Lesson 16.3: Aquatic Biomes

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

 

Lesson Objectives

Describe how aquatic biomes are divided into zones, and list types of aquatic organisms.

Identify marine biomes, and state which biomes have the highest biodiversity.

Name types of freshwater biomes, and describe how they differ from one another.

Introduction

Terrestrial organisms are generally limited by temperature and moisture. Therefore, terres- trial biomes are defined in terms of these abiotic factors. In contrast, most organisms that live in the water do not have to deal with extremes of temperature or moisture. Instead, their main limiting factors are the availability of sunlight and the concentration of dissolved nutrients in the water.

 

 

What Are Aquatic Biomes?

Aquatic biomes are biomes found in water. Water covers 70 percent of Earth’s surface, so aquatic biomes are a major component of the biosphere. However, they have less total biomass than terrestrial biomes. Aquatic biomes can occur in either salt water or freshwater. About 98 percent of Earth’s water is salty, and only 2 percent is fresh. The primary saltwater biome is the ocean. Major freshwater biomes include lakes and rivers.

 

 

Aquatic Zones

In large bodies of standing water (including the ocean and lakes), the water can be divided into zones based on the amount of sunlight it receives. There is enough sunlight for photo- synthesis only in - at most - the top 200 meters of water. Water down to this depth is called the photic zone. Deeper water, where too little sunlight penetrates for photosynthesis, is called the aphotic zone.

Surface water dissolves oxygen from the air, so there is generally plenty of oxygen in the photic zone to support organisms. Water near shore usually contains more dissolved nutrients than water farther from the shore. This is because most dissolved nutrients enter a body of water from land, carried by runoff or rivers that empty into the body of water. When aquatic organisms die, they sink to the bottom, where decomposers release the nutrients they contain. As a result, deep water may contain more nutrients than surface water.

Deep ocean water may be forced to the surface by currents in a process called '''upwelling.''' When this happens, dissolved nutrients are brought to the surface from the deep ocean. The nutrients can support large populations of producers and consumers, including many species of fish. As a result, areas of upwelling are important for commercial fishing.With these variations in sunlight, oxygen, and nutrients, different parts of the ocean or a lake have different types and numbers of organisms. Therefore, life in a lake or the ocean is generally divided into zones. The zones correlate mainly with the amount of sunlight and nutrients available to producers. Figure 1 shows ocean zones. Lakes have similar zones.

The littoral zone is the shallow water near the shore. In the ocean, the littoral zone is also called the intertidal zone.

 

 

Figure 16.10: The ocean is divided into many different zones, depending on distance from shore and depth of water. The pelagic zone is divided into neritic and oceanic zones based on distance from shore. Into what additional zones is the pelagic zone divided on the basis of water depth? What additional zones make up the benthic zone? (17)

 

The pelagic zone is the main body of open water farther out from shore. It is divided into additional zones based on water depth. In the ocean, the part of the pelagic zone over the continental shelf is called the neritic zone, and the rest of the pelagic zone is called the oceanic zone.

The benthic zone is the bottom surface of a body of water. In the ocean, the benthic

zone is divided into additional zones based on depth below sea level.

 

Aquatic Organisms

Aquatic organisms are classified into three basic categories: plankton, nekton, and benthos. Organisms in these three categories vary in where they live and how they move.

 

Plankton are aquatic organisms that live in the water itself and cannot propel them- selves through water. They include both phytoplankton and zooplankton. Phyto- plankton are bacteria and algae that use sunlight to make food by photosynthesis. Zooplankton are tiny animals that feed on phytoplankton.

Nekton are aquatic animals that live in the water and can propel themselves by swimming or other means. Nekton include invertebrates such as shrimp and vertebrates such as fish.

Benthos are aquatic organisms that live on the surface below a body of water. They live in or on the sediments at the bottom. Benthos include sponges, clams, and sea stars (see Figure 2).

