Sundarban Mangrove Wetland (A UNESCO World Heritage Site): A Comprehensive Global Treatise

By Santosh Kumar Sarkar

Sundarban Mangrove Wetland: A Comprehensive Global Treatise provides an illustrative account of the ecology, biology, conservation and management strategies of this endangered UNESCO World Heritage Site.

The book offers a comprehensive and accessible guide to a variety of wetland ecosystems, including endangered flora and fauna, the ecology and diversity of pelagic and benthic biota, the impact of multiple stresses on the biota, inorganic and organic pollutants in biotic and abiotic matrices and their remedial measures, the impact of climate change on mangrove plants, and their conservation and management strategies. Divided into seven chapters, the book presents a realistic summary of the wetland environment and its resources, citing individual case studies considering a host of topics of particular interest. Analysis of this unique wetland provides crucial comparisons with other wetlands and their status, environmental challenges and possible remedial measures.

Sundarban Mangrove Wetland is an in-depth and up-to-date account ideal for the student, teacher or researcher in marine biology & ecology, environmental science, marine geochemistry, marine pollution and ecotoxicology and wastewater treatment. Covering both fundamental and advanced aspects, the book is also useful for policy makers and those involved in coastal resource conservation and management.

• Presents an in-depth and illustrative accounting of an iconic tropical mangrove wetland in an intelligible and easy-to-understand manner
• Provides a unique look at the ecology, biodiversity and conservation and management of the Sundarban wetlands, along with the emerging ecological issues that may affect long-term sustainability
• Focuses on several case studies, considering microzooplankton and trace metals in the Sundarban wetlands

• Language: English
• Publisher: Elsevier Science
• Release date: Apr 13, 2022
• ISBN: 9780128173718

Santosh Kumar Sarkar

Dr. Santosh Kumar Sarkar is a Professor at the Department of Marine Science, University of Calcutta. Prof. Sarkar’s research focuses on the diverse ecological aspects of wetland environments in India and abroad. Including several major international collaborations, he has worked on the Ganges River Estuary and adjacent Sundarban Mangrove Wetland for 30 years, with over 100 articles published in peer-reviewed international journals. Prof. Sarkar’s unique knowledge and expertise in wetland ecosystems will be of immense benefit to anyone involved in coastal or marine management.

Book preview

Front Cover for Sundarban Mangrove Wetland (a UNESCO World Heritage Site) - A Comprehensive Global Treatise - 1st edition - by Santosh Kumar Sarkar

Sundarban Mangrove Wetland (a UNESCO World Heritage Site)

