Dynamic Sedimentary Environments of Mangrove Coasts

Dynamic Sedimentary Environments of Mangrove Coasts provides knowledge on the importance of sedimentary dynamics in managing mangrove forests. In the first part of the book, the editors seamlessly offer a general introduction of mangrove sedimentary dynamics. This leads into more in-depth information on soil surface elevation change, sea level rise, and the importance of sedimentary dynamics in the loss or gain of blue carbon. The book concludes the discussion of mangrove sedimentary dynamics by addressing the issues of climate change (e.g. sea level rise and blue carbon) on mangrove restoration and sediment.

This book will assist coastal managers and academics in addressing the gaps in mangrove restoration and coastal management. As such, it will be a valuable reference for advanced undergraduate students, graduate students, researchers, academics in the field of coastal restoration, and coastal management practitioners.

• Provides a state-of-the-art summary of research into sedimentary dynamics in mangrove forests
• Includes updates on issues of climate change-relevant to mangroves, such as blue carbon and sea level rise
• Presents scientific background and successful case studies for mangrove restoration that can solve problems relating to mangrove management

• Language: English
• Publisher: Elsevier Science
• Release date: Dec 5, 2020
• ISBN: 9780128175101

Book preview

Dynamic Sedimentary Environments of Mangrove Coasts

Editors

Frida Sidik

Institute for Marine Research and Observation, Ministry of Marine Affairs and Fisheries, Indonesia

Daniel A. Friess

Department of Geography, National University of Singapore, Singapore

Table of Contents

Cover image

Title page

Copyright

Contributors

Foreword

List of reviewers

Introduction

Part I. Biogeomorphological processes

Chapter 1. Biogeomorphic evolution and expansion of mangrove forests in New Zealand’s sediment-rich estuarine systems

1. Introduction

2. Biogeomorphology

3. Mangroves in New Zealand

4. Estuary evolution

5. Case study—sand flat to mangrove forest

6. Future shock—what’s the fate of New Zealand’s mangrove forests?

7. Summary and conclusions

Chapter 2. Mangroves: a natural early-warning system of erosion on open muddy coasts in French Guiana

1. Introduction

2. Mud dynamics and vulnerability of the wave-exposed mangrove coast of French Guiana

3. Status and dynamics of mangroves on a rapidly changing muddy coast

4. Mangrove-based early warning of coastal vulnerability to mud erosion

5. Concluding remarks

Chapter 3. Groundwater research in mangrove coastal ecosystems—new prospects

1. Introduction

2. Linkage between the hydrogeological and carbon cycle in mangrove ecosystems

3. Mangrove forest degradation and loss

4. Available techniques in SGD research

5. Summary

Chapter 4. Flow and sediment dynamics around structures in mangrove ecosystems—a modeling perspective

1. Introduction

2. General hydro- and sediment dynamics in mangrove ecosystems

3. State-of-the-art numerical modeling on hydro-/sediment dynamics in mangrove ecosystems

4. Disparate treatment of spatial scales

5. Conclusions and outlook

Chapter 5. Morphological plasticity and survival thresholds of mangrove plants growing in active sedimentary environments

1. Introduction

2. Colonization and biogeomorphic succession

3. Wind resistance and mechanical stability in unstable sediments

4. Root responses for mechanical stability

5. Plastic responses to sediment dynamics in mature forests

6. Concluding remarks

Chapter 6. Microbial communities in mangrove sediments

1. Microbial communities in mangrove sediments—setting the scene

2. Microbial communities, their drivers, and their role in element cycling

3. Microbial communities in mangroves

4. Techniques to depict microbial communities and the processes they drive

5. Conclusion and perspectives

Appendix 1: Excerpts from the R scripts (full script available under: https://gitlableibniz-zmt.de/vhe/microbial-communities-in-mangroves) used for the bibliographic review, showing the different syntaxes used to group the studies by microorganism types, habitat types and applied analytical approaches

Part II. Long-term sedimentary processes and sea-level rise

Chapter 7. The history of surface-elevation paradigms in mangrove biogeomorphology

1. Introduction

2. Surface elevation determines mangrove species distribution and growth (Paradigm 1)

3. Mangroves can modify their surface elevations (Paradigm 2)

4. Conclusions

Chapter 8. Radiocarbon dating of mangrove sediments

1. Introduction

2. Brief introduction to the principles of radiocarbon dating

3. Radiocarbon dating of mangrove sediments

4. Other quantitative dating approaches

5. Conclusions

Chapter 9. Australian mangroves through the Holocene: interactions between sea level, mangrove extent, and carbon sequestration

1. Introduction

2. Early phases of the postglacial marine transgression: 13,000–9000 BP

3. Late phases of the postglacial marine transgression: 9000–6000 BP

4. Estuary infill in the mid-late Holocene

5. Contemporary patterns of mangrove–saltmarsh interaction

6. 21st century sea-level rise acceleration and the challenges of coastal planning

7. Conclusions

Chapter 10. Responses of mangrove ecosystems to sea level change

1. Introduction

2. Factors contributing to mangrove resilience to sea level rise

3. Landward migration and sediment availability in arid and semiarid areas

4. New biogeochemical setting following mangrove migration

5. Plant productivity

6. Measuring mangrove resilience to sea level rise

7. Differential responses of mangrove types (estuarine/deltaic, fringing) to sea level rise

8. Regions with mangroves under threat from sea level rise

9. Conclusions

Chapter 11. Does geomorphology determine vulnerability of mangrove coasts to sea-level rise?

1. Introduction

2. Methods

3. Results and discussion

4. Conclusions

Part III. Blue carbon

Chapter 12. Environmental drivers of blue carbon burial and soil carbon stocks in mangrove forests

1. Introduction

2. Abiotic factors

3. Biotic factors

4. Conclusion

Chapter 13. Gaps, challenges, and opportunities in mangrove blue carbon research: a biogeographic perspective

1. Introduction

2. Mangroves as blue carbon sinks: stocks versus sequestration

3. Biogeography of blue carbon research in mangroves

4. Global trends in blue C in mangroves

5. Challenges and opportunities to improve mangrove blue C inventories and mapping

Chapter 14. State of biogeochemical blue carbon in South Asian mangroves

1. Introduction

2. Overview of past and current research in South Asia

3. Mangrove blue C in the Indian Sundarbans

4. Research gaps and future directions

Chapter 15. Quantity and quality of organic matter in mangrove sediments

1. Organic matter in mangrove sediments—setting the scene

2. Quantity versus quality of SOM

3. Origin of SOM

4. Mangroves worldwide—alike but distinct

5. Conclusion

Chapter 16. Relevance of allochthonous input from an agriculture-dominated hinterland for Blue Carbon storage in mangrove sediments in Java, Indonesia

1. Introduction

2. Material and methods

3. Results

4. Discussion

5. Conclusions

Chapter 17. Potential carbon loss in sediment through methane production during early development stage of mangrove regeneration in restored mangroves

1. Introduction

2. Study area and sampling sites

3. Sample collection and analytical methods

4. Results

5. Discussion

6. Conclusion

Appendix 1: soil surface CH4 fluxes

Appendix 2: CH4 fluxes from tree stem and soil surface in the literature

Chapter 18. Blue carbon storage comparing mangroves with saltmarsh and seagrass habitats at a warm temperate continental limit

