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The Biosphere

Aquatic Biomes

Aquatic biomes are often distinguished by their physical characteristics, such as temperature, availability of light, and salinity.

Aquatic biomes are characterized by physical factors such as salinity (salt content of water) and geologic features. They do not correlate with latitude, largely because a single aquatic biome, the ocean, takes up 75% of Earth's surface. The ocean greatly influences the biosphere (all living things and their environments on Earth). The ocean plays a huge role in the water cycle, as evaporation from the ocean surface provides most of Earth's precipitation. Marine photosynthetic plankton make the greatest contribution to atmospheric oxygen.

Freshwater makes up a small percentage of Earth's total water supply, but there are a variety of freshwater biomes (defined by climatic, physical, and biotic characteristics) because the speed of water flow and the productivity of the surrounding soil give these biomes different attributes. Freshwater biomes include lakes, rivers, and wetlands. The chemical makeup of a freshwater biome is linked with that of its surrounding soils and plant life. The close proximity of freshwater biomes to human development makes them especially vulnerable to pollution and overuse.

Both freshwater and marine biomes are stratified into layers. Photosynthetic organisms need sunlight, so they are restricted to the shallow photic zone, the upper portion of an aquatic environment where light can penetrate. The benthic zone is comprised of the waters from near the surface down to the deepest depths, where little, if any, light reaches. The body of water may be shallow or deep. These zones hold different organisms based primarily on the temperature and sunlight needs of the organisms. Shallow waters tend to be warm, and deep waters tend to be cold. The deep, dark, cold regions of the ocean generally have fewer organisms than the shallow regions. In the spring and fall, lakes often experience a turnover, the seasonal mixing that redistributes oxygen and nutrients in a body of water. Oxygen from the shallow parts of the lake is brought to the deep parts, and nutrients from the deep are brought to the surface.

Freshwater and Saltwater Zones

Both salt water and freshwater consist of distinct layers. In the salt water ocean, the benthic zone extends from the shore into the deep ocean. The photic zone is the region nearest the surface, receiving plenty of light. The aphotic zone lies below the photic zone, where less light penetrates. In freshwater reservoirs such as lakes, the photic zone lies along the surface, similar to the photic zone in the ocean. At the bottom of the reservoir is the aphotic zone, where little light penetrates.

Lakes, Rivers, and Streams

Lakes are standing bodies of water, usually freshwater. They are stratified; the shallow waters are warm, and the deep waters are cold. Salinity, oxygen, and nutrient levels vary among lakes. Some lakes are clear, low in nutrients, and high in oxygen. Urban and agricultural runoff can lead to eutrophication, an algal bloom caused by excessive nutrients, leading to a drop in dissolved oxygen content. The resultant murky, low-oxygen conditions may be harmful to oxygen-using organisms.

Process of Eutrophication

Eutrophication occurs in a body of water when excess nutrients (P and N) are added, causing rapid growth of photosynthetic organisms, such as phytoplankton. The water becomes murky, dissolved oxygen drops, and heterotrophic organisms can be suffocated. This is different from oligotrophic bodies of water that are high in oxygen content and low in nutrients.
Rivers and streams are flowing bodies of freshwater. As the water runs downhill, it picks up sediment and becomes warmer. Treated human wastewater and agricultural and urban runoff are large sources of pollution in freshwater. Damming and water use also disrupt rivers and streams as well as the organisms living in and near them.

Wetlands and Estuaries

Wetlands are areas that are completely saturated with water, either periodically or permanently. Wetlands are ecologically important because of their ability to filter pollution from water by trapping pollution in sediments at the bottom of the wetland. They are teeming with photosynthetic organisms and have high productivity. Wetlands are also vulnerable because they are often filled in or drained to make way for urban development.

An estuary is the location where a river meets the ocean. It is a place where fresh and salt water mix depending on the tides. Salty ocean water moves into and out of estuaries regularly. Many organisms that reside in estuaries must be able to tolerate a wide range of salinities, for example, mangrove trees and blue crabs. Nutrients flowing in from the river contribute to the high productivity of estuaries. Grasses and phytoplankton are common. Many species of invertebrates, fish, and birds reside in this biome. This makes estuaries a popular feeding place for humans and marine predators alike. Similar to wetlands, these highly productive biomes are in danger because of overuse by humans and habitat loss through urban development.

Intertidal Zone

Intertidal zones occur at the transition between land and sea. As the tide rises and falls, the intertidal zone is cyclically exposed to air and direct sun or submerged in water. Elevated portions of the zone are exposed to air more of the time, and lower portions are submerged in water more of the time. As a result, the distribution of species in the intertidal zone is stratified based on each organism's physiological constraints. Algae and sea grass are the primary producers in this biome. Organisms that are able to attach to rocks are common because they can avoid being swept away by the tide. In addition, the tide delivers food, so organisms can remain in one place. Sea anemones, urchins, and sea stars are commonly found in intertidal zones. This zone is the marine biome most accessible to humans. As a result, pollution and urban development have a huge impact on the intertidal zone.

Intertidal Zone

In the intertidal zone, species are stratified according to tide levels. Only species that can cling to rocks or can hide from the sun live in the areas that are exposed at certain times of day. Stringy seaweeds live where the tide reaches its highest point.

Open Ocean

The enormous open ocean covers most of Earth's surface. These waters are constantly mixed by ocean currents, driven by winds and differences in water density. Close to half of all global photosynthesis is completed by huge masses of plankton (tiny organisms) floating in the ocean. The open ocean is also home to large schools of fishes, sea turtles, dolphins, and whales. Despite the vast size of this biome, overfishing has had a major impact on fish populations. Regulations and catch limits have been successful in restoring the health of fish stocks in some cases.

Benthic Zone

The benthic zone is comprised of waters from near the surface down to the deepest depths, including some subsurface regions. Most of the seafloor is a dark, cold environment under moderate to high pressure. Most organisms cannot live here because they would be crushed by the pressure of the water. Species that do live in this zone do not contain air spaces in their bodies, which helps them avoid being crushed. Photosynthesis is limited to the portions of this zone that some light can reach. Other producers in the benthic zone are chemoautotrophic, using hydrogen sulfide as an energy source. A deep-sea hydrothermal vent, which is a deep-sea fissure releasing extremely hot fluid that contains hydrogen sulfide, can act as a source. Organisms near vents, such as the giant tube worm, are adapted to extreme heat and pressure. Other benthic organisms, such as sponges and sea stars, are adapted to the cold and dark environment.