WORLD ATLAS OF CORAL REEFS
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may out-compete them. These typically include algae
and sponges which may compete for space and light and
overgrow corals. It can also include algae living within the
plankton which can literally block out the light and increase
the turbidity to levels which the corals cannot survive.
Patterns across the reef
Where conditions allow, coral reefs form and continue
to thrive, marking out a colorftil barrier along many
coastlines and far out across the Pacific and Indian
Oceans. As individual coral reefs are examined more
closely, new and distinctive patterns emerge, formed by
the species which make up the reef community.
Moving across a reef from the beach to the open sea.
environmental conditions vary considerably. Close to the
shore there may be freshwater runoff, loose sediments of
sand or mud, and regular exposure to the air and sunlight
with the shifting tides. Further out conditions are shallow
and bright, but there may be little circulation of the water.
At the outer edge of the shallow reef, the waters change
dramatically. Waves may break on the reef top. Lower
down, the light diminishes rapidly with depth. Light and
depth, tides, water circulation, wave action, sediments,
nutrients, temperature variation and salinity all have a part
to play in determining which species are found where on a
reef, and clear zones have been recognized. A number of
these are illustrated in Figure 1.3.
Beach and intertidal communities
Although considered beautiful by millions of tourists,
beaches and other intertidal areas are among the harshest
communities for many species. Daily exposure to drying
air and hot bright sunshine is inimical to most marine
species, while regular or occasional soaking by saltwater
is equally difficult for most terrestrial species. Beaches
themselves are places of constantly shifting sediments,
offering no solid substrate on which to establish, and only
tiny interstices between the sediment particles for refuge.
The coastline is also the point at which terrestrial inputs
are at their most concentrated, with runoff, pollutants and
sediments greatly influencing life in some areas. Life on
sandy beaches is not abundant. There are many microscopic life forms within the sand, and a range of species,
notably crabs, patrol the shores for food. In rocky areas a
greater diversity of life occurs, notably molluscs, algae and
bryozoans, and a complex pattern of communities may be
found associated with tide pools and their position relative
to the tidal range. Mangroves are a group of highly adapted plants which thrive in intertidal waters. Ahhough
frequently associated with reefs they are somewhat restricted in where they can grow, and only build extensive
communities in areas where there are fine silts and muds,
particularly where there is some freshwater input.
Above: Mangroves are important intertidal communities in many reef areas. Center: The reef crest, ttie shallowest part
of the reef, northern Red Sea. Below: Coral diversity is highest on the reef slope, typically reaching a peak below the areas
of highest exposure to waves, but still in shallow areas where loss of light is not a limiting factor.
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S^"-'»!
may out-compete them. These typically include algae
and sponges which may compete for space and light and
overgrow corals. It can also include algae living within the
plankton which can literally block out the light and increase
the turbidity to levels which the corals cannot survive.
Patterns across the reef
Where conditions allow, coral reefs form and continue
to thrive, marking out a colorftil barrier along many
coastlines and far out across the Pacific and Indian
Oceans. As individual coral reefs are examined more
closely, new and distinctive patterns emerge, formed by
the species which make up the reef community.
Moving across a reef from the beach to the open sea.
environmental conditions vary considerably. Close to the
shore there may be freshwater runoff, loose sediments of
sand or mud, and regular exposure to the air and sunlight
with the shifting tides. Further out conditions are shallow
and bright, but there may be little circulation of the water.
At the outer edge of the shallow reef, the waters change
dramatically. Waves may break on the reef top. Lower
down, the light diminishes rapidly with depth. Light and
depth, tides, water circulation, wave action, sediments,
nutrients, temperature variation and salinity all have a part
to play in determining which species are found where on a
reef, and clear zones have been recognized. A number of
these are illustrated in Figure 1.3.
Beach and intertidal communities
Although considered beautiful by millions of tourists,
beaches and other intertidal areas are among the harshest
communities for many species. Daily exposure to drying
air and hot bright sunshine is inimical to most marine
species, while regular or occasional soaking by saltwater
is equally difficult for most terrestrial species. Beaches
themselves are places of constantly shifting sediments,
offering no solid substrate on which to establish, and only
tiny interstices between the sediment particles for refuge.
The coastline is also the point at which terrestrial inputs
are at their most concentrated, with runoff, pollutants and
sediments greatly influencing life in some areas. Life on
sandy beaches is not abundant. There are many microscopic life forms within the sand, and a range of species,
notably crabs, patrol the shores for food. In rocky areas a
greater diversity of life occurs, notably molluscs, algae and
bryozoans, and a complex pattern of communities may be
found associated with tide pools and their position relative
to the tidal range. Mangroves are a group of highly adapted plants which thrive in intertidal waters. Ahhough
frequently associated with reefs they are somewhat restricted in where they can grow, and only build extensive
communities in areas where there are fine silts and muds,
particularly where there is some freshwater input.
Above: Mangroves are important intertidal communities in many reef areas. Center: The reef crest, ttie shallowest part
of the reef, northern Red Sea. Below: Coral diversity is highest on the reef slope, typically reaching a peak below the areas
of highest exposure to waves, but still in shallow areas where loss of light is not a limiting factor.