Aquatic Organisms
Aquatic organisms are unevenly distributed and classified based on their zone of occurrence and mobility across zones. Broadly, they are grouped into the following five categories:
1. Neuston
- Organisms that live at the air-water interface. Some stay on top of the water (e.g., water striders), while others stay just below the surface and feed within water.
- Examples: Floating plants, beetles, back-swimmers.
2.Periphyton
- Organisms that attach to stems, leaves, or submerged surfaces emerging from the bottom mud.
- Examples: Sessile algae, attached animals
3.Plankton
- Microscopic free-floating organisms, unable to swim effectively against currents.
- Includes:
- Phytoplankton: Microscopic algae (plants).
- Zooplankton: Microscopic crustaceans, protozoans.
- Found in almost all aquatic ecosystems except fast-flowing waters.
4.Nekton
- Free-swimming organisms that can overcome water currents. Range in size from small swimming insects to large whales.
- Examples: Fish, squids, whales, swimming insects.
5. Benthos
- Organisms living at the bottom of the water body. Found in almost every aquatic ecosystem.
- Examples: Worms, mollusks, bottom-dwelling invertebrates.
Factors Limiting Productivity of Aquatic Habitats
Aquatic ecosystems are influenced by several limiting factors:
Sunlight
- Penetration of sunlight diminishes rapidly with depth.
- Determines plant distribution and defines:
- Photic Zone (Euphotic): Upper, well-lit layer where photosynthesis occurs. Both photosynthesis and respiration take place.
- Aphotic Zone (Profundal): Lower, dark layer where light is insufficient for photosynthesis. Only respiration occurs here.
Dissolved Oxygen (DO)
- Dissolved oxygen in water is much lower than in air (~10 ppm in freshwater).
Dissolved oxygen (DO) Dissolved oxygen (DO) is the amount of oxygen that is present in water. Water bodies receive oxygen from the atmosphere and from aquatic plants. Running water, such as that of a swift moving stream, dissolves more oxygen than the still water of a pond or lake. |
- Dissolved Oxygen enters water via:
- Diffusion across air-water interface.
- Photosynthesis by aquatic plants.
- Dissolved Oxygen is lost via:
- Respiration by fish, decomposers, zooplankton.
- Warmer water (less soluble oxygen, higher decomposition).
- Critical Levels: When DO falls below 3–5 ppm, many aquatic organisms cannot survive.
- Winterkill: In ice-covered lakes, lack of light stops photosynthesis but respiration continues, depleting oxygen and causing fish deaths when ice melts.
Transparency
- Suspended particles (clay, silt, plankton) make water turbid, reducing light penetration and photosynthesis.
Temperature
- Water temperature changes slowly due to its high specific heat.
- Aquatic organisms have narrow tolerance to temperature changes compared to terrestrial organisms.
- Even small increases in water temperature threaten aquatic life by enhancing decomposition and reducing DO.