Life in Water: Aquatic Biomes

Life in Water: Aquatic Biomes

The Biosphere and its Components

  • Biosphere: The total global ecosystem, encompassing all portions of Earth that support life. It includes:

    • Lithosphere: The solid outer part of Earth.

    • Hydrosphere: All the water on Earth.

    • Atmosphere: The gases surrounding Earth.

  • Levels of Biological Organization (from smallest to largest):

    • Individual

    • Population

    • Community

    • Ecosystem

    • Biome

    • Biosphere

Biomes

  • Biome: A categorical unit of the aquatic biosphere distinguished primarily by prominent physical characteristics such as salinity and water movement.

  • Major Categories of Aquatic Biomes:

    • Saltwater: High salinity environments (e.g., oceans, coral reefs, estuaries).

    • Freshwater: Low salinity environments.

      • Lentic: Standing water (e.g., lakes, ponds).

      • Lotic: Flowing water (e.g., rivers, streams).

The Hydrologic Cycle

  • Definition: The dynamic movement of water between various "reservoirs" within a global exchange system.

  • Water Distribution on Earth:

    • 71\% of Earth's surface is covered by water.

    • Ocean: Contains 97\% of all water.

    • Ice Caps & Glaciers: Hold 2\% of water.

    • Freshwater (liquid): Accounts for only 1\% of water.

  • Key Process: Evaporation is a crucial component driving the cycle.

Ocean Currents

  • Formation: Wind drives ocean currents, which form massive circulation systems known as gyres.

  • Influence: These currents significantly moderate regional temperatures and distribute heat.

  • Examples of Major Currents:

    • The Gulf Stream: A warm-water current that moderates temperatures in northwestern Europe.

    • The Kuroshio: A warm-water current bringing tropical waters to the southern islands of Japan.

    • The Oyashio: A cold-water current that cools the northeast coast of Asia.

    • The Labrador Current: Cools the maritime provinces of northeastern Canada.

    • The Humboldt Current: Brings cold Antarctic waters along the west coast of South America.

    • The Benguela Current: Cools waters off southwestern Africa.

    • The West Wind Drift: Cold currents that circle Antarctica.

Major Aquatic Biomes in Detail
Oceans

  • Major Ocean Basins:

    • Pacific Ocean: The largest, covering nearly 180 \, \text{million km}^2.

    • Atlantic Ocean: Over 106 \, \text{million km}^2.

    • Indian Ocean: Nearly 75 \, \text{million km}^2.

  • Geographic Complexity: Each major basin contains smaller seas along its margins and varies greatly in average depth, featuring underwater mountains and trenches.

  • Structure - Horizontal Zones:

    • Littoral (Intertidal) Zone: The shallow coastline region, exposed to air at low tide.

    • Neritic Zone: Extends along the continental shelf, relatively shallow but always submerged.

    • Oceanic Zone: The vast open ocean beyond the continental shelf.

  • Structure - Vertical Zones:

    • Epipelagic Zone: The surface layer, receiving sunlight.

    • Mesopelagic Zone: Below epipelagic, dim light.

    • Bathypelagic Zone: Deep, dark, and cold.

    • Abyssal Zone: Very deep ocean, extreme pressure.

    • Hadal Zone: The deepest parts, found in ocean trenches.

    • Benthic Zone: Refers to ocean floor habitats (can occur at any depth).

Kelp Forests

  • Characteristics: Submerged forests formed by large brown algae called kelp.

  • Location: Grow along rocky shores in regions where maximum water temperatures are less than 20^ ext{o}\text{C} (e.g., along the west coast of South America due to cold Humboldt Current, extending into subtropical regions).

Coral Reefs

  • Characteristics: Complex underwater ecosystems built by coral polyps.

  • Locations: Typically circle the planet in a band centered on the equator, absent from shores heavily silted by runoff from large rivers.

  • Formation Types:

    • Fringing Reef: Corals grow directly around young islands.

    • Barrier Reef: As an island subsides, corals continue to grow upward and outward toward the sea, forming a barrier separated from the land by a lagoon.

