Masters of the sea: believe it or not, you couldn't live without plankton. From jellyfish to tiny plants, these drifters power the planet.

You may know Plankton as SpongeBob's TV rival, the grouch who's always scheming to steal a top-secret formula. But beyond the cartoon town of Bikini Bottom, real-life plankton play a different role. First, TV Plankton is a loner: But real plankton are the most abundant life forms in the sea. Second, plankton are far from bossy; instead they go with the flow. "Plankton includes anything that can't swim against the current," explains Paul Joyce, dean of the Sea Education Association.

These lousy swimmers come in all sizes--from jellyfish with 100-foot-long tentacles to critters the size of shrimp. But most are invisible to the naked eye. Some of these tiny animals, plants, and algae (plantlike organisms that make their own food using sunlight, water, and carbon dioxide gas) measure a measly five micrometers (one millionth of a meter)!

So what's the role of plankton on Earth? Unlike TV Plankton's evil bid to take over the world, real-life plankton are caretakers of our planet. For one, plantlike plankton are the base of the ocean food web. That means they're the driving force behind an interconnected system where organisms eat other organisms to obtain the energy they need to survive. Plus, these plankton produce half of the world's oxygen.

But recent studies have scientists concerned. Plankton populations are diminishing. Some scientists blame this bad news on global warming (average increase in the temperature of Earth's atmosphere). Read on to find out why scientists are paying such close attention to plankton.

ITSY-BITSY APPETIZER

What does the largest animal on Earth eat? Supersize portions of supersmall plankton. "It's astounding to see a blue whale gulping huge amounts of water to filter it for plankton," says John McGowan, a marine biologist at Scripps Institution of Oceanography. After the 25 m (80 ft)-long whale fills its mouth with seawater, it spits the water out through a giant filter in the jaw called baleen. This deluxe strainer catches thousands of shrimplike plankton known as krill.

Krill, jellyfish, and other ocean-drifting animals are zooplankton (ZOE-uh-PLANK-ton). "Zooplankton can be delicate and come in a variety of colors," says McGowan. "Some have interesting life histories," he adds (see Nuts and Bolts, p. 11).

SALAD BAR

Zooplankton make an easy meal for marine life. But what do the tiny floating animals eat? They graze on drifting plants and algae known as phytoplankton that bask in sunlight near the ocean surface, or underwater within sunlight's reach. "They are the 'grass of the sea,'" says Joaquim Goes, a researcher at the Bigelow Laboratory for Ocean Sciences. Like land-based plants, phytoplankton thrive if given enough sunlight and nutrients. Only problem? For part of the year, these nutrients are locked at the bottom of the ocean--far from where phytoplankton hang out.

LOW LIFE

After marine life dies, it decays (breaks down) into nutrient-rich matter that sinks to the ocean depths. During hot summer months, these nutrients are trapped in the deep waters. That's because the sun heats the ocean's surface waters but can't reach its deeper waters. That keeps the deep water chilly. And since the warm surface water is less dense (amount of matter in a given volume) than the cold water, it floats above it. That creates a barrier, called a thermocline, between the warm and cold ocean layers. This barrier prevents the nutrients below from rising to the surface.

SEASONAL CYCLE

Being such hopeless swimmers, phytoplankton depend on a seasonal churning of the sea to bring nutrients from the ocean's depths to its surface. Luckily, as the ocean's top layers cool in the wintertime, the thermocline disappears. Then winds and waves can push the deep, nutrient-rich waters upward. This motion is called convective mixing. By spring, ample nutrients and sunlight at the ocean surface send phytoplankton abloom.

Dense patches of phytoplankton become an all-you-can-eat salad for zooplankton and small fish which in turn become meals for other marine life (see diagram, p. 11).

PLANKTON IN PERIL

For the past 50 years, McGowan has studied the California current, a deep-sea flow of water about 1,000 km (600 mi) off California's coast. What he's found is alarming. "There, the abundance of plankton has dropped by about 70 percent in the past 20 years," he says. That's had a ripple effect on the local species that depend on plankton for food: There's been a 70 percent decline in sea birds and a 50 percent drop in fish larvae. What's to blame? Global warming.

When you watch TV or ride a car, you burn fossil fuels (petroleum, coal, and natural gas), which release carbon dioxide (heat-trapping greenhouse gas) into the air. Like land-based plants, phytoplankton use carbon dioxide gas for photosynthesis (process of capturing the sun's energy to turn carbon dioxide and water into food). "But we're putting more carbon dioxide into the atmosphere than the plants in the ocean can take up," explains McGowan.

And when excess greenhouse gases collect in the atmosphere, air temperatures rise. This heats the ocean's surface, creating the thermocline barrier that blocks nutrients from cycling up. Without them, phytoplankton can't survive. "And without plankton, the ocean food web would collapse," warns McGowan.

Even worse? A decline in phytoplankton means poorer air quality. Here's why: After phytoplankton remove carbon dioxide from the atmosphere during photosynthesis, they release oxygen into the air. Believe it or not, phytoplankton are the world's largest oxygen source!

Who would have thought SpongeBob's archenemy could be so important in real life?

DID YOU KNOW?

The term "plankton" comes from the Greek word for "wanderer."

Scientists have found plankton in the clouds! Hurricanes sent them there.

Jellyfish, including the hydromedusan shown here, are plankton.

