Maarg Manthan · Topic 4.2

Aquatic Ecosystems and Factors Affecting Their Productivity

Environment & Ecology › Terrestrial and Aquatic Ecosystems · Topic 4.2

Aquatic ecosystems are ecosystems in which water is the main medium, and they cover about 70 percent of the Earth's surface. They are divided into freshwater ecosystems, with low salt content, such as ponds, lakes, rivers and wetlands, and marine ecosystems, with high salinity, such as oceans, coral reefs and estuaries. Aquatic organisms are grouped as plankton, which drift, nekton, which swim actively, and benthos, which live on the bottom, with neuston at the surface and periphyton attached to surfaces. The productivity of aquatic ecosystems depends on light penetration, temperature, nutrient availability, dissolved oxygen, salinity, water movement and turbidity. Light reaches only the upper euphotic zone, and nutrients are often the limiting factor, so upwelling zones and estuaries are highly productive.

5 min read · UPSC Prelims · Environment & Ecology · Terrestrial and Aquatic Ecosystems

At a Glance

Area About 70 percent of Earth
Two classes Freshwater, marine
Life forms Plankton, nekton, benthos
Light zone Euphotic
Limiting factor Often nutrients
Exam link UPSC Prelims, Environment & Ecology: Terrestrial and Aquatic Ecosystems

Where Does It Fit?

This topic follows the account of terrestrial ecosystems. It introduces water-based ecosystems, which the next topics on freshwater, marine ecosystems and plankton explain in detail. Previous topic: Terrestrial Ecosystems: Forest Ecosystem in India, Grasslands, Tundra, Deserts, Mountains. Next topic: Freshwater Ecosystem.

What Are the Types and Life Forms of Aquatic Ecosystems?

Category	Details
Freshwater	Salinity below 0.5 parts per thousand. Lentic (still): ponds, lakes, swamps. Lotic (flowing): rivers, streams. Wetlands
Brackish	Mix of fresh and sea water, as in estuaries and lagoons
Marine	Salinity about 35 parts per thousand. Open ocean, coastal waters, coral reefs, mangroves and deep sea
Plankton	Floating organisms carried by currents: phytoplankton (producers) and zooplankton (consumers)
Nekton	Free-swimming animals such as fish, squid, whales
Benthos	Organisms living on or in the bottom: worms, crabs, mussels, sea stars
Neuston and periphyton	Neuston live at the water surface; periphyton are attached to submerged surfaces

What Are the Zones and Factors Affecting Productivity?

Euphotic zone: sunlit upper layer where photosynthesis exceeds respiration, to about 100 to 200 m in clear ocean
Dysphotic zone: twilight layer with dim light
Aphotic zone: dark layer with no photosynthesis, depending on food falling from above or chemosynthesis
Light: decreases with depth and with turbidity; controls phytoplankton
Nutrients: nitrogen, phosphorus, iron and silica often limit growth; coastal and upwelling areas are rich
Temperature and stratification: warm surface layers separate from cold deep water by a thermocline, which limits nutrient mixing
Dissolved oxygen: needed by animals; cold water holds more oxygen
Salinity and currents: salinity decides species; currents and waves mix nutrients

What Else Should You Remember?

Compensation depth: depth where photosynthesis equals respiration
Upwelling: rising of cold nutrient-rich water, as off Peru, Somalia and western India
El Nino: weakens upwelling off Peru and reduces fish catches
Eutrophication: excess nutrients cause algal blooms and low oxygen
Turbidity: suspended particles reduce light
Secchi disc: a disc used to measure water clarity
BOD: biochemical oxygen demand, a measure of organic pollution
Hypoxia and dead zones: oxygen-poor zones such as in the Gulf of Mexico and Arabian Sea
Open ocean: low productivity per area but huge in total
Coastal waters: high productivity due to nutrient run-off
Coral reefs: highly productive in clear nutrient-poor water through recycling
Productivity ranking: estuaries, reefs and upwelling zones are most productive
Deep sea vents: chemosynthetic ecosystems
Iron fertilisation: proposal to add iron to oceans to boost plankton and absorb carbon, which is controversial
Fisheries: depend on productive zones
Blue economy: sustainable use of ocean resources
Ocean acidification: lowers pH and harms shell-forming organisms
Temperature rise: increases stratification and may reduce productivity
Link: see Freshwater Ecosystem and Marine Ecosystem and Marine Organisms
Link to plankton: see Plankton: Phytoplankton and Zooplankton
Caution: numbers like depths vary in sources
Estuaries: mix freshwater and sea water and are productive because rivers bring nutrients
Salinity classes: freshwater, brackish and saline
Marine zones: intertidal, neritic and oceanic zones, and pelagic and benthic realms
Dissolved gases: oxygen enters by diffusion and photosynthesis while carbon dioxide dissolves and forms carbonic acid
Role of rivers: rivers supply nutrients and sediments to seas
Coastal wetlands: mangroves, salt marshes and seagrass are carbon-rich
Primary producers: phytoplankton make about half of global photosynthesis
Fish production: most marine catch comes from the productive continental shelf
Tides and waves: mix water near the coast and supply nutrients and oxygen
Seasonal change: productivity of Indian seas peaks during the monsoon upwelling off the west coast
Exam tip: remember that open ocean has low productivity per unit area while estuaries and reefs have high
Wetland definition: Ramsar Convention defines wetlands broadly as marshes, fens, peatlands or water, natural or artificial, permanent or temporary, up to six metres deep at low tide
Chemosynthetic systems: hydrothermal vent communities depend on bacteria and not on light

