documented by several studies (section 3.3.2). However, there has been no comprehensive regional-scale assessment of historical changes in mangrove habitat. Mangroves provide a number of ecosystem services in estuaries, which include: trapping of fine-sediments and associated stormwater contaminants; carbon sinks; primary production; habitat for juvenile fish; roosting, feeding and breeding sites for birds; and protection of low-lying shorelines from wave erosion (section 3). However, there are community concerns about mangrove-habitat expansion in Auckland’s estuaries, which relate to amenity and aesthetic values and loss of other estuarine habitats. This has led to calls for more active management of mangroves in some estuaries. In response to the potential effects of estuary sedimentation on mangrove habitat extent, the Auckland Regional Council contracted NIWA to evaluate the potential for mangrove-habitat expansion in Auckland’s east-coast estuaries over the next 100 years (Figure 3.1). The present study relates specifically to likely changes in mangrove- habitat due to the deposition of catchment sediments in the study estuaries. This work does not address other potential ecological effects related to estuary sedimentation, such as changes in benthic community composition on tidal flats, which may occur irrespective of mangrove response to terrigenous sediment inputs. The present study draws on existing information which include: historical records and projections of future sea-level rise; sediment accumulation rates (SAR); and recent mangrove research in NZ estuaries.