the backreef lagoon. The early study of Bikini Atoll in the
Pacific Ocean by Munk and Sargent (1948) illustrated that
waves breaking on the reef drove strong currents
(0.25–0.5 m/s) that transported water over the reef and
into the lagoon. In a fringing reef setting in the Caribbean,
Roberts and Suhayda (1983) measured strong short duration cross-reef currents of >0.4 to 1.8 m/s. Cross-reef currents and volume flux change with the tide. Maximum
flux occurs at high tide, but during very low tides (extreme
neap tides) and/or off-reef winds, currents may be
reversed and water may flow off-reef from the lagoon.
Cross-reef flux estimates range from 0.01 to 2.21 m
2
/s
(Marsh et al., 1981; Pickard, 1986; Lugo-Fernández
et al., 1998, 2004; Gourlay and Colleter, 2005). Wavedriven flows contribute about 45–60% of the total flow
in some Pacific and Caribbean reefs (Pickard, 1986;
Lugo-Fernández et al., 2004). The remainder of the flow
can be attributed to tides, wind stress, and infragravity
waves (Lugo-Fernández et al., 1998).
Because of their small amplitudes and long wavelengths or periods (24 and 12 h), the astronomical tides
behave as shallow waters waves, the velocities of which
depend on amplitude and water depth. Tidal currents
reverse direction once or twice a day affecting acrossthe-reef flow into the lagoon through surge channels and/
or larger tidal channels. These linked processes are well
established for barrier reef lagoons and especially atoll
lagoons where tides impinge unaffected on them from
deepwater. If the tidal amplitude is large enough to expose
the reef crest-flat, then at low tide water-level gradient will
develop that drives an off-reef flux, which depends on the
degree of connectivity between the lagoon and adjacent
ocean (Yamamono et al., 1998). Infragravity waves are
other shallow waves affecting cross-reef currents. These
flows have not been well studied, but have been observed
on shelves with reefs (Roberts and Suhayda, 1983;
Wolanski, 1994; Lugo-Fernández et al., 1998).
Strong winds either reinforce or oppose the wave- and
tidal-driven flows depending on wind direction. In fringing reefs, these effects are accentuated because of the
small water depths and the accumulation of water against
the coast, which creates a pressure gradient that opposes
all inflow at the reef crest. Under such conditions, a twolayer flow may develop if the lagoon is large enough and
deep enough to accommodate such a two-layer system,
on-reef flow at the surface and off-reef flow near the bottom (Storlazzi et al., 2004). As lagoon–land geometries
become more complex and scales increase (W and L)
over-the-reef flow becomes less important for driving
lagoon circulation. As in the case of the Great Barrier Reef
Lagoon near Lizard Island, Frith and Mason (1986) found
that circulation was highly coherent with the trade wind
for about half the year, but exhibited much more variability (e.g., current reversals) after the trade wind season
ended. Wind shifts and tides accounted for this lagoon circulation variability.
Recently, Monismith et al. (2006) describe a cross-reef
flux driven by buoyancy created by temperature
differences between the outer reef and lagoon. A similar
buoyancy flux should operate on reefs with hypersaline
lagoons. Hypersaline conditions are prevalent throughout
the reef-lagoon systems of the Red Sea (e.g., Friedman,
1968; Meshal, 1987) and the Arabian Gulf (e.g., Kinsman,
1969; Gunatilaka et al., 1984) because of excessive
evaporation.
Inside the lagoon, cross-reef flows, tides, and winds
drive a circulation which is constrained by the lagoon’s
morphology. Figures 1a and 2 illustrate different shallow
reef-lagoon geometries and their effects on the lagoon circulation. In the case of a backreef lagoon with limited connectivity with the open ocean, a small width, and a shallow
depth, the flow is generally downwind and tends to accelerate as the lagoon outlet is approached (e.g., Roberts,
1980; Tamura et al., 2007; Figure 1). If the reef fronting
a shallow lagoon is discontinuous as schematically illustrated in Figure 2, localized circulation cells may develop.