    • Atoll: When the island completely subsides below sea level, coral growth may form a ring-shaped reef on top of the submerged island, enclosing a central lagoon.

Intertidal Zones

  • Definition: The area of the marine environment that is exposed to air at low tide and submerged at high tide.

  • Environmental Gradient: Tidal fluctuation produces a gradient of environmental conditions.

    • Supratidal Fringe: Most exposure to atmosphere, least inundation.

    • Upper Intertidal Zone: Less inundation than middle, more than supratidal.

    • Middle Intertidal Zone: Intermediate exposure and inundation.

    • Lower Intertidal Zone: Least exposure to atmosphere, most inundation.

    • Subtidal Zone: Always submerged, below the lowest tide mark.

Estuaries

  • Definition: Partially enclosed coastal bodies of brackish water, where freshwater from rivers and streams merges with saltwater from the ocean.

Salt Marshes

  • Characteristics: Herb-dominated coastal ecosystems.

  • Locations: Found at mid to high latitudes.

  • Landscape Features (carved by tidal ebb and flow):

    • Creek Levee: Elevated banks along tidal creeks.

    • Creek Bench: Flat areas adjacent to creek levees.

    • Secondary Low Marsh: Lower marsh areas.

    • Creek Bottom: The bed of the tidal creeks.

    • Salt Pan: Depressions in the marsh where water can evaporate, leaving behind salt.

    • Marsh Flat: Flat marsh area with a concave profile.

Mangrove Forests

  • Characteristics: Forested wetlands dominated by salt-tolerant trees and shrubs (mangroves).

  • Locations: Found in tropical and subtropical environments where minimum sea surface temperatures are greater than 16^ ext{o}\text{C}.

Rivers & Streams

  • Physical Structure:

    • Riffle: Shallow, fast-flowing sections with turbulent water, often over rocky substrates.

    • Pool: Deeper, slower-flowing sections, often with finer sediments.

    • Wetted Channel: Contains water year-round.

    • Active Channel: The area that is usually flooded at least once each year.

    • Riparian Zone: The terrestrial area adjacent to the river/stream, significantly influenced by the water body. Roots of riparian trees often draw water from groundwater.

    • Benthic Zone: The bottom substrate.

    • Hyporheic Zone: The transitional zone between the surface water and the true groundwater, where water flows from river channels into groundwater and vice-versa.

    • Phreatic Zone: The true groundwater table, below the hyporheic zone. Aquatic organisms live in all zones from the phreatic zone to the water column.

  • Major River Systems (Examples):

    • North America: Yukon, Mackenzie, Columbia, Missouri, Mississippi, Colorado, Rio Grande, St. Lawrence.

    • South America: Amazon, Orinoco, La Plata.

    • Europe/Asia: Rhine, Volga, Danube, Ob, Yenisey, Lena, Amu Darya, Indus, Ganges, Yangtze, Huang He, Amur, Mekong, Tigris, Euphrates.

    • Africa: Nile, Niger, Congo, Orange, Zambezi.

    • Australia: Murray-Darling.

Lakes

  • Examples of Major Lakes: Great Bear Lake, Great Slave Lake, Lake Athabasca, Lake Winnipeg, Great Lakes (Superior, Michigan, Huron, Erie, Ontario), Great Salt Lake, Lake Värnern, Lake Ladoga, Caspian Sea, Lake Balkash, Aral Sea, Lake Baikal, Lake Chad, Lake Tana, Lake Turkana, Lake Victoria, Lake Tanganyika, Lake Nyasa, Lake Titicaca.

  • Thermal Stratification (Vertical Zonation):

    • Littoral Zone: The shallow, near-shore area where sunlight penetrates to the bottom, allowing rooted aquatic plants to grow.

    • Limnetic Zone: The open, offshore, sunlit surface water layer where photosynthesis occurs.

    • Epilimnion: The warmest, top layer of water in a stratified lake, where sunlight penetrates and warms the water.

    • Metalimnion (Thermocline): The middle layer where temperature and other physical/chemical factors (like dissolved oxygen) change rapidly with depth.

    • Hypolimnion: The coldest, deepest layer of water, which is dark and may lack dissolved oxygen.