The importance of burrowing and leaf litter feeding crabs ...elib.suub.uni- The importance of burrowing

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  • 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

    I

    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

  • Table of content

    II

    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

  • Thesis abstract

    III

    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 pl