The Hydrosphere is the combined mass of all water found on, under, and over the surface of the Earth. It includes water in all three states: liquid (oceans, rivers, lakes), solid (ice caps, glaciers), and gas (water vapor in the atmosphere).
Importance of Hydrosphere
Essential for Life: Water is the fundamental solvent for all life processes. Organisms are composed mostly of water.
Climate Regulation: The ocean (a huge part of the hydrosphere) is a massive heat sink. It absorbs, stores, and transports heat around the globe, moderating global climate.
Habitat: Aquatic ecosystems (oceans, lakes, rivers) provide the habitat for a vast majority of Earth's biodiversity.
Geological Agent: Water is the primary agent of weathering and erosion, shaping the Earth's surface (e.g., carving canyons, building deltas).
Human Resource: We depend on it for drinking, agriculture (irrigation), industry, transportation, and energy (hydroelectric power).
Key Fact: The distribution of Earth's water is highly uneven.
~97.5% is Salt Water (in oceans).
~2.5% is Fresh Water.
And of that 2.5% fresh water:
~68.7% is locked in glaciers and ice caps.
~30.1% is groundwater.
~1.2% is other surface water (lakes, rivers, swamps).
This means only a tiny fraction of all water is easily accessible fresh water for human use.
2. Hydrological Cycle
The Hydrological (or Water) Cycle is the continuous, global process of water movement between the atmosphere, land, and oceans, driven by solar energy and gravity.
Diagram: A landscape showing the ocean, mountains, and land.
1. Evaporation: (From ocean/lakes) Solar energy turns liquid water into gas (water vapor).
2. Transpiration: Water vapor is released from plants. (Evapotranspiration = Evaporation + Transpiration).
3. Condensation: Water vapor in the air cools and changes back into liquid, forming clouds.
4. Precipitation: Water is released from clouds as rain, snow, sleet, or hail.
5. Collection/Runoff: Water that falls on land flows over the surface (surface runoff) into rivers, lakes, and oceans.
6. Infiltration/Percolation: Water soaks into the soil and moves downward to become groundwater.
3. Fresh Water Ecosystem
These are aquatic ecosystems with low salt concentrations (salinity < 1%). They are divided into two main types:
Lotic (Flowing Water) Ecosystems
Examples: Rivers, streams, creeks.
Key Characteristics:
Unidirectional Flow: Water moves in one direction (downhill).
Organism Adaptations: Organisms are adapted to not be washed away (e.g., flat bodies, strong attachments, strong swimmers).
Energy Source: In small, shaded streams, the main energy source is often allochthonous (comes from outside), like leaves falling into the water. In large rivers, it's autochthonous (from algae/plants within).
Lentic (Standing Water) Ecosystems
Examples: Lakes, ponds, bogs, marshes.
Key Characteristics:
Still Water: No unidirectional current.
Zonation: Lakes are often stratified (layered) based on light and depth:
Littoral zone: Shallow, near-shore, light penetrates to the bottom, rooted plants grow. High biodiversity.
Limnetic zone: Open, surface water, light penetrates, dominated by plankton (phytoplankton and zooplankton).
Profundal zone: Deep, open water, below the range of light penetration. No photosynthesis. Organisms (fish, decomposers) depend on organic matter "raining" from above.
Benthic zone: The bottom substrate (mud/sediment) of the entire lake.
4. Marine Ecosystem
These are the saltwater ecosystems of the oceans, covering over 70% of the Earth's surface. They are the largest of all ecosystems.
Key Characteristics:
High Salinity (Salt Content): Averages 3.5% (or 35 ppt).
Vast and Deep: Huge volume, creating many different habitats.
Zonation: Also zoned by light and depth.
Photic (Euphotic) Zone: The top layer (0-200m) where sunlight penetrates, allowing photosynthesis by phytoplankton.
Aphotic Zone: The vast, deep, dark layer below 200m where there is no light. Organisms here are adapted to high pressure and cold, and rely on detritus from above or chemosynthesis (at hydrothermal vents).
Specific Marine Ecosystems: Include highly diverse coral reefs, open-ocean pelagic zones, and deep-sea benthic zones.
5. Estuarine Ecosystem
An Estuary is a partially enclosed, coastal body of water where a freshwater river or stream meets and mixes with the saltwater of the ocean.
Key Characteristics:
Brackish Water: The water is a mixture of fresh and salt, and its salinity changes with the tides and river flow.
Highly Variable: Organisms must be adapted to constant changes in salinity, temperature, and water level.
Extremely High Productivity: Estuaries are among the most productive ecosystems on Earth. Rivers bring in a constant supply of nutrients, and the shallow, sunlit water promotes rapid plant growth (e.g., salt marshes, mangroves).
"Nurseries of the Sea": The high nutrient levels and protection from ocean predators make them critical breeding and feeding grounds (nurseries) for many commercially important species of fish, shrimp, and crabs.
6. Concept of Surface and Ground Water
Surface Water
This is any body of water found on the Earth's surface. It is the most visible and easily accessible water source.
Characteristics: It is replenished by precipitation (rain/snow) and surface runoff. It is vulnerable to evaporation and direct pollution.
Ground Water
This is water that is held underground in the soil and in the pores and fractures of rock formations.
Infiltration/Percolation: Water from precipitation soaks into the ground (infiltrates) and moves downward (percolates).
Water Table: The upper level of the "zone of saturation," which is the area underground where all pores are completely filled with water.
Aquifer: A permeable underground layer of rock or sediment (like sand or gravel) that can hold and transmit usable quantities of water. We pump water from aquifers using wells.
Key Issues:
Surface Water: Easily polluted by industrial discharge, agricultural runoff (fertilizers, pesticides), and sewage.
Ground Water: "Out of sight, out of mind." It can also be polluted (e.g., from leaking landfills or septic tanks) and is very difficult and expensive to clean. Over-extraction (pumping water faster than it is recharged) is a major global problem, causing water tables to drop and wells to run dry.