Prevención de riesgos y vigilancia de la salud

Light is a major factor that governs the abundance and distri-bution of organisms in the freshwater world. Aquatic plants trap light energy and through the process of photosynthesis they convert light energy to chemical energy trapped in food molecules. Using light energy, plants combine water with carbon dioxide to make a wide range of complex, carbon-rich substances, including carbohydrates, fats (lipids), and pro-teins. Photosynthesis occurs in those parts of the plant—in complex plants, typically the stem and leaves—that contain the light-trapping green pigment chlorophyll. Dissolved car-bon dioxide is usually plentiful in freshwater and so, of course, is water. So, lack of water and carbon dioxide rarely, if ever, limits a freshwater plant’s ability to photosynthesize.

However, lack of light does. Anything that blocks sunlight penetration in freshwater can limit photosynthesis. In addi-tion, plants need nitrogen- and phosphorus-rich nutrients to manufacture their wide range of carbon-rich products. Scarci-ty of these nutrients limits plants’ abiliScarci-ty to photosynthesize.

As the products of photosynthesis are the ultimate source of food for all freshwater organisms—including animals and microbes—the plant nutrient supply and the penetration of sunlight have a profound effect on the nature and abun-dance of organisms living in a stretch of water.

Among the products of photosynthesis, carbohydrates include glucose (which the plant breaks down to release chemical energy for a wide range of processes), starch (a stored form of glucose), and cellulose (a substance that forms the main component in the walls of plant cells). The fats or lipid products of photosynthesis form valuable stores of chemical energy; they are also vital constituents of the mem-branes that enclose plant cells and are widely distributed within them. Proteins, too, are major constituents of biologi-cal membranes. Biologibiologi-cal catalysts biologi-called enzymes speed up chemical reactions in cells and determine the overall func-tion of individual cells. Most enzymes are made of protein.

Animals, too, need carbohydrates, fats, and proteins, but whereas plants normally make their own, animals have to obtain them ready-made. When an animal eats a plant or another animal, it gains a supply of carbon-rich foods that it digests and then reassembles according to its own needs.

Ultimately, all organisms depend on plants and some forms of bacteria that make their own food from inorganic (not carbon-based) substances. Most of these organisms make their food using light energy, so light governs the availability of food. Light, of course, is also necessary for animals to see.

Water—even clear water—filters out light quite quickly. In clear freshwater, about 99 percent of the light that penetrates the water surface is filtered out by a depth of about 165 feet (50 m). Most lakes and rivers are much shallower than this, so in those with fairly clear water, the sunlight penetrates right to the river or lake bottom and plants can photosynthe-size there. However, many lakes and rivers are far from clear, and substances dissolved in the water, or particles suspended in it, absorb much of the penetrating light. In these circum-stances, most of the sunlight is filtered out within 16 feet (5 m) depth of water, and little or no photosynthesis occurs below this depth.

Water, of course, is much denser than air. At atmospheric pressure and close to water’s freezing point of 32°F (0°C) freshwater is about 700 times denser than air. One result is that water supports the bodies of underwater animals and plants, and they generally need less internal support—such as a skeleton in the case of animals or a system of supporting fibers in plants—than their land-living relatives. On the other hand, because it is dense, water is much harder to move through than air. Animals have to expend considerable energy to swim through water. Their bodies, as in the case of fishes, are usually hydrodynamic (streamlined) to minimize drag (resistance to motion).

Water, like other liquids, becomes less dense (lighter per unit volume) as its temperature rises. This means that water becomes less buoyant—it provides less support—as it warms.

However, water is unusual because it becomes less dense as its temperature nears freezing point. Water at about 39°F (4°C) is denser than water at temperatures below this, down to 32°C (0°C). Consequently, ice floats.

Fortunately for living things, water resists temperature change. It has a high specific heat capacity; specific heat is the quantity of heat required to raise the temperature of a unit mass of substance by one degree. It takes about five times as

much heat energy to raise the temperature of a given mass of water by 1°F (0.55°C) as it does to warm the same mass of dry soil through the same temperature range. This means that the land warms and cools more rapidly than the water in lakes and rivers. Over the course of a day or the duration of a year, the temperature fluctuations in water are much less than those in air or on land. In temperate regions, air tem-peratures can fluctuate by as much as 27°F (15°C) in a single day, while the temperature of a small pool is unlikely to change by more than 5.4°F (3°C). This temperature-buffering effect helps animal and plant life to survive in freshwater throughout the year, from the icy conditions of winter to the baking heat of summer.

All freshwater fishes and invertebrates (animals without backbones) are ectothermic (from the Greek ektos, meaning

“outside,” and therme for “heat”). This means their body tem-peratures are largely determined by their environment.

When the water chills, their bodies cool, and when it warms, their bodies follow suit. This in turn affects the rate at which biological functions take place. As a general rule, for temper-ate freshwtemper-ater plants and ectothermic animals subjected to temperatures between 41°F (5°C) and 68°F (20°C), an 18°F (10°C) rise in temperature doubles the rate of chemical reac-tions within the body. Life processes—such as digestion, res-piration, and movement—are faster at warmer temperatures within this range.

