sediments at the reef edge due to greater exposure to
longshore currents at the mouth of the embayment, where
ongoing seaward progradation is now constrained by the
same processes that have limited substantial fringing reef
development on the adjacent headlands.
A review of the available chronostratigraphic data
available for fringing reefs indicates that most began to
form soon after the sea inundated the substrate during
the postglacial transgression, and that the bulk of most
fringing reef frameworks were in place within a few thousand years (Kennedy and Woodroffe, 2002; Montaggioni,
2005; Hopley et al., 2007). Where later start-ups have
occurred it is likely that the substrate is only recently available, due perhaps to shoreline movement or migration of
shallow marine sediment deposits (Larcombe and Woolfe,
1999; Perry, 2005), and/or the environment is very marginal and reef growth is episodic. Although the details of
why some apparently suitable substrates are not colonized
by fringing reefs while some very marginal substrates are
remain unknown, the rapidity with which corals have colonized most suitable coastal substrates and have developed fringing reefs has been observed at sites of recent
volcanic activity (Umbgrove, 1930) and is demonstrated
by the weight of radiometric dates from basal units in
fringing reef cores (see Radiocarbon (
14
C): Dating and
Corals). On this basis it would seem logical to suggest
that substantial fringing reefs have probably already
established at most sites that they are likely to.
Influence of terrestrial runoff, sediments and
nutrients on fringing reefs
The environmental conditions required for coral growth
are necessary prerequisites for fringing reef development,
and where these conditions are not met or are marginal,
fringing reefs will be absent or poorly developed. At
a broad scale this is most markedly expressed on tropical
coasts near major rivers where massive freshwater plumes
and sediment flux have limited fringing reef development,
such as in the western Atlantic south of around 5
N where
the influence of the Amazon is felt, or adjacent to the
Mekong, Niger, Congo, and Irrawaddy Rivers where
fringing reefs are similarly lacking (Hopley, 1982). In
addition to controlling fringing reef distribution, where
reefs can grow, terrestrial runoff and associated sediments
and nutrients may also affect fringing reef development
and morphology.
Guilcher (1988) distinguished a range of fringing reef
“subtypes” according to the extent of development of
a deeper back-reef channel termed the “boat channel”
(see Boat Channel) – a depression less than 10 m deep
between the shore and the outer reef flat that is commonly
used for small boat traffic on reef coasts. It was Guilcher’s
view that the boat channel formed because coral growth
was suppressed relative to that closer to the outer reef
edge, most probably by freshwater, nutrients, and sediments shed from the adjacent landmass. Water may also
be ponded over the backreef in moats (see Moats), but
moats form where the ebbing tide is held over the reef flat
(see Reef Flats) rather than in depressions produced where
upward reef growth is inhibited. Guilcher (1988) proposed
that the simplest fringing reefs were shore-attached and
lacked a boat channel, and developed where terrestrial
impacts on reef growth were minimal, as occurs in semiarid areas such as in the Red Sea. As discussed in section
“Fringing reef growth and sea level,” this view is over simplistic as it does not consider the influence of relative sealevel history. Fringing reefs with incipient boat channels
comprise another subtype, with a shallow (<2 m deep)
navigable channel behind the reef crest, with scattered live
corals but generally only limited sediment accumulation
indicative of minor terrestrial runoff. These reefs are common in the Gulf of Aqaba, but reefs from the Seychelles,
Sri Lanka and Madagascar may also be included in this
subtype (Guilcher, 1988). Fringing reefs with welldeveloped boat channels – several meters deeper than
the reef flat and up to several hundred meters wide – can
also be recognized, for example, at Lord Howe Island in
the Tasman Sea (Figure 3), and along the southwest coast
of Madagascar. Finally, the most complex subtype
described by Guilcher (1988) is fringing reefs with multiple land-locked lagoons – which he attributes to the combination of a wide intertidal zone and macrotidal range.
The influence of the adjacent landmass on fringing reef
growth is well demonstrated on the GBR where fringing
reefs are the most common reef type (758 of 2,904 named
reefs [26.1%]) but significant development on the mainland coast is restricted to the rocky shores of the Whitsunday region, and then intermittently north of Cairns
(Hopley et al., 2007). Elsewhere, coastal sediment dynamics, high seasonal fluvial yields from large coastal
catchments, and high inshore turbidity associated with
wind-wave resuspension of the inshore sediment prism
(Woolfe and Larcombe, 1999) inhibit fringing reef initiation and growth (Hopley et al., 2007). Fine sediments
may be resuspended very quickly by waves – at Nelly
Bay, Magnetic Island, wave-driven turbidity increases of
three orders of magnitude up to 100 NTU (Nephelometer
Turbidity Units) have been measured in an hour (Orpin
et al., 2004) (see Turbid Zone and Terrigenous SedimentInfluenced Reefs). Most fringing reefs on the GBR (652
of 758 [86%]) occur on high islands offshore from the
mainland, although fringing reef development can be poor
on those closer inshore. Fringing reef development is also
poor on larger high islands such as Hinchinbrook Island
and Gloucester Island where runoff can be substantial,
but some large high islands do support good fringing reefs
and so other controls must also be involved (Hopley,
1982). Although fringing reefs are abundant on the
GBR they are generally relatively small (mean <1 km
2
),
and a significant number (213 of 758 [28.1%]) are defined
as “incipient” – indicating that they lack extensive reef flat
development (Figure 1b). Although substrate geometry
and other factors can restrict fringing reef size, the generally limited dimensions and development may also reflect
their vulnerability to episodic disturbances such as flood
434
FRINGING REEFS
longshore currents at the mouth of the embayment, where
ongoing seaward progradation is now constrained by the
same processes that have limited substantial fringing reef
development on the adjacent headlands.
