Freshwater Biomes - Advanced
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
Say Thanks to the Authors
Click http://www.ck12.org/saythanks
(No sign in required)
To access a customizable version of this book, as well as other
interactive content, visit www.ck12.org
CK-12 Foundation is a non-profit organization with a mission to
reduce the cost of textbook materials for the K-12 market both in
the U.S. and worldwide. Using an open-source, collaborative, and
web-based compilation model, CK-12 pioneers and promotes the
creation and distribution of high-quality, adaptive online textbooks
that can be mixed, modified and printed (i.e., the FlexBook®
textbooks).
Copyright © 2016 CK-12 Foundation, www.ck12.org
The names “CK-12” and “CK12” and associated logos and the
terms “FlexBook®” and “FlexBook Platform®” (collectively
“CK-12 Marks”) are trademarks and service marks of CK-12
Foundation and are protected by federal, state, and international
laws.
Any form of reproduction of this book in any format or medium,
in whole or in sections must include the referral attribution link
http://www.ck12.org/saythanks (placed in a visible location) in
addition to the following terms.
Except as otherwise noted, all CK-12 Content (including CK-12
Curriculum Material) is made available to Users in accordance
with the Creative Commons Attribution-Non-Commercial 3.0
Unported (CC BY-NC 3.0) License (http://creativecommons.org/
licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated
herein by this reference.
Complete terms can be found at http://www.ck12.org/about/
terms-of-use.
Printed: January 4, 2016
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Freshwater Biomes - Advanced
1
Freshwater Biomes Advanced
• Name types of freshwater biomes, and describe how they differ from one another.
What may be the most biologically diverse type of ecosystem?
These are wetland marshes in eastern Finland. Notice the abundance of vegetation mixed with the water. And
of course, where there are plants, there are animals. Wetlands are considered the most biologically diverse of all
ecosystems. Plant life found in wetlands includes mangrove, water lilies, cattails, black spruce, cypress, and many
others. Animal life includes many different amphibians, reptiles, birds, insects, and mammals.
Freshwater Biomes
Freshwater biomes occur in water that contains little or no salt. Freshwater biomes include standing water and
running water biomes.
Standing Freshwater Biomes
Standing freshwater biomes include ponds and lakes. Ponds are generally smaller than lakes and shallow enough for
sunlight to reach all the way to the bottom. In lakes, at least some of the water is too deep for sunlight to penetrate.
As a result, like the ocean, lakes can be divided into zones based on availability of sunlight for producers.
• The littoral zone is the water closest to shore. The water in the littoral zone is generally shallow enough for
sunlight to penetrate, allowing photosynthesis. Producers in this zone include both phytoplankton and plants
that float in the water. They provide food, oxygen, and habitat to other aquatic organisms. The littoral zone
generally has high productivity and high biodiversity.
1
www.ck12.org
• The limnetic zone is the top layer of lake water away from shore. This zone covers much of the lake’s surface,
but it is only as deep as sunlight can penetrate. This is a maximum of 200 meters. If the water is muddy or
cloudy, sunlight cannot penetrate as deeply. Photosynthesis occurs in this zone, and the primary producers are
phytoplankton, which float suspended in the water. Zooplankton and nekton are also found in this zone. The
limnetic zone is generally lower in productivity and biodiversity than the littoral zone.
• The profundal zone is the deep water near the bottom of a lake where no sunlight penetrates. Photosynthesis
cannot take place, so there are no producers in this zone. Consumers eat food that drifts down from above,
or they eat other organisms in the profundal zone. Decomposers break down dead organisms that drift down
through the water. This zone has low biodiversity.
• The benthic zone is the bottom of a lake. Near the shore, where water is shallow, the bottom of the lake
receives sunlight, and plants can grow in sediments there. Organisms such as crayfish, snails, and insects
also live in and around the plants near shore. The plants provide shelter from predatory fish as well as food
and oxygen. In deeper water, where the bottom of the lake is completely dark, there are no producers. Most
organisms that live here are decomposers.
The surface water of a lake is heated by sunlight and becomes warmer than water near the bottom. Because warm
water is less dense that cold water, it remains on the surface. When dead organisms sink to the bottom of a lake, they
are broken down by decomposers that release the nutrients from the dead organism. As a result, nutrients accumulate
at the lake’s bottom. In spring and fall in temperate climates, the surface water of a lake reaches the same temperature
as the deeper water. This gives the different water layers the same density, allowing them to intermix. This process,
called turnover, brings nutrients from the bottom of the lake to the surface, where producers can use them.
Lakes can be categorized on the basis of their overall nutrient levels, as shown in Table below. Oligotrophic lakes
have low nutrient levels, so they also have low productivity. With few producers (or other aquatic organisms), the
water remains clear and little oxygen is used up to support life. Biodiversity is low.
