The Properties of Lakes
The physical and chemical properties of a lake depend upon many factors. Such conditions include the shape of the basin that contains the lake, the climate of the region, the nature of the water flowing into the lake (for example, whether it is rich or poor in
nutrients), and the age of the lake. Despite these differences, there are generalizations that apply to many lakes.
Temperature has a great impact on the inhabitants of ponds and lakes. Many organisms thrive within a fairly narrow range of temperatures. In temperate ponds and lakes, organisms become dormant, sluggish, or may die when their surrounding water chills to about 33.8°F (1°C) or when water temperature rises above 77°F (25°C). Ponds, being smaller and shallower than lakes, more closely follow the temperature of the air than do lakes. Due to its high specific heat, water warms up and cools down more slowly than the atmosphere does, and any air temperature changes tend to be followed only sluggishly by water in lakes and ponds. Nevertheless, in a hot summer, the temperatures in small, unsheltered ponds-subjected to the baking rays of the Sun-can rise to more than 77°F (25°C), threatening the health and survival of many of the pond's inhabitants. Fish, for example, require moderately high levels of oxygen, and warm water contains comparatively less dissolved oxygen than cool. On hot, sunny days, distressed fish can sometimes be seen gulping air at the pond surface.
In lakes in temperate climates, freshwater "turns over" as it warms and cools with the changing seasons.
During the coldest parts of the winter, lake water freezes at the surface. The water just beneath the ice is chilly (about 34°F or 1°C), while deeper water is typically about 39°F (4°C). Freshwater at 39°F (4°C) is denser than water at lower temperatures, down to about 32°F (0°C), the temperature at which freshwater freezes. As a result, ice and chilly water float on top of slightly warmer water. The lake is stratified, with cold water lying above slightly warmer water with a steep temperature gradient (the thermocline) in between.
The daily duration and intensity of sunlight increases with the arrival of spring, and the lake's surface layer warms and any remaining ice melts. Spring storms help stir the lake's water so that the shallow and deep layers mix. By the middle of spring, most of the lake's water typically lies in the temperature range 39-43°F (about 4-6°C).
As spring gives way to summer, the days become longer, and the Sun is higher in the sky. Water at the lake surface warms as it absorbs increasing amounts of solar energy. By the middle of summer, the lake is once again two-layered, but now with less-dense warm water (temperature 59-68°F, or 15-20°C) floating above cool water (39-46°F, or 4-8°C), and with a thermocline lying between the two. The warm shallow layer is called the epilimnion (from the Greek root epi, meaning "upon," and limne, "lake") while the deeper, cool layer is the hypolimnion (from the Greek hypo, "under").
During the fall months, the air temperature drops and the epilimnion cools. Eventually, the temperature of the epilimnion becomes similar to that of the hypolimnion, and autumnal storms stir the water so

that the two layers mix and break down. As winter begins, the lake's water drops to a fairly uniform temperature, typically within the range 39-46°F (4-8°C).
Seasonal temperature shifts in the water column have a major effect on the distribution, activity, and productivity of organisms. In summer, for example, temperatures and light levels in the surface waters may be favorable for the growth of microscopic plants, but the nutrients they require are trapped in the hypolimnion. This starves the microscopic plants of nutrients, curbing their potential growth. Breakdown of lake stratification in spring and fall serves to circulate nutrients throughout the water column, and many lakes experience an explosion of phytoplankton growth at these times.
Lakes through Time
Many of the small- to medium-size lakes that exist today will probably disappear within the next few hundred or thousand years unless people decide to maintain them. People could do this by constructing dams to keep the water in and dredging the lakes to counter the buildup of sediment. Left to their own devices, most lakes undergo a natural aging process that last hundreds or thousands of years and ends in the death of the lake. The surrounding land eventually claims the lake by filling it with mud, sand, or silt, while land plants invade the edges of the former lake and gradually spread toward its center.
When a lake first forms, it usually fills with clear freshwater that is low in nutrients such as nitrates (a source of nitrogen) and phosphates (providing phosphorus) and contains few dissolved salts. The lack of nutrients limits the mass and variety of plant life that is able to colonize the lake. Likewise, the absence of sediment on the lake bottom means that there are few rooted plants. Most of the early colonizing plants are phytoplankton, microscopic algae that float in the water. Animals, one way or another, depend on plants for their food, so the lack of plants limits the growth of animals. The lake supports a relatively small population of animals, and the fish that grow best are those, such as trout, that feed on animals that fall into the lake or land on its surface (flying insects, for example). A lake at this stage is called oligotrophic (from the Greek oligos, meaning "small" and trophe, "nourishment"). Lakes that form at high altitudes fill with precipitation or with runoff that has had little opportunity to erode particles and dissolve solutes from the surrounding landscape. Such lakes often remain oligotrophic throughout their lives.
In lowland areas, however, over tens or hundreds of years, the water draining into a lake brings with it dissolved nutrients and salts and particles that settle as sediment on the bottom of the lake. The accumulating sediment makes the lake shallower. Freshwater plants take root in the sediment at the edges (margins) of the lake, gaining nutrients from the sediment. The plants grow toward the surface, but most remain submerged. The water is moderately clear, and sunlight penetrates deeply, so rooted plants can readily photosynthesize (make their own food by trapping sunlight energy). A wide range of animals, including fishes, graze the phytoplankton and the rooted plants, keeping the plant growth in check.

