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 1. 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.
Trophic Classification of Freshwater Lakes Type of Lake Nutrient Level Productivity Clarity of Water Oxygen Level
Oligotrophic Low Low High High
Mesotrophic Medium Medium Medium Medium
Eutrophic High High Low Low
Hypertrophic Very high Very high Very 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 8). The bloom blocks sunlight from submerged plants and other producers and negatively impacts most organisms in the lake.
Figure 16.17
The phytoplankton bloom on this lake blocks most sunlight from penetrating below the surface, creating a condition detrimental to many other aquatic organisms.
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 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 9, 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 16.18
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.
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 8 above. 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 10 (left photo). Still other plants grow in sediments on the bottoms of streams and rivers. Many of these plants—like the cattails in Figure 10 (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 16.19
The picture on the left shows a thick mat of duckweed floating on a river. The picture on the right shows cattails growing in sediments at the edge of a stream bed. Notice the cattails long, slender leaves, which reduce water resistance.
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.
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:
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, which are described earlier in this lesson. Wetland vegetation must be adapted to water-logged soil, which contains little oxygen. Freshwater wetland plants include duckweed and cattails (see Figure 10, above). 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 pla
nts need to survive.
They provide a safe, lush habitat for many species of animals.
Lesson Summary
Aquatic biomes are divided into zones based on factors such as water depth and amount of sunlight available for photosynthesis. Aquatic organisms include plankton, nekton, and benthos.
Marine biomes include neritic, oceanic, and benthic biomes. Intertidal zones, estuaries, and coral reefs are marine biomes with the highest biodiversity.
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.
Review Questions
In a large body of standing water, what is the photic zone?
State why the oceanic zone has a lower concentration of nutrients than the neritic zone.
Why is moving water a major challenge for organisms in the littoral zone of the ocean?
Why does the profundal zone of a lake have no producers?
A new species of bioluminescent fish has been discovered in the ocean. Which oceanic zone is most likely the home of this fish? Explain your answer.
A developer plans to extend a golf course into a riparian biome. Outline environmental arguments you could make against this plan.
Compare and contrast plankton, nekton, and benthos.
In the deep ocean far from shore, why might you find more dissolved nutrients at the bottom than at the surface?
Further Reading / Supplemental Links
Trevor Day, Lakes and Rivers. Chelsea House Publications, 2006.
Trevor Day, Oceans. Chelsea House Publications, 2006.
Stephen Hutchinson and Lawrence E. Hawkins, Oceans: A Visual Guide. Firefly Books, 2005.
Peter D. Moore, Wetlands. Chelsea House Publications, 2006.
David Sanger and John Hart, San Francisco Bay: Portrait of an Estuary. University of California Press, 2003
Susan L. Woodward, Biomes of Earth: Terrestrial, Aquatic, and Human-Dominated. Greenwood Press, 2003.
http://ridge.icu.ac.jp/gen-ed/biomes.html
http://estrellamountain.edu/faculty/farabee/biobk/BioBookcommecosys.html
http://ridge.icu.ac.jp/gen-ed/biomes.html
http://sfbay.wr.usgs.gov/
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/B/Biomes.html
http://www.davidsanger.com/san-francisco-bay-book/
http://www.oceanexplorer.noaa.gov/edu/curriculum/section5.pdf
http://www.waterencyclopedia.com/La-Mi/Life-in-Water.html
Vocabulary
abyssal zone
Part of the ocean floor that is under the deep ocean.
abyssopelagic zone
Water between 4,000 and 6,000 meters below sea level in the oceanic zone.
aphotic zone
Deep water in a lake or the ocean where too little sunlight penetrates for photosynthesis to occur.
bathyal zone
Part of the ocean floor that makes up the continental slope.
bathypelagic zone
Water between 1,000 and 4,000 meters below sea level in the oceanic zone.
benthic biome
Marine biome that occurs on the bottom of the ocean where benthos live.
benthic zone
Bottom surface of the ocean or a lake.
benthos
Aquatic organisms that live on the surface below a body of water.
coral reef
Underwater limestone structure formed by tiny invertebrate animals called corals.
epipelagic zone
Top 200 meters of water in the oceanic zone.
