38
Reef Environments
The impact of wave stress upon the geomorphological features and on the
biological community structure of reefs is extremely significant in regions
situated in the traditional paths of hurricanes (Stoddart 1962, 1974;
Hernandez et al. 1977; Dollar 1982). These are the Caribbean reefs, those
of the Philippines and of the southern GBR (Stoddard 1969; Woodley et al.
1981; Kjerfve and Dinnel 1983). In these regions the strong tropical cyclones
with pressure in the centre of 945-955 mm are recurring every 3-10 years.
After the passing of such a cyclone with waves more than 10 m high near the
reef edge not only the ramose but also the massive corals are swept off the
outer reef slope and flat. On the flat and in the lagoon huge ridges of rubble
appear. Inside the lagoon during such a storm the height of waves will reach
3-5 m. These waves smash up the fine ramose corals. Destruction of corals
proceeds not only through stress of water and waves but also by the moving
masses of rubble and broken-out coral heads. The recovery of reefs after
passing of a hurricane takes 5-10 years, but some reefs cannot repair the
damage even in 10 years (Stoddart 1974).
Most coral reef biotopes exist in conditions of permanent and intensive
hydrodynamical movement of water masses, caused by the interactions of
various multidirectional and fluctuating currents with a complex relief of
reef constructions. For the measurement of currents speed three main
methods were used: winches (Novozhilov 1980; Wolanski and Jones 1980;
Andrews and Gentien 1982), buoys, floats, empty bottles, drogues (Hamner
and Hauri 1981; Atkinson et al. 1981), and dyes (vor Arx 1948; Roberts et
al. 1975; Marsh et al. 1981; Atkinson et al. 1981; Oberdorfer and
Buddemeyer 1985). The best results were obtained by their combinations.
The turbulence exchange is quentified via analysis of the vertical
thermohaline structure of the water column and also with the use of dyes
and model analysis (Bode and Starck 1983).
The main driving forces in reef hydrodynamics are the oceanic currents,
wind and wave stress, the short-period oscillations of sea level caused by
tides, and by the long-period internal waves caused by the heterogeneity of
basic fields in tropical shelf areas (Atkinson et al. 1981; Frith 1981). Their
relative importance varies, being dependent upon the reefal morphometry,
weather conditions in the region, and on the seasonal successions in the tide
heights. Most reef regions are situated in areas of the wind-driven, reverse
or compensative oceanic currents, such as the trade wind currents,
equatorial counter-currents, and the East-Australian current. Their velocity
varies in shelf areas within 10 to 50 cm s -1. The interaction of an atoll
situated in open ocean with a dominating current results in the upwelling of
deep waters at its windward side. One of the causes of upwelling is the
divergence of the oncoming current near the reef by two flows encircling the
reef body (Hamner and Hauri 1981). The upwelling can be also induced by
the interactions of the currents encircling the windward reef slope, with its
systems of spoors at depth of 20-40m (Roberts et al. 1975; cf. 1.1. Fig.
2.2). The oncoming current and the upward flows going through the grooves
Reef Environments
The impact of wave stress upon the geomorphological features and on the
biological community structure of reefs is extremely significant in regions
situated in the traditional paths of hurricanes (Stoddart 1962, 1974;
Hernandez et al. 1977; Dollar 1982). These are the Caribbean reefs, those
of the Philippines and of the southern GBR (Stoddard 1969; Woodley et al.
1981; Kjerfve and Dinnel 1983). In these regions the strong tropical cyclones
with pressure in the centre of 945-955 mm are recurring every 3-10 years.
After the passing of such a cyclone with waves more than 10 m high near the
reef edge not only the ramose but also the massive corals are swept off the
outer reef slope and flat. On the flat and in the lagoon huge ridges of rubble
appear. Inside the lagoon during such a storm the height of waves will reach
3-5 m. These waves smash up the fine ramose corals. Destruction of corals
proceeds not only through stress of water and waves but also by the moving
masses of rubble and broken-out coral heads. The recovery of reefs after
passing of a hurricane takes 5-10 years, but some reefs cannot repair the
damage even in 10 years (Stoddart 1974).
Most coral reef biotopes exist in conditions of permanent and intensive
hydrodynamical movement of water masses, caused by the interactions of
various multidirectional and fluctuating currents with a complex relief of
reef constructions. For the measurement of currents speed three main
methods were used: winches (Novozhilov 1980; Wolanski and Jones 1980;
Andrews and Gentien 1982), buoys, floats, empty bottles, drogues (Hamner
and Hauri 1981; Atkinson et al. 1981), and dyes (vor Arx 1948; Roberts et
al. 1975; Marsh et al. 1981; Atkinson et al. 1981; Oberdorfer and
Buddemeyer 1985). The best results were obtained by their combinations.
The turbulence exchange is quentified via analysis of the vertical
thermohaline structure of the water column and also with the use of dyes
and model analysis (Bode and Starck 1983).
The main driving forces in reef hydrodynamics are the oceanic currents,
wind and wave stress, the short-period oscillations of sea level caused by
tides, and by the long-period internal waves caused by the heterogeneity of
basic fields in tropical shelf areas (Atkinson et al. 1981; Frith 1981). Their
relative importance varies, being dependent upon the reefal morphometry,
weather conditions in the region, and on the seasonal successions in the tide
heights. Most reef regions are situated in areas of the wind-driven, reverse
or compensative oceanic currents, such as the trade wind currents,
equatorial counter-currents, and the East-Australian current. Their velocity
varies in shelf areas within 10 to 50 cm s -1. The interaction of an atoll
situated in open ocean with a dominating current results in the upwelling of
deep waters at its windward side. One of the causes of upwelling is the
divergence of the oncoming current near the reef by two flows encircling the
reef body (Hamner and Hauri 1981). The upwelling can be also induced by
the interactions of the currents encircling the windward reef slope, with its
systems of spoors at depth of 20-40m (Roberts et al. 1975; cf. 1.1. Fig.
2.2). The oncoming current and the upward flows going through the grooves