1. Introduction

2. Materials and methods

3. Results

4. Discussion

5. Conclusion

Chapter 19. Mangrove carbon sequestration and sediment deposition changes under cordgrass invasion

1. Introduction

2. Cordgrass invasion status in Chinese mangroves

3. Biological factors affecting the sediment carbon flux

4. Vegetation–sediment dynamic interactions and the consequent sedimentary processes

5. Conceptual model and evaluation of regional carbon storage

6. Conclusions and perspectives

Part IV. Mangrove management and restoration

Chapter 20. A framework for the quantitative assessment of mangrove resilience

1. Introduction

2. Methods

3. Results and discussion

4. Conclusions

Chapter 21. Assessment of typhoon impacts and post-typhoon recovery in Philippine mangroves: lessons and challenges for adaptive management

1. Introduction

2. Materials and methods

3. Results and discussion

4. Summary: vulnerability, research/management implications, and recommendations

Chapter 22. Managing sediment dynamics through reintroduction of tidal flow for mangrove restoration in abandoned aquaculture ponds

1. Introduction

2. Mangrove–sediment–hydrodynamics interactions

3. Estuarine ecohydrology in mangrove restoration and rehabilitation

4. Implications of mangrove restoration: carbon abatement opportunity and climate change mitigation

5. Case study: hydrological recovery in abandoned ponds in Perancak, Bali

6. Lesson learned from the case study

Chapter 23. Impacts of forestry on mangrove sediment dynamics

1. Introduction

2. Global overview of soil dynamics and recovery in mangrove harvest regimes

3. Postharvest impacts on mangrove soils in different harvest regimes

4. Forestry case study: soil dynamics in a large-scale selective logging regime in the Bintuni Bay mangrove, West Papua, Indonesia

5. Overview and future considerations

Appendix

Conclusions

Index

Copyright

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Contributors

Janine B. Adams,     Nelson Mandela University, Port Elizabeth, Eastern Cape, South Africa

Edward J. Anthony

LEEISA, CNRS, IFREMER, Univ Guyane, Cayenne, French Guiana, France

CEREGE, Univ Aix Marseille, CNRS, Collège de France, INRAE, IRD, Aix-en-Provence, France

Virni Budi Arifanti,     Center for Research and Development of Social, Economy, Policy and Climate Change (P3SEKPI), Ministry of Environment and Forestry, Bogor, Indonesia

Philippa Ascough,     Scottish Universities Environmental Research Centre, NERC Radiocarbon Facility, East Kilbride, United Kingdom

Thorsten Balke,     School of Geographical and Earth Sciences, University of Glasgow, Scotland, United Kingdom

Sinegugu P. Banda,     Nelson Mandela University, Port Elizabeth, Eastern Cape, South Africa

Sara Beavis,     Fenner School of Environment and Society, Australian National University, Acton, ACT, Australia

Uta Berger,     Institute of Forest Growth and Forest Computer Sciences, Technische Universität Dresden, Tharandt, Germany

Elodie Blanchard,     AMAP, University of Montpellier, IRD, CIRAD, CNRS, INRAE, Montpellier, France

Donald R. Cahoon,     U.S. Geological Survey, Patuxent Wildlife Research Center, Beltsville, MD, United States

Luzhen Chen,     Key Laboratory of the Ministry of Education for Coastal and Wetland Ecosystems, College of the Environment and Ecology, Xiamen University, Xiamen, Fujian, China

Yining Chen,     Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, Zhejiang, China

Luiz Drude de Lacerda,     Instituto de Ciências do Mar, Universidade Federal do Ceará, Fortaleza, Ceará, Brazil

Stephen Eggins,     Research School of Earth Sciences, Australian National University, Acton, ACT, Australia

Joanna C. Ellison,     School of Geography, Planning and Spatial Sciences, University of Tasmania, Launceston, Tasmania, Australia

Hongyu Feng,     Key Laboratory of the Ministry of Education for Coastal and Wetland Ecosystems, College of the Environment and Ecology, Xiamen University, Xiamen, Fujian, China

Daniel A. Friess,     Department of Geography, National University of Singapore, Singapore

Antoine Gardel,     LEEISA, CNRS, IFREMER, Univ Guyane, Cayenne, French Guiana, France

Lucy Gwen Gillis,     ZMT - Leibniz Centre for Tropical Marine Research, Bremen, Germany

Christiane Hassenrück,     ZMT - Leibniz Centre for Tropical Marine Research, Bremen, Germany

Véronique Helfer,     ZMT - Leibniz Centre for Tropical Marine Research, Bremen, Germany

Katrin Huhn,     MARUM - Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany

Tapan Kumar Jana,     Department of Marine Science, The University of Calcutta, Kolkata, West Bengal, India

Tim C. Jennerjahn

Leibniz Centre for Tropical Marine Research, Bremen, Germany

Faculty of Geoscience, University of Bremen, Bremen, Germany

Jaime Leigh Johnson,     University of the Western Cape, Bellville, Cape Town, South Africa

Jeff Kelleway,     School of Earth, Atmospheric and Life Sciences, University of Wollongong, NSW, Australia

Ken W. Krauss,     U.S. Geological Survey, Wetland and Aquatic Research Center, Lafayette, LA, United States

Denny Wijaya Kusuma,     Institute for Marine Research and Observation, Ministry of Marine Affairs and Fisheries, Bali, Indonesia

Marine Le Minor

MARUM - Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany

Géosciences Rennes, Université de Rennes 1, CNRS, UMR 6118, Campus de Beaulieu, Rennes, France

Catherine E. Lovelock,     School of Biological Sciences, University of Queensland, St Lucia, QLD, Australia

Massimo Lupascu,     Department of Geography, National University of Singapore, Singapore

Richard MacKenzie,     Institute of Pacific Islands Forestry, Pacific Southwest Research Station, USDA Forest Service, Hilo, HI, United States

Paul Macklin

National Marine Science Centre, Southern Cross University, Coffs Harbour, NSW, Australia

Department of Chemical Analysis, Faculty of Mathematics and Natural Sciences, Universitas Pendidikan Ganesha, Singaraja, Bali, Indonesia

Graham B. McBride,     National Institute of Water and Atmospheric Research (NIWA), Hamilton, Waikato, New Zealand

Karen L. McKee,     U.S. Geological Survey, Wetland and Aquatic Research Center, Lafayette, LA, United States

Wei Jian Ong,     School of Geography, Planning and Spatial Sciences, University of Tasmania, Launceston, Tasmania, Australia

Mark Pritchard,     National Institute of Water and Atmospheric Research (NIWA), Hamilton, Waikato, New Zealand

Bayu Priyono,     Institute for Marine Research and Observation, Ministry of Marine Affairs and Fisheries, Bali, Indonesia

Christophe Proisy

French Institute of Pondicherry, Pondicherry, India

AMAP, University of Montpellier, IRD, CIRAD, CNRS, INRAE, Montpellier, France

Anusha Rajkaran,     University of the Western Cape, Bellville, Cape Town, South Africa

Jacqueline L. Raw,     Nelson Mandela University, Port Elizabeth, Eastern Cape, South Africa

Raghab Ray,     Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, Japan

Michael Roderick,     Research School of Earth Sciences, Australian National University, Acton, ACT, Australia

Kerrylee Rogers,     School of Earth, Atmospheric and Life Sciences, University of Wollongong, NSW, Australia

Judith Rosentreter,     Centre for Coastal Biogeochemistry, School of Environment, Science and Engineering Southern Cross University, Lismore, NSW, Australia

