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SOLAR POWERED

First of two parts

Illustration by DAVID WILLIAMS

From very far away, Earth is a bright, beautiful marble against the black velvet of deep space. The vivid blue of the ocean contrasts with the brilliant white of clouds and polar ice and the browns of the continents.

At that distance, there is no obvious visual signal of an Earth teeming with life, little sign of the presence of plants, or even the largest animals, whales.

Closer up, from low Earth orbit down to the altitudes where jet aircraft fly, the land shows evidence of life. We can distinguish forests, prairies, deserts. But the ocean still gives no strong indication that anything lives there . . . that is unless we start to think about one of its special features.

That feature is color.

Once we look more carefully, we see that the ocean is not all blue. Especially near the continents, different colors appear _ greens and browns, usually. Flying over Tampa Bay, we see waters that are always one or more of these other colors.

Do the colors mean something? Are living organisms somehow responsible? The answer is an emphatic yes. The greens, the browns are there because of the dominant form of life in the sea, which is not fish, whales, clams, lobsters, or any of the "well-known" marine animals. It is phytoplankton, the plants upon which virtually all other marine life depends for existence. In fact . . . if you are only aware of the well-known marine animals that Jacques Cousteau has made famous, you have missed about 99.99 percent of all the biological action in the sea.

Why? Because phytoplankton are extremely small, microscopic in fact, and the reason for their size has to do with how water absorbs light. This fact has enormous implications for how people use the marine ecosystems.

The sun is the key

An ecosystem is a collection of interacting organisms that is defined by the way energy flows through it. Energy underlies everything. For both land and ocean, the flow begins with the sun. Land plants and marine plants capture some of the solar energy striking the Earth and, through photosynthesis, use it to convert water and carbon dioxide into plant tissue. Chlorophyll molecules regulate the process, and oxygen gas is released to the air or to sea water as a byproduct. This oxygen helps to maintain Earth's atmosphere.

However, as the recent California brush fires dramatically show, plant tissue is not stable when oxygen is present. It burns, and the combustion can release large amounts of energy as heat. On land this energy release is usually much slower than in a forest or brush fire . . . fortunately! It also occurs slowly in the ocean as plant matter reacts with oxygen dissolved in seawater. Scientists explain all this by saying that the sun's energy is "stored" in plants and "flows" from the sun into plants and back out into the environment when reactions with oxygen occur.

This flow does not always go directly back into the environment. Animals called herbivores have evolved to take advantage of the situation by eating the plants (and the "stored" energy in plant tissue) and then forcing the combustion that is going to occur anyway to take place inside themselves in a slow, controlled process called respiration. They use the energy that is released to power their movements and their metabolism. All of their food (and its stored energy) is not burned up, however; a tiny fraction is broken into smaller molecules which recombine into new tissue needed for the growth, replacement and repair of existing tissues. Both land and ocean have herbivores, but the best known are grazers on land like deer, rabbits, moose, zebras, grasshoppers.

Other animals have evolved to obtain stored energy by eating the flesh of herbivores. Here again, most of the ingested tissue is burned to release its stored energy and a tiny fraction is turned into new tissue which still has stored energy. These animals are carnivores (meat eaters), and are represented on land by well-known "fierce" predators like the tiger or the hawk as well as lesser-known ones like the shrew. In this case, stored solar energy is said to "flow" from plants to animals to other animals.

Animals with a mixed diet are omnivores, a group that we humans belong to, along with other land animals like bears, raccoons, some birds, rats, etc. Omnivores get part of their energy one step closer to the sun than straight carnivores, who don't eat plants.

The combination of all the plants, herbivores, carnivores and omnivores in a given region of the land or the ocean is the food web of that region, and the base of the food web is plant matter. Food webs are usually complicated; the sun's energy can "flow" via many pathways _ plants to herbivores, plants to omnivores, plants to herbivores to carnivores, plants to herbivores to omnivores to carnivores, etc. The functioning of this complex flow of stored energy makes up the ecosystem of the region.

