i Anthropogenic impacts on Waituna Lagoon: Reconstructing the environmental history Sarai Cosgrove 2011 A dissertation submitted in partial fulfilment of the requirements for a Master of Science degree in Ecology at the University of Otago, Dunedin, New Zealand ii iii Abstract Waituna Lagoon is a shallow temperate coastal lagoon in Southland, New Zealand. The 1,350 hectare Waituna Lagoon and associated wetland system is of national and international importance for its unique ecology and the cultural values placed upon it. Waituna is opened to the sea ca. 1-2 times annually, during which time it becomes estuarine. This opening regime has been artificially managed since 1908, with an unknown impact on the lagoon. Since 2007, Waituna appears to have experienced rapid degradation and eutrophication, possibly due to intensification of farming in its catchment. It has rare, extensive beds of the seagrasses Ruppia megacarpa and Ruppia polycarpa, which have been lost from many similar systems due to eutrophication and sedimentation. Waituna Lagoon is currently under active management to preserve its unique ecology, and its restoration to a more natural state is being considered. This study aims to reconstruct Waituna Lagoon’s natural opening regime and Ruppia dynamics, as understanding the natural ecosystem is important for restoration. The hypotheses are: (1) that under a natural opening regime, environmental variations within Waituna Lagoon were less frequent but more extreme; (2) that Ruppia was not present in Waituna Lagoon under a natural opening regime; and (3) that Ruppia was not dominant in Waituna Lagoon under a natural opening regime. Three push cores (130, 83 and 64 cm length) were collected from the sheltered Shand Bay and from the wind-exposed, deepest site in the main lagoon. Analyses of sedimentary material included organic and water content, sediment density and grain size, 210Pb, charcoal, indicator pollen, foraminifera and macrofossils. Images of the cores were taken via x-ray and the Geotek Multi-Sensor Core Logger. 210Pb dating of the sediment was unsuccessful, but the approximate date of transition to an anthropogenically modified system was inferred to occur at ca. 50 cm depth in Shand Bay using profiles of indicator pollen and charcoal. The historical opening regime was reconstructed using strata of elevated sand content to indicate open (marine) phases and strata of elevated organic content to indicate closed (freshwater) phases. Hypothesis one was supported. Under a natural opening regime phases of marine and terrestrial influence appeared to be more pronounced, and iv potentially lasted for longer periods of time. After ca. 1900 the sediment data indicated a higher frequency of phase changes inhibiting lagoon equilibration. An exception was the most recent sediments, which suggested a marine-influenced phase. Ruppia inhabited Waituna Lagoon under a natural opening regime, and circumstantial evidence suggests that it may have been more dominant at times under the natural opening regime. Hypotheses two and three were rejected. Ruppia pollen was more abundant when the lagoon was open, possibly due to increased light availability. Sediment chronologies are not consistent across Waituna Lagoon. Sediment characteristics differ between the deeper, wind- and tide-exposed mid-lake and shallow, sheltered Shand Bay sites. Sediment strata in the cores from the two sites could not be correlated, indicating that sediment dynamics at the two sites differed markedly. Sediment deposition in the lagoon is highly episodic and appears to derive from diverse sources, making it difficult to accurately reconstruct timelines related to changes in sedimentation and lagoon conditions. v Acknowledgements “…No man is an island, entire of itself…” – John Donne (Meditation XVII) As supervisors, Marc Schallenberg from the Department of Zoology and Daphne Lee from the Department of Geology made invaluable contributions to this project. They were prompt and thorough with responses to questions and made valuable suggestions. This project could not have been completed without their generous assistance. The Department of Conservation was integral to this study. In addition to providing funding and initiating the project, various members of the Department of Conservation provided valuable information. Notable in this respect are Hugh Robertson and Brian Rance. Many people from the University of Otago contributed to this project. I was given an office in the Department of Zoology and laboratory space in the Departments of Zoology, Geology and Pharmacy. Kind assistance and support in the way of feedback on study design, assistance with laboratory work and specimen identification, and the provision of support and general information