Andre S. Rovai

Department of Oceanography and Coastal Sciences, College of Coast and Environment, Louisiana State University, Baton Rouge, LA, United States

Departamento de Oceanografia, Universidade Federal de Santa Catarina, Florianópolis, Santa Catarina, Brazil

Neil Saintilan,     Department of Environmental Sciences, Macquarie University, NSW, Australia

Severino G. Salmo III ,     Institute of Biology, College of Science, University of the Philippines Diliman, Quezon City, Philippines

Sigit D. Sasmito

Department of Geography, National University of Singapore, Singapore

Center for International Forestry Research (CIFOR), Jalan CIFOR, Situgede, Bogor, Indonesia

NUS Environmental Research Institute, National University of Singapore, Singapore

Juliet Sefton,     Department of Geography, Durham University, Durham, United Kingdom

Sahadev Sharma,     Institute of Ocean and Earth Sciences, University of Malaya, Kuala Lumpur, Selangor, Malaysia

Frida Sidik

Institute for Marine Research and Observation, Ministry of Marine Affairs and Fisheries, Bali, Indonesia

School of Biological Sciences, University of Queensland, St Lucia, QLD, Australia

Mériadec Sillanpää

Department of Geography, National University of Singapore, Singapore

Green Forest Product and Tech. Pte. Ltd., Research Department, Singapore

Irawan Sugoro,     National Nuclear Energy Agency of Indonesia, Jakarta, Indonesia

Sukristijono Sukardjo,     Research Center for Oceanography, Indonesian Institute of Sciences., Jakarta, Indonesia

I Gusti Ngurah Agung Suryaputra,     Department of Chemical Analysis, Faculty of Mathematics and Natural Sciences, Universitas Pendidikan Ganesha, Singaraja, Bali, Indonesia

Andrew Swales,     National Institute of Water and Atmospheric Research (NIWA), Hamilton, Waikato, New Zealand

Sartji Taberima,     Department of Soil Science and Land Resource, Faculty of Agriculture, Papua University, Manokwari, West Papua, Indonesia

Rui Xiang Teo,     Green Forest Product and Tech. Pte. Ltd., Research Department, Singapore

Robert R. Twilley,     Department of Oceanography and Coastal Sciences, College of Coast and Environment, Louisiana State University, Baton Rouge, LA, United States

Yaya I. Ulumuddin

Research Center for Oceanography, Indonesian Institute of Sciences., Jakarta, Indonesia

Fenner School of Environment and Society, Australian National University, Acton, ACT, Australia

Alejandra G. Vovides,     School of Geographical and Earth Sciences, University of Glasgow, Scotland, United Kingdom

Susan Vulpas,     Department of Environmental Science, American University, Washington D.C., United States

Romain Walcker,     Laboratoire Ecologie Fonctionnelle et Environnement, Univ Toulouse, CNRS, Toulouse, France

Raymond D. Ward

Centre for Aquatic Environments, School of the Environment and Technology, University of Brighton, Brighton, United Kingdom

Institute of Agriculture and Environmental Sciences, Estonian University of Life Sciences, Tartu, Estonia

Sarah Woodroffe,     Department of Geography, Durham University, Durham, United Kingdom

Ruhuddien Pandu Yudha,     Forestry Department, PT. Bintuni Utama Murni Wood Industries, Jakarta, Indonesia

Yihui Zhang,     Key Laboratory of the Ministry of Education for Coastal and Wetland Ecosystems, College of the Environment and Ecology, Xiamen University, Xiamen, Fujian, China

Martin Zimmer

ZMT - Leibniz Centre for Tropical Marine Research, Bremen, Germany

University of Bremen, Faculty of Biology/Chemistry, Bremen, Germany

IUCN-SSC Mangrove Specialist Group

Foreword

People see mangroves as a forest, as coastal vegetation that can withstand saline conditions and are well adapted to the harsh and dynamic intertidal environment. More generally, mangroves are an ecosystem with strong geomorphic, sedimentary, and biogeochemical links with adjacent coastal systems, such as mudflats. The long-term functioning of the mangrove ecosystem involves biogeomorphic interactions between mangroves, hydrodynamics, and sediments. The mangrove sedimentary system is very dynamic, creating an ecosystem that is able to act as a coastal stabilizer, carbon sink, and a nutrient-rich habitat that supports other ecosystems across the wider coastal landscape.

Recently, a growing awareness of mangrove in coastal protection and climate change has risen among policy makers and the general population to protect and restore mangroves. However, these laudable efforts often neglect the interaction between hydromorphodynamics and vegetation and other natural processes controlling the development of mangroves. The long-term survival of mangroves and the sustained provision of their ecosystem services are dependent on their ability to adapt to sea-level rise and other climate change impacts. Scientific knowledge of these key processes will help provide the evidence base from which policy makers and coastal managers can make suitable management and restoration decisions.

This book summarizes the latest conceptual and empirical research on sedimentary dynamics and their importance for managing mangrove forests. Part I covers the background knowledge of mangrove ecosystems and associated hydrological and sedimentary dynamics. These fundamental biogeomorphological processes have consequences on the role of mangroves in adapting to and mitigating the changing climate. Part II includes chapters on long-term sedimentary dynamics operating along mangrove coasts in response to local rises in sea level, focusing on the capacity of mangrove soil surfaces to potentially increase in elevation. Part III explores the blue carbon potential of mangrove sedimentary environments as a mitigation strategy for climate change. Both parts address the importance of the complexities of mangrove sedimentary dynamics for coastal management in the tropics and subtropics to ensure the future of mangroves. This understanding is linked to the successes and failures in mangrove management in addressing the issues of mangrove degradation and restoration (Part IV).

As the editors, we are grateful to many colleagues worldwide in mangrove science who contributed their ideas and work presenting the state of mangrove regions in this book. The Dynamic Sedimentary Environments of Mangrove Coasts encompasses a broad and multidisciplinary range of topics in mangroves around the globe, which would not be possible without the tremendous work and cooperation of the contributors. We thank them for their efforts that have remarkably shaped the direction and key messages of this book. We also greatly appreciate the reviewers who have provided valuable reviews and feedback that have strengthened the work presented here.

We hope that this book will help readers better understand the state of the art of research in mangrove sedimentary environments, and best utilize this knowledge and lessons learned to improve mangrove management in the face of global change.

Frida Sidik and Daniel A. Friess

List of reviewers

Fernanda Adame

Daniel M. Alongi

Aplena E. Bless

Clint Cameron

Luzhen Chen

Nicole Cormier

Cheryl Doughty

Daniel Friess

Glenn Guntenspergen

Erik Horstman

Mahmood Hossain

Catherine E. Lovelock

Cyril Marchand

David Neil

Jacqueline Raw

Tim Shaw

Frida Sidik

Lili Wei

Pim Willemsen

Erik Yando

Introduction

Frida Sidik ¹ , and Daniel A. Friess ² ,      ¹ Institute for Marine Research and Observation, Ministry of Marine Affairs and Fisheries, Bali, Indonesia,      ² Department of Geography, National University of Singapore, Singapore

Coastal mangrove forests serve as an important interface between the land and sea. Located in the intertidal zone, mangroves are generally distributed from mean sea level to highest spring tide, often found in the upper intertidal zone on many low-energy coasts, and can coexist with other coastal ecosystems such as seagrasses, salt marshes, and tidal flats (Alongi and Brinkman, 2011; Wolanski et al., 2009).