Most plankton are as small as a red blood cell! Just a teaspoon of seawater can contain millions of plankton.

WEB EXTRA

To learn more about plankton, visit this online exhibit: www.amonline.net.au/ exhibitions/ beyond/ index.htm

NUTS & BOLTS

Once a plankton, always a plankton? No way! Many lead double lives, beginning life as plankton but ending as an animal that moves on its own.

Take lobsters or crabs: They start life as free-floating larvae (immature forms), or zooplankton. Then they grow into crustaceans (animals with external skeletons, segmented bodies, and five or more pairs of legs) that can swim against a current. "About a third of all zooplankton are the larval stages of adult animals that live on the sea floor," says McGowan.

THE OCEAN FOOD WEB

Each yellow arrow leads from food to predator. In general, smaller creatures are food for larger critters. After marine life dies, it decays into nutrients that sink to the sea floor (red squiggles).

DID YOU KNOW?

* The copepod is Earth's most abundant animal. If all of these plankton were divided among the world's human population, each person would get 1 billion copepods!

* Not only is the plankton-feeding blue whale the largest animal on Earth, it's also the loudest. The call of the blue whale has been recorded at levels up to 188 decibels. This is louder than a jet plane, which reaches about 140 decibels! Called "whale songs," the calls have a low frequency (number of vibrations per second) and can travel miles underwater.

CRITICAL THINKING:

* Global warming is causing plankton populations to decrease. Have students think about how their daily activities contribute to increased temperatures on Earth and to plankton decline.

CROSS-CURRICULAR CONNECTIONS:

GEOGRAPHY: Research and then draw a map showing the migratory paths of the plankton-feeding blue whale.

RESOURCES

* Grolier search term: Plankton

* For more plankton information and lesson ideas, visit this Web site presented by NASA and Bigelow Laboratory for Ocean Sciences: www.bigelow.org/foodweb/

DIRECTIONS: On a separate piece of paper, write two essays using the given scenarios and the words provided in the parentheses.

1. You're a blue whale, and mealtime approaches. Describe your search for food.

(baleen, krill, zooplankton, phytoplankton)

2. You play a scientist in a disaster movie, and you've noticed the rapid decline of phytoplankton. Make a speech to warn the public of the potential dangers the decline may cause.

(ocean food web, photosynthesis, carbon dioxide, oxygen)

In "Masters of the Sea" (p. 8), you learned that lobsters start their lives as tiny, free-floating zooplankton. They remain plankton until they grow body parts that help them swim against currents. What happens to a lobster's body during its life journey? Explore the Web sites in "Lobster Tank" (below) to help you fill in the information about a lobster's stages of growth (below).

Answers should contain the following:

1. A blue whales use its baleen, a giant filter in the jaw, to strain for food. It eats mostly krill, a tiny shrimplike zooplankton. Zooplankton are animals that can't swim against the current. These drifting animals eat phytoplankton, or drifting plants and algae.

2. Phytoplankton are the base of the ocean food web, or the driving force behind an interconnected system where organisms eat other organisms to obtain the energy they need to survive. Without phytoplankton, the food web would collapse. Also, phytoplankton remove carbon dioxide, a heat-trapping gas, from the air. Like land plants, they use photosynthesis--the process of capturing the sun's energy to turn carbon dioxide and water into food. The process releases oxygen.

LOBSTER TANK:

* http://octopus.gma.org/lobsters/society.html

* www.enchantedlearning.com/subjects/invertebrates/ crustacean/Lobsterprintout.shtml

1. A newly hatched lobster larva has -- on its legs, which help it swim during the first month of its life. Usually found drifting within the top meter of the ocean surface, this drifter does not look like a lobster yet. It must molt, meaning it --, -- times before taking on a shape that resembles a mini adult.

2. When the youngster is between 2 weeks and 1 month old, its swimming abilities improve. It makes its way to the sea bottom to find a home. An ideal habitat has lots of --. That's because at this life stage, the lobster is tiny and vulnerable to -- like -- and --.

3. Until its first birthday, the lobster stays home, and finds food by using small appendages under the abdomen called -- that pump water into its living space. It hopes to nab small critters carried in by the water. For the following years, the growing crustacean keeps a low profile. For food, it depends mostly on tiny animals that drift down from above. During its first five years of life, the lobster outgrows its shell so often, it molts up to -- times.

4. When a lobster reaches adulthood, about five to seven years, it molts only about -- a year. This bigger and stronger lobster no longer needs to stay close to home. It can roam around to look for food. Three items on its menu: --, --, and --. Still, the lobster needs to be cautious of predators. That's why most of these critters are -- or active at --.

5. The lobster will keep molting and growing throughout its lifetime. Believe it or not, lobsters can live to be -- years old, and the biggest lobster ever caught weighed over -- pounds!

6. Draw a lobster in the space below. Match the following words to the corresponding body part: carapace, antennae, walking legs, swimmerets, tail fan.

Answer

1. feathery hairs; sheds it shell and grows a new one; three

2. hiding spaces; enemies or predators; two of the following: cod, sculpin, eelpout, sea robins, skates, octopi, other lobsters

3. swimmerets; 25

4. once; three of the following: shrimp, sand fleas, sea urchin crabs, flounder clams, worms, snails, mussels, lobsters, plants; nocturnal; night

5. 100; 44

6. See the Enchanted Learning site in "Lobster Tank."