Major Aquatic Biomes (College Board AP® Environmental Science): Study Guide

Freshwater biomes

Global distribution of freshwater biomes

• Freshwater biomes are found all over the world but make up only 2.5% of Earth's water supply

• They are located on every continent and include:

  • Streams

  • Rivers

  • Ponds

  • Lakes

Types of freshwater biomes

Streams and rivers

• Description:

  • Flowing freshwater, starting from springs or melting ice and snow

  • Flow rate can vary greatly depending on location and time of year

• Environmental aspects:

  • Naturally high oxygen levels due to constant movement

  • Temperature varies with location, altitude, and season

  • Nutrient levels are often lower near the source but increase downstream

Ponds and lakes

• Description:

  • Still freshwater bodies surrounded by land

  • Size ranges from small ponds to large lakes

• Environmental aspects:

  • Stratification occurs in deeper lakes with layers based on temperature

  • This usually includes:

    • warmer surface layers

    • colder bottom layers

  • Oxygen levels are usually higher near the surface and lower in deeper layers

Principal environmental aspects of freshwater biomes

• Vital resource for human populations:

  • Freshwater biomes provide drinking water, support agriculture (used for irrigation), and generate hydroelectric power

• Biodiversity:

  • Freshwater biomes support a wide range of organisms, including fish, amphibians, birds, and aquatic plants

• Nutrient cycling:

  • Freshwater biomes play a key role in cycling nutrients like nitrogen and phosphorus

  • Excess nutrients can lead to algae blooms, reducing oxygen levels and harming aquatic life

    • This process is known as eutrophication

• Human impacts:

  • Pollution from agriculture and industry affects water quality

  • Dam construction can alter water flow and disrupt ecosystems

Marine biomes

• Marine biomes cover about 70% of the Earth’s surface and have high salinity

  • They make up the largest group of biomes on Earth

• Marine biomes include:

  • Oceans

  • Coral reefs

  • Marshlands

  • Estuaries

Global distribution of marine biomes

Oceans

• There are five recognised oceans:

  • Pacific Ocean

  • Atlantic Ocean

  • Indian Ocean

  • Arctic Ocean

  • Southern Ocean (also known as the Antarctic Ocean)

Coral reefs

• There are two major categories of coral reefs:

  • Warm—found within warm, shallow tropical waters (near the equator)

    • Examples: Great Barrier Reef (Australia), Coral Triangle (Southeast Asia)

  • Cold—found in deep, nutrient-rich sea waters, where temperatures and salinity levels are more stable, compared to coastal waters

    • Typically found between 200 and 1000 meters below the surface

    • These reefs are not dependent on sunlight for energy, as they do not host photosynthetic algae like warm-water reefs

    • Instead, they rely on organic material sinking from surface waters for sustenance

Marshlands

• Located in coastal areas where land meets the sea

  • Examples: Everglades (United States), Sundarbans (India and Bangladesh)

Estuaries

• Found where freshwater rivers meet the ocean

  • Examples: Chesapeake Bay (United States), Amazon River estuary (Brazil).

Principal environmental aspects of marine biomes

Oceans

• Key aspects:

  • High salinity

  • Stratification of temperature and light

    • The photic zone (upper layer) supports most marine life due to sunlight for photosynthesis

    • The deep sea (aphotic zone) contains organisms adapted to darkness and high pressure

• Importance:

  • Oceans help to regulate global temperatures through ocean currents like the Gulf Stream

  • They provide habitats for a vast range of organisms, from plankton to whales

  • Algae supply a large portion of the Earth’s oxygen

    • They also take in carbon dioxide from the atmosphere

Coral reefs

• Key aspects:

  • High biodiversity

    • Coral reefs support 25% of marine species despite covering less than 1% of the ocean floor

• Importance:

  • Act as natural barriers, protecting coastal ecosystems and human populations against storm surges and coastal erosion

  • Essential for tourism and fishing industries

Marshlands

• Key aspects:

  • Also known as salt marshes

  • Where saltwater and freshwater meet in an estuary

  • Tidal wetlands with salt-tolerant plants

  • High biodiversity

    • Serve as feeding and breeding grounds for many species of fish and birds

• Importance:

  • Filter pollutants from water and store carbon in plant roots and soil

  • Protect coastal areas from flooding

Estuaries

• Key aspects:

  • Brackish water (mix of freshwater and saltwater) with fluctuating salinity

  • Serve as nurseries for fish and shellfish

• Importance:

  • Provide nutrient-rich habitats that support high biodiversity

  • Offer economic benefits, e.g. through fishing

Algae in marine biomes

• Microscopic algae like phytoplankton play a crucial role in Earth's carbon and oxygen cycles

  • Through photosynthesis, they absorb carbon dioxide (CO₂) from the atmosphere and use sunlight to convert it into energy, releasing oxygen (O₂) as a byproduct

• Oxygen production:

  • Phytoplankton produce about 50-80% of the Earth's oxygen

    • This means they are essential for maintaining breathable air

  • This oxygen is released into the water and atmosphere

    • This supports marine life and terrestrial organisms

• Carbon dioxide absorption:

  • Phytoplankton absorb large amounts of CO₂

    • This helps reduce greenhouse gas levels in the atmosphere

  • Some of this absorbed carbon becomes part of the food web when phytoplankton are consumed by other organisms

  • Some of this absorbed carbon is stored in the ocean depths when dead phytoplankton sink

    • This contributes to the ocean’s role as a carbon sink

Distribution of nonmineral marine natural resources

• Nonmineral marine natural resources include living resources such as:

  • Fish

  • Shellfish

  • Seaweed

  • Algae

• The global distribution of these resources is influenced by environmental factors, including:

  • Salinity

  • Depth

  • Turbidity

  • Nutrient availability

  • Temperature

• These resources are vital for:

  • Food security

  • Economic activities like fishing

  • Ecological balance within marine ecosystems

Factors affecting distribution of nonmineral marine natural resources

Salinity

• The salt concentration in water impacts the types of species that can survive in a given area

  • For example, fish like salmon can adapt to both saltwater and freshwater, allowing them to live in estuaries and oceans

  • Higher salinity areas, such as the Red Sea, often support unique species due to their specialized adaptations

Depth

• Different marine species live at varying depths, depending on the availability of light and pressure conditions

  • Surface waters: these zones support phytoplankton and small fish that rely on sunlight

  • Deep waters: these zones support species adapted to darkness and high pressure, like anglerfish and squid

    • The deep waters of the Atlantic Ocean are home to commercial species like cod

Turbidity

• Turbidity refers to how clear or murky the water is

  • This affects light penetration and photosynthesis

• Clear waters allow algae and seagrass to grow, supporting diverse marine life

  • For example, coral reefs in the Caribbean thrive in clear, low-turbidity waters

• Turbid waters, often found near river mouths, can support species like shrimp and catfish adapted to low visibility

Nutrient Availability

• Nutrients like nitrates and phosphates drive the productivity of marine ecosystems

  • Upwelling zones, where nutrient-rich water rises to the surface, are hotspots for marine life

Temperature

• Marine species are adapted to specific temperature ranges, which determine their distribution

  • Warm waters: support tropical species like tuna and coral

  • Cold waters: Support species like krill and cod