Their location in the intertidal zone means that mangrove forests interact with a range of processes that cross the terrestrial, coastal, and marine domains. Mangroves are a dynamic ecosystem, interacting with biophysical factors that operate over temporal scales from seconds to millennia, and at spatial scales from the individual tree to the global scale. In Dynamic Sedimentary Environments of Mangrove Coasts, we highlight key frontiers in mangrove sedimentary environments. Part I summarizes the state of the art of our knowledge of sedimentary processes influencing this important intertidal ecosystem, from microbial communities in mangrove sediments to the large-scale biogeomorphic evolution of mangrove coastlines. Part II takes a long-term and geological view of sedimentary processes in mangroves, and how they influence the vulnerability or resilience of mangroves to sea-level rise. Part III focuses on an increasingly important aspect of mangroves; how sedimentary processes contribute to the sequestration, accumulation, and storage of blue carbon. Finally, Part IV presents case studies that highlight the importance of sedimentary understanding for mangrove management, sustainable use, conservation, and restoration.

Mangrove distribution is a function of climate, biogeomorphology, and sedimentary processes

Mangroves are controlled by tidal patterns that shape mangrove forests into five principal geomorphic types: fringe, riverine, overwash, basin, and dwarf systems (Lugo and Snedaker, 1974). Woodroffe and Davies (2009) further defined mangroves based on morphological settings, which are river-dominated, tide-dominated, wave-dominated, composite river and wave-dominated, drowned bedrock valley, and carbonate settings. These classifications represent the linkages between mangroves and environmental settings (Lugo and Snedaker, 1974; Woodroffe and Davies, 2009).

The distribution of mangroves is dependent on a range of geomorphic and sedimentary processes, coastal geography, and climate (Duke et al., 1998; Osland et al., 2017). The global distribution of mangroves is constrained primarily by temperature, that limits the mangrove belt primarily (but not exclusively) to the tropical and subtropical latitudes, where air temperature are at least 20oC in the coldest month (Duke, 1992). Mangrove-temperature linkages are prominent in most subtropical mangrove coasts (e.g. America, eastern Australia, New Zealand, eastern Asia, and southeast Africa) which can result in expansion or contraction the extent of mangrove forests (Osland et al., 2017). Temperature, interplaying with rainfall, salinity, and tidal variation, contributes to mangrove species richness and biomass at the regional scale (Duke et al., 1998; Osland et al., 2017). Mangroves in wet tropics, especially in Indo-West Pacific, are often greatest in diversity and biomass, whereas arid mangrove areas are characterized by low species richness, scattered with specific tree architecture (Duke et al., 1998).

Within the broader boundary conditions provided by temperature, biophysical and sedimentary factors and their gradients become important factors driving mangrove distribution, as mangroves show species-specific tolerances and sensitivity to environmental changes (Duke et al., 1998; Woodroffe, 1992). Each mangroves species has a specific tolerance and response to gradients in salinity, soil type and chemistry, nutrient content, sediment supply, and hydrological regime, that enables them to survive in harsh environment (Alongi, 2009; Woodroffe and Davies, 2009). This suggests that, as coastal vegetation, mangroves are not a single group of species but rather a diverse range of mangrove plants structuring a forest with different levels of adaptation to a dynamic and changing environment (Smith, 1992). Vegetation community composition can thus change through time in association with the morphological evolution of shorelines (Woodroffe, 1992). The zonation also often matches with topographic contours that indicates the importance of tidal inundation as factor controlling the zonation (Duke et al., 1998). To this end, we currently focus on hydrogeomorphological processes of mangroves to place this book in context of sedimentary coasts.

Hydrogeomorphological processes can both facilitate and constrain the development of mangroves. For example, mangrove loss and regrowth have been observed simultaneously along the French Guiana mangrove coastline, which is attributed to natural processes of erosion and retreat (Anthony et al., 2013). This periodic morphological change represents the natural process of a mangrove shoreline reaching equilibrium with hydrodynamics and sediment transport (Anthony et al., 2013; Woodroffe and Davies, 2009). In most cases, mangroves actively contribute to the stabilization of land by capturing sediments, resulting in coastal accretion in the low-energy zone between the mud bank and the shore (Furukawa and Wolanski, 1996). In contrast, when mangroves receive a reduced sediment supply, they are not able to establish adequate coastal accretion due to lack of sediment trapping capacity, leading to coastline erosion (Willemsen et al., 2016).

Mangroves provide important ecosystem services to society

The natural processes controlling the interaction between morphodynamics and vegetation generate their potential as natural barriers that stabilize and protect the coastline from coastal hazards (Spalding et al., 2014). It has been claimed that mangroves can provide varying levels of protection against waves (Sánchez-Núñez et al., 2020), storm surges (Zhang et al., 2012), cyclones (Das and Vincent, 2009), and potentially some tsunamis (Danielsen et al., 2005). Functioning can only be understood by considering the interaction between trees, sediments, and hydrodynamic forcing, and their combined effects. For example, for the ecosystem service of land consolidation, the sediment, which is brought in from outside to the system (allochthonous) or produced within the ecosystem (autochthonous), is carried in suspension, trapped, and accumulated (Furukawa and Wolanski, 1996; Woodroffe, 1992). Over the course of time, the new sediments consolidate and become more compact, thus enabling mangrove propagules to colonize in the new land (Toorman et al., 2018). When mangroves grow, like all trees, they adapt and find their stability by developing an adaptive root system in the soil (McKee, 1996), spreading in horizontal direction to divide their weight over a larger area (Komiyama et al., 2008). The roots do not grow deeply due to anaerobic conditions and the low permeability of the mud, which is usually characterized by a dense sediment–water mixture dominated by clay particles (Furukawa et al., 1997; Gill and Tomlinson, 1977).

Mangrove forests also contribute to global climate regulation, due to the capacity of mangroves to sequester carbon (Howard et al., 2014; Sidik et al., 2019). Mangroves are productive coastal forests and have a large capacity for carbon storage, termed as blue carbon, which is captured from the atmosphere, transported materials from upstream and adjacent coastal environments, and organic material produced within the system (Alongi, 2014; Donato et al., 2011). Estimations of global mangrove carbon stocks vary from 4.19 to 10 Pg (Alongi, 2018; Hamilton and Friess, 2018). Unlike terrestrial forests, the largest carbon pool in mangroves is found in the soil, thus enhanced sediment accumulation is the key component to keep largest contribution of soil carbon to the total carbon stock of mangrove ecosystem, particularly minerogenic mangrove site (Alongi, 2018). Accumulation rates, sediment sources (allochthonous vs. autochthonous), and carbon storage capacity vary between and within sedimentary and geomorphic settings, resulting in a range of soil carbon stocks from <50 tons/ha to >500 tons/ha (Alongi, 2018; Lovelock et al., 2017; Rovai et al., 2018; Twilley et al., 2018). Recent estimates reveal that mangrove soils could store 2.6–6.4 Pg C in the top meter of the soil column, with Indonesia as the country with the greatest soil carbon stock (Atwood et al., 2017; Jardine and Siikamäki, 2014; Sanderman et al., 2018). These examples show the importance of understanding key sedimentary processes to better understand the creation of mangrove ecosystem services.