Why Does It Matter for the Exam?

Life forms: plankton, nekton, benthos
Zones: euphotic, aphotic
Factors: light, nutrients, oxygen
Current affairs link: ocean acidification and dead zones
Common errors: saying open oceans are highly productive per area or that plankton swim actively

As an analytical point, water ecosystems supply food, oxygen and climate regulation, but nutrient pollution and warming are changing their productivity. Managing land use in catchments is essential to protect rivers, lakes and coasts.

Why Is This Topic Important?

Types Fresh, marine
Life forms Three
Factors Light, nutrients

What Came Next?

The next topic covers freshwater ecosystems. Next topic: Freshwater Ecosystem.

Key People and Terms

Plankton Drifting organisms
Nekton Swimmers
Benthos Bottom dwellers
Euphotic zone Sunlit layer
Upwelling Rising nutrient water
BOD Oxygen demand of organic matter

Exam Corner

Points to Remember

Water covers 70 percent of Earth.
Plankton drift; nekton swim; benthos bottom.
Photosynthesis only in the euphotic zone.
Upwelling raises nutrients.
Marine salinity about 35 ppt.

Do Not Confuse With

Lentic and lotic Still versus flowing water
Euphotic and aphotic Sunlit versus dark
Plankton and nekton Drifters versus swimmers

Memory Hook

Plankton drift, nekton swim, benthos crawl; light and nutrients set the pace.

Mains Angle

Questions ask for life forms, zones and productivity factors. These points help.

Types Aquatic classes
Factors Productivity
Threats Pollution

A Question You May Face

An original practice question, not a past paper question.

“Describe the types of aquatic ecosystems and the factors that determine their productivity.”

How to Answer

Types Fresh, marine.
Life forms Three.
Factors Light, nutrients.
Threats Eutrophication.

GS Relevance

Prelims: types, life forms, zones, factors of productivity. GS Paper 3: ecology.

Frequently Asked Questions

What are aquatic ecosystems?

Aquatic ecosystems are ecosystems where water is the main medium of life. They are divided into freshwater ecosystems such as ponds, lakes, rivers and wetlands, and marine ecosystems such as oceans, coral reefs and estuaries. Together they cover about 70 percent of the Earth's surface and hold a large share of its biodiversity.

What are plankton, nekton and benthos?

Plankton are organisms that drift with water currents, including microscopic plants called phytoplankton and animals called zooplankton. Nekton are animals that swim actively against currents, like fish and whales. Benthos are organisms that live on or in the bottom of a water body, like worms, crabs and mussels.

What factors affect productivity in aquatic ecosystems?

Productivity depends on light penetration, availability of nutrients such as nitrogen, phosphorus, iron and silica, temperature, dissolved oxygen, salinity, turbidity and water movement. Light limits production to the upper euphotic zone, and in many places nutrients are the limiting factor, so areas of nutrient supply are highly productive.

What is upwelling?

Upwelling is the rising of cold, nutrient-rich water from deep layers to the surface, usually along coasts where winds push surface water away. It supports huge phytoplankton growth and rich fisheries, as off Peru, Somalia and the west coast of India. El Nino can weaken upwelling.

What is the difference between lentic and lotic ecosystems?

Lentic ecosystems are standing freshwater bodies, such as ponds, lakes and swamps, where water moves slowly. Lotic ecosystems are flowing freshwater bodies such as rivers and streams, where the current brings oxygen and carries nutrients, and the organisms are adapted to flow.

PYQ Practice — Statement Analysis

1 Photosynthesis in oceans occurs in the euphotic zone.

True

Where light reaches.

2 Nekton are organisms that drift passively.

False

They swim actively.

3 Upwelling zones are highly productive.