Geoscience Reference
In-Depth Information
Fig. 9.12
Intertidal sediments accumulating in and around the prop roots of
Rhizophora
sp. colonies. Note the abundant leaf and
seagrass litter accumulating on the substrate. Inhaca Island, Mozambique.
source for organic matter (Dittmar et al. 2001).
In settings characterized by limited tidal exchange
and by limited fluvial influence, organic matter
is largely retained within the environment and
represents a dominant sediment contributor.
Wood detritus breaks down much more slowly,
but is facilitated by the activities of wood-boring
teredinid molluscs.
Another important input into mangrove sedi-
ment derives from mangrove-associated shelly
faunas. Of these the most abundant are crabs,
molluscs and foraminifera (Fig. 9.11). Dominant
crab families are the Grapsidae and Ocypodidae
and individual species densities can be in the order
of 60 m
−2
. Numerous molluscs, including species
of barnacles, oysters, gastropods and bivalves,
also occur and are often dominant skeletal
contributors (Plaziat 1995), although there are
marked variations between environments (Plaziat
1974). Foraminifera are also abundant, although
abundance and diversity are strongly influenced
by local hydrodynamics, the seasonality of fluvial
influence and substrate elevation (Debenay et al.
2002). Although the skeletal remains of molluscs,
foraminifera and crabs accumulate with man-
grove sediments, much of this material is subject
to intense dissolution within acidic porewaters.
Although the processes of skeletal modification
and degradation in mangroves remain poorly
documented, the best preservation is likely in
areas where (i) rapid burial in fine-grained (low
permeability) sediments occurs, or (ii) either
sea water or carbonate muds buffer sediment
porewater acidity (Plaziat 1995).
9.2.5 Controls on mangrove sediment transport
and accumulation
9.2.5.1 Mangrove sediment transport
The mechanisms and rates of sediment trans-
port within and through mangrove systems are