Energy is "lost' along the way

A key feature of all ecosystems is the way energy is "lost" as solar energy flows through. A woman who is 80 years old has eaten tons of food during her life and yet never weighed more than, say, 150 pounds. What happened to all that food? Nearly all of it got burned up and turned into carbon dioxide, water and waste products which were released back into the environment. This was the only way she could get enough energy to stay alive. So, at any time during her life, the pounds of available food had to have been much greater than her weight _ roughly 10 times greater in fact _ since most of it was going to be destroyed for its energy. This is true for all humanity; if we are to survive, the weight of human food on Earth has to be roughly 10 times the combined weight of the people on Earth at all times.

The same general idea applies to all of the animals on Earth. The weight of Earth's plants is roughly 10 times the weight of the animals that eat them.

The weight of these herbivores is, in turn, roughly 10 times the weight of the carnivores that eat them. So as we go along food chains up the food web from plants to grazers to meat eaters, there is a drop off in biomass of about 90 percent at each step. The greater the number of steps, the greater the drop off _ 10 pounds of plants for 1 pound of grazers for a tenth of a pound of carnivores.

These ideas are critical to understanding how the interaction between light and water affects the ocean ecosystem (and makes it differ) from the land ecosystem.

Water absorbs sunlight fast

Picture in your mind the marvelous photographs of Earth looking back from an Apollo moon mission. The light rays that entered Apollo's camera had a roundabout journey. They came from the sun, struck the Earth and then were reflected back to the camera. They had to pass through the atmosphere twice _ about 70 miles since the atmosphere is 35 miles thick _ and there was still plenty of light to see Earth's surface features clearly.

Water is not that way. Even though a glass of pure water seems as transparent as air, water is an excellent absorber of light. By the time sunlight has penetrated only { inch, 30 percent of its energy is gone. At a depth of 30 feet, 80 percent of its energy has been lost. At 400 feet below even the clearest ocean surface, only about 1 percent of the sunlight is left. Since the ocean is 2.4 miles deep on average, the vast majority of the ocean is perpetually dark.

The only place where plants can live in the ocean is its upper sunlit "skin." If they sink down into the darkness, they die. They face a big problem because all surface ocean water eventually sinks. On land, plants attach to the ground and lift their leaves toward the sun on trunks or stems. Some marine plants (such as the sea grasses of Tampa Bay or the kelp beds off California) do something similar in waters where sunlight reaches the sea bed. But this occurs only in very shallow water along the fringes of the continents; it is useless in most of the ocean. Just imagine a sea grass leaf two miles tall!

The density of water provides some help. At sea level, a cubic foot of air weighs 1.2 ounces, but a cubic foot of water weighs 62 pounds. Water is 800 times denser than air, which causes it to exert a much larger upward buoyant force than air. Thus plants can float in water, as do many living things.

But that's not enough. Plants get waterlogged and sink. So in order to thrive throughout the entire ocean, plants evolved a very effective strategy: each one is a single cell. An entire plant with enough chlorophyll to reproduce itself is contained within a space only a few thousandths of an inch across. The arrangement maximizes the surface area of each plant, which in turn increases the resistance of the water to its sinking! The cells create spines on their outer surfaces to prevent sinking even further.

This single-celled feature is the principal reason the marine ecosystem is different: the plants at the base of the marine food web are extremely small. They are carried along by surface ocean currents and, as mentioned, are called phytoplankton (in Greek, "phyton" means plant, and "planktos" means drifter). Far out to sea, phytoplankton are widely dispersed, and the water appears a deep blue. In Tampa Bay and other near-shore waters where they are in high abundance, their chlorophyll makes the water a brownish-green. Phytoplankton are the principal cause of the color variations that indicate life in the sea.

Phytoplankton are small in size only

At 70 percent of the Earth's surface, the ocean is so large that marine phytoplankton are among the most important plants on Earth, ranking along with prairie grasses, pine trees or the Amazon rain forest. They have names such as diatom or dinoflagellate. Not many people know what a diatom is . . . or have ever seen one. A powerful microscope is required. Without it, we can only see the colors they generate. Here in Florida, a dinoflagellate called Gymnodinium causes poisonous red tides. Phytoplankton account for at least one-third of the photosynthesis on the surface of the Earth and provide the ocean's contribution to the maintenance of the oxygen in the atmosphere.

