About Coral Reefs/Comparing Oceans

Ask people where in the ocean the largest number of fish are concentrated, and most imagine the vivid, bustling coral reef. But consider the locations of the major fisheries of the world, which depend on netting large quantities of fish in each tow (cooler waters). In the temperate and cold seas there tend to be relatively few species of animals, but many individuals. In tropical waters, though the variety of life is staggering, there are far fewer individuals of each species.

Underwater coral reef scene Though cold ocean waters offer a smaller variety of habitats (mainly ocean floor or open water) for a few species, there's more than enough food to nourish huge schools of tuna, cod, and mackerel. Nutrients like phosphorus and nitrogen are carried by rivers to the sea. There they fertilize the cold waters and promote the growth of tiny plants called phytoplankton. These in turn feed tiny animals called zooplankton which are eaten by small fishes that are eaten by larger fishes and on up the food chain. Cold water also holds more dissolved oxygen and carbon dioxide, gases animals and plants need, than warm water can. (This is because molecules move faster in warmer water and literally bump the gases out of the water.) This rich soup of microscopic plants and animals and decaying plants‹the base of the ocean food chain‹makes cold northern waters appear murky.

In contrast, a tropical sea is crystal clear, with almost nothing suspended in the water to refract light or block our view. In other words, it's empty. A coral reef is like a city in a desert - an oasis - which may provide the only food and shelter for many miles around. Corals are adapted to these nutrient-poor environments, so they can tolerate only minute quantities of sediments or nutrients in the water. Even a quarter-inch of silt can smother the living coral polyps. High levels of nutrients, such as the phosphates and nitrates from sewage, stimulate blooms of green algae that soon overgrow the coral.

On tropical coral reefs, there are hundreds, even thousands, of different kinds of plants and animals, but the number of individuals of each species is limited. A single coral outcrop may be home to as many as 200 different species of fishes, but perhaps only two butterflyfish, which feed only on the tentacles of featherduster worms, claim this patch of reef. They will defend their territory from all others of their kind, which are competing for the same food and hiding places. Specializations in color, behavior, and diet allow animals to carve out their individual niches, or living space, on the reef. That specialization may take the form of unusual partnerships of symbiosis, in which two different species live together for the benefit of at least one, or of supporting two separate day and night communities of animals.

Gulf of Maine

Nova Scotia, New Brunswick, Maine, New Hampshire, and Massachusetts border "New England's own ocean," the Gulf of Maine. Shallow underwater areas of sand and gravel, Georges Bank and Brown Bank, form its eastern rim. These shallow depths almost cut the Gulf off from the rest of the Atlantic Ocean, restricting most of the flow of open ocean water to a narrow valley between the banks called the Northeast Channel. This separation is why the Gulf is sometimes called "a sea beside a sea." Fresh water flowing into the Gulf from many rivers makes it colder and less salty than the outer Atlantic.

A gyre, an ocean current that flows counterclockwise in the Gulf of Maine, circulates nutrients, food, and pollution throughout the Gulf, stirring them together like a spoon in a pot. Nutrients are washed in from the land or are churned up from the ocean floor to support vast quantities of phytoplankton, zooplankton, and huge schools of fish. In the summer, humpback, fin, and minke whales visit the Gulf of Maine to gorge on the rich food supply.

The special conditions that have contributed to the fertility of the Gulf of Maine are :

  • nutrients carried in by rivers
  • cold water from the Nova Scotia shelf, holding more dissolved gases like oxygen and carbon dioxide. The Gulf of Maine is categorized as sub-arctic with a wide range in water temperature.
  • shallow, continental shelf and banks for photosynthesis.
  • strong circulation of nutrients by the Gulf of Maine gyre, other currents, winds, strong tidal mixing, and seasonal overturn of deep and surface waters

Overfishing and coastal pollution that has drifted out to 200 miles from shore now threaten the fish stocks of Georges Bank, once one of the richest fishing areas in the world.