Such cells are primarily a product of shoreline currents
created by waves refracted through openings between reef
segments. These divergent currents at the shoreline create
cuspate shoreline morphologies. However, when W
increases to beyond narrow fringing reef-lagoon barrier
reef scale, these circulation cells break down and lagoon
circulation becomes wind-and tide-dominated (as
described by Wolanski, 1994).
Atoll lagoons
Atolls are a special type of reef-lagoon system. Early in
the study of oceanic processes and their impacts on reefs
and their lagoons, von Arx (1948) conducted
a groundbreaking study on lagoon circulation at Bikini
and Rongelap Atolls. Circulation in both lagoons was primarily wind-driven in the surface layer with overturning
and counter-rotating cells established in the deeper parts
of the lagoons. This two-layer flow is illustrated schematically in Figure 2. Strength of both surface and deeper
layer circulation was found to vary over a day with changing wind speed. Smaller variations were found to be
related to perturbations caused by wave-swell environments, tides, and changes in major ocean currents encountering the atolls. At both Rongelap and Bikini Atolls, over
2% of the lagoon volume was found to be transported into
and out of the lagoons on each tidal cycle.
Like their fringing reef and barrier reef counterparts,
over-the-reef flux of ocean water to the atoll lagoon is
driven by a water-level gradient established primarily by
the breaking of waves on the reef front. Atolls located in
monsoonal regions, such as the Maldives in the Indian
Ocean, receive winds and waves from two dominant
directions. It follows that lagoon circulation reversals will
occur at the monsoon frequency. An investigation of
Cocos (Keeling) islands by Kench (1994), an Indian
Ocean Atoll with a lagoon about 3–5 km diameter, determined that atoll structure controlled the pattern of energy
distribution and lagoon circulation. In this case, tidal currents drove lagoon circulation and water had a residence
LAGOON CIRCULATION
615
Pacific Ocean by Munk and Sargent (1948) illustrated that
waves breaking on the reef drove strong currents
(0.25–0.5 m/s) that transported water over the reef and
into the lagoon. In a fringing reef setting in the Caribbean,
Roberts and Suhayda (1983) measured strong short duration cross-reef currents of >0.4 to 1.8 m/s. Cross-reef currents and volume flux change with the tide. Maximum
flux occurs at high tide, but during very low tides (extreme
neap tides) and/or off-reef winds, currents may be
reversed and water may flow off-reef from the lagoon.
Cross-reef flux estimates range from 0.01 to 2.21 m
2
/s
(Marsh et al., 1981; Pickard, 1986; Lugo-Fernández
et al., 1998, 2004; Gourlay and Colleter, 2005). Wavedriven flows contribute about 45–60% of the total flow
in some Pacific and Caribbean reefs (Pickard, 1986;
Lugo-Fernández et al., 2004). The remainder of the flow
can be attributed to tides, wind stress, and infragravity
waves (Lugo-Fernández et al., 1998).
Because of their small amplitudes and long wavelengths or periods (24 and 12 h), the astronomical tides
behave as shallow waters waves, the velocities of which
depend on amplitude and water depth. Tidal currents
reverse direction once or twice a day affecting acrossthe-reef flow into the lagoon through surge channels and/
or larger tidal channels. These linked processes are well
established for barrier reef lagoons and especially atoll
lagoons where tides impinge unaffected on them from
deepwater. If the tidal amplitude is large enough to expose
the reef crest-flat, then at low tide water-level gradient will
develop that drives an off-reef flux, which depends on the
degree of connectivity between the lagoon and adjacent
ocean (Yamamono et al., 1998). Infragravity waves are
other shallow waves affecting cross-reef currents. These
flows have not been well studied, but have been observed
on shelves with reefs (Roberts and Suhayda, 1983;
Wolanski, 1994; Lugo-Fernández et al., 1998).