What are the animals that graze on phytoplankton, the "cattle of the sea," like? They have to be small, too (millimeter size, usually), in order to be able to grab and eat the tiny plant cells. They drift with the currents and are called zooplankton. In fact, most of the grazing "herds" of the ocean _ equivalent to deer, antelope, wildebeests, zebras, etc. on land _ are so small that a microscope is needed to identify them. (A well-known exception are Antarctic krill, which grow to about 2 inches.) Usually they have names like copepods and ostracods, which are derived from their scientific names. Most have no common names like "moose," "buffalo," or "krill" because non-scientists by and large don't even know they exist and have therefore never thought up common names for them. Yet, taken together, zooplankton are the principal herbivores over 70 percent of the Earth's surface.

How about the principal meat eaters or carnivores that prey on the principal herbivores in the ocean? They're also small, at least for part of their life cycles. Most stay small throughout their lives, such as the rather strange-looking worm, chaetognath, or inch-sized fish such as lantern fish or hatchet fish. Many are zooplankton that look like insects under a microscope. A few are the tiny larvae of much larger fish or shellfish, like tuna and snook or clams and lobsters, that live among and eat the copepods and other microscopic animals for a while shortly after hatching. Later they grow much larger and eat bigger animals or pieces of dead debris.

When they do, some reach a size that human beings can see (and catch), and therein lies a potentially huge problem about us getting food from the sea: overfishing. There is a serious risk that we will eventually kill and eat so many food fish that their populations will never recover.

Why? Two main reasons: marine food chains are too long, and we mostly get seafood by "hunting."

The best known marine animals that we catch and eat _ tarpon, grouper, sheepshead, redfish, sharks, and so on _ are almost all large adult carnivores who only get a share of the sun's energy after it has passed through a long series of episodes, or steps, in which a larger organism eats a smaller one. They are at or near the top level of the marine food web, usually with five to 10 feeding steps along the food chains between them and the tiny, single-celled phytoplankton that are the base of the web.

So, in order for a local Tampa Bay fisherman to be able to catch a 200-pound shark, the ocean in our region must contain 2,000 pounds of fish like kingfish, 20,000 pounds of fish like herring or anchovies, 200,000 pounds of microscopic zooplankton and 2-million pounds of phytoplankton. And the ocean must maintain and replace all those pounds of marine organisms so that there will be another 200-pound shark to be caught, or else the fisherman will not be successful next time out. It would be much easier for the ocean to provide 200 pounds of microscopic zooplankton; only 2,000 pounds of phytoplankton would be required instead of the 2-million required to sustain a shark. But the fisherman has no interest in zooplankton and doesn't know how to catch them.

Contrast this situation with the land. There, only two feeding steps are required for us to obtain food: plants and grazers. Food chains are very short. Two-million pounds of plants can support 200,000 pounds of cattle on land, for example, compared to only 200 pounds of shark or 2,000 pounds of kingfish in the sea. In terms of delivering food stuffs to people, the land is one-hundred to one-thousand times more efficient. And people can even partially overcome the loss of biomass from land plants to grazers by eating parts of the plants directly _ something we cannot easily do to phytoplankton.

The land has another advantage as a food source compared to the sea. We have learned to farm the land. We can maximize the yield of food plants growing on a given plot of ground by fertilization and weed or pest control. In the ocean, we still act as our Cro-Magnon ancestors did on the land 30,000 years ago; we simply hunt what nature supplies . . . and there are so many of us and we've gotten so good at hunting in the ocean that yields of food fish are shrinking. As recent examples, the World Wildlife Fund reported serious losses of Atlantic bluefin tuna, and the U.S. restricted the fishing of Atlantic haddock. Essentially, we have not learned to farm the sea. Until we do, our chances of overcoming the disadvantages of the long marine food chains will be slight indeed.

What then can we learn from this big "jump" from ocean color to seafood? It's all part of the story of life in the sea and human interaction with the ocean. Because of the physics of light in seawater, ocean colors in a sense symbolize the richness of marine life and its differences from life on land. The remarkably strong absorption of light has a major effect on the ocean ecosystem and places critical limits on our ability to use its living resources. We must understand and abide by these limits so that the ocean which dominates our planet's surface may remain both vital and delightful.

