Abstract of the water body and waves, a

Abstract

West African coastline has been receding at a very fast
pace in the last two decades but the role of mangroves on the coast has not
been estimated. Studies have shown that the complex interlocking root system
and composition of mangrove stand may provide coastal protection against storm
surges, tsunamis, coastal erosion, sea-level rise ,cyclones (Chang  al. 2006,Danielsen et al.2005,Kathiresan and
Rajendran 2005) but the temporal change in the mangroves composition and fauna may
impact the coastline dynamics.The study aims at simulating a set of selected
dominant species of mangrove trees from West Africa, interacting with different
crabs and snails (representing key players in structuring and driving
mangroves, their processes and services)  to understand how they impact coastal sedimentation/erosion.
This is a novel approach to experimentally gain information on the contribution
of species-specific mangroves interacting independently or with the benthic
fauna and how this translate to the overall effort of mangrove restoration
projects in West Africa.

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Scientific question/Objective 

Among the current emerging issues of mangrove studies is the combination of
field investigations, and experimental data to understand the relationship of how
different mangrove tree species in concert with benthic fauna (crabs or /mucus
secreting fauna) aid in sediment trapping and sedimentation. The novelty of
this approach is the focus on determining which mangrove tree species and fauna
to introduce in a future restoration project based on the different local
environmental conditions.Based on experimental quantification of how
different mangrove species (e.g., Rhizophora
sp. with aerial prop roots versus
Avicennia sp. with finger-like
aerial pneumatophores) affect sedimentation, how mangrove inhabitants (e.g.,
burrowing crabs or mucus-secreting snails) stabilize the trapped sediment, and
how this is influenced by the sediment type, the slope of the shoreline,
movement of the water body and waves, a modelling-approach
will implement virtual species with corresponding characteristics. Under
different simulated environmental conditions, the resulting ecosystem properties and processes
emerging from combining different functional groups of organisms (relative to
single-species simulation) will be quantified and fed into the developing
toolbox.

Methodology
Five individuals each of three different mangrove species (prop roots: Rhizophora mucronata; knee roots: Bruguiera gymnorhiza; pencil roots: Avicennia marina) will be planted in a
75×45 cm² container with artificial sediment (coarse-grained sandy growth substrate).
Mangrove trees will establish in these container either without animals or with
crabs, with snails, or with crabs and snails: three (functionally) different
species of mangrove crabs (large (e.g., Perisesarma
bidens) versus small (e.g., Uca sp.) burrowers versus tree-climbing (e.g., Armases elegans) crabs); three (functionally) different species of mangrove snails
(large (e.g., Terebralia sp.) versus small (e.g., Neritina sp) surface-dwellers versus
tree-climbing (e.g., Melampus coffea)
snails) of the same biogeographical region (i.e., the Western Atlantic).
Animals will be allowed to develop burrows (if any) and mucus trails.
Fine-meshed cages will cover the containers to prevent animal inhabitants from
escaping.

(1) Sediment particles of different sizes (finer than the growth
substrate) will be suspended in a reservoir from which the sediment-laden water
will be flushed over the setup at different flow rates (through pumps) to mimic
incoming water and quantify sediment-trapping by mangrove roots, crab burrows
or snail mucus. The incoming and settled sediment will be distinguished from
the growth substrate through its size upon sieving and quantified
gravimetrically (upon drying). After weighing, the particle size of the settled
sediment (as a function of flow rate) will be determined with a CASY
cell-counter (using sediment-free water as control).

(2) Upon settlement of fine-grained sediment, the
above-mentioned experimental setups will be exposed to strong water currents
and waves (through pumps and wave generators) to mimic incoming water and
quantify erosion versus its
prevention by mangrove roots or through sediment stabilization by crab burrows
or snail mucus. As above (1), the amount of eroded sediment of different
particle sizes will be quantified through sieving and weighing (upon drying)
from sediment-laden water that leaves the experimental containers.

(3) Field investigations will be conducted in three west African countries
(Ghana, Togo, and Benin) to experimentally quantify the width of forest, tree
density, tree height, soil texture, bathymetry in the location of the three
mangrove species selected for the purpose of this study.

Virtual Toolbox:

The virtual toolbox will contain a set of species that
interact in the constructed ecosystem with respect to sediment dynamics
(accretion versus erosion) and will
determine how it drives these processes through mutual feedback in a
self-organised way. A predefined set of functional responses will be defined
for each species through their development and reaction potential which can
then be used to design a targeted ecosystem with specific services. However, as
species reactions are context dependent, the ecosystem trajectories have to be
analysed for relevant species combinations to evaluate which potential is
realised in a given environmental situation. Thus, specific ecosystem processes
will be very closely linked with species-specific properties and dynamics in a
feedback system.

The focus on the resulting ecosystem processes and on
testing how this is influenced by different sets of interacting species
(initially different functional types of snails, crabs and mangroves)
underlines the design aspect. The definition of the species is closely
interlinked with the experimental approach and the available knowledge on
processes in mangrove system. The model design will utilise a generic and
modular approach with clearly defined (and documented) internal and external
interfaces (in ecological terms interactions/influence potential on ecosystem
properties or other organisms) to be able to easily add further species to the
toolbox when their definition is available.

Developing a generic species layout constitutes the
biggest challenge for the toolbox design and clearly distinguishes it from
previous modelling approaches which are usually confined to narrowly defined
system variability. The model will use an agent-based modelling approach
(Breckling et al. 2006, Grimm et al. 2005) which allows flexibility in
representing individual functions and the inclusion of spatial processes. A
combination with other modelling approaches (e.g. CA, ODE) is possible (Reuter
et al. 2008) were this is necessary in the description of the abiotic
processes.

The specific components comprise:

(1) Framework
Ecosystem

Development of a framework ecosystem defined by
ecosystem processes (indicators) which are influenced by species existence and
impact. This includes the spatial pattern of sediment processes (material
removal, clumping through mucus, sediment deposition) and generic interfaces
for exchange with species.

(2) Species
Toolbox

The species toolbox constitutes of a generic
declaration of a basic set of species with different functional types
resembling the species under studies. These types will be defined by their
ecological function and potential ecological service and described in detail by
their potential to occupy different ecological niches (e.g. habitat), and their
activity repertoire (processes describing the interaction with other species,
interrelation with the framework ecosystem).

(3) Specification
and Scenario Simulation

The data from the experiments together with general
knowledge on mangrove systems will be used to specify species-driven processes
and their responses to the environment. The species specification will be
entered into a data base allowing an easy choice of species with selected
contributions to ecosystem processes.

Scenario simulations will comprise the combination of
different species (functional types) to identify respective resulting ecosystem
processes and related ecosystem services under the influence of changing
environmental situation.

4) Further steps will
include the addition of life-history processes and population dynamics.

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