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per metre creek length per spring tide, or a vert-
ical accretion rate of around 0.1 cm yr
−1
. Krauss
et al. (2003) have documented highest sediment
settling rates (11.0 mm yr
−1
) around prop roots,
as opposed to pneumatophores (8.3 mm yr
−1
),
although the latter are most effective in terms
of sediment retention. Sediment trapping is
also aided where bacterial and/or algal mats
develop on substrate surfaces. Pelletization of
fine sediment by benthic detritivores will also
bind sediment and limit sediment entrainment
and (re)export.
often less than 0.5 cm yr
−1
(with a maximum
of around 1 cm yr
−1
). Sediment accumulation
rates are, however, highly variable both within
and between environments, as well as season-
ally (Saad et al. 1999). Such differences reflect,
in large part, variations in sediment supply and
the local processes of sediment transport and
reworking. Viewed at a relatively simplistic level,
narrow mangrove-fringed shorelines are likely
to exert much less influence on sediment bud-
gets than wide mangrove shorelines (Wolanski
1995), although few sufficiently detailed budget
studies have been undertaken to enable detailed
understanding of the net inputs/outputs of sedi-
ment in different mangrove settings.
Different mangrove environments, however,
can be identified that have very different sedi-
ment substrates and distinct processes of
sediment supply and accumulation. The Grijalva-
Usumacinta delta, Mexico, for example, forms a
complex network of mangrove-fringed lagoons,
channels and interdistributary basins (Thom
1967). The delta is developed along a microtidal
coastline and is dominated by fluvial inputs (Fig.
9.14a). Fluvial discharge is, however, highly sea-
sonal and during the dry season saline wedges
develop within the creeks that may extend up to
30 km inland. Sediment inputs are dominated
by fluvially derived inorganic sands, silts and
clays, and organic material produced
in situ
from
litter-fall. In contrast, the mangroves developed
in the vicinity of Coral Creek, north-east Austra-
lia are associated with an ebb-tide-dominated
estuary, which receives very little fluvial fresh-
water or sediment input (Fig. 9.14b; Grindrod
& Rhodes 1984; Wolanski et al. 1992). As in
the previous example, the predominant sediment
substrate type across the mangrove is an inter-
tidal organic mud, derived primarily from
in situ
breakdown of organic material. Terrigenous sedi-
ments are only important in the upper reaches of
creek networks. Mangroves also develop along
carbonate-rich shorelines, such as those around
the Gulf of St Vincent in southern Australia
(Butler et al. 1977). The mangroves develop on
mixed carbonate-siliciclastic sediments, although
marked variations in sediment content occur
across the mangrove (Fig. 9.14c).
9.2.5.3 Mangrove sediment reworking
Although small-scale effects of physical sedi-
ment reworking and transport influence grain
entrainment and transport, biological rework-
ing also occurs. Much of this is attributed to the
activities of crabs, which rework sediment dur-
ing feeding and burrow construction. Grapsidae
crabs feed primarily on leaf litter, whereas
Ocypodidae (fiddler crabs) are detritivores and
ingest sand grains in order to remove organic
material. This latter group produce large numbers
of faecal and pseudofaecal pellets, with sedi-
ment turnover rates of up to 0.5 kg m
−2
day
−1
(Hogarth 1999). Although this has an important
effect on retexturing sediments, the generation
of extensive, interconnecting burrow networks
also provides a conduit for groundwater flow
(Wolanski et al. 1992). Flow velocities in these
burrows may be up to 30 mm s
−1
with 1000 -
10,000 m
3
of water flow per tidal cycle per
square kilometre (Ridd 1996).
9.2.6 Variations in mangrove sediment
accumulation
It is now widely accepted that mangroves tend
to follow rather than initiate sediment accumu-
lation (Woodroffe 1992), however, once estab-
lished the mangroves may enhance sediment
accumulation by facilitating the trapping of
sediment around roots and pneumatophores. A
review of recent literature on short-term vertical
accretion rates by Ellison (1998) suggests that
sediment accretion rates within mangroves are