Threats to mangroves and their ecosystem services

While mangroves provide various ecosystem services to coastal communities, they continue to decline due to natural forces and human disturbances (Alongi, 2002). A number of studies have presented the evidence that the loss and degradation of mangroves over the last decades is due in large part to mangrove conversion to aquaculture, particularly in Southeast Asia (Thomas et al., 2017) where the largest proportion of mangrove is found globally (Bunting et al., 2018). Clearing mangroves for aquaculture ponds is also reported in South America, South Asia, and Africa, as this practice has been widespread in these regions (Feka and Ajonina, 2011; Ferreira and Lacerda, 2016; Paul et al., 2017; Rahman et al., 2010). However, mangroves also experience a number of other drivers of land cover conversion that vary regionally in importance, include oil palm in Southeast Asia, rice in Southeast Asia and West Africa, and overextraction in East Africa (Friess et al., 2019) These anthropogenic drivers of mangrove loss often interact with natural processes, such as erosion and climate change impacts, that can exacerbate mangrove loss (Gopal, 2012).

Mangrove loss and degradation have multiple potential effects, ranging from loss of protection to adjacent ecosystems, such coral reefs (Hayden and Granek, 2015), and coastal communities (Badola and Hussain, 2005; Barbier, 2016), depleted seafood resources (Barbier, 2000; Ruitenbeek, 1992) to increases in CO2 emissions (Lovelock et al., 2017; Pendleton et al., 2012). In recent years, the latter has become a concern in climate change discussions as widespread mangrove deforestation could result in the release of 0.45 Pg CO2e yr −¹ to the atmosphere (Pendleton et al., 2012). Having mangrove forests with the greatest soil carbon stock, Indonesia is also the country with the highest potential CO2 emissions due to mangrove loss, followed by Malaysia, United States, and Brazil (Atwood et al., 2017). Therefore, estimations of soil carbon stocks have become increasingly important to bring attention to the threats of deforestation, which could yield potential CO2 emissions from soils of ∼7.0 Tg CO2e yr −¹(Alongi, 2018; Atwood et al., 2017).

The role of mangroves in shaping geomorphological processes also influences their potential resilience to climate change. The capacity of mangroves to accelerate their rate of soil elevation represents one potential adaptation to the rising sea level (Cahoon and Guntenspergen, 2010). Sediment trapping can result in soil accretion that, in combination with biotic processes, contributes to surface and subsurface soil change (Cahoon et al., 2006; Friess et al., 2019). Contemporary changes of this vertical movement of coastal surfaces can be measured by rod surface elevation table–marker horizon method (RSET-MH), which has been used globally across a range of wetland ecosystems such as mangroves and salt marshes. Over longer time scales, the process of sediment deposition associated with sea-level fluctuation in mangrove coasts contributes to broader geomorphological evolution and is commonly detected by paleoecological reconstruction and radioisotope analysis (Wolanski et al., 2009; Woodroffe, 1992).

Using the sedimentary properties of mangroves to promote their conservation and restoration

Unlike terrestrial forests, although the main carbon in mangroves is found in the soil, it is often excluded from carbon credits (Wylie et al., 2016). The inclusion of the soil component in conservation and restoration programmes can assist in prioritizing management of sites in ways to preserve existing carbon sinks or increase carbon sequestration (Macreadie et al., 2017; Sidik et al., 2018, 2019). Recognizing the value of blue carbon, a growing awareness has risen among policymakers, stakeholders, and communities to protect and restore mangroves (Thomas, 2014). Mangrove restoration for carbon abatement is plausible but tends to fail if the underlying factors that cause the initial degradation or death of the habitat are not first addressed (Lewis et al., 2016). Selecting a successful approach for mangrove restoration requires knowledge of ecohydrology and other processes operating in this dynamic sedimentary environment (Anthony and Goichot, 2019; Lewis, 2005). Hydrodynamic processes can alter the establishment and early development of mangroves, from propagule dispersal to recruitment of sapling stage (Krauss et al., 2008); however, this knowledge is often neglected in restoration and plantation projects, leading to low levels of success (Iftekhar, 2008). Therefore, a key challenge is the translation of science into a format that the public and policymakers can understand and implement in mangrove management.

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Part I

Biogeomorphological processes

Outline

Chapter 1. Biogeomorphic evolution and expansion of mangrove forests in New Zealand's sediment-rich estuarine systems

Chapter 2. Mangroves: a natural early-warning system of erosion on open muddy coasts in French Guiana

Chapter 3. Groundwater research in mangrove coastal ecosystem—new prospects

Chapter 4. Flow and sediment dynamics around structures in mangrove ecosystem—a modeling perspective

Chapter 5. Morphological plasticity and survival thresholds of mangrove plants growing in active sedimentary environments

Chapter 6. Microbial communities in mangrove sediments

Chapter 1: Biogeomorphic evolution and expansion of mangrove forests in New Zealand’s sediment-rich estuarine systems

Andrew Swales, Mark Pritchard, and Graham B. McBride     National Institute of Water and Atmospheric Research (NIWA), Hamilton, Waikato, New Zealand

Abstract

Mangrove forests (Avicennia marina) occupy New Zealand's northern estuaries, which range in size from 1 to 750   km². Contrary to the global pattern of forest loss, areal expansion of New Zealand's mangrove forests has averaged 4%   yr −¹ since the 1930s. These modern mangrove forests have colonized intertidal flats as they have progressively accreted above mean sea level (MSL), following catchment deforestation (mid–late 1800s) and soil erosion resulting in 10-fold increases in estuary sedimentation rates. The fate of these mangrove ecosystems under rising sea levels depends on maintaining intertidal sedimentary environments. Data from 18 estuaries show that potential mangrove habitat (i.e., intertidal area above MSL) is well predicted by an exponential relationship with the ratio of catchment annual sediment load to estuary tidal-prism volume (r²   =   0.69, P   <   .001). Coastal embayments have the smallest areas of intertidal habitat above MSL due to limited fluvial sediment supply. Barrier-type estuaries have the largest intertidal areas above MSL and have also experienced the largest increases in mangrove forest habitat. A case study is presented from the Firth of Thames, where mangroves have colonized some 11   km² of rapidly accreting intertidal flat since the early 1960s. Seedling recruitment is largely governed by tidal and stochastic variations in water level and substrate disturbance by waves. Sediment delivery to the mangrove forest is controlled by spring-tide inundation coupled with onshore winds that resuspend intertidal muds. Surface-elevation gain is constrained by sediment desiccation and compaction during the summer. Simulations of future sea-level rise and sediment-supply scenarios indicate that intertidal habitats in small tidal creeks (0.1-km fetch [F]) and estuaries (1-km F) will maintain their elevation relative to MSL at relatively low sediment-supply rates. By contrast, intertidal flats in the largest estuaries (10-km F) will be vulnerable to inundation by rising seas. Sediment-supply rates twofold higher than historically will be required to sustain these larger systems. The potential for landward retreat of mangrove forests will be limited by storm-defence infrastructure.

