Geoscience Reference
In-Depth Information
These may be characterized by different man-
grove species depending upon species tolerance
to environmental stress.
Avicennia
, for example,
are most resistant to low and high temperatures,
and to high soil salinities, whereas
Rhizophora
are resistant to low temperatures (Plaziat 1995).
Along arid shorelines, the major environmental
gradients relate to salinity, which increases
landward due to high evaporation rates. In
these environments mangroves thus often form
narrow fringes of dwarf
Avicennia
(
and those derived from reef-associated benthic
organisms and calcareous algae. Coral reef
framework is composed of two main construc-
tional components, the skeletons of hermatypic
corals (primary framebuilders) and a diverse
array of associated calcareous encrusting faunas
(secondary framebuilders). Calcareous encrusters
(crustose coralline algae, encrusting forms of
bryozoans and foraminifera, and serpulids) pro-
duce multiple crusts on the dead surfaces of coral
skeletons (Martindale 1992) and help bind and
stabilize the reef framework (Rasser & Riegl
2002). Breakdown of these primary and secondary
framework contributors (and thus framework-
related sediment production) is facilitated by
physical and biological activity.
Physical (storm) disturbance results in frag-
mentation and transport of coral framework,
and generation of coral rubble (Fig. 9.8), which
in turn can be degraded by physical reworking to
produce fine coral sand/silt (Fig. 9.9). However,
the release of framework carbonate into sedi-
ment results primarily from bioerosion (a term
used to describe biological substrate erosion;
Neumann 1966). Bioerosion is facilitated by
a wide range of reef-associated faunas, includ-
ing fish and echinoids, and endolithic forms of
sponges, bivalves and worms (Fig. 9.8; Hutchings
1986). Framework degradation and sediment
production by fish and echinoid species results as
a by-product of the search for food. Parrotfish
and surgeonfish, for example, have heavily cal-
cified mouthparts and bite off chunks of coral
substrate, which is excreted as fine sand (Fig. 9.9;
Gygi 1975). Similarly, echinoids such as
Diadema
sp. have heavily calcified feeding apparatus
enabling them to remove coral skeleton during
feeding (Fig. 9.8). As a by-product, they pro-
duce abundant carbonate-rich faecal pellets
(Fig. 9.9; Scoffin et al. 1977).
Significant degradation of framework also
results from the activities of endolithic boring
organisms. These include specific groups of
sponges, bivalves and worms (Bromley 1978;
Perry 1998a). These organisms, which use either
physical and/or chemical processes to excavate
tunnels/chambers within dead coral skeleton,
produce boreholes
1 m high).
In contrast, dense, sprawling forests develop on
large areas of deltaic mud and organic-rich sub-
strates along subequatorial shorelines (Smith
1992). In these cases, the dominant
Rhizophora
and
Avicennia
plants may reach heights of 40 m
(Plaziat 1995).
<
9.2
SEDIMENT SOURCES AND SEDIMENT ACCUMULATION
PROCESSES
9.2.1 Sources and characteristics of coral reef
sediments
Sediments that accumulate on and around coral
reefs derive from a range of sources. These
include:
1
skeletal sediments - the calcareous remains
of reef framework-building and reef-associated
organisms;
2
non-skeletal sediments - grains produced by
physico-chemical induced carbonate precipita-
tion;
3
allochthonous sediments - grains derived
from terrestrial sources and which may be either
natural or anthropogenic in origin.
The relative abundance of these sediment
contributors varies both within and between
environments.
9.2.1.1 Skeletal sediments
Typically the most abundant constituent of
reef-related sediments are the skeletal remnants
of calcareous reef organisms. These can be sub-
divided into sediments produced by the break-
down of carbonate framework contributors,
>
1 mm in diameter and are