The color of life

Let's see what happens to the enormous amount of energy absorbed, used and lost as energy makes its way through Tampa Bay _ fueling life and giving color to the waters.

1 This story begins millions of miles away where the sun gives off energy that travels through space, makes its way through the atmosphere and warms the waters of all the Earth's oceans.

2 Most of the sun's energy is simply absorbed by water, but a small amount is trapped and absorbed by the billions of single-cell plants that make up most of life in the sea. These single-cell organisms are known as phytoplankton. They are the most numerous and most important plants on Earth.

3 The next step in the food web is zooplankton. The grazers of the sea, these tiny creatures are the principal herbivores. They feed on the single-cell phytoplankton.

4 The carnivores of the sea start out small too because their food, the zooplankton, is small. As they continue to grow, these carnivores eat larger organisms until, after many steps in a food chain, we start seeing fish we might catch on hooks or in nets.

5 As the carnivores grow, so do their enemies _ in size, but not in number. Grouper eat fish and sharks eat grouper, each passing on a fraction of the sun's energy gathered by billions of phytoplankton, zooplankton and carnivores along the way.

6 By the time the grouper makes its way to our dinner table, the sun's energy has passed through many levels of the food web. Between each level, most of the energy is lost so it took literally billions of sea creatures and plants to help that grouper to your sandwich.

FAST FACT

Sunlight is energy. In fact the energy in the sunlight falling on Tampa Bay every year equals 15 100-megaton hydrogen bombs.

FAST FACT

A typical quart of Tampa Bay water contains 960,000 individual plants called phytoplankton.

FAST FACT

A 200-pound shark requires at least 140 square miles of Tampa Bay water or 470 square miles of Florida coastal water to supply enough food to survive.

FAST FACT

Because whales are at the top of the food web, the total mass of whales is extremely small compared to plankton. An ocean without whales could survive. An ocean without plankton would be doomed.

The unseen world

From a distance sea water may look lifeless, but through a microscope a world of small, strange creatures live their lives and give sea water its varying colors.

Grass of the sea

A diatom, a common phytoplankton you might find in Tampa Bay water. It is a microscopic single-cell plant that must float near the water's surface where light is available for photosynthesis.

Cattle of the sea

A copepod, one of many herbivores that graze on phytoplankton. Copepods produce oil to help them float and stay at the surface among the phytoplankton.

Sea-wolves

The arrow worm is a carnivore commonly found in sea water. This strange-looking creature stays small throughout its life but other carnivores start small and grow large.

ABOUT THE SERIES:

Lessons from the Sea is a collaborative project between the University of South Florida's Marine Science Department, the Florida Institute of Oceanography, the Times' Discovery section and Newspaper In Education programs, as well as dozens of school science teachers who have been striving to improve science education in local schools. Special thanks go to Carmen Kelley, science teacher at Dixie Hollins High School, Dana Wetzel, who teaches at Estero High School in Fort Myers, and professor Kent Fanning, all of whom volunteered their efforts. The contents of the teaching packet distributed to schools in conjunction with this project was supplied in part by professor Gregg Brooks of Eckerd College and the many teachers who contributed to his book Coastal Geology and Water Resoures of Florida. _ Chris Lavin, Discovery Editor

ABOUT THE WRITER:

Kent A. Fanning, 52, is professor in the Marine Science department of the University of South Florida. He was born in Oklahoma, grew up in Denver and graduated from the Colorado School of Mines. He received a doctorate in oceanography from the University of Rhode Island and joined USF in 1973. He is a father of two and lives with his wife, Jane, in St. Petersburg.

ABOUT THE ARTIST:

David Williams, 33, has been a news artist for the Times for seven years. Previously he worked for the Lexington (Ky.) Herald-Leader. He is a graduate of Eastern Kentucky University. He is a native of Ashland, Ky. He lives in St. Petersburg with his wife, Charlotte, who is assistant city editor for the Times' North Pinellas editions.

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