Keywords

Mangroves; Estuaries; Biogeomorphology; Sediment supply; Surface elevation; Models; Sea level; New Zealand

Acknowledgments

1. Introduction

2. Biogeomorphology

3. Mangroves in New Zealand

3.1 Geographic distribution, environmental settings, and forest types

3.2 Historical patterns

3.3 Biophysical drivers

4. Estuary evolution

5. Case study—sand flat to mangrove forest

5.1 Environmental setting

5.2 Mangrove-seedling recruitment

5.3 Biogeomorphic evolution

5.4 Surface-elevation dynamics

6. Future shock—what’s the fate of New Zealand’s mangrove forests?

6.1 Elevation capital and future fate

6.2 Estuaries—predicting the shape of things to come

6.3 Legacy sediments

7. Summary and conclusions

References

1. Introduction

Mangrove forests form a major component of intertidal habitats on river deltas, muddy shores, and estuaries of the tropical Indo-Pacific, Atlantic-East Pacific Regions, and in the high-latitude temperate estuaries of Australasia and southern Africa (Giri et al., 2011; Woodroffe et al., 2016). The largest mangrove forests occur in the fluvial sediment–rich and meso- and macrotidal settings where accumulating fine sediment has formed extensive deltas and low-gradient mudflats (Woodroffe et al., 2016). These fluvial sediment mangrove forests contrast markedly with forests that develop in autochthonous systems, such as oceanic low islands and karst coasts where peat development and/or carbonate production are the major sedimentary processes controlling mangrove forest evolution (Snedaker, 1995; McKee et al., 2007).

Mangrove forest loss intensified during the 20th century primarily due to activities associated with aquaculture, land reclamation, and overutilization by growing human populations and today occupy an estimated 85–138   ×   10³   km² (Giri et al., 2011; Hamilton and Casey, 2016; Richards and Friess, 2016). There is some evidence that rates of mangrove forest loss have declined particularly in Southeast Asia over the last decade through improved management and restoration efforts (Richards and Friess, 2016). The extent of remaining mangrove forests, however, represents less than half of that which existed ~50   years ago (Spiers, 1999; Spalding et al., 2010; Giri et al., 2011).

Although the world's largest mangrove forests occur in fluvial sediment–rich settings, studies based on observations of the biogeomorphic evolution of these systems over annual to decadal time scales are rare (Walsh and Nittrouer, 2004; Swales et al., 2015, 2019; Woodroffe et al., 2016; Sidik et al., 2016). Many of the sediment-rich mangrove forests of New Zealand's northern estuaries have developed relatively recently (i.e., during the 20th century) so that we have accurate records of the environmental changes. These data include mapping of mangrove-habitat expansion and sedimentary records for a range of estuarine geomorphologies. Research undertaken over the last decade has identified key processes controlling mangrove ecology and provided new insights into the role of mangrove forests in the geomorphic evolution of these estuaries.

In this chapter, we identify the key biophysical processes controlling mangrove ecology and the role of mangrove forests in the geomorphic evolution of New Zealand's sediment-rich northern estuaries. We draw on the historical record of mangrove-habitat expansion over the last several decades and insights from sedimentary records for a range of estuary types based on their geomorphology. A case study of the biogeomorphic evolution of a mangrove forest that has occurred in a large coastal embayment (Firth of Thames) since the early 1960s is presented. Finally, using data and models, we discuss how New Zealand's mangrove forests are likely to respond to future changes in sediment supply and rates of sealevel rise (SLR).

2. Biogeomorphology

Biogeomorphology (or ecogeomorphology) focuses on the interface between the earth and biological sciences, which considers the two-way feedbacks between ecological and geomorphic processes and how these processes influence ecosystems and landform evolution (Viles, 1988; Naylor et al., 2002; Wheaton et al., 2011). Although the term biogeomorphology was first coined by Viles (1988), early ecologists "recognised the need for the documentation of  geomorphic form and process" (e.g., Cowles, 1899, 1901; Clements, 1916) to explain species distributions (Hupp et al., 1995). Integrated studies explicitly exploring the two-way interactions between ecology and geomorphology began in the 1950s (Hall and Smith, 1955; Olsen, 1958; Hack and Goodlett, 1960; Imeson, 1976; Alpert, 1985).

The role of biology in landscape evolution in terrestrial (Kirby, 1995; Phillips, 1995; Goodson et al., 2002; Dietrich and Perron, 2006; Osterkamp and Hupp, 2010) as well as marine environments (D'Alpaos et al., 2007; Murray et al., 2009; Corenblit et al., 2011; Marani et al., 2010, 2013; Coco et al., 2013; Van Maanen et al., 2015) is an emerging theme in contemporary geomorphology. Dietrich and Perron (2006) posed the following question in relation to landscape evolution: "If life had not arisen, would the tectonic and climatic processes that drive uplift and erosion of landscapes be significantly different? Bird (1986) posed this question on the role of mangroves in the geomorphic evolution of estuaries: whether mangroves promote sedimentation, and thereby the evolution of depositional landforms, or whether they simply occupy sites that have become ecologically suitable, moving in to colonise (and possibly thereafter to stabilise and protect) an intertidal morphology that would have formed independently in their absence (Bird, 1986). Geomorphologists and ecologists have sought to address this question for over a century (Vaughan, 1909; Davis, 1940; Chapman and Ronaldson, 1958; Scoffin, 1970; Thom et al., 1975; Vann, 1980; Bird, 1986; Walsh and Nittrouer, 2004; McKee, 2011). The prevailing scientific consensus regarding the biogeomorphic evolution of mangrove forests in fluvial sediment–rich systems is that mangroves only colonize intertidal flats when local environmental conditions become ecologically suitable" (Bird, 1986), with substrate elevation and stability being key factors (Balke et al., 2015; Oh et al., 2017). Once established, do mangroves in turn enhance sedimentation relative to unvegetated tidal flats through biophysical feedbacks (Fig. 1.1)?

Over the last decade or so, computational models of coastal wetland biogeomorphology have been developed that explicitly account for the two-way coupling of coastal wetlands with physical processes (i.e., hydrodynamics, sediment transport, and deposition) and resulting landscape evolution occurring over a range of time scales (D'Alpaos et al., 2007; Murray et al., 2009; Marani et al., 2010, 2013; Horstman et al., 2015; Van Maanen et al., 2015; Willemsen et al., 2016). Exploratory/abstracted modeling approaches are increasingly being used to overcome the limitations of deterministic (bottom-up) numerical models. A critical issue that remains to be solved, however, is how to accurately simulate long-term geomorphological evolution over meaningful spatial and temporal scales (Coco et al., 2013). Various biophysical processes may also operate over a range of time scales (Phillips, 1995). Plant communities respond to seasonal environmental drivers, whereas landscape evolution may be strongly influenced by infrequent episodic events. This begs the question to what extent do plant communities and geomorphology interact (Phillips, 1995). In the case of mangroves, Woodroffe et al. (2016) have argued that there is still insufficient understanding of the ecology and morphodynamics of mangrove forest sedimentary systems to confidently predict their long-term evolution using existing models. The experimental and observational data, including measurements of processes and long-term geomorphic evolution that are required to underpin and validate these biogeomorphic models of mangrove systems, are still rare. These fundamental questions and knowledge gaps about the role of mangroves in the geomorphic evolution of estuaries have been explored for a New Zealand estuarine system (Firth of Thames) using observations and modeling across a range of time scales (i.e., from discrete storm events to decades) (Swales et al., 2015, 2016, 2019). The major findings of this research are described in the case study section.

Figure 1.1 Conceptual model of processes influencing mangrove forest biogeomorphology in fluvial sediment–rich estuaries. Key processes controlling sedimentation, surface elevation gain, and mangrove forest development (blue (light gray in print version) boxes) and associated environmental drivers (green (dark gray in print version) boxes). 