A Comprehensive Global Treatise

Santosh Kumar Sarkar

Department of Marine Science, University of Calcutta, Kolkata, India

Table of Contents

Cover image

Title page

Copyright

Dedication

Foreword

Preface

Acknowledgments

Abbreviations and acronyms

Chapter 1. Resource conservation and management

Abstract

Chapter Outline

1.1 Classification and characteristics of coastal wetlands

1.2 Mangrove flora and fauna

1.3 Mangroves goods and ecosystem services

1.4 Mangroves: A carbon source and sink

1.5 Remote-sensing techniques for mangrove mapping

References

Chapter 2. Ecology and diversity of biota in Sundarban regions

Abstract

Chapter Outline

2.1. Distribution and diversity of microbial communities

2.1.1 Bacterial communities

2.1.2 Fungal communities

2.1.3 Conclusion

2.2. Distribution and diversity of phytoplankton of Sundarban

2.2.1 Introduction

2.2.2 Classification of plankton

2.2.3 Phytoplankton community in Sundarban regions

2.2.4 Phytoplankton cell volume in Indian Sundarban

2.2.5 Molecular characterization of phytoplankton

2.2.6 Phytoplankton community in tropical coastal regions

2.2.7 Phytoplankton responses to climate change

2.2.8 Conclusion

2.3. Distribution and diversity of microzooplankton (Tintinnida: Ciliata) of Sundarban: a case study

2.3.1 Introduction

2.3.2 Materials and methods

2.3.3 Results and discussion

2.3.4 Conclusion

2.4. Distribution and diversity of mesozooplankton of Sundarban: a case study

2.4.1 Introduction

2.4.2 Materials and methods

2.4.3 Results and discussion

2.4.4 Conclusion

2.5. Distribution and diversity of dominant macrobenthos

2.5.1 Introduction

2.5.2 Cnidarian

2.5.3 Annelida

2.5.4 Arthropoda

2.5.5 Mollusca

2.5.6 Echinodermata

2.5.7 Miscellaneous groups

2.5.8 Conclusion

2.6. Distribution and diversity of Ichthyofauna

2.6.1 Introduction

2.6.2 Morphology and taxonomy of marine fish

2.6.3 Distribution of marine fish

2.6.4 Impact of anthropogenic threats on fish

2.6.5 Conclusion

2.7. Distribution and diversity of avifauna

2.8. Distribution and diversity of herpetofauna

2.9. Distribution and diversity of mammals

Chapter 3. Pollution in abiotic matrices and remedial measures

Chapter Outline

3.1. Distribution of trace metals in sediment core of Sundarban: a case study

3.1.1 Introduction

3.1.2 Material and methods

3.1.3 Results and discussion

3.1.4 Statistical analyses

3.1.5 Status of trace metal contamination in mangrove sediments in India and abroad

3.1.6 Conclusion

Further reading

3.2. Mercury and methylmercury in sediment cores of Sundarban: a case study

3.2.1 Introduction

3.2.2 Materials and methods

3.2.3 Results and discussion

3.2.4 Remedial measures for mercury contamination

3.2.5 Conclusion

Further reading

3.3. Dissolved trace metals in coastal regions of Sundarban: a case study

3.3.1 Introduction

3.3.2 Material and methods

3.3.3 Results and discussion

3.3.4 Carcinogenic risk assessment

3.3.5 Comparative account of dissolved metal concentrations

3.3.6 Conclusion

Further reading

Chapter 4. Trace metal bioaccumulation

Chapter Outline

4.1. Trace metals in mesozooplankton of Sundarban: a case study

4.1.1 Introduction

4.1.2 Materials and methods

4.1.3 Results and discussion

4.1.4 Conclusion

4.2. Trace metal accumulation in biota of Sundarban: a case study

4.2.1 Introduction

4.2.2 Materials and methods

4.2.3 Results and discussion

4.2.4 Conclusion

4.3. Mercury in human hair and relation to fish consumption in Indian Sundarban: a case study

4.3.1 Introduction

4.3.2 Materials and methods

4.3.3 Results

4.3.4 Discussion

Chapter 5. Emerging contaminants and organic micropollutants

Abstract

Chapter Outline

5.1 Introduction

5.2 Persistent organic pollutants in sediment of Sundarban: Case studies

5.3 Conclusion

References

Chapter 6. Phytoremediation of trace metals by mangrove plants

Abstract

Chapter Outline

6.1 Accumulation of trace metals by mangrove plants in Sundarban: a case study

6.2 Remediation mechanisms

6.3 Material and methods

6.4 Phytoremediation efficiency

6.5 Results and discussion

6.6 Bioconcentration and translocation factors

6.7 Statistical analyses

6.8 Conclusion

References

Chapter 7. Mangroves and climate change: a global issue

Chapter Outline

7.1. Mangrove wetland vulnerability to climate change

7.1.1 Introduction

7.1.2 Climate change threats

7.1.3 Conclusion

Further reading

7.2. Extinction risk of mangroves and geographic areas of global concern

7.2.1 Introduction

7.2.2 Regional and national rates and causes of loss

7.2.3 Impact of mangrove forest degradation on biodiversity

7.2.4 Conclusion

Further reading

7.3. Mangroves as a protection from extreme climatic events

7.3.1 Introduction

7.3.2 Role of mangroves in mitigating coastal disasters

7.3.3 Conclusion

Further reading

7.4. Conservation of mangrove wetland: strategies and future challenges

7.4.1 Introduction

7.4.2 Legislative protection of mangrove forests

7.4.3 Nonlegislative protection of mangroves

7.4.4 Conclusion

Further reading

Glossary

Weblink for further reading

Index

Copyright

Elsevier

Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands

The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom

50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States

Copyright © 2022 Elsevier Inc. All rights reserved.

No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions.

This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein).

Notices

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein.

ISBN: 978-0-12-817094-6

For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Candice Janco

Acquisitions Editor: Louisa Munro

Editorial Project Manager: Rachel Pomery/ Sara Greco

Production Project Manager: Sruthi Satheesh

Cover Designer: Mark Rogers

Typeset by MPS Limited, Chennai, India

Dedication

Dedicated to all those deeply involved in conservation, management, and wise use of the Mangrove Wetlands

Foreword

Jörg Rinklebe, Head of the Laboratory of Soil- and Groundwater-Management Institute of Soil Engineering, Waste- and Water Science, Faculty of Architecture and Civil Engineering University of Wuppertal, Wuppertal, Germany

Mangroves are a fascinating group of halophytic plants, the botanical amphibians of seashore, providing a broad array of benefits that span the realm of regulating, provisioning, cultural and supporting ecosystem services and thus deserves global attention. The book "Sundarban Mangrove Wetland (a UNESCO World Heritage Site): A Comprehensive Global Treatise" provides an illustrative resource of this iconic transboundary tidal wetland dealing with an array of multidisciplinary topics, from basic features to advanced and emerging issues, such as concepts of mangrove biology and ecology, impact of pollution and climate change, and conservation and management strategies in local, regional, and global scales. In tropical and subtropical regions, mangrove forests cover an area of approx. 200,000 km² around the world. However, a substantial reduction in their biomass is noticed, especially in developing countries such as Brazil, China, India, Indonesia, Nigeria, and the Philippines. Hence the book is timely to address the extent, status, present strategies, and future challenges in managing and conserving the mangrove communities to ensure their sustainability and conservation. The up-to-date coverage of the book is integrated with a stimulating, recent overview of critical key components of mangrove environments with a global perspective, citing examples from diversified mangrove ecosystems with unique ecological strategies.

Formed of alluvial deposits delivered directly by Ganges-Brahmaputra- Meghna Rivers, the globally important Sundarban is a continuous tract of man- grove forests, providing a unique representation for comparisons to be made with other wetlands, environmental challenges, and remedial strategies. The author has prioritized the key environmental and ecological issues like rapid anthropogenic climate change, global warming, and extreme natural disasters on mangrove communities on a global scale in a lucid and palpable manner. Those activities which threaten their ongoing survival are identified and suggestions are offered to minimize their effects on these coastal intertidal plant communities. This book would be an ideal source of scientific information to graduate and advanced students, academicians, researchers, scientists, practitioners, stakeholders, and many more involved in wetland ecosystem research, beneficial for effectively protecting the fragile wetland environment. I wish to extend my thanks to Prof. S. K. Sarkar, author of the book, for this timely and invaluable contribution. I hope it will be widely read and used.

Preface

Mangroves are one of the most human-affected vegetated ecosystems along the sheltered coastlines in tropical, subtropical, and some warm temperate regions, supporting unique biodiversity and providing important ecosystem services to a large section of the coastal communities across the globe. The proximate human-induced factors such as urbanization, land-use change, resource overexploitation, aquaculture, and agriculture have caused their exponential decrease and threaten their global future. At broader scales, mangrove forest health and expansion are also impacted by complex and cumulative interactions of many natural factors such as relative sea-level rise and fluctuations in sea-level linked to climate oscillations, cyclone activities, temperature, and precipitation changes with important implications for the vulnerability of the coastal populations who rely on mangrove resources.

Considering the current degradation scenarios of the true mangroves and mangrove associates communities in regional and global scale, I have been inspired to present the book entitled "Sundarban Mangrove Wetland (a UNESCO World Heritage Site): A Comprehensive Global Treatise" to focus on the key emerging issues being concerned on a global scale. Sundarban wetland (straddling India and Bangladesh) is considered as the world’s largest transboundary mangrove ecosystem, harboring a rich and varied array of floral and faunal assemblages. This unique wetland provides an excellent representative for comparisons to be made with other wetlands in the context of environmental challenges and remedial strategies.

The book has been divided into five sections and structured into seven chapters. Each chapter has its own identity, relevance, and importance and is beneficial for a wide audience to get access to fundamental and realistic conception to advanced knowledge of the mangrove extent, structure and status, present strategies, and future challenges in conservation and management strategies. The first part of the book outlines the coastal wetland ecosystem followed by an exhaustive account of the mangrove diversity, current status, challenges, and management on a global perspective together with a brief description of the endangered species in mangrove regions of the world. This chapter also considers some important relevant topics such as mangrove goods and economic services, carbon storage efficiency, and application of remote sensing in mangrove management. The second part stresses on the ecology and diversity of the microbial to mammal communities. The third part covers a detailed account of the inorganic and organic micro- pollutants in biotic and abiotic matrices, citing case studies from Indian Sundarban wetland. The fourth part accentuates on the phytoremediation efficiency of trace metal(loid)s by dominant native mangrove plant species of Indian Sundarban and illustrates their potential in the context of pollution indices. The final part stresses the impacts of climate change on mangrove communities on a global scale and options for management strategies for sustainable exploitation, economic sustainability, and future research for development.

The book is intended to serve as a useful and upgraded reference source for a large section of people, directly involved in wetland research and management at the regional and global scale. I trust that this book would provide better understanding and stimulate greater interest in wise use of wetlands, their protection, and restoration at the grassroots level and would be useful in generating innovative research and implementation ideas. Constructive comments and suggestions for improvement of the book would be gratefully appreciated.