Birds and mammals, however, can regulate their tempera-ture internally, usually keeping their body temperatempera-tures some-where in the region of 100°F (38°C), considerably warmer than their usual surroundings. Birds and mammals are endothermic (from the Greek endon for “within”), and their body temperature alters little over the normal range of water temperatures between 41°F (5°C) and 68°F (20°C).

Water’s temperature also affects its ability to hold dissolved substances. Solids that dissolve in water usually do so more readily at warm temperatures than cool ones. The opposite trend applies to gases that dissolve in water. Oxygen is twice as soluble in water near its freezing point than it is at 86°F (30°C). Oxygen is a constituent of air and vital to most organisms because they need it for respiration, and most

freshwater organisms gain their oxygen by extracting it from the surrounding water. For fishes and invertebrates, high water temperatures can pose a problem. Warm water temper-atures speed up life processes, causing animals to demand more oxygen, but at the same time the water contains less dissolved oxygen. Under such conditions, animals often move to cooler parts of the lake to avoid the oxygen-shortfall problem. If dissolved oxygen becomes scarce, fish may resort to gulping air at the water surface.

Wind exerts a great effect on lakes, particularly large lakes.

Strong winds blowing in the same direction for any length of time generate a series of waves. These stir the water near the surface and help to oxygenate the water. Winds can be cool-ing or warmcool-ing, and the wind enhances the rate at which heat energy is either added to the lake or removed from it.

Winds also pile up water on the downwind side of the lake.

All these wind-driven factors influence the distribution of organisms in the lake.

Winds generate water currents (flows of water). The stronger the wind, and the longer it blows in a given direc-tion, the stronger the surface current it produces. Because water is so much denser than air and so difficult to shift, strong winds produce water currents that flow relatively slowly. When moving water piles up at the downwind side of the lake, it cannot flow back along the surface in the direc-tion it has come, because the water flowing in behind it blocks the way. Instead, it moves either sideways or down-ward. This effect creates currents beneath the surface that flow in the opposite direction to the surface current. These subsur-face currents rarely penetrate deeper than 65 feet (20 m) even in the deepest lakes (an exception is Lake Baikal: see “Lake Baikal,” pages 79–83).

Other types of water currents arise because of water’s ten-dency to rise when warm and sink when cool (except near water’s freezing point). The most obvious effect occurs when cool air chills the lake’s surface water, causing it to sink. This sets up convection currents, with cool water sinking and warm water rising in a circular pattern. Such movements can bring nutrient-rich water close to the lake surface, encourag-ing the growth of phytoplankton (microscopic driftencourag-ing algae).

In rivers, water flow tends to be unidirectional (one-way) and much stronger than in lakes. Flowing water delivers oxy-gen and food, but water that is flowing too strongly will wash animals downstream. Different species of animals are adapt-ed to survive in different speadapt-eds of water flow (see “Adapta-tions for life in running water,” pages 95–96).

Water is heavy. Its density (mass per unit volume) is high—

about 8.3 pounds per U.S. gallon (1 kg/L). Being so dense, a column of water exerts hundreds of times more pressure than an equivalent column of air. The air pressing down on Earth’s surface is several miles thick, and the pressure it exerts is de-fined as 1 atmosphere. A column of water about 33 feet (10 m) high exerts a similar pressure. Descending from the water sur-face, where the pressure is 1 atmosphere, the pressure becomes 2 atmospheres by 33 feet (10 m) beneath the surface and 3 atmospheres by 65 feet (20 m) down.

The pressure inside an aquatic organism is about the same as that in the surrounding water. Most of an organism is liq-uid, and small changes in the depth at which an organism swims pose little problem. However, gases change markedly in volume with changes in pressure. A doubling of pressure will halve the volume of a gas, so the air-filled lungs of a human swimmer at the surface will decrease to half this vol-ume when he or she dives to a depth of 33 feet (10 m). They will return to their original size when she surfaces.

Problems arise when animals living at high-pressure depths rise in the water column. Gases dissolved in the blood expand and tend to bubble out. This is not a problem if the ascent is slow, but if it is rapid, the gas bubbles can block small blood vessels, causing pain and even death. A condi-tion called “the bends,” in which the human body is wracked with pain, causing the diver to bend over in an attempt to relieve it, is produced in this way. The diver breathes pressur-ized air and when he rises in the water column too quickly, dissolved nitrogen bubbles out of the blood, causing recog-nizable symptoms, which, in severe cases, can prove fatal.

The reduction in water pressure during ascent causes other problems. For example, when a fish is raised too quickly from deep water, the air in its swim bladder, a buoyancy control sac, expands and can burst.

Finally, the attraction between water molecules creates sur-face tension that gives water an obvious sursur-face film, almost like a skin, at its boundary with air. Water has the highest sur-face tension of any liquid except the metal mercury. For some creatures, water’s surface film is their habitat (see “On the surface,” pages 96–97).