The second marine biome is composed of coral reefs. Near shorelines, coral
reefs provide specialized situations where life thrives. Coral reefs are actually built
by the lime-rich excrement of tiny colonial animals called coral polyps. These
reefs are found worldwide in shallow, warm waters. The presence of reef struc-
tures allows a multitude of symbiotic relationships between plants and animals
fostering significant diversity of life forms.
The third marine biome is found in Earth’s estuaries. These are constricted ocean
arms into which freshwater streams debouch and are characterized by the mixing of
freshwater with saltwater. A large number of plant and animal species can be found
and, typically, biomass and biodiversity are much higher than in the open ocean.
Nutrients emanating from rivers and the various salt contents up and down the estu-
ary make for an energetic chain of life with reeds and other plants anchored in the
shallows and ocean fish coming into protected waters to spawn.
Another class of aquatic biomes is associated with fresh water. The freshwater
class includes lakes, ponds, and streams. Combinations of size and flow conditions
make for rich biodiversity. Particularlyfecund in the production of biomass are
wetlands (swamps and marshes) that are covered by shallow water for all or part
of the year. They are delineated with trees dominating swamps and grasses domi-
nating marshes.
Land biomes roughly correspond to world climate zones. For clarity, world
biome maps are usually limited to a dozen or less biomes. Most plant and animal
species are not exclusive to single biomes although they tend to be more or less34 Biomes
Coral Reefs
Coral reefs are an amazing interplay between the organic and inorganic realms. Coral reefs
result from the excrement of trillions of various species of small animals known as coral pol-
yps (generally less than 5 mm [2 in] long). Living in colonies, the polyps excrete calcium car-
bonate, which becomes rocklike. These rigid structures protect the polyps against waves,
tides, and predators. The reef grows from the seafloor upward and can make for a complex
seafloor bottom with quiet water and sanctuary for small animals and plants. Coral reefs
are among the most biodiverse places on Earth. Geographically, there are two requirements
for coral reefs: (1) shallow seawater less than 50 m (160 ft) deep; and (2) water temperatures
above 20°C (68°F). Coral reefs are found in all the shallow tropical waters of the world. The
largest reef is the Great Barrier Reef (actually a complex of many thousands of individual reefs
off of the northeastern coast of Australia); it has an extent of 2,600 km (1,600 mi). Fossilized
reefs found underwater near cold places such as Greenland, indicating a history of continental
drift/change in climate. Additionally, mountains such as the Guadalupe Mountains of Texas
(Permian) and the Hochkonig Massif of Austria (Triassic) were once reefs, hinting at the enor-
mous changes wrought byplate tectonics.