 

 

Figure 16.11: This sea star, or starfish, is an example of a benthic organism. The tiny white projections on the bottom surface of the sea star allow it anchor to, or slowly crawl over, the bottom surface of the ocean. (16)

 

 

Marine Biomes

Marine biomes are aquatic biomes found in the salt water of the ocean. Major marine biomes are neritic, oceanic, and benthic biomes. Other marine biomes include intertidal zones, estuaries, and coral reefs.

 

Neritic Biomes

Neritic biomes occur in ocean water over the continental shelf (see Figure 1). They extend from the low-tide water line to the edge of the continental shelf. The water here is shallow, so there is enough sunlight for photosynthesis. The water is also rich in nutrients, which are washed into the water from the nearby land. Because of these favorable conditions, large populations of phytoplankton live in neritic biomes. They produce enough food to support many other organisms, including both zooplankton and nekton. As a result, neritic biomes have relatively great biomass and biodiversity. They are occupied by many species of invertebrates and fish. In fact, most of the world’s major saltwater fishing areas are in neritic biomes.

Oceanic Biomes

Oceanic biomes occur in the open ocean beyond the continental shelf. There are lower concentrations of dissolved nutrients away from shore, so the oceanic zone has a lower density of organisms than the neritic zone. The oceanic zone is divided into additional zones based on water depth (see Figure 1).

 

The epipelagic zone is the top 200 meters of water, or the depth to which enough sunlight can penetrate for photosynthesis. Most open ocean organisms are concentrated in this zone, including both plankton and nekton.

The mesopelagic zone is between 200 and 1,000 meters below sea level. Some sun- light penetrates to this depth but not enough for photosynthesis. Organisms in this zone consume food drifting down from the epipelagic zone, or they prey upon other organisms in their own zone. Some organisms are detrivores, which consume dead organisms and organic debris that also drift down through the water.

The bathypelagic zone is between 1,000 and 4,000 meters below sea level. No sunlight penetrates below 1,000 meters, so this zone is completely dark. Most organisms in this zone either consume dead organisms drifting down from above or prey upon other animals in their own zone. There are fewer organisms and less biomass here than in higher zones. Some animals are bioluminescent, which means they can give off light (see Figure 3). This is an adaptation to the total darkness.

The abyssopelagic zone is between 4,000 and 6,000 meters below sea level. The hadopelagic zone is found in the water of deep ocean trenches below 6,000 meters. Both of these zones are similar to the bathypelagic zone in being completely dark. They have even lower biomass and species diversity.

 

Benthic Biomes

Benthic biomes occur on the bottom of the ocean where benthos live. Some benthos, including sponges, are sessile, or unable to move, and live attached to the ocean floor. Other benthos, including clams, burrow into sediments on the ocean floor. The benthic zone can be divided into additional zones based on how far below sea level the ocean floor is (see Figure 1).

 

The sublittoral zone is the part of the ocean floor that makes up the continental shelf near the shoreline. The water is shallow enough for sunlight to penetrate down to the ocean floor. Therefore, photosynthetic producers such as seaweed can grow on the ocean floor in this zone. The littoral zone is rich in marine life.

The bathyal zone is the part of the ocean floor that makes up the continental slope. It ranges from about 1,000 to 4,000 meters below sea level. The bathyal zone contains no producers because it is too far below the surface for sunlight to penetrate. Although

 

 

Figure 16.12: The anglerfish lives in the bathypelagic zone. The rod-like structure protruding from the anglerfish’s face is tipped with bioluminescent microorganisms. The structure wiggles like a worm to attract prey. Only the ”worm” is visible to prey in the total darkness of this zone. (26)

 

consumers and decomposers live in this zone, there are fewer organisms here than in the sublittoral zone.

The abyssal zone is the part of the ocean floor in the deep open ocean. It varies from

about 4,000 to 6,000 meters below sea level. Organisms that live on the ocean floor in this zone must be able to withstand extreme water pressure, continuous cold, and scarcity of nutrients. Many of the organisms sift through sediments on the ocean floor for food or dead organisms.

