Abiotic Factors in Lakes

Transcription

1 4.1 Abiotic Factors in Lakes There is considerable variation among aquatic ecosystems. Swamps, marshes, ponds, lakes, rivers, and marine ecosystems all have distinct abiotic factors that require specific adaptations by their communities. That diversity is also evident within each of the ecosystems. From the surface a lake may appear to be pretty much the same everywhere, but be the surface the amount of light available, the water temperature, and oxygen levels can all vary. Not surprisingly, the organisms you can find in each area also differ greatly. The Structure of Lakes and Ponds Figure 1 shows a cross section of a typical lake. The littoral zone is the area extending out from the lakeshore to the point where plants rooted in the bottom of the lake can no longer be found. Aquatic plants that grow to the surface, such as bulrushes and water lilies, take hold where the littoral zone is shal. In slightly deeper areas, plants that are rooted to the bottom but completely submerged may thrive. Beyond the littoral zone is the limnetic zone, the area of the open lake where there is enough light for photosynthesis to occur. The most common form of organism within the limnetic zone is called plankton. The word plankton is used to describe both autotrophic and heterotrophic littoral Figure 1 A cross section showing the three main zones of a lake. Note that the depth of the boundary between the limnetic zone and the profundal zone varies in each lake. Living things such as algae and undissolved solids can both block light. microorganisms. Heterotrophic plankton (invertebrate animals) feed on the autotrophic plankton (tiny plants and algae). Both kinds of plankton are food for consumers in the er trophic levels, such as fish, tadpoles, and birds. The region beneath the limnetic zone, where there is not enough light for photosynthesis to occur, is called the profundal zone. (This zone is not found in ponds.) In most lakes, the only source of nutrients in the profundal zone is the rain of dead plants and animals that falls from the limnetic zone. This detritus is sly broken down by bacteria or consumed by other bottom-dwelling invertebrates and fish, called detritus feeders. limnetic Limit of effective light penetration profundal 126 Chapter 4

2 The decay of this falling organic matter has important consequences for the ecosystem. Bacteria use oxygen to decompose detritus, reducing the amount of oxygen available in the water. In the absence of sunlight and plants to replenish the oxygen, oxygen levels could be reduced to very levels. The only larger organisms that survive are those that can tolerate oxygen levels; they include some invertebrates, and a very few fish species such as carp. water oxygen temperature levels water depth 0 loon trout catfish (a) An oligotrophic lake. Deeper lakes tend to be cooler. Cold water can hold more dissolved gases (including oxygen) than warm water. Changes in Lake Ecosystems There are two kinds of lake. Oligotrophic lakes are typically deep and cold. Lake Baikal in Russia (which is very deep more than 1600 m at the deepest point and holds more water than any other lake), and Lake Superior (much of which is over 200 m deep) are prime examples. Nutrient levels are in such lakes, limiting the size of producer populations. Because there are limited numbers of only a few kinds of organisms, the water is usually very clear. Eutrophic lakes are generally shal and warmer, and have an excellent supply of nutrients. Many species of photosynthetic organisms find these abiotic conditions very favourable. As a result, the water of eutrophic lakes is often murky. In general, oligotrophic lakes gradually become eutrophic. Eutrophic lakes become increasingly shal, eventually filling in and becoming dry land. This evolution from oligotrophic, to eutrophic, to land is called eutrophication and may take hundreds or even thousands of years. Figure 2 shows the eutrophication of a lake. Humans sometimes accelerate eutrophication by adding to lakes nutrient-rich substances such as human wastes, fertilizers in the runoff from agricultural land, and other household and industrial products. water oxygen temperature levels water depth 0 duck turtle catfish frog (b) Soil sediment and organic material falling to the bottom of the lake gradually make the lake shaler. As it becomes shaler, its profundal zone sly disappears, until eventually sunlight can reach the lakebed. Lake temperatures rise and oxygen levels drop. Organisms that require er levels of oxygen begin to disappear. water oxygen temperature levels water depth 0 heron frog racoon (c) The lake continues to become shaler and warmer. Plants can grow at all levels, and the warmth encourages the growth of plankton. When the plants die, decomposers return their nutrients to the lake, using oxygen in the process. Later in the eutrophication process, the lake will become a marsh and then dry land. Figure 2 The eutrophication of a lake, as seen through changes in the community (summer conditions) Sustaining Aquatic Ecosystems 127