Satellite images of phytoplankton density in the Gulf of Maine show high levels of phytoplankton with a seasonal variation. In the winter there is relatively less phytoplankton because of the mixing of the ocean layers that occurs in the winter and because of the lower levels of sunlight. Phytoplankton levels increase in the spring as nutrients wash into the Gulf of Maine and as daylight increases, and then decrease in the summer as organisms eat the phytoplankton. In the fall there is a second increase in phytoplankton levels with increased overturning of the water.

The cooler water temperature of the Gulf of Maine shows up on infrared satellite images. These images also reflect the seasonal change in water temperature in the Gulf of Maine.

Caribbean waters

Tropical waters like those of the Caribbean Sea are warm, clear, shallow, and very salty. The Caribbean Sea is an homogenous body of water about 500 miles wide by about 1,200 miles long. The Caribbean Current flows from east to west, with localized gyres and countercurrents. In 1983-84, the near-universal mortality of long-spined sea urchins (Diadema antillarum) illustrated just how homogenous the area was. Within one year, nearly 99% of all the sea urchins were killed by some as-yet unknown pathogen.

Scientists from 22 Caribbean Basin nations are cooperating on regional projects to research such things as the effect of hurricanes on fish kills and coral destruction, mass mortalities of marine organisms, larval distribution patterns, and sea level rise.

Satellite images show the Caribbean to be devoid of phytoplankton compared to the Gulf of Maine. Virtually no seasonal variation in phytoplankton levels occurs there, with the exception of areas that are affected by run-off from rivers such as the Amazon. After the rainy season, the Amazon River washes nutrients many miles out into the sea. That runoff promotes phytoplankton blooms. This natural process is exacerbated by increased erosion caused by rain forest cutting. Satellite images show that the sea surface temperature of the Caribbean is warm throughout the year, as would be expected of an area just above the equator.

"City fish" vs. "Country fish"

Cold-water fish are like country fish: they live close to the earth, dress in drab colors, and follow the rhythms of the seasons. Gulf of Maine fish tend to be full-bodied and streamlined, built for swimming long distances in large schools. Or they may be adapted for living on the ocean floor. "Groundfish" is the term that describes fish, such as cod, haddock, and flounder, which live on or near the bottom. Cold-water fishes, like those in the Gulf of Maine, tend to be dull-colored (brown, black, or speckled) to match the ocean floor and the seaweed, or silvery to reflect the light under water (typically, schooling fish are silver). Bottom dwelling fish may have "chin whiskers" or barbels to feel and taste food (cod have these), or may have side (pelvic) fins adapted for touching and tasting what's on the ocean floor (sea robins have these). Other bottom dwellers may have skin that resembles seaweed (sea raven, goosefish)

Coral reef fish are like city fish: they live in tight quarters at well-defined levels, they are colorful, they are active night and day, and they form complex and sometimes unusual associations and specializations. Coral reef fish live crowded together in a reef, so they have developed specializations in color or behavior (territorial, symbiotic, nocturnal, etc.) or in design (venomous, needle-shaped) to help them carve out their niche in the reef. Coral reef fishes are often disc-shaped, flattened from side to side, so they can slip into crevices in the reef. Coral reef fishes tend to be brightly colored to attract a mate, to advertise their services (such as a cleaner fish), to warn others away because they are territorial or venomous (such as a lionfish), or simply to blend in with the neon colors of the reef.

Schools of cold water fishes range across broad expanses of ocean, while the city dwellers of the coral reef compete for cramped quarters. Reef fishes occupy many different levels, like apartment dwellers in high-rise buildings. Once an animal sets up housekeeping in one area, it rarely moves unless it is forcibly evicted by a new tenant. So many creatures crowd into the coral reef that they must maintain two different shifts to accommodate all the residents. The coral reef community pulses both day and night. Although there are diurnal (active in daytime) and nocturnal (active in nighttime) communities in cold seas, the differences between day and night are not as striking. There, the rhythm of life - feeding, migration and birth - seems more attuned to the slower pace of the changing seasons.

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