TABLE 1.1: Trophic Classification of Freshwater Lakes
Type of Lake
Oligotrophic
Mesotrophic
Eutrophic
Hypertrophic
Nutrient Level
Low
Medium
High
Very high
Productivity
Low
Medium
High
Very high
Clarity of Water
High
Medium
Low
Very low
Oxygen Level
High
Medium
Low
Very low
Acid rain is another cause of low productivity in lakes. Acid rain falling into a lake causes the lake water to become
too acidic for many species to tolerate. This results in a decline in the number and diversity of lake organisms. This
has happened to many lakes throughout the northeastern United States. The water in the lakes is very clear because
it is virtually devoid of life.
Lakes with high nutrient levels have higher productivity, cloudier water, lower oxygen levels, and higher biomass
and biodiversity. Very high nutrient levels in lakes are generally caused by contamination with fertilizer or sewage.
The high concentration of nutrients may cause a massive increase in phytoplankton, called a phytoplankton bloom
(see Figure 1.1). The bloom blocks sunlight from submerged plants and other producers and negatively impacts
most organisms in the lake.
Running Freshwater Biomes
Running freshwater biomes include streams and rivers. Streams are generally smaller than rivers. Streams may start
with surface runoff, snowmelt from a glacier, or water seeping out of the ground from a spring. If the land is not
flat, the water runs downhill. The water joins other streams and then rivers as it flows over the land. Eventually, the
water empties into a pond, lake, or the ocean.
Some species living in rivers that empty into the ocean may live in freshwater during some stages of their life cycle
2
www.ck12.org
Chapter 1. Freshwater Biomes - Advanced
FIGURE 1.1
The phytoplankton bloom on this lake
blocks most sunlight from penetrating below the surface, creating a condition detrimental to many other aquatic organisms.
and in salt water during other stages. For example, salmon are born and develop in freshwater rivers and then move
downstream to the ocean, where they live as adults. In contrast, some eels are born and develop in the ocean and
then move into freshwater rivers to live as adults.
Compared with standing water, running water is better able to dissolve oxygen needed by producers and other aquatic
organisms. When a river rushes over a waterfall, like the one in Figure 1.2, most of the water is exposed to the air,
allowing it to dissolve a great deal of oxygen. Flowing water also provides a continuous supply of nutrients. Some
nutrients come from the decomposition of dead aquatic organisms. Other nutrients come from the decomposition of
dead terrestrial organisms, and other organic debris such as leaves, that fall into the water.
FIGURE 1.2
Flowing water forms a waterfall on the
South Yuba River in Nevada County, California.
As the water falls through the
air, it dissolves oxygen needed by aquatic
organisms.
3
www.ck12.org
Algae are the main producers in running freshwater biomes. If water flows slowly, algae can float suspended in
the water, and huge populations may form, like the phytoplankton bloom in Figure 1.1. If water flows rapidly,
algae must attach themselves to rocks or plants to avoid being washed away and generally cannot form very large
populations.
Plants are also important producers in most running water biomes. Some plants, such as mosses, cling to rocks.
Other plants, such as duckweed, float in the water. If nutrient levels are high, floating plants may form a thick mat
on the surface of the water, like the one shown in Figure 1.3 (left photo). Still other plants grow in sediments on
the bottoms of streams and rivers. Many of these plants—like the cattails in Figure 1.3 (right photo)—have long
narrow leaves that offer little resistance to the current. In addition to serving as a food source, plants in running
water provide aquatic animals with protection from the current and places to hide from predators.
FIGURE 1.3
The pond on the left has a thick mat of duckweed plants. They cover the surface of the water and use sunlight
for photosynthesis. The cattails on the right grow along a stream bed. They have tough, slender leaves that can
withstand moving water.
Consumers in running water include both invertebrate and vertebrate animals. The most common invertebrates
are insects. Others include snails, clams, and crayfish. Some invertebrates live on the water surface, others float
suspended in the water, and still others cling to rocks on the bottom. All rely on the current to bring them food and
dissolved oxygen. The invertebrates are important consumers as well as prey to the many vertebrates in running
water. Vertebrate species include fish, amphibians, reptiles, birds, and mammals. However, only fish live in the
water all the time. Other vertebrates spend part of their time on land.
The movement of running water poses a challenge to aquatic organisms, which have adapted in various ways. Some
organisms have hooks or threadlike filaments to anchor themselves to rocks or plants in the water. Other organisms,
including fish, have fins and streamlined bodies that allow them to swim against the current.</pre>
The interface between running freshwater and land is called a riparian zone. It includes the vegetation that grows
along the edge of a river and the animals that consume or take shelter in the vegetation. Riparian zones are very
important natural areas for several reasons:
4
www.ck12.org
•
•
•
•
Chapter 1. Freshwater Biomes - Advanced
They filter pollution from surface runoff before it enters a river.
