interpreting intertidal sea-level indicators, especially coral
microatolls (see Microatoll) (Figure 1a). Moating also has
important implications for sedimentation on reefs. It controls the tidal windows when tidal currents are active,
and modifies reef flat depths and thus where reef top
waves may entrain and transport sediments.
Bibliography
Hopley, D., 1982. The Geomorphology of the Great Barrier Reef:
Quaternary Development of Coral Reefs. New York: Wiley.
Cross-references
Algal Rims
Great Barrier Reef Committee
Microatoll
Moats
Reef Flats
MOATS
Scott Smithers
James Cook University, Townsville, QLD, Australia
Synonyms
Ponds; Reef top pools
Definition
Moats are areas of water held over intertidal reef surfaces
above the open ocean level during lower tides. Moats form
when ebb-tide drainage is impeded behind biologically
constructed or sedimentary structures. The former includes
reef crest algal rims and algal terracettes. Sedimentary structures include storm ridges or ramparts (commonly of coral
shingle) and minor ridges of sand and gravel. Shallow
moats may also pool in subtle reef flat depressions.
Moats behind algal rims can be impressive,
impounding water well above the ocean tide, especially
in areas of high tidal range. For example, a high algal
rim extending > 2 m above the MLWS tide level encircles
Redbill Reef on the central Great Barrier Reef, and holds
water just 8 cm below MSL during low tides (Hopley,
1982). Moats of this type may cover several km
2
. Smaller
algal terracettes are common on the seaward slopes of
algal rims and form shallower (< 20 cm) moated ponds
typically 1–100 m
2 in size.
Moats confined by sedimentary structures may be less
permanent in their construction and effect. For example,
unconsolidated rubble and shingle ramparts tend to leak
initially and are vulnerable to reworking. Although water
remains moated above open water, small breaches can episodically lower water levels. Over time compaction, infill
of voids with finer sediments, and basal cementation
may improve the damming efficiency of these structures, but usually at a lower height than immediately
post-emplacement.
Moats occur on most reef flats but are best developed
where the tidal range is large, algal rims thrive, and coral
growth and storm frequency provide enough rubble and
depositional events. Emergent back reef, as produced by
late Holocene relative sea level fall, is also excellent substrate for moat development.
Moats are important habitats that may support a diverse
reef flora and fauna above the ocean low tide level. Moderately sized and cemented/encrusted moats are more
physically stable and typically harbour the most diverse
communities, especially where moat water is well flushed
daily. Small shallow moats tend to be less permanent, and
are vulnerable to biologically limiting thermal and salinity
extremes.
Several other features are referred to as moats with
varying degrees of correctness. Moated boat channels –
containing water of navigable depth – occur on some reef
flats, either immediately behind algal rims where surging
waves and sediment flux limit reef growth or, on fringing
reefs, nearer to the shore where terrestrial runoff achieves
the same effect. Although in each case water may moat
behind the algal rim, the deeper water is chiefly
a function of locally restricted vertical accretion rather
than ponding over the reef flat. Maldivian cay shorelines
oscillate with monsoonal reversals and similarly constrain
upward reef growth near the cay below that of the reef
edge. The deeper zone separating the reef rim and cay
formed by this process has also been called a moat.
Bibliography
Hopley, D., 1982. The Geomorphology of the Great Barrier Reef:
Quaternary Development of Coral Reefs. New York: Wiley.
Cross-references
Algal Rims
Boat Channel
Fringing Reefs
Maldives
Moating
Reef Flats
Shingle Ridges
MOLLUSCS
José H. Leal
The Bailey-Matthews Shell Museum, Sanibel, FL, USA
Definition
The Mollusca are soft-bodied animals that usually have
a hard, protective shell. They constitute the most diverse
marine phylum and the most diverse group of coral-reef
organisms (Paulay, 1997). Coral reefs provide substrata,
protection, and food for molluscs (Caterall, 1998; Morton,
1983). In turn, molluscs influence reef growth and internal
biological processes. Some gastropods prey directly on
live coral tissue, causing, in some cases, a significant
712
MOATS
microatolls (see Microatoll) (Figure 1a). Moating also has
important implications for sedimentation on reefs. It controls the tidal windows when tidal currents are active,
and modifies reef flat depths and thus where reef top
waves may entrain and transport sediments.
