Copyright is owned by the Author of the thesis. Permission is given for a copy to be downloaded by an individual for the purpose of research and private study only. The thesis may not be reproduced elsewhere without the permission of the Author. A record of natural and human- induced environmental change from Lake Horowhenua A thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Earth Science School of Agriculture and Natural Environment, Massey University, Palmerston North, New Zealand Celeste Bevins 2019 Lake Horowhenua at sunset. Abstract Lake Horowhenua is a hypertrophic turbid lake located in the western coastal plain of the lower North Island of New Zealand. In order to effectively restore or manage modified systems such as Lake Horowhenua, an understanding of past environmental change and natural variability is essential to provide a benchmark for ‘natural’ conditions. Cores from the bed of Lake Horowhenua have been analysed to reconstruct a detailed environmental record for the last c. 4,200 cal yr BP. Prior to lake formation, the area now occupied by the lake was subject to fluvial deposition from the Ohau River sometime prior to 7,500 cal yr BP. Dune transgression began in the region c. 7,700 cal yr BP at the very earliest, and drainage of the small streams and springs was impeded, allowing for the formation of a proto lake. A tidal surge up the Hokio Stream may have occurred c. 7,100 cal yr BP. Clastic delivery into the lake from the inflowing streams was high from 4,200 cal yr BP until sometime around 3,200 cal yr BP. The lake then transitioned into a phase of increased autochthonous sedimentation. During this phase, stable bottom water oxygenation, high water quality, and little catchment disturbance dominated. The pollen record indicates that the pre- human vegetation was a lowland podocarp-hardwood forest, dominated by Dacrydium cupressinum. There is some evidence of vegetation response to long-term climate change associated with increasing intensification of the westerly circulation regime through the mid to late Holocene. Proxy evidence implies human arrival to the Lake Horowhenua area occurred c. 519-486 cal yr BP, and land clearance was initiated on the plains inducing erosion within the catchment. However, sedimentation rates did not change from pre-human levels and water quality remained high. European arrival c. 1840 AD saw a further acceleration of land clearance on the plains and in the uplands, the introduction of exotic plant species for forestry, increased pastoral land for agriculture, and a reduction in water quality. Aquatic weed growth began at 1898 AD and was prolific from 1985 AD. High nutrient levels are evident from c. 1985 AD. Sedimentation rates and erosion increased significantly to 0.34 cm a-1 compared to pre-human (0.08 cm a-1) and Māori settlement (0.07 cm a-1). i Acknowledgements First and foremost, I would like to thank the Muaūpoko Tribal Authority and the Lake Horowhenua Trust for access to Lake Horowhenua and their permission to study this taonga. I would also like to thank Noel Procter for access via his land. I would sincerely like to thank my supervisors, Dr Kat Holt and Dr Jon Procter (Massey University) for giving me the opportunity to study Lake Horowhenua. A huge thanks to Kat for her guidance on this thesis, and introducing me to the art of pollen counting and paleoenvironmental change. I would like to thank Massey University staff for their support in the lab, field, and during discussions. Thanks to David Feek for his help and strength during coring operations. A further thanks to Dr Anja Moebis, Ian Furkert, Dr Paramsothy Jeyakumar, Dr Qinhua Shen, and Julie Palmer for their help with answering all my questions. Thanks to Marcela Gonzalez for helping with the MP-AES. Thanks to Dr Alastair Clement for the discussions on the sand deposit. Thanks to Piet Verburg and Max Gibbs (NIWA), and Marcus Vandergoes (GNS Science). I am grateful for their advice and the discussions on this project. I would like to express my gratitude to Andrew Rees (VUW) for arranging for me to come to VUW, answering my questions on grain size analysis, and helping with the age-depth model. Thanks to Julia Peters, Marozane Spammers, and Wendy Payne for proofreading. Thanks to Amber Brine for photoshopping my cores back together, and Hannah Walters for helping in the lab. I would like to thank the Graduate Women Manawatū (GWM) Charitable Trust, Massey University and the Geography Department for their financial support. This work was supported through a GWM Postgraduate Scholarship, Massey University Masterate Scholarship, and the Geography Student Research Fund. This research was funded by MBIE’s Vision Matauranga Capability Fund. I would like to thank my extended family and friends, particularly Mum, Dad, Amber, and Tyler, and their families. They never left it more than a month between visits and took my mind off things when it was needed most. Lastly, and most importantly, I dedicate this thesis to Liam Parkins, my husband, whose support both emotionally and financially allowed me to leave my job and find my passion. Without him, this would have been a pipe dream. ii Table of Contents Page Abstract ........................................................................................................................................... i Acknowledgements ....................................................................................................................... ii Table of Contents ......................................................................................................................... iii List of Figures ............................................................................................................................... vi List of Tables ................................................................................................................................ ix Chapter 1: Introduction ............................................................................................................. 1 1.1 General introduction ................................................................................................................ 1 1.2 Motivation for research ............................................................................................................ 2 1.3 Research aims and objectives ................................................................................................. 2 1.4 Thesis structure ........................................................................................................................ 3 Chapter 2: Background information .......................................................................................... 4 2.1 Regional and geological setting ............................................................................................... 4 2.2 Climate ..................................................................................................................................... 5 2.3 Lake description ....................................................................................................................... 6 2.4 Development of Levin and Horowhenua ................................................................................. 7 2.5 Vegetation ................................................................................................................................ 8 2.6 Environmental degradation of Lake Horowhenua ................................................................. 12 2.7 Literature review .................................................................................................................... 15 2.7.1 Introduction ............................................................................................................ 15 2.7.2 Measuring baseline conditions and establishing reference conditions ................... 15 2.7.3 Natural variability .................................................................................................. 17 2.7.4 Human arrival in New Zealand .............................................................................. 20 2.7.5 Anthropogenic influences and evidence of impacts on lakes and surrounding environs in New Zealand ................................................................................................ 22 2.7.6 Summary ................................................................................................................ 24 Chapter 3: Methodology ............................................................................................................ 25 3.1 Coring .................................................................................................................................... 25 3.2 Sand deposit ........................................................................................................................... 28 3.3 Core stratigraphy .................................................................................................................... 29 3.4 Chronology ............................................................................................................................ 29 3.4.1 Radiocarbon dating ................................................................................................ 29 3.4.2 Lead-210 and cesium-137 concentration dating .................................................... 32 3.5 Bulk density measurements ..................................................................................................