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.
hadal zone
Part of the ocean floor that is in deep ocean trenches.
hadopelagic zone
Water of deep ocean trenches below 6,000 meters in the oceanic zone.
intertidal zone
Narrow strip along the coastline of the ocean that falls between high- and low-tide water lines.
limnetic zone
Top layer of deep water in a lake, down to the depth that sunlight penetrates.
littoral zone
Shallow water near the shore of a lake or the ocean.
marine biome
Aquatic biome found in the salt water of the ocean.
mesopelagic zone
Water between 200 and 1,000 meters below sea level in the oceanic zone.
nekton
Aquatic animals that live in the water itself and can propel themselves by swimming or other means.
neritic biome
Marine biome that occurs in ocean water over the continental shelf.
neritic zone
Part of the pelagic zone over the continental shelf.
oceanic biome
Marine biome that occurs in ocean water beyond the continental shelf.
oceanic zone
Part of the pelagic zone beyond the continental shelf.
pelagic zone
Main body of open water away from shore in a lake or the ocean.
photic zone
Depth of water in a lake or the ocean to which sunlight can penetrate and photosynthesis can occur.
plankton
Aquatic organisms that live in the water itself and cannot propel themselves through water.
profundal zone
Deep water in a lake near the bottom where no sunlight penetrates.
riparian zone
Interface between running freshwater and land.
sublittoral zone
Part of the ocean floor that makes up the continental shelf.
turnover
Process in which different layers of lake water intermix and bring nutrients from the bottom to the surface.
upwelling
Process in which deep ocean water is forced to the surface by currents, bringing dissolved nutrients from the bottom to the surface.
wetland
Area that is saturated or covered by water for at least one season of the year.
Points to Consider
Next we discuss community interactions. Abiotic factors such as water depth affect organisms in aquatic biomes. Organisms in all biomes are also affected by biotic factors, which include their interactions with other species.
How do you think different species interact?
What types of relationships do you think different species might have with eachother?
How could these relationships affect the evolution of the species involved?
Lesson 16.4: Community Interactions
Lesson Objectives
State the significance of the community in ecology, and list types of community interactions.
Define predation, and explain how it affects population growth and evolution.
Describe competition, and outline how it can lead to extinction or specialization of species.
Define symbiosis, and identify major types of symbiotic relationships.
Describe ecological succession, and explain how it relates to the concept of a climax community.
Introduction
Biomes as different as grasslands and estuaries share something extremely important. They have populations of interacting species. Moreover, species interact in the same basic ways in all biomes. For example, all biomes have some species that prey on other species for food. Species interactions are important biotic factors in ecological systems. The focus of study of species interactions is the community.
What Is a Community?
In ecology, a community is the biotic component of an ecosystem. It consists of populations of different species that live in the same area and interact with one another. Like abiotic factors, such as climate or water depth, species interactions in communities are important biotic factors in natural selection. The interactions help shape the evolutio
n of the interacting species. Three major types of community interactions are predation, competition, and symbiosis.
Predation
Predation is a relationship in which members of one species (the predator) consume members of other species (the prey). The lions and cape buffalo in Figure 1 are classic examples of predators and prey. In addition to the lions, there is another predator in this figure. Can you find it? The other predator is the cape buffalo. Like the lion, it consumes prey species, in this case species of grasses. Predator-prey relationships account for most energy transfers in food chains and webs (see the Principles of Ecology chapter).
Figure 16.20
An adult male lion and a lion cub feed on the carcass of a South African cape buffalo.
Types of Predators
The lions in Figure 1 are true predators. In true predation, the predator kills its prey. Some true predators, like lions, catch large prey and then dismember and chew the prey before eating it. Other true predators catch small prey and swallow it whole. For example, snakes swallow mice whole.
CK-12 Biology I - Honors Page 72