Acknowledgments

I owe an enormous debt of gratitude to the following academicians and researchers who have shared their expertise and provided me with their constructive suggestions, further clarifications, and valuable comments on one or more chapters: Drs. J. Biswas and Swati Das (Senior Research Fellow), Kalyani University, India; K.K. Satpathy, Indira Gandhi Center of Atomic Research (IGCAR), Tamil Nadu, India; R. Kundu, Saurashtra University, India; M. Mohan, P. Ragavan, Jayant Kumar Mishra, and Mohanraju Raju, Department of Ocean Studies and Marine Biology, Pondicherry University, India; Ramaswamy Babu Rajendran, Department of Environmental Biotechnology, Bharatidasan University, Tamil Nadu, India; B.P. Dash, Fakir Mohan University, Odisha, India; R. Pushponathan, Senior Scientist, ICAR, India; Dr. Sarangi, Indian Space Research Organization, India; Jiang-Shiou Hwang, National Taiwan Ocean University, Keelung, Taiwan; Simonetta Corsolini, University of Siena, Italy; Krishna Das, University of Liege, Belgium; Karla Pozo, Masaryk University, Czech Republic; Emmanoel V. Silva-Filho, Fluminense Federal University, Brazil; Jason Kirby, Commonwealth Scientific and Industrial Research Organization (CSIRO), Australia; M.P. Jonathan, (CIIEMAD) (IPN), Mexico; and Prof. K. Ponnambalam, Systems Design Engineering, University of Waterloo, Canada.

For the contribution of photographs/images used in different chapters of the book, I am immensely grateful to the following scientists/researchers: Dr. R. Nagarajan, Department of Applied Geology, Faculty of Engineering and Science, Curtin University, Malaysia; Dr. M.P. Jonathan, Mexico; Rahul Kundu, Saurashtra University, India; Dr. Bisnu Prasad Dash, Fakir Mohan University, India; A. Gopalakrishnan, Assistant Professor (Aquatic Animal Health & Coastal Livelihood Security), Faculty of Marine Sciences, Center of Advanced Study in Marine Biology, Annamalai University, India; Dr. C. Sivaperuman, ZSI, A & N Islands Scientist-E & Officer-in-Charge, Zoological Survey of India (ZSI), Andaman & Nicobar Regional Center, India; Kantharajan G., Aquatic Environment and Health Management Division, ICAR-Central Institute of Fisheries Education, Mumbai, India; Prof. P.M. Mohan and Dr. V. Ragavan, Department of Ocean Studies and Marine Biology, Pondicherry University, Brookshabad Campus, India; Ms. Swati Das, Kolkata; Prof. S. Das, Institute of Economic Growth, University of Delhi Enclave, Delhi, India; Profs. K. Sridhar and K. Sharatchandra, Department of Biosciences, Mangalore University, Karnataka, India; Prof. M. Monirul H. Khan, Professor of Zoology, Jahangirnagar University, Savar, Dhaka, Bangladesh; Eliete Zanardi Lamardo and Roxanny Helen de Arruda-Santos, Universidade Federal de Pernambuco—UFPE, Dept Oceanografia—Laboratório OrganoMAR Av Arquitetura s/n—Cidade Universitária—Recife Pernambuco, Brazil; Simonetta Corsolini, Department of Physics, Earth and Environmental Sciences, Ecotoxicology and Remote Regions, University of Siena, Italy; Dr. Eusebio Cano, University of Jaen, Spain; Billy K.Y. Kwan, Department of Biology and Chemistry, City University of Hong Kong, Hong Kong; S.G. Cheung, College of Fisheries and Life Science, Shanghai Ocean University, China; Paul K.S. Shin, State Key Laboratory in Marine Pollution, City University of Hong Kong, Hong Kong; Dr. Jessica Keller, Center for Marine and Environmental Studies, University of the Virgin Islands, United States; Dr. Carl C. Trettin, Center for Forested Wetlands Research, USDA Forest Service, United States; Dr. J.R. Dolan, Center National de la Recherche Scientifique (CNRS), Marine Microbial Ecology, France; Ahmad Fitri Aziz and Frances Hii Dai Sze, Faculty of Resource Science and Technology, Universiti Malaysia Sarawak (UNIMAS), Malaysia; Dr. Gilvan Yogui, Universidade Federal de Pernambuco, Centro de Tecnologia e Geociências, Departamento de Oceanografia, Brazil; São João River Estuary (Rio de Janeiro State, Brazil); Prof. Paulo Antunes Horta, Laboratório de Ficologia, Departamento de Botânica, Centro de Ciências Biológicas, Universidade Federal de Santa Catarina, Florianópolis, SC, Brasil; Dr. Carolyn Lundquist, Principal Scientist-Marine Ecology, National Institute of Water & Atmospheric Research Ltd (NIWA), Hamilton, Sayam U. Chowdhury, Assistant Coordinator, Spoon-billed Sandpiper Task Force, Prof. Norman Duke, James Cook University, Australia. The excellent cover photos of mangrove flowers were graciously provided by Prof. P. M. Mohan, Department of Ocean Studies and Marine Biology, Pondicherry University, India.

I would like to express my sincere thanks and gratitude to the Editors of the following journals/Book for extending me the copyright in reproducing the published matter (tables and images used in the book): (a) Taprobanica—The Journal of Asian Biodiversity (b) International Society for Mangrove Ecosystems (ISME), Okinawa, Japan and International Tropical Timber Organization (ITTO), Yokohama, Japan. Edited by H.T. Chan; (c) Indian Journal of GeoMarine Sciences (IJMS), New Delhi, India and (d) The State of the World’s Mangroves 2021. Global Mangrove Alliance (e) Marine and Freshwater Research, CSIRO Publishing, Australia (f) Remote Sensing (MDPI) and World Atlas of Mangroves by Spalding et al., (2010), Published with ISME, ITTO and project partners FAO, UNESCO-MAB, UNEP-WCMC and UNU-INWEH.

I am indebted to Prof. Dr. Md. Abul Mansur, Department of Fisheries Technology, Bangladesh Agricultural University, Bangladesh for identifying of marine fish; Prof. Patricia Luciano Mancini, Museu de Zoologia, Universidade de São Paulo, São Paulo, SP, Brazil for identifying the mangrove bird cattle egret (Bubulcus ibis); and Prof. S. K. Chakraborty, Department of Zoology, Vidyasagar University, West Bengal, India for identification of benthic macrofauna.

The incredible support and assistance rendered by five of my former research scholars, namely, B. D. Bhattacharya, R. Choudhury, D. Rakshit, S. Mitra and P. Mondal for preparing the manuscript are greatly appreciated. My daughter Prathama Sarkar has extended excellent technical support in preparing tables, figures and fulfilled other requirements in preparing the manuscript. I wish to extend my sincere thanks to my wife Mrs. Manjushree Sarkar for bearing all family problems and her continuous support, cooperation and patience in completing the work.

