Hydrodynamics and Thermohaline Features of Reef Waters
41
the barrier reefs the rate and especially the directions of currents, which are
dependent upon the wind stress and the tide flows through the passages
between the barriers, nevertheless are dominated by the influence of the
main general oceanic current, which prevails in the shelf area. Thus in the
lagoon of GBR the dominating direction of the current (SSE) coincides with
the direction of the here dominating East-Australian current. In the lagoon
of the barrier reef along New Guinea near Port Moresby the direction of
current is determined by the here dominating oceanic SE current (Moore
1981). The rates of surface currents in lagoons of barrier reefs vary within
5-30cms- 1 (Orr 1933; Roberts et al. 1975; Cresswell and Greig 1978;
Andrews 1983a).
The vertical thermal stratification in reef waters is practically absent even
in deep semi-closed lagoons (see below). This proves that there is a high
level of vertical turbulence in their water columns. The zone of the most
active turbulence is situated near the outer windward reef slopes. Measured
by stroboscopic photography of a patch of dye, the coefficient of turbulence
up the windward reef slope was more than 2 x 10 2 cm 2 s -1 at depths of
20-30m at a current velocity of 20cms- 1 (Roberts 1977). But locally this
coefficient in the vicinity of reefs can be even more - up to 3-5 X
10 2 cm 2 S-I. In deeper layers the turbulence is usually more intensive, it
being enhanced by the interaction of currents with elements of reef
constructions. For example, near the reef off Koetivi Island (Seychelles) the
turbulence coefficients near the surface were 0.5-1.7 x 10 2 and near the
bottom 3.5 x 10 2 cm 2 s- 1 (Novozhilov 1980). The turbulence coefficients
were maximal during the periods of incoming tide.
The residence time of water in the lagoons of the ring reef depends on the
heights of tides, rates of currents, as well as upon their morphometry. Its
estimations were based upon measurements of the rate of water transport
across the flat and through the channels. In the semi-closed lagoon of the
Bikini atoll during one tide cicle some 4% of the total water volume in it is
exchanged. The residence time for this atoll was estimated to be within 40
days and the time for complete mixing of the water column by currents in its
60 m deep lagoon 6-12 days (von Arx 1948). The longest exchange rates
were recorded in the shallow lagoon of the One Tree ring reef - less than 1
week (Ludington 1979; Frith 1981). At the Enivetok atoll the rate was 30
days, in the Canton atoll 50 days (Smith and Jokiel 1978), in the Tarawa
atoll 10-15 days (Kimmerer and Walsh 1981) and in the elevated Fanning
atoll, which has only one narrow pass, about 8 months (Gallagher et al.
1971; Atkinson et al. 1981). The exchange of water between lagoon and the
surrounding ocean proceeds not only via surface hydrodynamics, but also
through the porous walls of reef via the moving groundwaters (Lam 1974;
Rougerie 1985). Their flows are induced by the fluctuations of sea-level
during the tides. The flows of groundwaters inside the lime body of the reef
dissolve the carbonates and thus perforate it, creating its permeability
(Johannes 1980). The most permeable for the underground passage of
41
the barrier reefs the rate and especially the directions of currents, which are
dependent upon the wind stress and the tide flows through the passages
between the barriers, nevertheless are dominated by the influence of the
main general oceanic current, which prevails in the shelf area. Thus in the
lagoon of GBR the dominating direction of the current (SSE) coincides with
the direction of the here dominating East-Australian current. In the lagoon
of the barrier reef along New Guinea near Port Moresby the direction of
current is determined by the here dominating oceanic SE current (Moore
1981). The rates of surface currents in lagoons of barrier reefs vary within
5-30cms- 1 (Orr 1933; Roberts et al. 1975; Cresswell and Greig 1978;
Andrews 1983a).
The vertical thermal stratification in reef waters is practically absent even
in deep semi-closed lagoons (see below). This proves that there is a high
level of vertical turbulence in their water columns. The zone of the most
active turbulence is situated near the outer windward reef slopes. Measured
by stroboscopic photography of a patch of dye, the coefficient of turbulence
up the windward reef slope was more than 2 x 10 2 cm 2 s -1 at depths of
20-30m at a current velocity of 20cms- 1 (Roberts 1977). But locally this
coefficient in the vicinity of reefs can be even more - up to 3-5 X
10 2 cm 2 S-I. In deeper layers the turbulence is usually more intensive, it
being enhanced by the interaction of currents with elements of reef
constructions. For example, near the reef off Koetivi Island (Seychelles) the
turbulence coefficients near the surface were 0.5-1.7 x 10 2 and near the
bottom 3.5 x 10 2 cm 2 s- 1 (Novozhilov 1980). The turbulence coefficients
were maximal during the periods of incoming tide.
The residence time of water in the lagoons of the ring reef depends on the
heights of tides, rates of currents, as well as upon their morphometry. Its
estimations were based upon measurements of the rate of water transport
across the flat and through the channels. In the semi-closed lagoon of the
Bikini atoll during one tide cicle some 4% of the total water volume in it is
exchanged. The residence time for this atoll was estimated to be within 40
days and the time for complete mixing of the water column by currents in its
60 m deep lagoon 6-12 days (von Arx 1948). The longest exchange rates
were recorded in the shallow lagoon of the One Tree ring reef - less than 1
week (Ludington 1979; Frith 1981). At the Enivetok atoll the rate was 30
days, in the Canton atoll 50 days (Smith and Jokiel 1978), in the Tarawa
atoll 10-15 days (Kimmerer and Walsh 1981) and in the elevated Fanning
atoll, which has only one narrow pass, about 8 months (Gallagher et al.
1971; Atkinson et al. 1981). The exchange of water between lagoon and the
surrounding ocean proceeds not only via surface hydrodynamics, but also
through the porous walls of reef via the moving groundwaters (Lam 1974;
Rougerie 1985). Their flows are induced by the fluctuations of sea-level
during the tides. The flows of groundwaters inside the lime body of the reef
dissolve the carbonates and thus perforate it, creating its permeability
(Johannes 1980). The most permeable for the underground passage of