Biotic components of this diagram adapted from Cahoon, D.R., Guntenspergen, G.R., January–February 2010. Climate change, sea-level rise and coastal wetlands. Natl. Wetl. Newsl., 8–12.

3. Mangroves in New Zealand

3.1. Geographic distribution, environmental settings, and forest types

Mangrove forests composed of monospecific stands of the gray mangrove Avicennia marina var australasica occur in the numerous estuaries of northern New Zealand. This mangrove is also known as Mānawa, which is the name given to A. marina by the indigenous Māori people who colonized New Zealand around 1300 AD (Wilmshurst et al., 2008). This subspecies also occurs in the estuaries and on the coasts of southeastern Australia, New Caledonia, and Lord Howe Island (Duke, 1991). The southern latitudinal limit of mangroves in New Zealand is at 38.08°S (de Lange and de Lange, 1994), which is close to the southernmost limit for mangroves globally (i.e., 38.9°S, Corner Inlet, South Victoria, Australia; Bird, 1986). The present southern latitudinal limit for mangroves in New Zealand occurs at the Kawhia (west coast) and Ohiwa Harbours (east coast) (Fig. 1.2). The southern limit for mangroves in Australasia is controlled by low winter air temperatures and the frequency of lethal frosts (e.g., Chapman and Ronaldson, 1958; Saintilan et al., 2009). Physiological limitations at low air temperatures may also be causal factors (Walbert, 2002; Beard, 2006). The North Island's biogeography and oceanography also appear to limit the dispersal of mangrove propagules along the coast (de Lange and de Lange, 1994). Although studies of A. marina propagule dispersal show that they can travel tens of kilometers or more (e.g., Clarke, 1993), they may not remain viable for more than several days (de Lange and de Lange, 1994). Furthermore, the relatively small number of, and distance between, estuaries located south of mangroves present southern limit suitable for colonization are effective barriers to mangroves increasing their range in New Zealand.

Figure 1.2 Distribution of Avicennia. marina mangroves in the estuaries of New Zealand's upper North Island: (A) The present southern latitudinal limit of mangroves is 38°S and occurs at Kawhia Harbour on the west coast and Ohiwa Harbour on the east coast; (B) location of estuaries on Auckland's east coast and the Firth of Thames.

New Zealand's A. marina mangrove forests occur in a range of estuarine geomorphic settings (Fig. 1.3). The Roy et al. (2001) classification scheme for south-eastern Australian estuaries is also broadly applicable to New Zealand estuaries and describes mangrove distribution based on the geological maturity of the sedimentary system (i.e., degree of infilling) and relative dominance of tides and waves in controlling water circulation and sediment transport. New Zealand mangroves mainly occur in barrier-enclosed and headland-enclosed estuaries, with inlets restricted by rocky headlands, and coastal embayments, typically with small catchments relative to estuary area (Hume and Herdendorf, 1988; Morrisey et al., 2010). These estuaries developed in drowned river valleys and coastal lowlands as they were flooded by rising sea levels during the most recent postglacial marine transgression (Hume, 2003). They range in size (i.e., high-tide area) from less than 1–~750   km² (NZ Estuary Classification, Hume et al., 2007).

New Zealand's mangrove forests include several functional types that are described by Lugo and Snedaker's (1974) classification. Although developed originally for carbonate-sediment environments, this scheme is suitable for low-diversity mangrove systems (Woodroffe, 1992), such as those that occur in New Zealand and southeast Australia. Common forest types include (1) fringe and (2) riverine forests composed of relatively tall trees that flank mid-to-intertidal flats and river/tidal channels, respectively. Trees in these forests typically have longer hydroperiods and receive more nutrients and sediment compared to interior (3) scrub or (4) basin forests. Trees in these interior forests grow at higher elevations and/or more remote from tidal flats and channels where environmental conditions are typically less favorable for tree growth. Nutrient-addition experiments in New Zealand's mangrove forests have shown that tree growth at interior scrub forest sites are N limited, whereas fringe forests show little evidence of nutrient limitation (Lovelock et al., 2007, 2010). The reduced hydrological connectivity (Lovelock et al., 2007) and sediment/pore-water properties (e.g., pH, oxidation state) of mangrove forest interiors is also likely to impart less favorable conditions for plant growth.

Figure 1.3 Examples of New Zealand's Avicennia. marina mangrove estuaries: (A) southern Firth of Thames, Waikato Region (coastal embayment, 37.25°S, 175.40°E); and (B) Waiwera estuary (headland/barrier-enclosed 37.54°S, 174.69°E), (C) Whangapoua (barrier enclosed, 36.14°S, 175.41°E), and (D) Orewa estuary (barrier enclosed, 36.60°S, 174.69°E), Auckland Region.

3.2. Historical patterns

Mangrove forest loss occurred in New Zealand estuaries due to human activities over the last century or so associated with land reclamation for port infrastructure, urban development and agriculture, road and causeway construction, landfills, and stock grazing. Historical changes (both losses and gains) in the extent of New Zealand's mangrove forests have not been accurately quantified because substantial habitat change occurred prior to systematic aerial photographic surveys that began in the 1930s (Morrisey et al., 2010).

Contrary to the global pattern of ongoing mangrove forest loss, rapid expansion of mangrove forests has occurred in many New Zealand estuaries. Rates of mangrove-habitat expansion since the 1930s have averaged 4.1%   yr −¹ (range −0.2%–20.2%   yr −¹) in terms of the intertidal area occupied. This rate of habitat expansion is some twofold higher than for temperate Avicennia mangrove forests of southeastern Australia (average 2.1%   yr −¹; range 0.7%–9.1%   yr −¹) (Morrisey et al., 2010). In New Zealand, mangrove-habitat expansion has primarily occurred by colonization of unvegetated intertidal flats seaward of existing forests (Burns and Ogden, 1985; Ellis et al., 2004; Morrisey et al., 2010; Lundquist et al., 2014; Swales et al., 2015; Horstman et al., 2018). Mangrove forests presently occupy about 260   km² of intertidal flats in New Zealand estuaries (Spalding et al., 2010). This habitat expansion has coincided with large-scale catchment deforestation from the mid-1800s, resulting in increased soil erosion and formation of intertidal flats as the rate of estuary infilling accelerated (e.g., Swales et al., 1997, 2002a,b).

Estuary infilling, associated with a shift from sand- to mud-dominated systems, resulted in the reduction in the areal extent of subtidal habitats and loss or degradation of ecosystems. This degradation has been characterized by loss of plants and animals sensitive to increased water turbidity, reduced light levels, and fine sediment deposition (e.g., seagrass meadows, filter-feeding bivalves) (Inglis, 2003; Thrush et al., 2004). For example, extensive areas of mature mangrove forest had developed in the Kaipara Harbour (Northland NZ) by the 1920s and infrequently inundated by spring tides (Ferrar, 1934). Historical accounts of settlers dating back to the 1860s describe white-sand beaches fringing the shoreline. These accounts indicate that mangroves were not widespread in the large tidal rivers of the northern Kaipara. Slash and burn agriculture resulting in catchment disturbance was practiced by Māori over the several hundred years prior to European arrival. Sedimentary records suggest that Māori land use activities had a minor impact on these northern estuaries. Background sediment accumulation rates (SAR) over the several thousand years prior to European settlement averaged 0.1–1   mm/yr (e.g., Hume and McGlone, 1986; Sheffield et al., 1995; Swales et al., 1997, 2002a,b; Thrush et al., 2004; Oldman et al., 2009). These background SAR were an order of magnitude lower than the sedimentation that followed large-scale catchment disturbance due to timber extraction, mining, and the development of pastoral agriculture from the mid-1800s. Historical records suggest that mangrove forests formed in estuaries as they infilled with eroded soils in the decades following large-scale catchment deforestation (Swales et al., 2011) (Fig. 1.4).

