the morphology of the corals themselves (Wells, 1957;
Rosen, 1975; Geister, 1977; Pichon, 1978; Chappell,
1980; Done, 1982; Done, 1983). Most of these studies
have found that shallow coral assemblages show
a distinct zonation as wave energy, and hydrodynamic disturbance varies with depth and margin exposure. For
example, Done (1982) found that shallow communities
changed with a decreasing cross-shelf wave exposure in
the central Great Barrier Reef. Individual reefs also
showed the same pattern, with shallow-wave-adapted
communities being replaced along leeward margins by
upward shifts in the positions of deeper communities.
Growth form of corals also show similar zonation patterns
in response to depth and exposure-related variation in
wave energy (Pichon, 1978; Chappell, 1980; Madin and
Connely, 2006). Such zonation patterns have led to
a clear distinction between rough-water reefs exposed to
trade-wind-, swell-, and storm-generated wave fields and
calm-water reefs protected from them (Geister, 1977,
1980).
A comparison of zonation schemes, however, shows
that the coral assemblages in each zone can be highly variable on both inter-reefal and regional scale, and there is
a significant overlap between the members of each assemblage zone (e.g., Done, 1982). Despite this “coral-scale”
variability, the geomorphology of these reefs is very similar. Indeed, equally similar reef structures are produced
in areas with order-of-magnitude differences in species
abundances, and even where reefs have depauperate
species numbers, their basic geomorphology is identical
(e.g., Glynn et al., 1996). This consistency implies that
gradients in wave exposure not only control the main
environmental tolerances of shallow coral assemblages
but also the form of the structures they build (Graus
et al., 1977; Graus et al., 1984, 1985; Graus and
Macintyre, 1989). It therefore follows that reefs might be
better classified according to their geomorphic form
(Wells, 1957).
Until very recently, a synoptic view of reef geomorphology showing the impact of varying ecological processes has not been possible, and previous attempts to
summarize information have had to rely on parochial
descriptions of what was accessible or visible at the time
(e.g., Hopley, 1982; Guilcher, 1988). Following this
approach, vast areas of modern reefs have been inadequately characterized and, with some exceptions, quantitative differences and the similarities between different
regions have gone undocumented. This deficient situation
is about to change. The recent advent of widely available
orthorectified satellite imagery (Google Earth) and rapid
development and cost reduction in geographic information systems, multi-beam sonar, LiDAR, and other
remote-sensing technologies are making large-scale morphometric quantification of reefs feasible (Naseer and
Hatcher, 2001; Andrefouet et al., 2001; Storlazzi et al.,
2003; Naseer and Hatcher, 2004). As such, we are on the
cusp of being able to provide a more holistic view of modern coral-reef form and a development that will provide
timely insight into the resilience and resistance of reefs
to human-induced global environmental change.
In this entry, I examine satellite imagery using Google
Earth along with local descriptions of reefs and attempt
to summarize what is presently known about the geomorphic zonation of well-ordered, rough-water reefs, outline
their formative processes, and assess their development.
Standard geomorphic reef zonation
A comparison of well-ordered, rough-water reefs shows
a remarkable degree of conformity between the size and
character of their major morphological features
(Figure 1). All of them consist of five first-order or “standard” zones: a calm reef lagoon, with varying number of
patch reefs; a largely barren, shallow reef flat strewn with
detritus; a narrow, gently sloping reef front exposed to
waves and swells; a steeper reef slope bearing corals to as
deep as 100 m; and a less steeply inclined fore-reef apron,
where coral and reef detritus accumulate. The boundaries
between these standard zones are delineated by simple
slope breaks. The break between the lagoon and the reef flat
is either a sand slope or a small reef scarp. The break
between the level reef flat and the gently sloping reef front
occurs at the reef crest. Similarly, the shelf break separates
the gently sloping reef front from the steeper reef slope, and
the break between the reef slope and fore reef is the transition into slopes that reflects the angle of sediment repose.
Each of these standard and easily defined reef zones can
in turn be subdivided into commonly recurring secondorder zones or subzones, which may not be present on all
reefs and, if they are present, may vary between margin,
area, and region. As a consequence, their boundaries can
be more difficult to define.
Lagoon
With their calm, stable environments, and depth ranges
that are usually within the range of coral growth, lagoons
potentially represent an ideal and extensive habitat for
reefs (Figure 2). The actual variation in lagoonal reef
development however is striking, and even in areas where
lagoonal reefs are prolific, such as the Tuamotus, there are
examples where reefs are completely absent (Adjeroud
et al., 2000; Paulay and Kerr, 2001).
Patches, knolls, and pinnacles
Lagoons with significant reef development show two
main types of subcircular structures, patch reefs and
knolls. Patch reefs reach mean sea level and rise from
substrates <20 m deep; where the substrate is deeper,
they are termed pinnacles. Knolls are submerged reef
patches but can also develop into pinnacles where they
have >20 m relief (Stoddart, 1969). Patches, knolls, and
pinnacles can develop in a dispersed pattern or can
become connected to form linear ridges with various
arrangements (e.g., Maxwell, 1968; Hopley, 1982).