Strong winds either reinforce or oppose the wave- and
tidal-driven flows depending on wind direction. In fringing reefs, these effects are accentuated because of the
small water depths and the accumulation of water against
the coast, which creates a pressure gradient that opposes
all inflow at the reef crest. Under such conditions, a twolayer flow may develop if the lagoon is large enough and
deep enough to accommodate such a two-layer system,
on-reef flow at the surface and off-reef flow near the bottom (Storlazzi et al., 2004). As lagoon–land geometries
become more complex and scales increase (W and L)
over-the-reef flow becomes less important for driving
lagoon circulation. As in the case of the Great Barrier Reef
Lagoon near Lizard Island, Frith and Mason (1986) found
that circulation was highly coherent with the trade wind
for about half the year, but exhibited much more variability (e.g., current reversals) after the trade wind season
ended. Wind shifts and tides accounted for this lagoon circulation variability.
Recently, Monismith et al. (2006) describe a cross-reef
flux driven by buoyancy created by temperature
differences between the outer reef and lagoon. A similar
buoyancy flux should operate on reefs with hypersaline
lagoons. Hypersaline conditions are prevalent throughout
the reef-lagoon systems of the Red Sea (e.g., Friedman,
1968; Meshal, 1987) and the Arabian Gulf (e.g., Kinsman,
1969; Gunatilaka et al., 1984) because of excessive
evaporation.
Inside the lagoon, cross-reef flows, tides, and winds
drive a circulation which is constrained by the lagoon’s
morphology. Figures 1a and 2 illustrate different shallow
reef-lagoon geometries and their effects on the lagoon circulation. In the case of a backreef lagoon with limited connectivity with the open ocean, a small width, and a shallow
depth, the flow is generally downwind and tends to accelerate as the lagoon outlet is approached (e.g., Roberts,
1980; Tamura et al., 2007; Figure 1). If the reef fronting
a shallow lagoon is discontinuous as schematically illustrated in Figure 2, localized circulation cells may develop.
Such cells are primarily a product of shoreline currents
created by waves refracted through openings between reef
segments. These divergent currents at the shoreline create
cuspate shoreline morphologies. However, when W
increases to beyond narrow fringing reef-lagoon barrier
reef scale, these circulation cells break down and lagoon
circulation becomes wind-and tide-dominated (as
described by Wolanski, 1994).
Atoll lagoons
Atolls are a special type of reef-lagoon system. Early in
the study of oceanic processes and their impacts on reefs
and their lagoons, von Arx (1948) conducted
a groundbreaking study on lagoon circulation at Bikini
and Rongelap Atolls. Circulation in both lagoons was primarily wind-driven in the surface layer with overturning
and counter-rotating cells established in the deeper parts
of the lagoons. This two-layer flow is illustrated schematically in Figure 2. Strength of both surface and deeper
layer circulation was found to vary over a day with changing wind speed. Smaller variations were found to be
related to perturbations caused by wave-swell environments, tides, and changes in major ocean currents encountering the atolls. At both Rongelap and Bikini Atolls, over
2% of the lagoon volume was found to be transported into
and out of the lagoons on each tidal cycle.
Like their fringing reef and barrier reef counterparts,
over-the-reef flux of ocean water to the atoll lagoon is
driven by a water-level gradient established primarily by
the breaking of waves on the reef front. Atolls located in
monsoonal regions, such as the Maldives in the Indian
Ocean, receive winds and waves from two dominant
directions. It follows that lagoon circulation reversals will
occur at the monsoon frequency. An investigation of
Cocos (Keeling) islands by Kench (1994), an Indian
Ocean Atoll with a lagoon about 3–5 km diameter, determined that atoll structure controlled the pattern of energy
distribution and lagoon circulation. In this case, tidal currents drove lagoon circulation and water had a residence
LAGOON CIRCULATION
615