This rapid expansion of mangrove habitat in New Zealand estuaries has also polarized coastal communities over the last decade or so (Harty, 2009; Horstman et al., 2018). Environmental groups have advocated protection of mangrove habitat, whereas a growing number of community groups have formed with a major purpose being to lobby local government agencies to manage the expansion and/or reduce the area of mangrove habitat in estuaries (Harty, 2009; Lundquist et al., 2014; Horstman et al., 2018). Mangrove removal campaigns have typically not realized anticipated ecological benefits such as reestablishment of seagrass meadows and shellfish beds (Bulmer et al., 2017; Horstman et al., 2018). Environmental management agencies also increasingly recognize that mangrove-habitat expansion is a symptom of estuary infilling resulting from land use activities that have increased soil erosion in catchments.

Figure 1.4 Example of tidal-creek infilling and mangrove-habitat expansion (Mangemangeroa, east Auckland), 1863 to 2016. View looking downstream from the Whitford Road Bridge. Major environmental changes include lateral growth of the intertidal flats and associated reduction in channel size, consistent with progressive loss of tidal prism and development of a mangrove forest of the intertidal flats. 

Photo credits: Bridge over the Mangemangeroa Creek near Howick, 1860s. John Kinder photographs. MSS & Archives 2009/7, item PH 233. Special Collections, University of Auckland Libraries and Learning Services.

3.3. Biophysical drivers

Mangrove forests occupy the intertidal zone above mean sea level (MSL) (Galloway, 1982; Bird, 1986; Ellison, 1993), although the position of the lower elevation of mangrove forests varies about MSL due to local conditions. For example, mangrove forests occupy most of their potential habitat down to MSL in sheltered tidal creeks and smaller fetch-limited estuaries in New Zealand (Auckland Region) estuaries (Swales et al., 2009). The lower elevation limit for A. marina forests can be decimeters higher than MSL due to wave exposure and increasing tidal range (e.g., Swales et al., 2009; Balke et al., 2015; Barker et al., 2015).

The approximate MSL minimum for mangrove forest distribution is related to the underlying physiological tolerance of mangroves to emersion. Regular daily exposure at low tide enables seedlings growing in muddy substrates in particular to maintain an adequate oxygen supply to their tissues (Clarke and Hannon, 1970; Curran et al., 1986; Hovenden et al., 1995). A corollary of this physiological constraint on mangrove establishment is that the geomorphic evolution of estuaries and coastal sedimentary landforms exerts a first-order control on mangrove forest ecology through sediment transport, deposition, and resulting vertical accretion of intertidal flats. Mangroves colonize intertidal flats only after they become ecologically suitable (Bird, 1986). Estuarine sedimentation builds intertidal flats that provide habitat that is potentially suitable for mangrove colonization. Therefore, although intertidal flat development provides the opportunity for mangrove forest establishment or expansion, seedling recruitment depends on biophysical factors that control the likelihood of propagule survival and substrate disturbance. Key factors include wave exposure and timing of propagule production relative to changes in tidal-flat hydroperiod (i.e., frequency and duration of tidal inundation) that occur during the spring–neap tidal cycle, stability of the substrate and bed-level dynamics (e.g., Lovelock et al., 2010; Balke et al., 2015). Biostabilization of intertidal sediments by microorganisms can reduce the susceptibility of intertidal-flat sediment to erosion (e.g., Widdows et al., 2004), thereby promoting propagule establishment. Predation of propagules (e.g., Clarke and Kerrigan, 2002) will reduce recruitment.

Sea-level variations in estuaries over seasonal to decadal time scales will also influence recruitment success. These variations are also superimposed on the long-term trend of SLR induced by climate warming (Nerem et al., 1998; Church and White, 2011; Watson et al., 2015). In the Pacific Region, major interannual and decadal cycles include El Niño-Southern Oscillation and the Inter-decadal Pacific Oscillation. These large-scale processes result in decimeter-scale variability in MSL and wind climate from year to year (Goring and Bell, 1999; Hannah, 2004). Crucially during the summer months, when mangrove propagules are produced, lower than normal sea levels during El Niño events and calm or offshore winds coinciding with spring-to-neap tidal phases provide disturbance-free windows of opportunity over several days or more (Balke et al., 2015) for stranded mangrove propagules to establish on intertidal substrates (Thom, 1967; Chapman, 1976; Clarke and Myerscough, 1993; Balke et al., 2015; Lovelock et al., 2015a). Over longer time scales (i.e., decades), the biogeomorphic evolution of mangrove forests is also influenced by feedbacks that influence sedimentation processes (e.g., negative feedback between surface elevation and hydroperiod, Swales et al., 2015; Horstman et al., 2015), sediment supply, and accommodation space (Walsh and Nittrouer, 2004; McKee et al., 2007; Alongi, 2008; Swales et al., 2016; Willemsen et al., 2016; Woodroffe et al., 2016). Thus, biophysical processes across a range of spatial and temporal scales exert a strong influence on mangrove forest evolution in estuaries and on muddy coasts. Over geological to historical time scales, estuary infilling results in large-scale environmental changes. Subtidal areas and water-depth decrease and as a result hydrodynamic conditions, sediment properties and accumulation rates, geomorphological characteristics, and ecosystems change.

4. Estuary evolution

New Zealand's estuaries have progressively infilled with fluvial and marine sediments over the last 6000–7000   years since their formation following the last ice age. Stages of development range from youthful systems that retain a substantial proportion of their original tidal volume to mature estuaries dominated by river discharge. In semimature estuaries, sediment infilling is usually associated with the expansion of accreting intertidal flats that progressively displace subtidal basins. The rate of estuary infilling primarily reflects the original volume of the tidal basin, rate of sediment supply, and changes in sea level. Evidence from global salt marsh sedimentary records indicate that low rates of sea-level change (i.e., tenths of mm/yr) persisted until as recently as the early 20th century. Both sedimentary and tide-gauge records show that average rates of SLR have increased to the order of mm/yr in the modern era (Church et al., 2013). The biogeomorphic evolution of estuaries can be conceptualized as developmental stages (youth, middle, and old age) that are characterized by a progressive shift from subtidal to intertidal systems (Fig. 1.5).

Figure 1.5 Biogeomorphic evolution of estuaries. The estuaries that have formed during the Holocene follow a cycle of development determined by the size (volume) of the ancestral basin/flooded river valley, the rate of sediment input, and trapping efficiency of the system. The sediment trapping efficiency typically reduces as estuaries infill due to reduction in accommodation volume and increased efficiency of sediment remobilization by fetch-limited waves.

Tidal ranges in barrier-enclosed estuaries and lagoons are reduced by sand deposits at their inlets, so that tidal currents are weak and wind waves and wind-driven circulation control sediment transport on tidal flats (Roy et al., 2001). In northern New Zealand, many drowned river valley estuaries have reached an advanced