Some people go whale watching. Some people long to swim with dolphins. Others earn their livelihood fishing for giant tuna in a vast ocean. These marine animals capture our attention and our imagination. We have a connection to all the living things of the ocean, from the microscopic floating plants that supply us with the oxygen we breathe to the huge blue whale that fills its belly with a ton of krill. Microscopic or oversized, plant or animal, from muddy shoreline to deep ocean floor, the ocean's living things attest to its endless variety, its biodiversity. Scientists say that there may be millions more species than we know swimming, floating, and crawling in the deep oceans and as yet unseen by human eyes. With the aid of submersible technology, entire new ecosystems are being discovered. Each ecosystem consists of a community of living things that interact with one another in complex relationships in unique conditions of water temperature, salinity, chemical composition, and currents.
Far below the surface of the ocean, where no sunlight reaches, hot water laced with chemicals spews out of cracks in the ocean floor. These cracks (hydrothermal vents) occur most often along the mid-ocean ridge, where Earth's crustal plates are spreading apart. Water reaching temperatures of four hundred degrees Celsius and chemical compounds such as hydrogen sulfide billow out from the vents. At certain vents, as the hot, sulfide-rich water comes in contact with cold seawater, metal sulfides precipitate out. The chemicals pile up into structures that resemble chimneys, which scientists call "black smokers." Scientists have found one black smoker that is as tall as a fifteen-story building. Can living things survive in such a place? The answer is yes.
In 1977, scientists aboard the submersible Alvin, exploring five thousand feet below the surface of the Pacific, saw large, four-foot-tall tube worms, some with bright red plumes, living around a hydrothermal vent. Later laboratory investigation revealed that the unusual worms had no digestive system but instead contained about 285 billion bacteria per ounce of tissue! In this sunless world, a type of sulfur-loving bacteria was the worms' food source. Clouds of bacteria, appearing white in the lights of the sub, were able to use hydrogen sulfide as an energy source. In most other food chains, plants convert carbon dioxide into food using sunlight during photosynthesis. These peculiar bacteria were able to convert hydrogen sulfide into food during chemosynthesis. Also found around the vents, feeding on the water rich in chemosynthetic bacteria, were certain kinds of clams and mussels. At this great depth and pressure, some species of octopus prey upon these shelled invertebrates. But when the hot water and chemicals coming from the vent slow down to a trickle, the animals disappear.
In the past twenty years, more than three hundred species have been identified in this unique environment. Similar vent organisms have been discovered at the base of the continental shelf, where the ocean water is sulfide-rich but not hot, as in the hydrothermal vents. These "cold seeps," as they are called, illustrate how little we know about the productivity of the ocean bottom. As far as scientists can tell, hydrothermal vents and cold seeps have not yet been affected by human activities.
Far away in a tropical ocean is another distinctive marine ecosystem. One in four marine species on our planet lives in a coral reef, an underwater world like no other with its colorful variety of swimming and floating animals making their way among the branching corals. The food chain of the coral reef begins with photosynthetic algae, microscopic organisms that use sunlight to make food. Most of the algae live in harmony within the tiny coral animals themselves. Reef-building corals secrete a hard, stony shell of calcium carbonate that builds up over time and provides the habitat for reef animals. Colorful invertebrates such as the coral shrimp feed on algae and detritus around the coral, where they in turn may become dinner for small fish. Large, sleek, and silvery barracuda patrol the outer reef, preying on smaller fish such as the butterfly fish. However, there is trouble in this paradise: pollution from pesticides, sewage, and soil run-off has damaged many reefs in the Caribbean and Pacific. The practice of dynamite fishing to stun fish and capture them for the aquarium trade has devastated reefs in parts of Asia, the South Pacific, and Africa.
Perhaps not as familiar to us is the frigid water of the polar ocean. The food chains of the polar ocean also begin with algae, including symmetrically shaped diatoms with hard silicate shells. Algae are eaten by tiny invertebrate animals, including shrimplike krill. In the ocean around Antarctica, krill are an important food source, eaten by a diverse group of animals including fish, baleen whales, and Adélie penguins. The penguins are in turn preyed upon by leopard seals. The top predator of the Antarctic is the killer whale, which eats penguins and seals. Thus overfishing of krill in polar waters may jeopardize not only krill, but whales, seals, and penguins too.
Along more temperate seacoasts, kelp forests form another unique ecosystem. Kelp are brown algae that can grow as much as sixty centimeters in one day, ultimately reaching as long as eighty meters. Tiny crustaceans called copepods are among the animal plankton that feed on the floating algae and detritus in the Pacific along the California coast. Larger invertebrates, such as sea urchins and abalone, graze on the kelp and are in turn eaten by sea otters. The overharvesting of kelp and a decrease in water quality have impaired the productivity of these ecosystems. Discharge from nuclear power plants on the California coast raises the water temperature just enough so that more sea urchins and abalone survive and grow, eating kelp and diminishing the size of kelp beds. Understanding the various marine ecosystems helps us to better understand the important connections among marineorganisms and suggests how much we still have to learn about the oceans. This understanding also raises warning flags about the necessity of monitoring human activity to keep these connections from being severed and to protect marine biodiversity.