Bibliography
Hopley, D., 1982. The Geomorphology of the Great Barrier Reef:
Quaternary Development of Coral Reefs. New York: Wiley.
Cross-references
Algal Rims
Great Barrier Reef Committee
Microatoll
Moats
Reef Flats
MOATS
Scott Smithers
James Cook University, Townsville, QLD, Australia
Synonyms
Ponds; Reef top pools
Definition
Moats are areas of water held over intertidal reef surfaces
above the open ocean level during lower tides. Moats form
when ebb-tide drainage is impeded behind biologically
constructed or sedimentary structures. The former includes
reef crest algal rims and algal terracettes. Sedimentary structures include storm ridges or ramparts (commonly of coral
shingle) and minor ridges of sand and gravel. Shallow
moats may also pool in subtle reef flat depressions.
Moats behind algal rims can be impressive,
impounding water well above the ocean tide, especially
in areas of high tidal range. For example, a high algal
rim extending > 2 m above the MLWS tide level encircles
Redbill Reef on the central Great Barrier Reef, and holds
water just 8 cm below MSL during low tides (Hopley,
1982). Moats of this type may cover several km
2
. Smaller
algal terracettes are common on the seaward slopes of
algal rims and form shallower (< 20 cm) moated ponds
typically 1–100 m
2 in size.
Moats confined by sedimentary structures may be less
permanent in their construction and effect. For example,
unconsolidated rubble and shingle ramparts tend to leak
initially and are vulnerable to reworking. Although water
remains moated above open water, small breaches can episodically lower water levels. Over time compaction, infill
of voids with finer sediments, and basal cementation
may improve the damming efficiency of these structures, but usually at a lower height than immediately
post-emplacement.
Moats occur on most reef flats but are best developed
where the tidal range is large, algal rims thrive, and coral
growth and storm frequency provide enough rubble and
depositional events. Emergent back reef, as produced by
late Holocene relative sea level fall, is also excellent substrate for moat development.
Moats are important habitats that may support a diverse
reef flora and fauna above the ocean low tide level. Moderately sized and cemented/encrusted moats are more
physically stable and typically harbour the most diverse
communities, especially where moat water is well flushed
daily. Small shallow moats tend to be less permanent, and
are vulnerable to biologically limiting thermal and salinity
extremes.
Several other features are referred to as moats with
varying degrees of correctness. Moated boat channels –
containing water of navigable depth – occur on some reef
flats, either immediately behind algal rims where surging
waves and sediment flux limit reef growth or, on fringing
reefs, nearer to the shore where terrestrial runoff achieves
the same effect. Although in each case water may moat
behind the algal rim, the deeper water is chiefly
a function of locally restricted vertical accretion rather
than ponding over the reef flat. Maldivian cay shorelines
oscillate with monsoonal reversals and similarly constrain
upward reef growth near the cay below that of the reef
edge. The deeper zone separating the reef rim and cay
formed by this process has also been called a moat.
Bibliography
Hopley, D., 1982. The Geomorphology of the Great Barrier Reef:
Quaternary Development of Coral Reefs. New York: Wiley.
Cross-references
Algal Rims
Boat Channel
Fringing Reefs
Maldives
Moating
Reef Flats
Shingle Ridges
MOLLUSCS
José H. Leal
The Bailey-Matthews Shell Museum, Sanibel, FL, USA
Definition
The Mollusca are soft-bodied animals that usually have
a hard, protective shell. They constitute the most diverse
marine phylum and the most diverse group of coral-reef
organisms (Paulay, 1997). Coral reefs provide substrata,
protection, and food for molluscs (Caterall, 1998; Morton,
1983). In turn, molluscs influence reef growth and internal
biological processes. Some gastropods prey directly on
live coral tissue, causing, in some cases, a significant
712
MOATS