Some open—ocean organisms are strong swimmers—think of sharks and whales, for example. But plankton go with the flow.

The name "plankton" is from the Greek for "wanderer." Phytoplankton are photosynthetic organisms, and zooplankton are animals. Some are plankton-for-life, while others, such as fish larvae, grow into swimmers.

Most plankton are microscopic: copepods are tiny crustaceans that could fit on the head of a pin. But some drifters such as the lion's mane jellyfish can be huge—as big around as a child's wading pool.

A free-floating lifestyle poses problems for plankton and other drifters. They can't control where they travel, and it's hard to evade predators. Far from shore, nutrient concentrations tend to be low. The ultimate challenge for drifters is simply to stay afloat to feed and not sink out of sight.

Long-distance pollutants
If there's nowhere to hide from predators in the open ocean, there's also no refuge from pollution. The same currents that move drifters around the open ocean also move pollutants.

	Up, Up and Away Giant tubeworms live on volcanic vents that can shut down without warning. To perpetuate the species, they send their larvae on voyages of discovery. Like hot-air balloonists, larvae lift off in hot-water columns rising from the vent, to reach currents that sweep them away-hopefully to descend toward hospitable new homes. © Woods Hole Oceanographic Institution

Catastrophic oil spills make headlines. A bigger problem gets far less press: out on the open ocean, tankers pump their bilges and flush their ballast tanks. These routine procedures spill far more oil than tanker accidents do each year. Contamination is a particular problem along major shipping channels.

One group of drifters that's especially hard-hit by spilled oil are the neuston—organisms that live on or under the water's surface. Some organisms are neuston throughout their life cycle; others, such as fish eggs and larvae, are temporary members of the community. Even the thinnest layer of petroleum floating on the sea surface can be deadly for neuston.

Oil isn't the only open-ocean pollutant. Many toxic substances are transported—either by ocean or atmospheric currents—far from centers of civilization. The pesticide DDT—still used legally in many nations—can be measured in the body fat of Antarctic penguins. At the opposite end of the globe, Arctic polar bears, poised at the top of the food chain, "bioaccumulate" toxic PCBs in their bodies at levels two billion times the concentration in seawater.

	The Portuguese man-of-war (Physalia physalis) can drift over enormous distances powered by surface currents and "sail" power. Although each man-of-war begins life from a single larva, the adult form is actually a colony of individuals, each with a specific task. Some are responsible for flotation, others capture prey with stinging cells and still others digest food.

Personal Flotation Devices
There's more sunlight for photosynthesis at the sea surface. That means phytoplankton need to maintain their position in this surface zone-and so do the zooplankton that feed on them. How do drifters keep from sinking out of sight?

Some drifters have bodies shaped to resist sinking. Phytoplankton are often very flat-or form long chains. Other phytoplankton and many fish larvae increase their overall surface area with long spines or other projections that create drag.

Other drifters have built-in buoyancy compensators. Diatoms store lighter-than-water droplets of oil. Colonial siphonophores, relatives of jellyfish, divide and conquer: certain members of the colony literally support the others by functioning as gas-filled flotation devices, keeping the colony afloat and acting as a sail.

Some zooplankton swim strongly enough to make daily vertical migrations. When the Sun's up, they let themselves sink into dimly lit waters, where they avoid the sharp eyes of predatory fish. Later, under cover of night, they head toward the surface where their own food is concentrated.