Unit Eight. The Living Environment
Freshwater ecosystems (lakes, ponds, rivers, and wetlands) are distinct from both ocean and land ecosystems, and they are very limited in area. Inland lakes cover about 1.8% of the earth’s surface and rivers, streams, and wetlands about 0.4%. All freshwater habitats are strongly connected to land habitats, with marshes and swamps (wetlands) constituting intermediate habitats. In addition, a large amount of organic and inorganic material continually enters bodies of freshwater from communities growing on the land nearby (figure 36.18a). Many kinds of organisms are restricted to freshwater habitats (figure 36.18b,c). When they occur in rivers and streams, they must be able to attach themselves in such a way as to resist or avoid the effects of current or risk being swept away.
Figure 36.18. Freshwater ecosystems.
(a) In this stream in the northern coastal mountains of California, as in all streams, much organic material falls or seeps into the water from communities along the edges. This input is responsible for much of the stream's biological productivity. Organisms such as this speckled darter (b) and this giant waterbug with eggs on its back (c) can only live in freshwater habitats.
Like the ocean, ponds and lakes have three zones in which organisms live (figure 36.19a): a shallow “edge” zone (the littoral zone), an open-water surface zone (the limnetic zone), and a deep-water zone where light does not penetrate (the profundal zone). Also, lakes can be divided into two categories, based on their production of organic material. In oligotrophic lakes (figure 36.19b), organic matter and nutrients are relatively scarce. Such lakes are often deep, and their deep waters are always rich in oxygen. Oligotrophic lakes are highly susceptible to pollution from excess phosphorus from such sources as fertilizer runoff, sewage, and detergents. Eutrophic lakes, on the other hand, have an abundant supply of minerals and organic matter (figure 36.19c). Oxygen is depleted at the lower depths in the summer because of the abundant organic material and high rate at which aerobic decomposers in the lower layer use oxygen. These stagnant waters circulate to the surface in the fall (during the fall overturn, as discussed below) and are then infused with more oxygen.
Figure 36.19. Characteristics of ponds and lakes.
(a) Ponds and lakes can be divided into three zones based on the types of organisms that live in each. A shallow "edge" (littoral) zone lines the periphery of the lake where attached algae and their insect herbivores live. An open-water surface (limnetic) zone lies across the entire lake and is inhabited by floating algae, zooplankton, and fish. A dark, deep-water (profundal) zone overlies the sediments at the bottom of the lake. The profundal zone contains numerous bacteria and wormlike organisms that consume dead debris settling at the bottom of the lake. Lakes can be oligotrophic (b), containing scarce amounts of organic material, or eutrophic (c), containing abundant amounts of organic material.
Thermal stratification, characteristic of the larger lakes in temperate regions, is the process whereby water at a temperature of 4°C (which is when water is most dense) sinks beneath water that is either warmer or cooler. Follow through the changes in a large lake in figure 36.20 beginning in winter 1, where water at 4°C sinks beneath cooler water that freezes at the surface at 0°C. Below the ice, the water remains between 0° and 4°C, and plants and animals survive there. In spring 2, as the ice melts, the surface water is warmed to 4°C and sinks below the cooler water, bringing the cooler water to the top with nutrients from the lake’s lower regions. This process is known as the spring overturn.
In summer 3, warmer water forms a layer over the cooler water that lies below. In the area between these two layers, called the thermocline, temperature changes abruptly. You may have experienced the existence of these layers if you have dived into a pond in temperate regions in the summer. Depending on the climate of the particular area, the warm upper layer may become as much as 20 meters thick during the summer. In autumn 4, its surface temperature drops until it reaches that of the cooler layer underneath—4°C. When this occurs, the upper and lower layers mix—a process called the fall overturn. Therefore, colder waters reach the surfaces of lakes in the spring and fall, bringing up fresh supplies of dissolved nutrients.
Figure 36.20. Spring and fall overturns in freshwater ponds or lakes.
The pattern of stratification in a large pond or lake in temperate regions is upset in the spring and fall overturns. Of the three layers of water shown in midsummer (lower right), the densest water occurs at 4°C. The warmer water at the surface is less dense. The thermocline is the zone of abrupt change in temperature that lies between them. In summer and winter, oxygen concentrations are lower at greater depths, whereas in the spring and fall, they are more similar at all depths.
Key Learning Outcome 36.10. Freshwater ecosystems cover only about 2% of the earth's surface; all are strongly tied to adjacent terrestrial ecosystems. In some, organic materials are common, and in others, scarce. The temperature zones in lakes overturn twice a year, in spring and fall.