Once the lake ages and becomes very rich in nutrients, with its water green for most or all of the year and its bottom thickly covered in sediment, it is said to be eutrophic (derived from the Greek for "well nourished"). Eutrophic lakes are usually defined as those that have a hypolimnion that is depleted of oxygen in summer. This effect is caused by high levels of organic matter that sink from the epilimnion and decay in the hypolimnion, encouraging bacterial growth that strips the water of much of its oxygen.
In aging lowland lakes, submerged freshwater plants such as broad-leaved pond weeds gradually become replaced by those with floating leaves, such as water lilies and water hyacinth. The process of plants colonizing the edges of the lake continues. The plants accumulate sediment around their roots and stems, and when they die back each winter, their rotted remains contribute further to the sediment. Emergent plants-those that grow from the lakebed and through the water surface-speed up the drying out of the lake. Because their leaves release water vapor drawn from the lake bottom, emergent plants vastly increase the surface across which evaporation takes place. Emergents include reeds and arrowhead plants that grow in the edges and shallows of the lake. Eventually, sediment accumulates sufficiently so that land-living plants, such as tussock sedges, grow in the accumulating water-saturated sediment at the edges of the lake, and they replace the freshwater plants. Submerged plants meanwhile colonize areas closer to the center of the lake, later to be succeeded by reeds and other emergent plants. Once emergent plants are growing across most of its area, the lake's days are numbered. Within a matter of decades it will most likely become wetland (soil that is saturated with water for at least part of the year) or even dry land. Most or all of the clear patches of water disappear.
The transition from lake to bog and from bog to firm ground, as a result of encroaching land vegetation, is evident on all continents in regions from temperate to tropical. Geologists find the remains of thousands of former lakes as sediment deposits filling depressions in the underlying rock.
The gradual replacement of one type of biological community by another in a given location, in an orderly series over time, is called an ecological succession. The disappearance of a lake and its replacement by forested dry land over hundreds or thousands of years is a good example of ecological succession.
In forested areas with cool temperate climates, sphagnum mosses are commonly the pioneer colonizers of the edges of lakes. At the lake margins, sphagnum mosses grow upward to the light, depriving lower layers of moss the available sunlight. The light-starved moss beneath dies and creates peat (a brown layer of partially decayed plant matter). Gradually, the accumulating peat turns the edges of the lake into a spongy bog, which rises higher and begins to dry out as the peat layer grows. Rushes colonize the margins of the lake. Over decades, the sphagnum mosses progressively grow toward the center of the lake, and the area of open water shrinks. Shrubs and trees take root in what was once boggy ground.
Eventually, the succession from an aquatic community, through boggy conditions, to dry land penetrates to the center of the former lake. Forest has replaced open water. Similar successions, but involving different types and species of plants, occur in lakes in other climates.

Many factors cause lakes to disappear. Climatic changes can reduce the level of precipitation in a particular region so that lakes and rivers dry up. In limestone regions, prolonged drought starves the ground of water and sinkhole ponds and lakes dry up. Sudden geological events can drain the water from a lake or divert the water flow so that the lake no longer fills. In limestone country, erosion or subsidence can create an outlet that causes a sinkhole lake to drain into an underground cavern within a matter of days or hours. The lake can disappear overnight. An erupting volcano or a rumbling earthquake can alter the contours of the landscape, building a volcanic cone or causing the ground to fracture and sink, so that flowing water takes new paths. Human interference can divert the supply of water that would otherwise fill lakes and rivers, as in the case of the shrinking Aral Sea.
Lakes
Objectives- learn properties of lakes and how they change over a period of time, whether it is throughout the year or over a period of years
Go to the webpage to view images of water layers during the different seasons (lake turnover) and the various layers plants are found at.
1. How did many natural lakes form in the northern part of North America?
2. Describe how many natural lakes form in southern North America?
3. Describe the temperature layers in a lake in the winter and why?
4. Define thermocline.
5. What happens to the temperature layers in a lake in the spring?
6. Describe what happens to the temperature layers in the summer and why?

7. Define epilimnion
8. Define hypolimnion
9. Describe what happens to the temperature layers in a lake in the fall.
10. How does a lake turnover benefit a lake?
11. What do you think will happen to the oxygen levels at the lower levels of a lake in the summer time?
12. Describe the lifecycle of a lake.