Finally I would like to express my sincere thanks and immense gratitude to Elsevier production staff members who are directly involved to the successful completion of the project, namely, Candice Janco (Publisher), Louisa Munro (Acquisitions Editor), Sruthi Satheesh (Production Project Manager), and Praveen Anand (Copyrights Coordinator) and Mark Rogers (Cover Designer). I particularly wish to extend my sincere thanks and gratitude to Pomery Rachel & Sara Greco, Editorial Project Managers, who oversaw the project work and kept it moving along on schedule. The book could not have arrived at its present stage without their keen interest, guidance, encouragement, and enormous efforts— thanks to their patience and understanding.

Abbreviations and acronyms

AEWA Agreement on the Conservation of African-Eurasian Migratory Waterbirds

CBMM Community-based mangrove management

CBOs Community-based organizations

CBD Convention on biological diversity

CEC Chemicals of emerging concern

CITES Convention on the International Trade of Endangered Species

CRZ Coastal regulation zone

DRR Disaster risk reduction

EDCs Endocrine disrupting chemicals

EIA Environmental impact assessment

ENSO El Niño-Southern Oscillation

EWS Early warning system

GDP Gross domestic product

GHG Greenhouse gases

GMA Global Mangrove Alliance

ICZM Integrated coastal zone management

IPCC The Intergovernmental Panel on Climate Change

IUCN International Union for Conservation of Nature and Natural Resources

IVI Importance Value Index

MAB Man and Biosphere Programme

NDVI Normalized Difference Vegetation Index

NGO Nongovernmental organization

NMBHWB National Mission on Biodiversity and Human Well-Being

PES Payment for environmental service

PhACs Pharmaceutically active compounds

REDD+ Reduced emissions from deforestation and forest degradation plus carbon sequestration from forest enhancement

RSP Regional Seas Programme

SLR Sea-level rise

SSR Sea surface temperature

UAVs Unmanned aerial vehicles

UNFCCC United Nations Framework Convention on Climate Change

WAC World Heritage Convention

Chapter 1

Resource conservation and management

Abstract

Mangroves are one of the most potential tidal wetlands characterized by their hydrological, ecological, and geological features. They form the diversified and biologically productive ecosystem, populated with heterogeneous groups of plant taxonomy. The transboundary Sundarban mangrove wetland (89°02′ to 89°55′E and 21°30′ to 22°30′N) is situated on the Ganges–Brahmaputra–Meghna river network. This represents the largest continuous tract of mangrove forest in the world, spanning across Bangladesh (62%) and India (38%). The chapter gives an illustrative account of the diversity and distribution patterns of mangroves in India and other coastal regions across the world, along with their anomalous biogeographical patterns, mangrove landform classification and their morphological structure and adaptation strategies. In addition, the mangrove ecosystem goods and services, carbon storage efficiency and application of remote sensing for mangrove mapping have been discussed. A haven for rich biodiversity, Sundarban harbors several rare and globally threatened plants and animals. The overall common key threats for mangrove ecosystems are land-use changes, overexploitation of natural resources, chemical pollution from point and diffusive sources, reduced freshwater supply and silt deposition. Both India and Bangladesh should implement bilateral monitoring programs to resolve those emerging problems and formulate necessary management strategies to restore this diversified and iconic mangrove ecosystem.

Keywords

Biodiversity; endangered species; mangroves; mangrove adaptation; overexploitation; tropical cyclone; Sundarban wetland

Chapter Outline

Outline

1.1 Classification and characteristics of coastal wetlands 1

1.2 Mangrove flora and fauna 3

1.2.1 Characteristics of mangrove plant species 3

1.2.2 Characteristics of temperate mangroves 5

1.2.3 Floristic composition of mangroves in India 5

1.2.4 Mangrove community in other global regions 38

1.2.5 Global mangrove cover 48

1.2.6 Biogeographical division 50

1.2.7 Classification of mangrove landforms 53

1.2.8 Structure and adaptation strategies of Mangroves 55

1.2.9 Endangered species in mangrove regions of the world and their conservation strategies 59

1.2.10 Conclusion 69

1.3 Mangroves goods and ecosystem services 70

1.4 Mangroves: A carbon source and sink 78

1.5 Remote-sensing techniques for mangrove mapping 84

References 92

1.1 Classification and characteristics of coastal wetlands

Wetlands are the biologically productive distinct ecosystems in the world, exhibiting enormous diversity relating to their topography, climate, geographical location, water chemistry, and other relevant factors including human stresses. According to Cowardin et al. (1979), the primary goal of wetland classification is to impose boundaries on natural ecosystems for the purpose of inventory, evaluation and management. There is no standardized definition of a wetland mainly due to its wide local and regional differences in geomorphic settings, geographic locations, vegetation, ecological characteristics, and gradational or transgressive boundaries between its dry and wet environments. Ramsar Convention or Convention on Wetlands (Ramsar, Iran, 1971) under Article 1.1 defined wetlands as areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary with water that is static or flowing, fresh/brackish or salt, including areas of marine water, the depth of which at low tide does not exceed 6 m. Wetlands are recognized as biological supermarkets providing extensive volumes of food, water and shelter to a large section of animal groups of diverse feeding guilds (including endangered and threatened species) who spend significant periods of time or all of their life cycles.

Two primary wetland types can be considered: (1) coastal or tidal wetlands (inundated by tides, e.g., Sundarban in the Ganges–Brahmaputra delta; found along the Atlantic, Pacific, Alaskan and Gulf coasts) and (2) inland or nontidal wetlands, common on floodplains along rivers and streams (riparian wetlands). These have again classified into several other distinct forms based on their origin, vegetation, nutrient status and thermal characteristics, such as marine (coastal wetlands), estuarine (including deltas, tidal marshes, and mangrove swamps), lacustrine (lakes) (along rivers and streams), and palustrine/nontidal wetlands (marshy swamps and bogs), on the basis of their hydrological, ecological, and geological characteristics. Wetlands are efficient for reducing contaminants as they have unique filtering capabilities for retaining excess inorganic and organic nutrients and some pollutants, and also reduce sediment load that would hinder the egg development of fish and amphibians.

Coastal or tidal-wetland ecosystems define an area of land that is either seasonal or relatively permanent, inundated with fresh, brackish, or saline water. They form a unique ecological habitat located at the land–sea interface under specialized geomorphic and hydrological settings (such as mudflats, coastal beaches, sand bars, rocky shorelines, estuarine salt marshes, mangrove swamps and forests, seagrass meadows). These offer unique habitat for most biologically diverse ecosystems and play an irreplaceable and significant role in regional ecosystems (Gao et al., 2020) in terms of providing ecosystem goods and services in multifaceted ways.

