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The Importance of Burrowing and Leaf Litter Feeding Crabs for the Ecosystem Functioning of Mangrove Forests

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The Importance of Burrowing and Leaf Litter Feeding Crabs for the Ecosystem Functioning of Mangrove Forests The importance of burrowing and leaf litter feeding crabs for the ecosystem functioning of mangrove forests Dissertation submitted by Nathalie Pülmanns In partial fulfilment of the requirements for the degree of doctor of natural sciences (Dr. rer. nat.) Faculty of Biology/Chemistry University Bremen Germany March 2014 The present thesis has been realized from July 2011 until March 2014 at the Leibniz Center for Tropical Marine Ecology in Bremen. Board of Examiner: Reviewer: Prof. Dr. Ulrich Saint-Paul Leibniz Center for Tropical Marine Ecology, Bremen, Germany Reviewer: Prof. Dr. Juliane Filser Umweltforschung/- Technologie, Universität Bremen, Germany Examiner: Prof. Dr. Matthias Wolff Leibniz Center for Tropical Marine Ecology, Bremen, Germany Examiner: Dr. Karen Diele Edinburgh Napier University, Great Britain Members: Alejandra Sepúlveda Lozada PhD student at the Leibniz Center for Tropical Marine Ecology, Bremen, Germany Members: Constanze Bark Bachelor student at the University of Bremen Thesis supervisors: Dr. Karen Diele - Edinburgh Napier University, Great Britain Dr. Ulf Mehlig - Universidade Federal do Pará, Bragança, PA, Brazil Dr. Inga Nordhaus - Leibniz Center for Tropical Marine Ecology, Bremen, Germany Prof. Dr. Ulrich Saint-Paul - Leibniz Center for Tropical Marine Ecology, Bremen, Germany Table of content Table of content Table of content ...................................................................................................................... I Thesis abstract ......................................................................................................................III Zusammenfassung ................................................................................................................ V Chapter 1: General Introduction ............................................................................................ 9 Mangrove ecosystems ....................................................................................................... 9 Ecosystem function and functioning................................................................................... 9 Crabs as ecosystem engineers and keystone species ......................................................10 Burrowing crabs................................................................................................................12 Objectives.........................................................................................................................13 Chapter 2: Environmental settings ........................................................................................17 Study area ........................................................................................................................17 Climate .............................................................................................................................17 Study organism ................................................................................................................19 Chapter 3: Desalting by artificial burrows ..............................................................................23 Introduction .......................................................................................................................24 Material and methods .......................................................................................................26 Results .............................................................................................................................31 Discussion ........................................................................................................................36 Conclusion........................................................................................................................40 Chapter 4: CO2 release by Ucides cordatus burrows ............................................................41 Introduction .......................................................................................................................42 Material and methods .......................................................................................................43 Results .............................................................................................................................50 Discussion ........................................................................................................................55 Conclusion........................................................................................................................61 Chapter 5: Crab removal experiment ....................................................................................63 Introduction .......................................................................................................................64 Material and methods .......................................................................................................66 Results .............................................................................................................................71 I Table of content Discussion ........................................................................................................................82 Conclusion ...........................................................................................................................90 Chapter 6: Thesis synopsis ..................................................................................................91 The influence of U. cordatus on sediment processes ........................................................91 The role of U. cordatus in ecosystem functioning of Brazilian mangrove forests ...............91 The importance of mangrove crabs for ecosystem processes and ecosystem functioning 93 Conclusion and outlook ....................................................................................................95 Acknowledgments ................................................................................................................97 References ...........................................................................................................................98 List of figures ......................................................................................................................116 List of tables .......................................................................................................................119 List of appendices ..............................................................................................................121 Appendix ............................................................................................................................122 Eidesstattliche Erklärung ....................................................................................................135 II Thesis abstract Thesis abstract Mangrove forests are endangered in times of urbanization and climate change. The destruction of mangrove forests may result in a loss of species, ecosystem functions and thereby ecosystem goods and services. Studies have emphasized the importance of burrowing and leaf litter feeding crabs for functioning of mangrove ecosystems. Crabs rework the sediment by constructing and maintaining burrows and process leaf litter, which is otherwise exported by the tide. Thus, crabs can influence sediment characteristics, energy and nutrient cycling and, consequently, other ecosystem components such as tree growth. Yet, the impact of crab activity on ecosystem processes, and subsequently on ecosystem functioning, is unclear. Furthermore, previous studies focused on smaller fiddler crabs or crab species from the Indo-West Pacific region. Larger burrowing crabs, which are predominant in the Atlantic-East Pacific region, are less well studied, making generalization of the role of mangrove crabs difficult. This thesis aims to better understand the role of large burrowing and leaf litter feeding crabs, using Ucides cordatus (Ucididae) as a model organism, in sediment processes (chapters 3 and 4) and, based on these processes, in the ecosystem functioning of a Brazilian mangrove forest (chapter 5). Results of these studies are evaluated in the thesis synthesis (chapter 6) to understand the importance of U. cordatus, but also of burrowing and leaf litter feeding crabs in general, for mangrove ecosystems. The first experiment was a microcosm experiment exposing Rhizophora mangle L. seedlings to simulated spring-neap tide cycles. Within this setting, the ability of artificial burrows with one opening to facilitate salt release was studied, and it was determined if desalting improved seedling growth. Salt accumulated in sediment due to water uptake of seedling roots. Sediment salinity and seedling growth in plant containers with and without artificial burrows were recorded over six months. Additionally, sediment salinity data were collected in a field experiment to determine if areas with a high U. cordatus burrow density (mainly burrows with one opening) coincided with areas with low sediment salinity. Therefore, sediment salinity and burrow density were measured in rooted and non-rooted gap areas between trees. In the second field study, CO2 released from U. cordatus burrows and the reduction potential in the burrow wall sediment, as one factor influencing CO2 release, were measured. The contribution of crab respiration to the amount of CO2 released from burrows was estimated in the laboratory. As a third field experiment, a one year
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