Freshwater ecosystems are classified according to water flow. Lotic ecosystems are characterized by water that is moving continually from the earth or over its surface. Springs, brooks, streams, and rivers are lotic ecosystems, and their most important physical factor is water flow. Water flow, or current, is influenced by the amount of water in the stream or river, and this varies with rainfall patterns, snow melt, and other seasonal changes. When the water level of a stream or river increases, its current becomes faster and stronger.

Another characteristic is the ecosystem’s oxygen and nutrient content. Lotic ecosystems typically have high oxygen levels because water flowing over rocks, logs, and other objects causes turbulence that oxygenates the water. The more rapid and turbulent the flow, the more oxygen the water carries through the system.

A third characteristic is the production of organic matter. In lotic ecosystems, most of the production comes from surrounding land habitats and is carried into the stream in runoff from rainfall. Fallen leaves from dense, overhanging vegetation can add substantial amounts of organic matter. This matter enters the stream, where it becomes a food source for other organisms. The nutrient content of a lotic ecosystem is largely determined by the terrain and vegetation through which the stream or river flows. Because most lotic ecosystems are relatively shallow and have long shorelines, terrestrial production can usually supply the energy needs of the ecosystem. Fast-flowing streams and rivers do not support large communities of plankton because these microscopic organisms are washed away by the flow of water.

The lecture discusses lentic ecosystems. Summarize the points made in the lecture, and explain how they contrast with specific points made in the reading passage.

Do you agree or disagree with the following statement?

The world will be a better place 100 years from now.

Use specific reasons and details to support your opinion.

Key points:

• The lecture states that lentic ecosystems have standing or stationary water; the water does not flow. This contrasts with the point in the reading that lotic ecosystems have water that is moving continually.

• The lecture states that in lentic ecosystems, the oxygen content varies with the depth of the water. This contrasts with the point in the reading that lotic ecosystems typically have high oxygen levels because water flowing over objects oxygenates the water.

• The lecture states that in lentic ecosystems, a greater share of the production of organic matter occurs in the body of water. This contrasts with the point in the reading that in lotic ecosystems, most of the production of organic matter comes from surrounding land habitats.

Now listen to part of a lecture on the topic you just read about.

Now we’ll turn our attention to another class of freshwater ecosystem: standing bodies of fresh water—such as lakes, ponds, and marshes. These are called lentic ecosystems, and they differ from lotic ecosystems in a number of ways.

First, in lentic ecosystems, the water does not flow as it does in rivers and streams. While the water of lakes is subject to the movement of wind and waves, as well as some current, it is essentially a stationary body of water, not one that flows over the earth from one point to another.

Second, in lentic ecosystems the oxygen content of the water is not the same throughout the entire body of water. The oxygen content varies according to the depth of the water. Oxygen is produced during photosynthesis, which occurs only in the upper levels of a lake, where algae and other microscopic plants live. The deepest parts of a lake are often depleted of oxygen. This is because the depths of a lake are where the decomposition of organic matter occurs, and decomposition actually decreases the oxygen supply.

Third, in contrast to lotic ecosystems, a much greater share of organic production in lentic ecosystems occurs in the body of water itself. Most of the organic matter is produced by algae floating near the surface. Shallow, warm lakes are nutrient–rich and highly productive, while deep, cold lakes are less so. Microscopic plankton thrive in warmer water near the surface. Rooted and floating aquatic plants flourish in the shallow water close to shore.