Coastal wetlands are recognized as blue carbon ecosystems as they are extremely potential at storing carbon compared to tropical forests. These valuable and sensitive environments are threatened globally by multiple sets of factors, leading to serious ecological and economic consequences. These are threatened globally by several human-induced stresses including land-use change, human development, urbanization, resource extraction, mismanagement, diversion of freshwater for irrigation network, intrusion of invasive species as well as impact of climate (Erwin, 2009; Eslami Andargoli et al., 2009; Friess et al., 2019; Schummer, 2019; Sievers et al., 2021). In addition, the impact of global warming, especially sea level rise (SLR) is a matter of great concern when wetlands generally tend to migrate landward to occupy former uplands. All such environmental changes can induce changes in hydrological regime, impacting the biogeochemical processes which control several important wetland ecosystem services. Perusal of literature reveals the loss ranges from 70% to 80% in some countries for the last 50 years (Duarte, 2002; Duke et al., 2007; Frayer et al., 1983; Meng et al., 2016; Walpole & Davidson, 2018; Wolanski, 2007). Hence July 26 each year is celebrated as the International Day for the Conservation of the Mangrove Ecosystem keeping in mind to save this endangered and vulnerable biome.

The artificial or constructed wetlands (CW), on the other hand, are low-cost engineered systems and act as a biofilter to treat/remove a range of pollutants, such as persistent organic pollutants, heavy metals, originated from municipal, industrial and agricultural effluents or storm water runoff through physical, chemical, and biological processes. In a constructed mangrove wetland in central Thailand, Boonsong et al. (2002) had experimentally shown that the mangroves in natural and artificially plantation forest systems were highly efficient in treating municipal wastewater. Recently, it has been demonstrated that CW has the efficiency to improve the water quality in Fenhe River in Qingxu County in north China’s Shanxi Province.

To reduce wetland loss and deterioration and value the ecosystems, Davies et al. (2020) proposed a Universal Declaration of the Rights of Wetlands, and Ritesh et al. (2020) suggested that wetlands need to be placed within a social–ecological framing. The Ministry of Environment, Forest and Climate Change, India has notified the new Wetland Conservation Rules which impose strict regulations on construction of new industries or their further expansion as well as disposal of debris within the wetlands.

1.2 Mangrove flora and fauna

1.2.1 Characteristics of mangrove plant species

Mangroves are taxonomically diverse highly evolved flowering terrestrial plant groups (assemblages of salt-tolerant trees and shrubs) chiefly in tropical and subtropical latitudes across the globe. Mangrove forest, mangrove swamp, tidal forest, and mangal refer to the whole community or association dominated by these tidal halophytic plants. These tidal forests are the characteristic vegetation of sheltered coastlines (60%–75% of the coastline of the earth’s tropical regions) in a warmer and more humid environment. The mangrove swamps grow under a multiple set of environmental factors (Woodroffe et al., 1992) and occur most extensively on low-energy zones and form the highly diverse and biologically productive ecosystems to the shore.

Getting acclimatized in latitudinal distribution, they form two distinct communities: the Old World Mangroves or Mangal vegetatives, containing about 60 plant species, are chiefly composed of Avicennia marina, Bruguiera gymnorrhiza, Kandelia candel, Aegiceras corniculatum, and R. mucronata. In contrast, the less rich New World Mangal provides only ~10 species (such as A. germinas and R. mangle). It is worth referring to A. marina (grey mangrove), a pioneer group of dominant mangrove that flourishes throughout the Middle East, Southern Asia, and the south of the United States.

On the basis of ecological functions within the community, mangroves can be broadly categorized into the following three types with the following specific characteristics: (1) Red mangrove (Rhizophora mangle) can grow up to 70–80 ft in a soft-muddy environments, characterized by large, strong and tough arching prop roots; (2) black mangrove (Avicennia germinans), an important component of the marsh ecosystem, provided with pencil-like aerial roots evolved from long, horizontal cable roots; and (3) white mangroves (Laguncularia racemosa): small tree or shrub occupying the higher land, grows rapidly in rich soils with a height of 50 ft; easily differentiated from other mangrove species by its leaves and root systems.

Mangrove plants are generally categorized into (1) the true mangroves or eumangroves are true halophytes, strictly grown in the mangrove environment. They are characterized with distinct adaptive morphological features such as (a) respiratory or aerial roots to provide stability to the plant as well as aerating devices to survive in water-logged or anoxic soil, specialized dispersal devices (viviparous germination, a characteristic of Rhizophoraceae) (Fig. 1.1A) and physiological processes to live in hypersaline conditions through salt gland formation, (b) existing exclusively in the intertidal zones and not extending into terrestrial communities, (c) physiological adaptation for salt exclusion and/or salt excretion, and (d) taxonomic isolation from terrestrial counterparts. Examples are Rhizophora apiculata (Fig. 1.1B), K. candel, Ceriops tagal, B. gymnorrhiza, A. corniculatum Sonneratia caseolaris; and (2) the mangrove associates or back mangroves are glycophytes as they have limited salt-tolerance mechanisms. They have their prime occurrence in a terrestrial or aquatic habitat besides present in the mangrove ecosystem (Duke et al., 1998; FAO, 2007; Jayatissa et al., 2002; Macintosh et al., 2002). These are associated with coastal communities and are dispersed by sea currents, such as Hibiscus tiliaceus (coastal cottonwood) and Ipomoea pes-caprae (Beach morning glory). Physiologically and ecologically, true mangroves differ from mangrove associates that occur almost exclusively in mangrove habitats.

Figure 1.1 (A) Rhizophora sp., showing the viviparous seedlings, (B) Rhizophora apiculata anchoring the loose soil by prop roots at Pichavaram wetland. Courtesy A. Gopalakrishnan.

1.2.2 Characteristics of temperate mangroves

The high latitude and low diversity mangrove assemblages are quite pronounced when the structural complexity and biological diversity reached its maximum at the equator and subsequently diminished toward the north and south (Ellison, 2002). The temperate mangrove forests present in southeastern Australia, northern New Zealand, and South Africa’s eastern coast predominantly occur in estuaries, whereas the tropical systems occur in river deltas or muddy coasts. Rates of loss of mangrove forests are estimated at 1%–2% year−1 as observed in the tropics (Duke et al., 2007; McLeod et al., 2011). The temperate mangrove forests are currently expanding in New Zealand, southern Australia, the United States (Florida), South Africa, Japan, and Brazil in recent decades (Giri et al., 2011; Morrisey et al., 2010).

They can be distinguished in several ways, most remarkably in lower floral and faunal diversity and density and average smaller plant height (not exceeding >4 m), and productivity generally declines with increasing latitude. The plant community is predominated by the monospecific stand of A. marina, by possessing specialized physiological and reproductive adaptive strategies to sustain and grow up in such regions. The following environmental stresses are chiefly concerned for lower species diversity: (1) forest frequency duration and/or severity, (2) severe competition of mangrove with cooccurring salt-marsh plants, (3) inhibition of reproductive events with decreasing temperature, and (4) lack of suitable habitat for mangrove growth and favorable conditions for propagate dispersion.