were provided by many people to whom I am indebted. In alphabetical order, the contributors are as follows: Gerry Closs and Stephen Wing from the Division of Ecology; Sean Fitzsimons from the Department of Geography; Bob Dagg, Ewan Fordyce, Gary Wilson, Jamie Howarth and Simone Hicks from the Department of Geology; Candida Savage from the Department of Marine Science; Mike Broughton from the Department of Pharmacy; and Hamish Spencer, Karen Judge, Kate Beer, Ken Miller, Matthew Downes and Melanie Young from the Department of Zoology. Nicky McHugh from the Department of Zoology was phenomenally helpful, teaching me how to handle toxic chemicals and use numerous pieces of equipment in the lab. I was assisted in seed identification by Bev Clarkson, Bill Lee, Jamie Wood and Peter Johnson from Landcare Research; and the photography and identification of Leptospermum scoparium pollen by Liz Kennedy and Ian Raine of GNS Science. Environment Southland and the Department of Conservation kindly let me use their data on the opening regime and water level at Waituna Lagoon; and Chris Jenkins of Environment Southland answered many initial questions. vi Locals who live adjacent to Waituna Lagoon and/or frequently use it for recreational purposes were overwhelmingly supportive of this study. It was very encouraging to be met with such enthusiasm and genuine interest. Their passion for the project was contagious. Chris Owen was extremely generous, giving a day of his time and the use of his pontoon to enable our sampling, and even jumping in to help extract the cores. Many thanks to my friends, family and flatmates for the good times, the support and the crazy times. Crystal Symes went above and beyond the call of duty, spending a very long day as my field assistant right before her final exams and providing advice on statistical analyses. Sophie White and Trish Cosgrove kindly provided feedback on drafts. To all of these people and many more who offered time, assistance, smiles and hugs – thank you. vii Table of contents Abstract iii Acknowledgements v Table of contents vii List of tables xi List of figures xii 1. General introduction 15 1.1 Lake and wetland restoration 15 1.2 Paleolimnology 17 2. New Zealand’s coastal lagoons, estuaries and wetlands during the Holocene 19 2.1 Introduction 19 2.2 Formation 19 2.3 Vegetation 23 2.4 Salinity 25 2.5 Sedimentation 27 2.6 Historical anthropogenic impacts on sedimentation in New Zealand’s aquatic ecosystems 29 3. Waituna Lagoon and wetlands 35 3.1 Geology and hydrology 37 3.2 Human activities associated with Waituna Lagoon 41 3.3 Flora 44 3.4 Fauna 47 3.5 Recent degradation 48 3.6 Aim and hypotheses 49 viii 4. Methodology 53 4.1 Sampling sites and core collection 53 4.2 Sediment extrusion 56 4.3 Organic and water content 56 4.4 Sediment grain size 56 4.5 210Pb 57 4.6 Geotek Multi-Sensor Core Logger 57 4.7 Charcoal 57 4.8 Pollen 58 4.9 Foraminifera 58 4.10 Macrofossils 60 4.11 Data processing and analysis 60 5. Results 63 5.1 Sediment chronology 63 5.2 Hypothesis one: Opening regime 67 5.3 Hypotheses two and three: Ruppia 74 5.4 Intra-lagoon variation 77 6. Discussion 81 6.1 Inferred sediment chronology 81 6.2 Hypothesis one: Opening regime 83 6.3 Hypotheses two and three: Ruppia 88 6.4 Intra-lagoon variation 92 6.5 Potential limitations 94 6.6 Recommendations for future paleolimnological studies 96 6.7 Summary: A brief environmental history of Waituna Lagoon 98 7. References 101 ix Appendix A. Detailed methodology and rationale 131 A.1 Sampling sites and core collection 131 A.2 X-ray densitometry 132 A.3 Sediment extrusion 132 A.4 210Pb 133 A.5 Charcoal 133 A.6 Pollen 135 A.7 Foraminifera 141 A.8 Macrofossils 142 Appendix B. Additional results 145 B.1 Core imaging 145 B.2 210Pb 146 B.3 Geotek Multi-Sensor Core Logger 149 B.4 Pollen 151 B.5 Macrofossils 153 Appendix C. Ecology of select taxa found at Waituna Lagoon 155 C.1 Ammonia beccarii 155 C.2 Dacrycarpus dacrydioides 156 C.3 Dacrydium cupressinum 156 C.4 Daphnia carinata 157 C.5 Leptospermum scoparium 158 C.6 Myriophyllum triphyllum 159 C.7 Pinus radiata 160 C.8 Pteridium esculentum 161 C.9 Ruppia megacarpa and polycarpa 161 C.10 Weinmannia racemosa 164 x Appendix D. Utility of the methods 165 D.1 X-ray densitometry 165 D.2 Organic and water content 166 D.3 Sediment grain size 166 D.4 210Pb 167 D.5 Geotek Multi-Sensor Core Logger 167 D.6 Charcoal 168 D.7 Pollen 169 D.8 Foraminifera 170 D.9 Macrofossils 171 xi List of tables Table 1 Sites used in the meta-analysis of sedimentation in New Zealand aquatic systems with average sedimentation (mm/yr) 31 Table 2 Waituna Lagoon opening regime (1972-1995, 2002-2010) 41 Table 3 Summary table of the cores and proxies used for analysis 55 Table 4 Correlations between indicators of the opening