A review of the available chronostratigraphic data
available for fringing reefs indicates that most began to
form soon after the sea inundated the substrate during
the postglacial transgression, and that the bulk of most
fringing reef frameworks were in place within a few thousand years (Kennedy and Woodroffe, 2002; Montaggioni,
2005; Hopley et al., 2007). Where later start-ups have
occurred it is likely that the substrate is only recently available, due perhaps to shoreline movement or migration of
shallow marine sediment deposits (Larcombe and Woolfe,
1999; Perry, 2005), and/or the environment is very marginal and reef growth is episodic. Although the details of
why some apparently suitable substrates are not colonized
by fringing reefs while some very marginal substrates are
remain unknown, the rapidity with which corals have colonized most suitable coastal substrates and have developed fringing reefs has been observed at sites of recent
volcanic activity (Umbgrove, 1930) and is demonstrated
by the weight of radiometric dates from basal units in
fringing reef cores (see Radiocarbon (
14
C): Dating and
Corals). On this basis it would seem logical to suggest
that substantial fringing reefs have probably already
established at most sites that they are likely to.
Influence of terrestrial runoff, sediments and
nutrients on fringing reefs
The environmental conditions required for coral growth
are necessary prerequisites for fringing reef development,
and where these conditions are not met or are marginal,
fringing reefs will be absent or poorly developed. At
a broad scale this is most markedly expressed on tropical
coasts near major rivers where massive freshwater plumes
and sediment flux have limited fringing reef development,
such as in the western Atlantic south of around 5
N where
the influence of the Amazon is felt, or adjacent to the
Mekong, Niger, Congo, and Irrawaddy Rivers where
fringing reefs are similarly lacking (Hopley, 1982). In
addition to controlling fringing reef distribution, where
reefs can grow, terrestrial runoff and associated sediments
and nutrients may also affect fringing reef development
and morphology.
Guilcher (1988) distinguished a range of fringing reef
“subtypes” according to the extent of development of
a deeper back-reef channel termed the “boat channel”
(see Boat Channel) – a depression less than 10 m deep
between the shore and the outer reef flat that is commonly
used for small boat traffic on reef coasts. It was Guilcher’s
view that the boat channel formed because coral growth
was suppressed relative to that closer to the outer reef
edge, most probably by freshwater, nutrients, and sediments shed from the adjacent landmass. Water may also
be ponded over the backreef in moats (see Moats), but
moats form where the ebbing tide is held over the reef flat
(see Reef Flats) rather than in depressions produced where
upward reef growth is inhibited. Guilcher (1988) proposed
that the simplest fringing reefs were shore-attached and
lacked a boat channel, and developed where terrestrial
impacts on reef growth were minimal, as occurs in semiarid areas such as in the Red Sea. As discussed in section
“Fringing reef growth and sea level,” this view is over simplistic as it does not consider the influence of relative sealevel history. Fringing reefs with incipient boat channels
comprise another subtype, with a shallow (<2 m deep)
navigable channel behind the reef crest, with scattered live
corals but generally only limited sediment accumulation
indicative of minor terrestrial runoff. These reefs are common in the Gulf of Aqaba, but reefs from the Seychelles,
Sri Lanka and Madagascar may also be included in this
subtype (Guilcher, 1988). Fringing reefs with welldeveloped boat channels – several meters deeper than
the reef flat and up to several hundred meters wide – can
also be recognized, for example, at Lord Howe Island in
the Tasman Sea (Figure 3), and along the southwest coast
of Madagascar. Finally, the most complex subtype
described by Guilcher (1988) is fringing reefs with multiple land-locked lagoons – which he attributes to the combination of a wide intertidal zone and macrotidal range.
The influence of the adjacent landmass on fringing reef
growth is well demonstrated on the GBR where fringing
reefs are the most common reef type (758 of 2,904 named
reefs [26.1%]) but significant development on the mainland coast is restricted to the rocky shores of the Whitsunday region, and then intermittently north of Cairns
(Hopley et al., 2007). Elsewhere, coastal sediment dynamics, high seasonal fluvial yields from large coastal
catchments, and high inshore turbidity associated with
wind-wave resuspension of the inshore sediment prism
(Woolfe and Larcombe, 1999) inhibit fringing reef initiation and growth (Hopley et al., 2007). Fine sediments
may be resuspended very quickly by waves – at Nelly
Bay, Magnetic Island, wave-driven turbidity increases of
three orders of magnitude up to 100 NTU (Nephelometer
Turbidity Units) have been measured in an hour (Orpin
et al., 2004) (see Turbid Zone and Terrigenous SedimentInfluenced Reefs). Most fringing reefs on the GBR (652
of 758 [86%]) occur on high islands offshore from the
mainland, although fringing reef development can be poor
on those closer inshore. Fringing reef development is also
poor on larger high islands such as Hinchinbrook Island
and Gloucester Island where runoff can be substantial,
but some large high islands do support good fringing reefs
and so other controls must also be involved (Hopley,
1982). Although fringing reefs are abundant on the
GBR they are generally relatively small (mean <1 km
2
),
and a significant number (213 of 758 [28.1%]) are defined
as “incipient” – indicating that they lack extensive reef flat
development (Figure 1b). Although substrate geometry
and other factors can restrict fringing reef size, the generally limited dimensions and development may also reflect
their vulnerability to episodic disturbances such as flood
434
FRINGING REEFS