1.2.3 Floristic composition of mangroves in India

The coastline of peninsular India (~7500 km long) has been undergoing morphological changes throughout the geological past and can be divided into the east and west coasts and island chains based on their geomorphological settings. The mangroves occur along the coastal districts on the east and west coast within the intertidal to tidal, supratidal, or subaerial deltaic zones and vary locally according to geomorphological, climatological, topographical, edaphic, and biological factors. Mangroves are distributed along Indian coastlines among 12 maritime states and union territories (69°–89.5°E and 7°–23°N) (Fig. 1.2), comprising ~3% of the world’s mangroves. West Bengal harbors major (45.31%) mangrove cover while 1 km² of mangrove cover is recorded from Puducherry (FSI, 2015).

Figure 1.2 Map showing the distribution of mangroves in Indian subcontinent (mangrove area cover in Km² (A) and corresponding no. of mangroves (n) shown in parenthesis).

It has been reported that the mangroves cover in the Sundarban has shrunk by more than 2 km²—from 2214 to 2112.11 km²—between 2017 and 2019 (FSI, 2019). A total of 125 species including 39 species of true mangroves and 86 species of mangrove associates are recorded from India (Kathiresan, 2010). The maximum species diversity is recorded from Odisha with a total of 101 species while Gujarat harbors 40 species (Kathiresan, 2010).

Mandal and Naskar (2008) have classified the India’s mangrove habitat into the following three broad categories:

1. Deltaic mangrove: characterized by high tidal range by mighty rivers; present along the mouth of different major estuaries on the east coast of India (such as deltas of the Ganges, Brahmaputra, Mahanadi, Krishna, Godavari, and Cauvery rivers) and two gulfs (Gulf of Kutch and Gulf of Khambhat) in the west coast. With an area of 2560 km², this covers ~58% of the total Indian mangroves.

2. Estuarine and Backwater mangroves: characterized by low tidal range, made by typical funnel-shaped estuaries; constituting ~29% along the west coast (Arabian Sea) (such as estuaries of the Indus, Narmada and Tapti Rivers or intertidal coastlines, minor river mouths, sheltered bays and backwater areas, creeks and neritic inlets of these areas).

3. Insular or Island mangroves: generally present at the mouths where rivers are seen to border the open sea. It covers about 16% of the total mangrove area, found along the shallow but protected intertidal zones of bay islands, Lakshadweep atoll and Andaman and Nicobar Islands (ANI) (FSI, 2015; Mandal & Naskar, 2008).

The deltaic mangrove forests located in the east coast of India (~4700 km²) are floristically diverse and grown luxuriantly along the maritime states such as Tamil Nadu, Andhra Pradesh, Odisha, West Bengal, and ANI. The West coast (850 km²) extends from Kerala, Karnataka, Goa, Maharashtra, and Gujarat and also includes the coral atolls of Lakshadweep Islands. The extensive and diversified mangroves of ANI are possibly best developed in India in terms of their density and growth (Mandal & Naskar, 2008). The mangroves in the eastern coast cover are comparatively larger (~70%) and more widespread than predominantly localized west coast (30%) because the east coast terrain has a gentle slope as plains with extensive flats for mangrove colonization compared with the steep gradient along the west coast. Moreover, the east coast is equipped with a good number of large estuaries with deltas formed by runoff and deposition of sediments, especially along the northern portion (West Bengal and Odisha coast such as Krishna, Cauvery, and Ganges). In contrast, the west coast has funnel-shaped estuaries characterized by steep slopes and with an absence of deltas. This leads to higher salinity levels than the east coast, resulting in a mangrove community less diverse and smaller in size too.

1.2.3.1 Mangroves in Sundarban wetland

1.2.3.1.1 Physiographic set up and characterization

The Sundarban tidal wetland forest (21°31′ to 22°30′N and 88°10′ to 89°51′E) occupies in the fluvio-marine megadelta of the Ganga, Brahmaputra, and Meghna (GBM) rivers in the Bay of Bengal (BoB)—one of the geochemically youngest and tectonically active potential river basins in the world. This single largest contiguous block of tidal mangrove forest globally, typically recognized as moist tropical serial forest, presents an excellent example of land–sea interface with a dynamic and complex physiographical and geomorphological processes. The forest (spanning India and Bangladesh) covers an area of approximately 10,000 km², of which 62% lies within Bangladesh (6017 km²) and 38% in neighboring country India (4246 km²). The geographical limits are the river Hooghly (a distributary of River Ganges) in the west and river Balawaswar (known as Ichamati or Raimangal in Bangladesh) in the east, the Bay of Bengal in the south, and the imaginary Dampier-Hodges line in the north (Government of West Bengal, India, 2013; www.sadepartmentwb.org/Introduce.html) (Fig. 1.3).

Figure 1.3 Mapped distribution of the mangrove ecosystem of the Indian Sundarban, showing the key rivers, the minimum convex polygon enclosing all occurrences of mangroves (orange line: extent of occurrence), and all occupied (> 1%) 10 × 10 km grid cells (dark grey cells: area of occupancy). Also shown is part of the mangrove ecosystem of the Bangladesh Sundarban. Distribution data current at 2016 from the Global Mangrove Watch. From Ocean Data Viewer (unep-wcmc.org). https://data.unep-wcmc.org/datasets/45.

The recorded total area of the Bangladesh part of the Sundarban Mangrove Forests (BSMF) is ~6017 km² as evidenced from the multispectral SPOT satellite data, out of which ~4267 km² is forest landmass and over 115 km² is marshes within a network of 450 rivers (Aziz et al., 2017; Iftekhar & Islam, 2004; Rahman & Asaduzzaman, 2010). The BSMF is intersected by seven main north to south flowing rivers, and a conglomeration of mudflats, coastal dunes, creeks and channels, flat marshy islands, tidal estuaries, river channels, inlets, and thick mangrove swamps is the unique morphological features of Sundarban, where the individual components are interdependent. The entire Sundarban Reserve Forest (SRF) covering an area of 139,519 ha, is protected and declared as a World Heritage site, proscribed under natural criteria IX and X and IUCN/SSC management category IV for sanctuary and II for the national park status (UNEP, 2011). The BSMF is largely populated by saline-tolerant or freshwater-loving species such as Heritiera fomes and Nypa fruticans, whereas in the Indian part of the Sundarban, low stands of Excoecaria agallocha, A. corniculatum and Ceriops decandra are common. Three wildlife sanctuaries are situated in BSMF of high socioecological importance, namely Sundarban South (36,970 ha), West (71,502 ha), and East (31,226 ha) (Fig. 1.4) (Rahman & Asaduzzaman, 2010). The wetland has been acclaimed as a wetland of international importance under the Ramsar Convention in 1992.

Figure 1.4 Map of the Sundarban Reserve Forest, Bangladesh.

