Describe the geographic patterns of life in the open ocean.

Because the oceans are so large and so deep, open-ocean, or pelagic, ecosystems are the most geographically extensive ecosystems on the planet. The open waters (also called the high seas) cover about 50% of Earth’s surface in total. Our knowledge of the open ocean, however, is still in its infancy. Scientists are working to understand what lives there, how pelagic ecosystems function, and how people are influencing them.

We begin with two geographic approaches to examining the open waters: (1) tracing the daily movements of organisms vertically through layers of the ocean and (2) following the periodic movements of organisms horizontally across the ocean’s surface.

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Much of the open ocean’s surface is a biological desert by day, but it is full of life at night, when organisms migrate upward from the safety of the dark depths to feed at the surface. These daily vertical migrations, or “commutes,” represent the largest synchronized animal movements on Earth. All types of organisms rise to the surface each night, including zooplankton, jellyfish, fish, and squid. Some organisms, such as jellyfish, take hours to reach the surface, spending more time in migrating than in feeding.

Many other organisms make a “reverse commute” to feed. In other words, they live at the lighted ocean surface but dive into the depths for their food (Figure 10.26).

During the last decade, thanks to new GPS tagging and tracking technology, scientists have begun to piece together where pelagic migratory organisms go. Scientists are using a variety of electronic tagging techniques on migratory species to reveal where they migrate, when they migrate, and why they migrate. Figure 10.27 traces the migrations of two species, the great white shark and the blue-fin tuna.

No sunlight reaches below the mesopelagic zone, which ends at a depth of 1,000 m (3,280 ft) (see Figure 10.6). Most of the open ocean is therefore pitch black and perpetually just above freezing. Pressures at these depths are extremely high because of the immense weight of water above.

There is, however, light at these depths. Ninety percent of organisms in the deep sea make their own light. Bioluminescence is the production of light through chemical means by living organisms. Bioluminescence has evolved independently among unrelated groups of deep-sea organisms at least 40 times. It is used to attract mates and prey and to hide and escape from predators (Figure 10.28).

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Scientific study of life in the deep ocean is a particularly challenging endeavor. The tremendous pressures of the deep have created a formidable barrier to scientific investigation, and staffed missions are very expensive. Robotic submersibles and trawls, which are far less costly, have been among the most important sources of scientific information on deep-ocean ecosystems (Figure 10.29).

The abyssal plains are relative biological deserts. Rising from these plains are biological islands in the form of seamounts and hydrothermal vents. Collectively, seamount ecosystems are larger in area than any terrestrial biome. Together, they make up an area approximately the same size as the continent of South America. These ecosystems, however, are largely inaccessible to scientific study and barely understood by scientists (Figure 10.30).

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Like winds around a high mountain, ocean currents flow over and around large seamounts. These currents may accelerate and become turbulent, stirring up nutrients from the seafloor and bringing them to the sunlit surface. The nutrients stimulate phytoplankton blooms, which support other organisms. Many migratory species seek out and congregate above seamounts, including sharks, tuna, whales, and seabirds.

Like seamounts, hydrothermal vent communities are rich biological islands on the seafloor. Hydrothermal vent communities are unique ecosystems found at hydrothermal vents, volcanic hot springs that emit mineral-rich water. Seawater circulates through fissures in the crust and becomes heated to over 400°C (750°F) before emerging from the seafloor. The water becomes a stew of acids, sulfides, methane, and carbon dioxide as the rocks dissolve in it. Specialized bacteria metabolize these chemicals by the process of chemosynthesis, and other organisms feed on those bacteria. These hydrothermal vent communities get their energy from geothermal energy rather than solar energy. If the Sun were to stop shining, these organisms would probably not be affected, while the rest of the biosphere would disappear. The locations of some hydrothermal vents are shown in Figure 10.31.

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All marine ecosystems, whether on the continental margins or in the open ocean, support fisheries of one type or another. A fishery is a region where fish are caught for human consumption. The livelihoods of about a half billion people, mostly in economically developing countries, are dependent on small-scale, local, and sustainable fishing. Sustainable fishing means that the amount of fish taken is equal to or less than the reproductive rate of the fish population.

Unsustainable fishing occurs when fish are taken out of a fishery faster than the population can reproduce, causing the fish population to decline. Due to unsustainable fishing, about 30% of the world’s fisheries have collapsed, defined here as being reduced to 10% of their former fish populations.

A growing human population, coupled with advances in industrial fishing technology in the last five decades, is to blame for global fishery declines. The global fish catch peaked in 1989 at 90 million tons and has since leveled off or declined. The decline is not due to conservation efforts—there are now more fishing boats in the ocean than ever. Rather, it is due to the fact that there are fewer fish in the oceans now than in the past.

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Drift net fishing, trawling, longline fishing, and seining are four methods commonly employed in industrial fishing. Drift net fishing suspends large nets in the upper reaches of the ocean. Trawling drags nets through the water column or along the seafloor. Longline fishing employs thousands of baited hooks on lines up to 80 km (50 mi) in length. Seining encircles fish and traps them in a large net. These fishing methods invariably kill other marine species (Figure 10.32). Bycatch consists of unwanted organisms, such as birds and sea turtles, caught by industrial fishing methods. These organisms are usually thrown back to sea dead. Bycatch is a serious problem that threatens sea turtles, albatross, whales, dolphins, and many species of fish.

Approximately one-third of the world’s open-water shark species (over 100 species) are considered endangered or critically endangered due to forces such as habitat degradation and unsustainable fishing. Because sharks are keystone species in many marine ecosystems, healthy functioning of those ecosystems is compromised when sharks are lost. Worldwide, an estimated 75 million sharks are killed every year only for their fins, which are used to make shark fin soup (Figure 10.33).

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