Aquatic Ecosystems
We have already studied terrestrial ecosystems — life on land.
Now, let’s dive (literally) into the aquatic world, where life follows a completely different set of rules.
When we say aquatic ecosystem, we simply mean:
“A community of plants and animals living and interacting within a water body.”
This includes everything — from a tiny pond in your village to the vast Pacific Ocean.
🧭 Classification of Aquatic Ecosystems
Aquatic ecosystems are divided based on salinity — that is, how much salt the water contains.
| Type | Salt Content | Examples | Key Features |
|---|---|---|---|
| Freshwater Ecosystem | < 5 ppt (parts per thousand) | Rivers, lakes, ponds, springs | Low salinity, supports diverse life |
| Marine Ecosystem | ≥ 35 ppt | Seas, oceans | High salinity, covers 70% of Earth |
| Brackish Water Ecosystem | 5–35 ppt | Estuaries, mangroves, salt marshes | Mixture of sea and freshwater |
Now, understand these one by one.
(a) Freshwater Ecosystems
These are water bodies found on land with very low salt concentration.
They are further divided into two categories:
- Lentic Ecosystems – still or static water
→ e.g., lakes, ponds, bogs, swamps - Lotic Ecosystems – running or flowing water
→ e.g., streams, rivers, springs
These systems are continuously refreshed by rainfall and surface runoff, keeping them low in salinity.
(b) Marine Ecosystems
These include oceans and seas — vast, saline water bodies covering about three-fourths of Earth’s surface.
- Average salinity ≈ 35 ppt
- Home to enormous biodiversity — from microscopic plankton to whales.
- Major roles:
- Regulate climate by storing and distributing heat.
- Act as carbon sinks.
- Provide food, minerals, and oxygen (through phytoplankton).
(c) Brackish Water Ecosystems
These are transitional zones — where freshwater and seawater meet.
Examples:
- Estuaries (river mouths mixing with sea water)
- Mangroves
- Salt marshes
Salinity: between 5–35 ppt
Such ecosystems are nutrient-rich and act as nurseries for many fish species — that’s why mangroves are called the cradle of marine life.
🐠 Aquatic Organisms
All aquatic life can be classified based on their position and movement within the water column.
| Type | Habitat / Movement | Example | Description |
|---|---|---|---|
| Neuston | At the air–water interface | Water striders, floating plants | Live on or just below the surface |
| Periphyton | Attached to stems, leaves, or stones | Sessile algae, small invertebrates | Grow on submerged surfaces |
| Plankton | Floating, microscopic organisms | Algae, diatoms, protozoa | Carried by water currents |
| Nekton | Active swimmers | Fish, whales, squid | Can overcome water currents |
| Benthos | Bottom dwellers | Crabs, snails, worms | Live on or in the sediments |
☀️ Factors Controlling Productivity in Water Bodies
Just as in terrestrial ecosystems, certain abiotic factors decide how productive an aquatic ecosystem can be — i.e., how much life it can support.
Let’s study the main ones.
(a) Sunlight
Sunlight = Energy Source for Photosynthesis.
But in water, light doesn’t penetrate deeply.
Its availability depends on:
- Turbidity (suspended particles like silt, clay, plankton)
- Water depth
Based on how far light penetrates, we divide water into two zones:
| Zone | Light Availability | Characteristics |
|---|---|---|
| Photic (Euphotic) Zone | Receives ≥1% of surface light | Photosynthesis possible; high productivity |
| Aphotic (Profundal) Zone | No sufficient light | No photosynthesis; decomposers dominate |
So, if you dive deeper — it gets darker, colder, and less oxygenated.
Life becomes scarce.
(b) Dissolved Oxygen (DO)
Just as land animals need air, aquatic organisms need oxygen dissolved in water.
- Average DO in freshwater ≈ 10 parts per million (ppm) —
which is 150 times lower than air! - Oxygen enters water via:
- Air–water interface
- Photosynthesis by aquatic plants
It escapes through:
- Respiration of organisms
- Decomposition of organic matter
❄️ Winterkill Phenomenon
In extremely cold regions, when lakes freeze:
- The ice layer blocks sunlight → no photosynthesis.
- But respiration continues, using up oxygen.
- Eventually, DO levels fall, and fish die due to suffocation.
➤ This is called Winterkill.
🌍 Impact of Global Warming on DO
- Warm water holds less oxygen.
- It also speeds up decomposition, consuming oxygen faster.
- Hence, higher temperature → lower DO → aquatic deaths.
When DO drops below 3–5 ppm, many fish and zooplankton cannot survive.
That’s why you often see fish kills in summer or polluted lakes.
(c) Temperature
- Water acts as a temperature buffer, so changes are slower than in air.
- However, aquatic organisms have narrow tolerance limits.
- Even a small rise in water temperature (due to global warming) can be fatal.