Composed of a cluster of about 102 miracle islands and a part of the Bay of Bengal Marine Ecoregion of the World, the Indian Sundarban mangrove ecosystem is classified as habitat type 12.7 Marine Intertidal—Mangrove Submerged Roots and 1.7 Forest-Subtropical/Tropical Mangrove Vegetation Above High Tide Level under the IUCN Habitats Classification (Version 3.10) (Spalding et al., 2007). It is situated in the low-elevated coastal zone, characterized by a complex network of tidal creeks and divided into core, buffer, and transition zones for resource management (Fig. 1.5). The core zone consists of the Indian Sundarban National Park (area of 1330.12 km²), the critical tiger habitat, and does not allow any anthropogenic activities to preserve the major habitats of diverse flora and fauna including the endangered ones. Recognizing the importance and uniqueness of Sundarban and its vast capacity of sustaining an excellent biodiversity, UNESCO declared the core region of the mangrove as Indian Sundarban Biosphere Reserve (SBR) in 1989 for protection and conservation of mangrove flora and fauna including the Royal Bengal Tiger from anthropogenic disturbances, and part of the Bangladeshi portion was declared a separate world heritage site in 1997. Area outside the core zone is designated as the buffer zone and consists of the Sajnekhali Wildlife Sanctuary with an area of 362.33 km². The transition area of 5705 km² along the northern boundary of the reserve is mainly mangrove reclaimed zone, where agriculture and coastal aquaculture are extensively practiced by about 4 million local people almost through the year. The Indian side of Sundarban received the prestigious Wetlands of International Importance tag under the Ramsar Convention on Wetlands in 2002 (Ramsar site no. 2370) and the Bangladesh counterpart received the Ramsar tag way back in 1992.

Figure 1.5 Map of Indian Sundarban delta, showing the location of 16 sampling sites encompassing the North & South 24 Pgs districts of West Bengal, India.

1.2.3.1.2 Mangrove species composition

Sundarban is acclaimed as the world’s largest transboundary repository of mangroves and mangrove associates flora and fauna. The ecoregions harbor diversified mangrove assemblages, supporting ~44% of global mangrove species (Alongi, 2009). The substantial variability in hydrological parameters, large tidal amplitude and gently sloping coastline, topographic heterogeneity, and their interactions, have resulted in this transboundary wetland into biologically productive and taxonomically diverse ecoregions. Through evaluation of time series Landsat satellite imagery data, Halder et al. (2021) have identified a decline of 3.76% aerial extent of Indian Sundarban mangrove forest between 1990 and 2019, this was mainly due to the cumulative impact of the followings features: (1) a measurable reduction in freshwater reaching the mangroves mainly due to oversiltation of the major tributaries of the Ganges and thus carries little quantities of freshwater, (2) diversion of freshwater in the upstream region, (3) construction of upstream embankments for diversion of irrigation water leading decline in freshwater availability altering the regional hydrological balance, (4) Bengal Basin is tilting toward the east because of neo-tectonic movement over the past millennium, resulting greater freshwater inflow to the Bangladesh Sundarban and less salinity compared to Indian side, (5) reduction of silt deposition, and (6) severe human pressures—particularly land acquisition by reclamation for settlement and expansion of agriculture and shrimp farms. Recently Mondal et al. (2021) had interpreted that frequent coastal hazards, sea-level rise, and ever-increasing anthropogenic pressure have complicated the growth and regeneration of mangroves in Indian Sundarban.

Majority of the distributaries of river Ganges have silted up and carry very little quantity of freshwater except during monsoon season. As a result, the stenoecious mangroves species, A. alba and E. agallocha are gradually replacing economically important H. fomes and Sonneratia caseolaris that require regular supply of freshwater for their growth (Gopal & Chauhan, 2006).

Ahmed, Ataullah et al. (2018) and Ahmed, Thompson et al. (2018) recorded 46 species belonging to 26 families and 41 genera in BSMF, out of which 19 were distinguished as true mangroves and the rest were mangrove associates species. Leguminosae and Rhizophoraceae were the dominant families represented by 5 species each. Three invasive species, namely, Blume alacera, Catharanthus roseus, Wedelia chinensis were identified which might alter the benthic community structure. The overall species diversity (H), species richness (d) and evenness (e) of SMF was found 3.81, 9.10, and 0.47, respectively. The dominant mangrove species in the Bangladesh Sundarban ecoregions comprises H. fomes, E. agallocha, Avicennia spp., Xylocarpus mekongensis, X. granatum, Sonneratia apetala, B. gymnorrhiza, B. parviflora, B. sexangula, Ceriops candellana, C. roxburghiana, C. tagal, C. decandra, A. corniculatum, R. apiculata, R. mucronata, and N. fruticans. Representatives of true and dominant mangrove plants and plant organs in Sundarban ecoregions have been presented in Fig. 1.6A–L along with the mangrove associates species (see Fig. 1.6M–Q).

Figure 1.6 Representative of mangrove plant and mangrove associates species from Sundarban wetland: (A) Avicennia alba, (B) Avicennia marina inflorescence, (C) Avicennia officinalis inflorescence, (D) Aegiceras corniculatum inflorescence, (E) Leaves of Aegialitis rotundifolia, (F) Bruguiera showing drooping propagule, (G) Excoecaria agallocha inflorescence, (H) Leaves of Heritiera fomes, (I) Lumnitzera racemosa, (J) Leaves and young propagule of Rhizophora apiculata, (K) Sonneratia apetala, (L) Sonneratia caseolaris fruit, (M) Acanthus ilicifolius, (N) Ipomoea pes-caprae natural habitat in the sand dunes, (O) Suaeda maritima, (P) Suaeda nudiflora, (Q) Nypa fruticans. A to P: Photos taken by the author; Courtesy Sayam U. Chowdhury, Assistant Coordinator, Spoon-billed Sandpiper Task Force; Q: Courtesy Sayam U. Chowdhury, Assistant Coordinator, Spoon-billed Sandpiper Task Force.

Iftekhar et al. (2008) observed that H. fomes, E. agallocha, and C. decandra jointly cover 95% of the forest area and H. fomes and E. agallocha have maintained their dominance over large portions of the mangrove forest. The maximum importance value index was also recorded in H. fomes (48.08) by Ahmed, Ataullah et al. (2018) and Ahmed, Thompson et al. (2018). A comprehensive list of mangrove and mangrove associates species from Sundarban ecoregions, with brief note of their dispersal mechanisms and conservation status has been presented in Table 1.1. The mangrove forests in Indian Sundarban during high tide and low tide have been shown in Fig. 1.7A and B, respectively.