The hadal zone is the ocean floor below 6,000 meters in deep ocean trenches. The only

places where organisms are known to live in this zone are at hydrothermal vents, where invertebrates such as tubeworms and clams are found. They depend on microscopic archaea organisms for food. These tiny chemosynthetic producers obtain energy from chemicals leaving the vents (see the Principles of Ecology chapter).

 

Intertidal Zone

The intertidal zone is a narrow strip along the coastline that falls between high- and low- tide water lines. It is also called the littoral zone (see Figure 1). A dominant feature of this zone is the regular movement of the tides in and out. In most areas, this occurs twice a day. Due to the tides, this zone alternates between being under water at high tide and being exposed to the air at low tide. An intertidal zone is pictured in Figure 4.

 

 

 

Figure 16.13: These pictures show the Bay of Fundy off the northeastern coast of Maine in North America. The picture on the left shows the bay at high tide, and the picture on the right shows the bay at low tide. The area covered by water at high tide and exposed to air at low tide is the intertidal zone. (24)

The high tide repeatedly brings in coastal water with its rich load of dissolved nutrients. There is also plenty of sunlight for photosynthesis. In addition, the shallow water keeps large predators, such as whales and big fish, out of the intertidal zone. As a result, the intertidal zone has a high density of living things. Seaweeds and algae are numerous, and they support many consumer species, either directly or indirectly, including barnacles, sea stars, and crabs.

Other conditions in the intertidal zone are less favorable. For example, there are frequent shifts from a water to an air environment. There are also repeated changes in temperature and salinity (salt concentration). These changing conditions pose serious challenges to marine organisms. The moving water poses yet another challenge. Organisms must have some way to prevent being washed out to sea with the tides. Barnacles, like those in Figure 5, cement themselves to rocks. Seaweeds have rootlike structures, called holdfasts, which anchor them to rocks. Crabs burrow underground to avoid being washed out with the tides.

 

Figure 16.14: Barnacles secrete a cement-like substance that anchors them to rocks. (5)

 

Other Marine Biomes

The intertidal zone has high biodiversity. However, it is not the marine biome with the highest biodiversity. That distinction goes to estuaries and coral reefs. They have the highest biodiversity of all marine biomes.

An estuary is a bay where a river empties into the ocean. It is usually semi-enclosed, making it a protected environment. The water is rich in dissolved nutrients from the river and shallow enough for sunlight to penetrate for photosynthesis. As a result, estuaries are full of marine life. Figure 6 shows an estuary on the California coast near San Francisco.

 

 

Figure 16.15: This satellite photo shows the San Francisco Estuary on the California coast. This is the largest estuary on the lower west coast of North America. Two rivers, the Sacramento and the San Joaquin, flow into the estuary (upper right corner of photo). The estuary is almost completely enclosed by land but still connected to the ocean. (4)

 

A coral reef is an underwater limestone structure produced by tiny invertebrate ani- mals called corals. Coral reefs are found only in shallow, tropical ocean water. Corals secrete calcium carbonate (limestone) to form an external skeleton. Corals live in colonies, and the skeletal material gradually accumulates to form a reef. Coral reefs are rich with marine organisms, including more than 4,000 species of tropical fish. Figure 7 shows a coral reef in the Hawaiian Islands.

 

 

Figure 16.16: Colorful fish swim in warm, shallow ocean water near a coral reef off the Hawaiian Islands. (2)

 

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.

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 phyto- plankton, 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 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.

Table 16.2: Trophic Classification of Freshwater Lakes

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. (28)

 

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 organ- isms. (3)

 

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. (21)

 

Consumers in running water include both invertebrate and vertebrate animals. The most common invertebrates are insects. Others include snails, clams, and crayfish. Some inverte- brates 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 ri- parian 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 plants 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

 

Further Reading / Supplemental Links

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?

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 pene- trates.

 

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 swim- ming 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?