Dispersed patches or knolls have a density continuum
from widely spaced to a point where they connect with
470
GEOMORPHIC ZONATION
Rosen, 1975; Geister, 1977; Pichon, 1978; Chappell,
1980; Done, 1982; Done, 1983). Most of these studies
have found that shallow coral assemblages show
a distinct zonation as wave energy, and hydrodynamic disturbance varies with depth and margin exposure. For
example, Done (1982) found that shallow communities
changed with a decreasing cross-shelf wave exposure in
the central Great Barrier Reef. Individual reefs also
showed the same pattern, with shallow-wave-adapted
communities being replaced along leeward margins by
upward shifts in the positions of deeper communities.
Growth form of corals also show similar zonation patterns
in response to depth and exposure-related variation in
wave energy (Pichon, 1978; Chappell, 1980; Madin and
Connely, 2006). Such zonation patterns have led to
a clear distinction between rough-water reefs exposed to
trade-wind-, swell-, and storm-generated wave fields and
calm-water reefs protected from them (Geister, 1977,
1980).
A comparison of zonation schemes, however, shows
that the coral assemblages in each zone can be highly variable on both inter-reefal and regional scale, and there is
a significant overlap between the members of each assemblage zone (e.g., Done, 1982). Despite this “coral-scale”
variability, the geomorphology of these reefs is very similar. Indeed, equally similar reef structures are produced
in areas with order-of-magnitude differences in species
abundances, and even where reefs have depauperate
species numbers, their basic geomorphology is identical
(e.g., Glynn et al., 1996). This consistency implies that
gradients in wave exposure not only control the main
environmental tolerances of shallow coral assemblages
but also the form of the structures they build (Graus
et al., 1977; Graus et al., 1984, 1985; Graus and
Macintyre, 1989). It therefore follows that reefs might be
better classified according to their geomorphic form
(Wells, 1957).
Until very recently, a synoptic view of reef geomorphology showing the impact of varying ecological processes has not been possible, and previous attempts to
summarize information have had to rely on parochial
descriptions of what was accessible or visible at the time
(e.g., Hopley, 1982; Guilcher, 1988). Following this
approach, vast areas of modern reefs have been inadequately characterized and, with some exceptions, quantitative differences and the similarities between different
regions have gone undocumented. This deficient situation
is about to change. The recent advent of widely available
orthorectified satellite imagery (Google Earth) and rapid
development and cost reduction in geographic information systems, multi-beam sonar, LiDAR, and other
remote-sensing technologies are making large-scale morphometric quantification of reefs feasible (Naseer and
Hatcher, 2001; Andrefouet et al., 2001; Storlazzi et al.,
2003; Naseer and Hatcher, 2004). As such, we are on the
cusp of being able to provide a more holistic view of modern coral-reef form and a development that will provide
timely insight into the resilience and resistance of reefs
to human-induced global environmental change.
In this entry, I examine satellite imagery using Google
Earth along with local descriptions of reefs and attempt
to summarize what is presently known about the geomorphic zonation of well-ordered, rough-water reefs, outline
their formative processes, and assess their development.
Standard geomorphic reef zonation
A comparison of well-ordered, rough-water reefs shows
a remarkable degree of conformity between the size and
character of their major morphological features
(Figure 1). All of them consist of five first-order or “standard” zones: a calm reef lagoon, with varying number of
patch reefs; a largely barren, shallow reef flat strewn with
detritus; a narrow, gently sloping reef front exposed to
waves and swells; a steeper reef slope bearing corals to as
deep as 100 m; and a less steeply inclined fore-reef apron,
where coral and reef detritus accumulate. The boundaries
between these standard zones are delineated by simple
slope breaks. The break between the lagoon and the reef flat
is either a sand slope or a small reef scarp. The break
between the level reef flat and the gently sloping reef front
occurs at the reef crest. Similarly, the shelf break separates
the gently sloping reef front from the steeper reef slope, and
the break between the reef slope and fore reef is the transition into slopes that reflects the angle of sediment repose.
Each of these standard and easily defined reef zones can
in turn be subdivided into commonly recurring secondorder zones or subzones, which may not be present on all
reefs and, if they are present, may vary between margin,
area, and region. As a consequence, their boundaries can
be more difficult to define.
Lagoon
With their calm, stable environments, and depth ranges
that are usually within the range of coral growth, lagoons
potentially represent an ideal and extensive habitat for
reefs (Figure 2). The actual variation in lagoonal reef
development however is striking, and even in areas where
lagoonal reefs are prolific, such as the Tuamotus, there are
examples where reefs are completely absent (Adjeroud
et al., 2000; Paulay and Kerr, 2001).
Patches, knolls, and pinnacles
Lagoons with significant reef development show two
main types of subcircular structures, patch reefs and
knolls. Patch reefs reach mean sea level and rise from
substrates <20 m deep; where the substrate is deeper,
they are termed pinnacles. Knolls are submerged reef
patches but can also develop into pinnacles where they
have >20 m relief (Stoddart, 1969). Patches, knolls, and
pinnacles can develop in a dispersed pattern or can
become connected to form linear ridges with various
arrangements (e.g., Maxwell, 1968; Hopley, 1982).
Dispersed patches or knolls have a density continuum
from widely spaced to a point where they connect with
470
GEOMORPHIC ZONATION