Aquatic environments make up the largest portion of the biosphere, covering approximately 75% of the Earth's surface.
Water Volume in the Biosphere
97.5% is in oceans.
2.5% is freshwater, including glaciers, polar ice caps, lakes, rivers, and groundwater.
Categorization of Aquatic Biomes
Based primarily on abiotic characteristics such as:
Salinity
Depth
Currents
Main Categories of Aquatic Environments
Divided based on salinity:
Freshwater
Saltwater (or marine)
Examples of Aquatic Biomes
Lakes
Reefs
Rivers
Pelagic zones
Benthic zones
Estuaries
Intertidal zones
Hydrologic Cycle
All aquatic biomes are directly or indirectly linked as components of the hydrologic cycle.
Hydrologic Cycle Components
Precipitation over land and ocean
Transport of water vapor
Solar energy over land
Movement of water vapor by wind
Evaporation from the ocean
Infiltration through the soil
Transpiration from land
Surface runoff and groundwater
Abiotic Factors Affecting Aquatic Ecosystems
Solar radiation
Temperature
Nutrient availability
Common Characteristics of Aquatic Biomes
More moderate temperatures compared to terrestrial biomes.
Light penetration decreases rapidly with depth.
Nutrients tend to concentrate at the bottom.
Stratification in Aquatic Biomes
Many aquatic biomes are stratified into zones or layers defined by:
Light penetration
Temperature
Depth
Light Penetration and Stratification
Stratification (vertical structure) based on light penetration:
Photic Zone: The upper zone where light penetrates and photosynthesis can occur.
Aphotic Zone: The intermediate zone where light does not penetrate.
Benthic Zone: The bottom zone where decomposition is most active.
Lake Stratification and Zonation
Littoral Zone
Limnetic Zone
Photic Zone
Pelagic Zone
Benthic Zone
Aphotic Zone
Marine Stratification and Zonation
Pelagic Zone
Photic Zone (0-200m)
Neritic Zone
Oceanic Zone
Aphotic Zone (200m-2,000-6,000m)
Benthic Zone
Intertidal Zone
Continental Shelf
Continental Barrier
Coral Reefs
Corals are colonial animals.
Corals secrete an external skeleton composed of calcium carbonate (CaCO_3) on which they grow.
Corals feed on zooplankton suspended in the water.
Zooplankton covers only 10% of their energy needs.
The remaining 90% comes from organic molecules produced by symbiotic zooxanthellae (protists) via photosynthesis.
Coral reefs consist of calcareous structures formed by the accumulation of coral skeletons.
Due to the mutualistic relationship with photosynthetic zooxanthellae, the distribution of corals is limited to clear waters with depths less than 45 meters (148 feet) where there is sufficient light.
Precipitation of calcium, necessary for the formation of the calcareous exoskeleton of corals, occurs when:
Water salinity and temperature are high (between 20°C and 28°C / 68°F and 82°F).
The concentration of CO_2 is low.
Coral reefs are found in warm, shallow waters near coasts in tropical and subtropical zones (between the Tropic of Cancer and the Tropic of Capricorn).
Coral reefs constitute less than 1% of the total area of the world's oceans, but they provide a great diversity of habitats and contain a great diversity of species.