Along the Coast
Coastal areas are often among the most dynamic and productive of environments. They are in many ways the definition of living on the edge. Waves crash against rocky cliffs or roll onto sandy shores. Estuaries exhale the last breath of river systems, the frenetic riverine pace yielding to a sprawling mixture of fresh- and saltwater.
In warm estuarine waters of the tropics, hot and humid mangrove forests are a hybrid of terrestrial and marine: above the water, they host insects, birds, monkeys, alligators and large mammals like deer and even tigers; beneath the water’s surface, fish swim among mangrove trees’ thick roots, which are covered with filter feeders like oysters, mussels, and anemones, while the muddy banks are inhabited by amphibians and crabs.
Whereas mangroves occupy the boundary between saltwater and fresh, and ocean and land, seagrasses form submerged beds, or meadows, in the sandy floor in shallow coastal seas. The plants’ extensive roots anchor the meadows firmly into the sand, making seagrass meadows safe shelter for invertebrates and fish; the fact that some plants can grow as much as a centimeter per shoot per day means they are able to withstand the attentions of a variety of herbivorous grazers, from sea urchins to turtles, manatees and dugongs.
In cool, nutrient-rich temperate waters, giant seaweeds called kelp reach upwards from the sea floor beyond the low tide, growing by as much as sixty centimeters a day as they race to be close enough to the surface where they can use the Sun’s light to photosynthesize. When conditions are suitable, they can cover vast areas: the coast of California alone has approximately 18,000 hectares of giant kelp growing along its coast. Kelp forests provide a sheltered calm amid the chaos of the pounding surf, and many organisms use the thick blades as a safe shelter for their young from predators or even rough storms. As a result, kelp forests support a greater variety and higher diversity of plants and animals than almost any other ocean community.
In contrast to kelp forests, coral reefs require clear, shallow waters with an ideal temperature range of between 20 and 30 degrees C. The reefs themselves are structures that have formed over hundreds, thousands or even millions of years by countless tiny organisms called polyps, which produce skeletons of calcium carbonate. Reef-building corals contain symbiotic, microscopic, photosynthesizing algae called zooxanthellae; the polyps provide the algae with carbon dioxide, and the zooxanthellae use sunlight to convert it into oxygen and carbohydrates. The algae are so small that there may be as many as two million of them in each square centimeter of coral tissue, making them by some distance the most abundant species on reefs, although far from the only ones.
Although coral reefs occupy only approximately 0.1 percent of the surface of earth, one-third of all known marine species live on them, and the total number of reef-dwelling species may number a million or more. Certainly, ecologists believe that they support a great number of species per unit area than any other marine system, and may in fact be the most diverse system on Earth. A study of a 15,000 hectare region in the Philippines documented over 5,000 species of molluscs, most of them tiny and observed just once. As testimony to this abundance and diversity, coral reefs are frequently referred to as ‘rainforests of the sea’.
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