3 Seasonal Variations in Canadian Lakes The unique property of water plays a crucial role in Canadian lakes. As water cools, just like other substances, it becomes more dense. However, as water cools be 4 C a strange thing happens it starts becoming less dense (Figure 3). This is why ice floats, forming a layer on top of cold water, and why the est layer of water in a lake or in the ocean often has a temperature of 4 C. Seasonal variations in a lake are shown in Figure 4. Winter During the winter, many of our lakes are covered by ice and snow. This prevents atmospheric oxygen from dissolving in the water. Under the ice the water is arranged in layers, according to its density. The least dense water, at or slightly above 0 C, is near the surface. The densest water, at 4 C, is found at the bottom. No matter how cold the air becomes above the ice, this structure remains the same, although the ice will get thicker if the air remains cold. If the ice is wind-bn and transparent, light can penetrate into the water, supporting photosynthesis in the liquid water be. However, if the ice freezes to a greater thickness than normal, or if the ice is covered in thick snow, light can no longer penetrate, and the organisms under the ice are in trouble. The level of dissolved oxygen in the water may drop until it is not enough to support some organisms. Because fish are particularly sensitive to dissolved oxygen concentrations, the result could be a massive die-off of some fish species. In shal lakes, particularly in the Arctic, ice may form right to the bottom, virtually eliminating most life forms every winter. Spring Spring brings storms and the melting of the ice. Oxygen can now pass from the air into the water. Wind stirs the water, creating waves that increase the surface area and so the rate at which oxygen can dissolve. As the cold surface water warms, it eventually reaches a temperature of 4 C, at which point it begins to sly sink through the less dense water beneath it, carrying its precious supply of oxygen with it. The mixing process that results is called the spring turnover. Summer As surface water warms above 4 C, it will no longer sink, because it is less dense than the cooler water be. Just as in winter, layers of water are set up, with the densest water at the bottom. If you swim in a lake during summer you can experience these layers. By aling your feet to descend sly through the water, you will encounter colder regions. Density (g/ml) Effect of Temperature on the Density of Water Temperature ( C) Figure 3 Unlike most substances, water becomes less dense as it cools be 4 C. 128 Chapter 4

4 The upper level of a lake, which warms up in summer, is called the epilimnion. The er level, which remains at a temperature, is called the hypolimnion. Between these two levels is the thermocline, a narrow zone in which the temperature drops rapidly from warm to cold. Because the epilimnion and hypolimnion do not mix, there is little movement of oxygen from the surface to the depths during summer. Organisms living in the hypolimnion must rely on oxygen reserves brought down during the spring turnover. The epilimnion has a different oxygen problem. The ability of water to hold dissolved gases is inversely proportional to the temperature of the water: the warmer the water, the less dissolved oxygen it can hold (Figure 5). During a hot spell, a lake that is fairly shal may lose so much oxygen that some species, such as lake trout, will die. Figure 4 Temperature and the variable density of water both play important roles in seasonal changes in Canadian lakes. Wind direction Wind direction Spring turnover Summer epilimnion thermocline Fall As temperatures begin to drop in the fall, surface water begins to cool. Once again, as the surface water reaches a temperature of 4 C, it sinks down through the lake. This fall turnover renews oxygen levels at er levels, and breaks up the summer thermal layers. Wind direction hypolimnion Fall Solubility of oxygen mg/l Solubility of Oxygen in Water Temperature ( C) Figure 5 Solubility of oxygen in water turnover Winter ice epilimnion thermocline hypolimnion Sustaining Aquatic Ecosystems 129