Table 1.1

Important Taxonomic keys for dominant mangrove species have been explained as follows: Avicennia marina can be distinguished from other species of this genus by its elliptic-oblong or elliptic-ovate leaves and apiculate fruits (2) Heritiera fomes can be distinguished from its shinning silvery undersurface of leaves and subglobose fruits with longitudinal and transverse ridges (3) H. kanikensis is closely allied to H. fomes but is readily recognized from other two species of Heritiera by its rough globose fruits devoid of any ridges or crest (4) H. littoralis: distinguished from H. fomes by its larger leaves with oblique bases, 8-12 cm long smooth ellipsoid fruits with a well-developed wing on the outerside and a keel on the inner side. H. Fomes (local name: Sundari in Bengal) could be a vicariant species (or an ecotype ?) of H. Littoralis. Global status of each species referred as per Polidoro et al. (2010) and status in Indian context as per Kathiresan (2008). CR, Critically Endangered; EN, Endangered; VU, Vulnerable; NT, Near Threatened; LC, Least Concern; DD, Data Deficient; VU, Vulnerable; LR, Lower risk; NA, Not assessed; SAT, salt accumulating type; SET, salt excreting type.

aDenote the mangrove associates while the others are true mangroves.

Figure 1.7 (A) Mixed mangrove forest in Sundarban coastal region; (B) Mangrove forest showing the stands of Rhizophora sp., in Indian Sundarban.

As a consequence of possible climate change impact, Dasgupta et al. (2017) estimated the significant variations in the standing stock of predominant mangrove species of Bangladesh Sundarban, which would of course have a regressive impact on the forest-based livelihoods. The authors noted varied patterns of gain and loss as a function of climate change induced aquatic salinity: significant depletion for H. fomes (high-value timber species), notable gains for E. agallocha; modest changes for A. alba, A. marina, A. officinalis, Ceriops decandra, and S. apetala; and mixed results for species combinations.

To explore the efficiency of Avicennia officinalis in diversified habitat conditions, Alam et al. (2020) analyzed the genetic diversity of the species and explored the low salt-adapted ecotype (growing in low-salinity zone) and high salt-adapted ecotype (growing in medium-salinity and high-salinity zones) in Sundarban mangrove forest, Bangladesh. The adaptive flexibility of the species has great ecological consequences to cope with the salinity regime changes due to the impact of climate change in the near future.

1.2.3.1.3 Zonation of mangroves

Zonation patterns are the species or species groups forming discrete bands parallel with the shore in response to varying total inundation. Mangrove zonation at any specific area is complex, influenced by cumulative impact of edaphic, hydrographic, geological, and meteorological components and varies with the physical, chemical, and biological interactions occurring in that area. The spatial distribution of mangrove trees and shrubs in a zone of Sundarban do not always seem to be homogeneous, but often occurring randomly intermingled, hence cannot be considered typical for that specific zone. This anomaly is chiefly due to human interference disrupting the original distribution of plants along ecological gradients, such as soil salinity and tidal amplitudes. Usually, it is observed that the vegetation is thriving under a larger tidal influence. However, the correlation between the tide level and the land surface is ever changing and the raised surface becomes flat over the time/years when the intensity of submergence becomes insignificant. In such conditions vegetation cannot remain in the zone and revert to a mosaic condition (Ranwell, 1972). In the Sundarban environment, a similar phenomenon exists, finally resulting in a vegetation complex incorporating the heterogeneous component. However, homogeneity prevails exclusively in the mudflats of eastern sector of Sundarban as the seagrass Porterasia coarctata (family Poaceae) remains the major representative and similarly mangrove associates such as Acanthus, Suaeda, Ipomoea, and Salicornia (Fig. 1.6M–P) in the western sector of the wetland.

1.2.3.2 Mangroves in east coast of India

1.2.3.2.1 Mangroves in Mahanadi Delta

Mangrove forests in Mahanadi delta region (latitude: 20°18′ to 20°32′ N and longitude: 86°41′ to 86°48′E) is located between Barunei mouth to Mahanadi mouth (Paradeep), covering an area of 69 km² mangrove cover in 1973 (Pattanaik & Prasad, 2011). Subsequently, Ghosh et al. (2015) identified a trend of increase as well as decrease in dense and open mangrove forest, respectively within a span of three decades, mainly concerned for large-scale encroachment. The mangrove community is highly enriched comprising 34 true mangrove species, dominated by A. officinalis, A. marina, S. apetala, E. agallocha, and R. mucronata. In this delta several physiognomic types have been identified including tall dense mangrove forest with H. fomes; open mangrove thickets with palms (Phoenix paludosa); scattered mangrove undershrubs with grassy halophytes (Suaeda, Salicornia).

1.2.3.2.2 Mangroves in Bhitarkanika

Formed by the rich alluvial deposits of the Brahmani, Baitarani and Dhama rivers, Bhitarkanika wildlife sanctuary (latitude: 20°40′ to 20°48′ N and longitude: 86°45′ to 87°50′E) is the second-largest mangrove wetland in India, covering an area of 197 km² (FSI, 2017). The wetland has been acclaimed as the Ramsar site due to its unique ecological, geomorphological and biological characteristics.

So far, 28 true mangroves and 4 mangrove associates plant species have been recorded, dominated by E. agallocha, H. littoralis, A. officinalis, and Cynometra ramiflora (Basha, 2018). During 1973 the mangrove coverage was estimated to be 180 km² by Reddy et al. (2007), comprising both dense (147 km²) and open (33 km²) mangrove forests. No change in dense mangrove cover was reported during 2004 but open mangrove forest decreased to 18 km². However, the total mangrove cover (combination of dense mangrove and open mangrove) was increased to 197 km² due to proper restoration and rehabilitation programs (FSI, 2017). Significant pressure on Bhitarkanika mangroves is related to increasing aquaculture, pollution impact, industrialization, storm surges, and frequent cyclones (Kadaverugu et al., 2021).

The Gahirmatha Beach (only marine wildlife sanctuary of Odisha) is the separating line between the Bhitarkanika National park and the Bay of Bengal, recognized as the world’s most important largest mass nesting beach for Olive Ridley Sea Turtles (Lepidochelys olivacea). The sanctuary harbors endangered wildlives such as saltwater crocodiles (SWCs) (Crocodylus porosus), Fishing Cat, and Sambar, together with diverse avian communities (both residential and migratory birds).

1.2.3.2.3 Mangroves in Krishna wetland

Spread across an area of 137 km² in Guntur and Krishna district of coastal Andhra Pradesh, the mangrove community of the wetland (latitude 15°50′ to 15°55′ N and longitude: 80°45′ to 80°50′E) is composed of 23 species of 14 families (Madhusudhana Rao et al., 2015), out of which 11 species are true mangroves and 12 species are mangrove associates. Gamage and Smakhtin (2009) demonstrated that conversion of mangrove wetland to aquaculture leads to shoreline retreat in Krishna delta. According to an FSI report (2017), the mangrove swamp of river Krishna has decreased from 158 km² in 2013 to 137 km² with 50 km² of moderately dense mangrove forest and 87 km² of open mangrove forest. Decline of mangroves are related to human-induced stresses such as extensive aquaculture activities, cutting of mangrove trees for timber/coal, scanty freshwater supply, hyper salinity, upstream industrial contaminants, and coastal corridor development.

1.2.3.2.4 Mangroves in Godavari wetland

Situated in east Godavari district of Andhra Pradesh (latitude: 16°30′ to 16°55′N and longitude: