ABE - Major Types of Freshwater Regions (Lesson)

Major Types of Freshwater Regions

Typically, if the salt concentration in a body of water is less than 1%, the water is considered fresh. Plants and animals in freshwater regions are adjusted to the low salt content and would not be able to survive in areas of high salt concentration (i.e., ocean). Streams and rivers, ponds and lakes, and wetlands are all different types of freshwater regions. Specific characteristics for each of these regions are explained below.  

Ponds and Lakes

Ponds and lakes range in size from just a few square meters to thousands of square kilometers. Several regions are remnants of the Pleistocene glaciation. Many ponds are seasonal, lasting just a couple of months (such as sessile pools) while lakes may exist for hundreds of years or more. Ponds and lakes may have limited species diversity since they are often isolated from one another and from other water sources like rivers and oceans. Lakes and ponds are divided into three different "zones" which are usually determined by depth and distance from the shoreline.

At the edges of the lake or pond is found the littoral zone. Because it can absorb the sun's heat and it is typically shallow, the warmth provides a habitat for a fairly diverse community, which can include several species of algae (like diatoms), rooted and floating aquatic plants, insects, crustaceans, fishes, and amphibians. Only the egg and larvae stages of insects are found in this zone. The vegetation and animals living in the littoral zone are food for other creatures such as turtles, snakes, and ducks.

The limnetic zone is near-surface open water surrounded by the littoral zone. This zone is well lit (like the littoral zone) and is dominated by zooplankton and phytoplankton. Plankton are small organisms that play a crucial role in the food chain. Without aquatic plankton, there would be few living organisms in the world, and certainly no humans. A variety of freshwater fish also occupies this zone.

Plankton have short life spans—when they die, they fall into the deep-water part of the lake/pond, the benthic zone. This zone is much colder and denser than the other two. Little light penetrates all the way through the euphotic zone into the benthic zone. The fauna are heterotrophs, meaning that they eat dead organisms and use oxygen for cellular respiration.

Temperature variation occurs seasonally. During the summer, the temperature can range from 4° C near the bottom to 22° C at the top. During the winter, the temperature at the bottom can be 4° C while the top is 0° C (ice). A narrow zone called the thermocline is located between the two layers. In this layer, the temperature of the water changes rapidly. Mixing between the top and bottom layers (due to winds) occurs in the spring and fall. This causes a more uniform 4 degrees C. This mixing also circulates oxygen throughout the lake. Of course, there are many lakes and ponds that do not freeze during the winter, thus the top layer would be a little warmer.

Streams and Rivers

These are bodies of flowing water moving in one direction. Streams and rivers can be found everywhere—they get their start at headwaters, which may be springs, snowmelt, or even lakes, and then travel all the way to their mouth, usually another water channel or the ocean.

These streams and rivers are stratified into vertical zones, extending from surface water through groundwater. The salt and nutrient content increases from the headwaters to the mouth. Stream headwaters are usually richer in oxygen. Rivers may also contain substantial oxygen, except where there has been organic enrichment. Headwater streams are often narrow with rocky bottoms. The temperature is cooler at the source than it is at the mouth. The water is also clearer, has higher oxygen levels, and freshwater fish such as trout and heterotrophs can be found there. Towards the middle part of the stream/river, the width increases, as does species diversity—numerous aquatic green plants and algae can be found.

The downstream reaches of the river's area are wide and meandering with silt bottoms. If streams flow through grasslands or deserts then they may be rich in algae and aquatic plants, but if they flow through temperate or tropical forests, then leaves and other organic matters are the primary source of food. In rivers, organic matter consists of dissolved and highly fragmented material.

Lakes

Lakes are standing bodies of water that can range from small ponds to really large lakes. The deeper it gets, the less light there is, which causes stratification. The salt content, oxygen concentration, and nutrient content vary among lakes and can change by season. Oligotrophic lakes are nutrient-poor and are oxygen-rich while eutrophic lakes are nutrient-rich. Oligotrophic lakes have less surface area relative to their depth than eutrophic lakes. The rate of photosynthesis is higher in eutrophic lakes than in oligotrophic. In the littoral zone aquatic plants live, and phytoplankton and cyanobacteria live in the limnetic zone. Also in the limnetic zone, small drifting animals graze on phytoplankton. Invertebrate animals live in the benthic zone. Fish live in all zones of lakes with sufficient oxygen.

Wetlands

Wetlands are an area covered with water for a long enough period to support aquatic plants. The high organic production and decomposition in wetlands cause the water and soil to be periodically low in dissolved oxygen. Basin wetlands develop in shallow basins. River wetlands develop along shallow and periodically flooded banks of rivers and streams. Fringe wetlands occur along the coasts of large lakes and seas. Wetlands are the most productive biomes on Earth. They have water-saturated soils that favor the growth of plants. Wetlands are homes to invertebrates, crustaceans, aquatic insect larvae, dragonflies, otters, alligators, and owls.

 

 

Hurricane Katrina and the Destruction of Wetlands around New Orleans

Click on the title below to watch the video then answer the following questions to check for understanding.

Wetland Destruction Video Links to an external site.

• Explain how wetlands act as a buffer against the ocean's force when a hurricane approaches the coastline.
• Why do some people think it is important to rebuild the marshes that were damaged during Hurricane Katrina?
• Do you think there should be restrictions on building in wetlands? Explain your reasoning.

 

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