They help keep river water clear by trapping sediments.
They protect river banks from erosion by running water.
They help regulate the temperature of river water by providing shade.
Wetlands
A wetland is an area that is saturated or covered by water for at least one season of the year. Freshwater wetlands are
also called swamps, marshes, or bogs. Saltwater wetlands include estuaries. Wetland vegetation must be adapted
to water-logged soil, which contains little oxygen. Freshwater wetland plants include duckweed and cattails (see
Figure 1.3). Some wetlands also have trees. Their roots may be partly above ground to allow gas exchange with the
air. Wetlands are extremely important biomes for several reasons.
•
•
•
•
•
They store excess water from floods and runoff.
They absorb some of the energy of running water and help prevent erosion.
They remove excess nutrients from runoff before it empties into rivers or lakes.
They provide a unique habitat that certain communities of plants need to survive.
They provide a safe, lush habitat for many species of animals.
Restoring Wetlands
More than 100,000 acres of wetlands are being restored in the Northern California Bay Area, but how exactly do we
know what to restore them to? Historical ecologists are recreating San Francisco Bay wetlands that existed decades
ago. To learn more, see Wetlands Time Machine at http://www.kqed.org/quest/television/view/416 .
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/139427
For more than 100 years, south San Francisco Bay has been a center for industrial salt production. Now federal and
state biologists are working to restore the ponds to healthy wetlands for fish and other wildlife. Salt marshes are rich
habitats that provide shelter and food for many species, some of which are endangered or threatened. See From Salt
Ponds to Wetlands at http://www.kqed.org/quest/television/from-salt-ponds-to-wetlands for additional information.
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/139427
San Francisco Bay: A Unique Estuary
An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a
free connection to the ocean. Estuaries can be thought of as the most biologically productive regions on Earth, with
very high biodiversity. Estuaries are zones where land and sea come together, and where fresh and salt water meet.
5
www.ck12.org
The San Francisco Bay is one of the great estuaries of the world. See Saving the Bay - San Francisco Bay: A
Unique Estuary at http://www.youtube.com/watch?v=clZz2OjE5n0&playnext=1&list=PL0AAA2B9B3E9F6AB3
and San Francisco Estuary Institute at http://www.sfei.org for further information. The complete Bay covers about
1,600 square miles (4,160 square kilometers), including sub-bays, estuaries, and wetlands. The main part of the Bay
measures 3 to 12 miles (5 to 20 km) wide (east-to-west), and about 60 miles (97 km) north-to-south. It is the largest
Pacific estuary in the Americas.
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/441
Vocabulary
• estuary: Bay where a river empties into the ocean.
• freshwater biome: Biome such as a lake or river that has water with little or no salt.
• limnetic zone: Top layer of deep water in a lake, down to the depth that sunlight penetrates.
• profundal zone: Deep water in a lake near the bottom where no sunlight penetrates.
• riparian zone: Interface between running freshwater and land.
• wetland: Area that is saturated or covered by water for at least one season of the year; swamps, marshes
and/or bogs whose soil is saturated.
Summary
• Freshwater biomes may be standing water biomes, such as lakes, or running water biomes, such as rivers.
Wetlands are biomes in which the ground is saturated or covered by water for at least part of the year.
Practice
Use this resource to answer the questions that follow.
• http://www.hippocampus.org/Biology → Non-Majors Biology → Search: Freshwater Biomes
1.
2.
3.
4.
5.
6.
6
Why are freshwater biomes considered "limited"?
Describe a wetland. Give examples of wetlands.
What are the 3 zones of ponds and lakes?
Compare the littoral zone to the profundal zone.
What types of organisms can thrive in the headwaters of streams and rivers?
Describe the changes that occur when a river empties into a lake or ocean.
www.ck12.org
Chapter 1. Freshwater Biomes - Advanced
Review
1. Why does the profundal zone of a lake have no producers?
2. A developer plans to extend a golf course into a riparian biome. Outline environmental arguments you could
make against this plan.
3. Discuss the importance of wetlands.
References
1. Dr. Jennifer L. Graham/U.S. Geological Survey. http://gallery.usgs.gov/photos/06_30_2010_hLc5FSq11Y
_06_30_2010_11#.T767jMV62Sp . Public Domain
2. Michael Nevins/U.S. Army Corps of Engineers. http://commons.wikimedia.org/wiki/Image:South_Yuba_R
iver_waterfall.jpg . Public Domain
3. Duckweed: Jonathan Jordan; Cattails: User:JoJan/Wikimedia Commons. Duckweed: http://www.flickr.
com/photos/jjordan64816/7251615240/; Cattails: http://commons.wikimedia.org/wiki/File:Girona_009.JPG
. Duckweed: CC BY 2.0; Cattails: CC BY 3.0
7