Aquatic Biomes

Terrestrial organisms are generally limited by temperature and moisture. Therefore, terrestrial 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 photosynthesis 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. • 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.

Figure 1: 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?

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 themselves through water. They include both phytoplankton and zooplankton. Phytoplankton 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 2: 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.

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 sunlight 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.

Figure 3: 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.

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 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.

Intertidal Zone

The intertidal zone is a narrow strip along the coastline that falls between high- and lowtide 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 4: 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.

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 5: Barnacles secrete a cement-like substance that anchors them to rocks.

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 6: 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.

• A coral reef is an underwater limestone structure produced by tiny invertebrate animals 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 7: Colorful fish swim in warm, shallow ocean water near a coral reef off the Hawaiian Islands.

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 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 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.

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 8: 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 9: 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 10: 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. 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.