5 Individual Variations in Lakes Every lake has its own special conditions (Table 1). These conditions determine which species can live in them. For example, each species of fish has a particular level of tolerance for oxygen. Trout, which require levels of dissolved oxygen, can be found at all levels of a lake in spring and fall, but in summer are confined to regions where more oxygen is available. Unlike trout, perch can can live in shal lakes that become warm in summer, because they can tolerate er levels of dissolved oxygen. The most productive part of a lake is the littoral zone, the area where algae and plants take advantage of the sunlight to carry out photosynthesis. The size of the littoral zone is determined by the depth of a lake and the slope of its lakebed, both of which are individual to each lake. That said, there are some general similarities among lakes within geographic regions. In Central Canada, extending through Quebec and Ontario into Nunavut and the Northwest Territories, most lakes were formed when glaciers gouged out basins in the granite bedrock of the Canadian Shield. Central Canadian lakes are often deep. Because minerals in granite do not dissolve well, these lakes also tend to be in nutrients. As a result, most are oligotrophic. Lakes in the Atlantic region are also mostly glacial in origin, but the bedrock in this region is more varied, as it originated from rock layers in ancient mountains. As a result, some lakes have more nutrients available than others, and the lakes may be oligotrophic or eutrophic. Lakes in the Prairie provinces were also formed by glaciers, but mostly in thick layers of gravel and sand. This varied base is richer in soluble nutrients, making the ecosystems of these lakes more productive. The lakes tend to be fairly shal and collect sediments more rapidly than the shield lakes. They are usually classified as eutrophic. Arctic lakes and ponds have conditions that distinguish them from the lake ecosystems of the south. Because of the temperatures at the peak of summer, many of the Arctic lakes experience only one turnover. Some lakes may not even thaw. Our largest lakes run in a curve from Great Bear Lake in the north to Lake Ontario in the southeast. They lie on the boundary between the glacial deposit to the south and west and the Central Canadian lakes to the north and east. They are considered to be oligotrophic, due to their depth and water temperatures. The major exception is Lake Erie, which is fairly shal. Agricultural and industrial pollutants Table 1 Dissolved Oxygen in Two Lakes (Summer) Dissolved oxygen (mg/l) Region Depth (m) Oligotrophic Eutrophic epilimnion epilimnion thermocline hypolimnion hypolimnion hypolimnion Try This Activity Did You Know? The word rival comes from the Latin word rivus (stream). The first rivals were people who lived along the same stream and competed for the same water. Undissolved Solids Rain carries soil and other solids into surface water where they can remain suspended, creating turbidity (cloudiness or murkiness) that limits the penetration of sunlight and reduces photosynthesis. Solids that are deposited as sediment on the bottom of the body of water can also affect ecosystems. Large volumes can bury bottom-feeding animals and the eggs of fish. Large amounts of falling sediment also make life generally unpleasant for filter feeders such as oysters. (a) Using filter paper and other materials, design a procedure (including safety precautions) to determine how much undissolved solid material there is in water samples from three different sources. Have your materials list and procedure approved by your teacher before starting. (Shake each of the samples before each test.) (b) Present your results (including your data) in a written report. 130 Chapter 4

6 plus human wastes have accelerated the eutrophication of Lake Erie, resulting in striking changes in the communities that inhabit the lake. Joint action by Canada and the United States appears to have sed this process considerably. Understanding Concepts 1. In your own words, define the terms oligotrophic and eutrophic. 2. Explain why you would expect to find different organisms in the limnetic, littoral, and profundal zones of a lake. In your answer, refer to the abiotic factors in each zone. 3. Explain why a shal lake tends to be warmer than a deep lake in summer. 4. In your own words, describe the changes that happen in a lake from summer to winter. 5. Describe what you would expect to happen to oxygen levels in the hypolimnion of a lake over the summer months. 6. Cold water holds more dissolved gas than warm water. Cold water also tends to collect in the er levels of a lake. However, in summer oxygen levels in a lake can be est in the warm surface water of a lake. Explain why. 7. Predict what would happen to a lake that experienced no seasonal changes in temperature. Make a diagram showing the temperature and oxygen levels in the water of the lake after many years of little change in surface air temperatures. 8. Use Figure 2 to answer the foling questions. (a) What happens to the depth of a lake over time? (b) Explain how the change in mean water temperature is related to the depth of the lake. (c) How would the change in lake temperatures affect the types and numbers of plants found in the area? (d) Describe changes in the species and populations of fish you would expect to find in a lake that progresses through the three stages of eutrophication. (e) Explain why turtles might be found in the second stage, but not the first. (f) Speculate about why loons are replaced by ducks. 9. Using the terms you ve learned in this section, describe a local lake or pond. Making Connections 10. A good fisher knows where to find fish. Catfish are less active than trout. In the summer months, which of these fishes would you expect to find in the hypolimnion and the epilimnion? Give your reasons. 11. Make diagrams showing how building a road along the shore of a lake would affect lake ecosystems. What measures could the road-builders take to reduce damage to organisms in the lake? 12. A factory is built along the shore of a lake, resulting in a gradual reduction of the number of algae in the lake. Years later, visitors notice that the lake is clearer than it used to be. Does this mean that the lake is less polluted than it once was? Explain your answer. Exploring 13. (a) Using soda pop, beakers, water, and other materials you choose, design a demonstration (including safety precautions) that shows the relationship between water temperature and the amount of dissolved gas. Have your teacher approve your materials list and design before you begin. (b) How did you measure the amount of dissolved gas in the soda pop? (c) Present your data in a graph and interpret your findings. Work the Web Various groups across Canada are working to reduce the effects of pollution on water systems. Choose one such group and report on their progress in a poster. To research antipollution groups, visit and fol the links from Science 10, 4.1. Challenge 2 Before you can assess the quality of water, you should ask three important questions: What will the water be used for? What are possible sources of pollutants? What can be measured? What would measures of the level of dissolved oxygen and undissolved solids indicate about water quality? Sustaining Aquatic Ecosystems 131