Geoscience Reference
In-Depth Information
1
allochthonous - sediments derived from out-
side the mangrove and either terrestrial (often
fluvial) or shallow marine in origin;
2
autochthonous - sediments produced
in situ
and which include both organic litter and skeletal
material.
Sediment accumulation is influenced by a range
of physical and ecological factors that control
transport, settling and reworking potential.
sands, and 50,000 t yr
−1
of suspended sediment).
As a result, the estuary is dominated by net
landward flood-tide sediment transport which
accumulates in the upper estuary.
9.2.4.2 Autochthonous sediments
In addition to externally sourced (fluvial or
marine) sediment, significant amounts of sedi-
ment may also derive from
in situ
sources, the
most important of which are organic litter and
mangrove-associated skeletal faunas (Fig. 9.11).
Most mangroves are characterized by high rates
of primary productivity and, as a result, large
amounts of organic material, in the form of leaf
litter along with decaying roots and branches,
accumulate in the sediment (Fig. 9.12). Rates of
leaf litter production (a commonly used proxy for
organic matter production; Hogarth 1999) range
from 5 to 15 t ha yr
−1
. Production rates are, how-
ever, dependent upon both season and setting.
At Darwin Harbour (Australia) Woodroffe et al.
(1988) measured the highest litter production
rates (up to 1400 g m
−2
yr
−1
) within tidal creek
settings beneath tall (13 m high)
Avicennia
sp.
trees. In nearby marginal hinterland settings,
production of leaf litter near small (
9.2.4.1 Allochthonous sediments
A significant proportion of the sediment that
accumulates within mangroves derives from
either terrestrial (mainly fluvially sourced) or
nearshore settings. Such sediment may be moved
either as suspended or bedload material (see
Chapter 1) and comprises either clastic, carbon-
ate or organic material. The relative importance
of sediment sources and of the import/export of
sediment varies considerably between settings
and depends in part on the relative importance
of tidal versus fluvial influence (Fig. 9.11). In the
microtidal Richmond River estuary (Australia)
92-99% of the annual suspended sediment load
is derived from fluvial inputs, 90% of which
enters the estuary during short (2 week) sea-
sonal flood events (Hossain & Eyre 2002). Only
1-2% of estuarine sediment is derived from
the continental shelf. In contrast, the Fly River
estuary (Papua New Guinea) occurs in a meso/
macrotidal setting. Despite high fluvial discharge
rates (6000 m
3
s
−1
), there is a net inflow of sus-
pended sediment from coastal waters which
exceeds fluvial discharge by around 10 times
(equivalent to 40 t s
−1
; Wolanski et al. 1998).
The seasonality of river flow is also an import-
ant control on the net transport of allochthonous
sediment in many fluvially influenced settings.
The mesotidal Normanby River estuary in north-
ern Australia is, for example, characterized by
long dry seasons (8 -10 months) with short
wet seasonal flow events (Bryce et al. 1998).
Although seasonal flood events are significant,
with flood-driven sediment transport estimated
to range from 6000 to 32,000 t per event, this
is insufficient to remove all of the annual sand
influx to the estuary (15-30,000 t yr
−1
of bedload
2 m high)
Ceriops
sp. was only 300 g m
−2
yr
−1
. At all sites,
litter production rates were highest during the
wet season.
Leaf litter may be broken down either by
microbial action or during crab feeding, exported
by tidal or river currents, or incorporated into
the sediment (Hogarth 1999). In a study from
Australia, Robertson et al. (1992) illustrated
the spatial variability that occurs within man-
groves in terms of reworking of organic matter.
In lower intertidal areas, litterfall averages
556gm
2
yr
−1
, of which 71% is exported by
tidal currents, 28% broken down by crabs and
around 1% lost to microbial action. In contrast,
within high intertidal settings, only 33% of the
509gm
−2
yr
−1
produced is exported by tides,
34% broken down by crabs and 33% decom-
posed by microbial activity. In settings with a
high tidal range and/or extensive fluvial activity,
significant export of organic material may thus
occur, with the mangrove acting as a major
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