Although the shallow seas occupy 8 percent of the ocean’s surface, they are brimming with life including 90 percent of the world’s commercial fisheries.

Most shallow seas occur along continental shelves — underwater extensions of the continents stretching about 50 miles with exceptions like the Pacific in South America (less than a half a mile) and off the Arctic coast of Siberia which extends some 450 miles. Water depths rarely exceed 650 feet.

In order for these waters to be productive and support myriad life forms they require sunlight and nutrients particularly nitrogen and phosphorous. This in turn enables phytoplankton to grow and comprise about 90 percent of primary production in the ocean powering the entire marine food chain.

Tropical seas are located between the Tropics of Cancer and Capricorn. And while these crystal clear waters are bathed in sunlight they are mostly deficient in nutrients. Nitrogen and phosphorous are found deep within the ocean, but in the tropics a lack of surface winds precludes little if any mixing of the upper and lower ocean waters.

Coral reefs are found in tropical waters above 68 degrees and are some of the most productive habitats in the ocean rivaling even that of tropical rainforest productivity.

So what makes the coral reefs so rich?

In order to overcome a lack of nutrients corals evolved an exquisite symbiotic relationship with zooanthellae — a photosynthetic algae that lives within the coral polyps.

Coral is made up of billions of polyps, resembling miniature sea anemones a couple millimeters long. Individual polyp’s live in a small cup-shaped skeleton of calcium carbonate, which it makes. Thousands of years of accumulation enable calcium carbonate to form coral reefs.

Polyps draw energy from the algae living in their tissue and in return they supply algae with nitrogen and phosphorous. Moreover, all the polyps waste is recycled by algae as a part of the photosynthesis process.

Fringe reefs occur as a narrow band along the coastline. The Red Sea has the longest single reef on the globe at over 2,500 miles.

Barrier reefs follow the coast but exist farther out to sea sometimes extending 60 miles away from land; lagoons and beds of sea grass protect them. Australia’s Great Barrier Reef covers about 87,000 square miles and is visible from outer space.

Charles Darwin discovered the third type of reef — an atoll which has rings of coral around a central lagoon ranging from a half-a-mile across to more than 19 miles wide. Atolls are not always close to continental shelves and are prevalent in the Indian and Western Pacific oceans.

Coral reefs are easily the most complex of all marine ecosystems. The biological diversity on the Indonesian archipelago reef is breathtaking at over 2,800 fish species and 450 kinds of coral.

The temperate seas, just beneath the polar seas, are amongst the most productive of all the shallow seas. Plankton blooms in massive annual events supporting colossal numbers of fish, seabirds and whales. 

When autumn occurs most will depart, some traveling more than 3,700 miles to equatorial whales nursing grounds.

Why are the temperate seas so rich?

It is due to the quantity of nutrients available in the surface layers of the ocean, and this is where photosynthesis takes place. The higher latitudes have powerful storms where the winds stir-up the water bringing deep layer nutrients to the surface. Also, rivers carrying nutrients replenish water near the coastline. Incidentally, these are where the most productive fishing grounds on the globe are located.

Winter storms inject new energies into the seas. And the phytoplankton blooms feed immense numbers of tiny animals called zooplankton, which feed jelly-fish, herring, sardines and anchovies.

Upwelling currents bringing nutrients to the phytoplankton are found predominantly along the west coasts of continents and in South America the Humboldt Current feeds anchovies that provide food for dusky dolphins, fur seals, sea lions, sharks and Magellanic penguins.

In the summertime, Alaska’s Aleutian Islands are home to 10 million breeding seabirds. Phytoplankton feeds 4.4 billion pounds of pollock, which in turn feeds birds, salmon, atka mackerel, herring and humpback whales.

Temperate sea continental shelves are fertile because waves stir-up the nutrients from the shallow bottoms (usually no deeper than 650 feet) and rivers constantly feed these seas with surface nutrients.  Sandy and muddy sea floor bottoms are inhabited by bottom feeding burrowers like lugworms and trumpet worms.

Continental shelves that have rocky bottom substrates are home to underwater jungles. One of the most surreal ecosystems I’ve visited is the giant kelp forests’ of Southern California. Two hundred foot-tall plants with holdfast or root-like structures are the ocean’s equivalent to terrestrial rainforests.


Dr. Reese Halter is a public speaker and conservation biologist. His latest book is “The Incomparable Honeybee and the Economics of Pollination,” Rocky Mountain Books. Contact him through

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