Maarg Manthan · Topic 4.3

Freshwater Ecosystem

Environment & Ecology › Terrestrial and Aquatic Ecosystems · Topic 4.3

Freshwater ecosystems have a salt content of less than 0.5 parts per thousand and make up less than 1 percent of the Earth's surface, though they hold a large share of its species. They are of two main types: lentic, standing water such as ponds, lakes and swamps, and lotic, flowing water such as rivers and streams, together with wetlands, which are in between. A lake has three zones: the littoral zone near the shore with rooted plants, the limnetic zone of open water where light reaches, and the profundal zone of deep water without light. Rivers change from a fast, cold, oxygen-rich upper course to a slow, warm lower course. Lakes undergo eutrophication, an enrichment with nutrients that raises algal growth and lowers oxygen, and are classed as oligotrophic, mesotrophic or eutrophic.

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

At a Glance

Salinity Below 0.5 ppt
Still water Lentic
Flowing water Lotic
Lake zones Littoral, limnetic, profundal
Problem Eutrophication
Exam link UPSC Prelims, Environment & Ecology: Terrestrial and Aquatic Ecosystems

Where Does It Fit?

This topic follows the account of aquatic ecosystems. It describes lakes, ponds and rivers in detail before the next topic on marine ecosystems. Previous topic: Aquatic Ecosystems and Factors Affecting Their Productivity. Next topic: Marine Ecosystem and Marine Organisms.

What Are the Features of Lakes, Ponds and Rivers?

Ecosystem	Features
Pond	Small, shallow, still water where light often reaches the bottom; often seasonal. Rich plant life
Lake	Large standing water body with zones. Littoral zone: shallow shore with rooted plants and many animals. Limnetic zone: open water with phytoplankton and fish. Profundal zone: deep dark area with decomposers
Thermal stratification	In temperate lakes, a warm epilimnion, a thermocline and a cold hypolimnion form in summer; they mix in autumn and spring (turnover)
River	Flowing water from source to mouth. Upper course: cold, fast, oxygen-rich, rocky bed, fewer nutrients. Middle: wider and slower. Lower: slow, warm, muddy, nutrient-rich with delta
Stream life	Streamlined or attached animals, such as trout and stonefly larvae in clean upper reaches
Wetland	Areas of marsh, fen or peatland, permanently or seasonally flooded
Productivity	Ponds and shallow lakes are productive, while deep lakes are less so

What Are Lake Trophic Types and Eutrophication?

Oligotrophic lake: nutrient-poor, clear, high oxygen, low productivity
Mesotrophic lake: moderate nutrients
Eutrophic lake: nutrient-rich, high algal growth, low transparency and oxygen
Eutrophication: enrichment by nitrogen and phosphorus from fertilisers, sewage and detergents; causes algal blooms, oxygen loss and fish kills
Cultural eutrophication: human-caused, faster than natural ageing
Lake ageing: lakes fill with silt and organic matter over time and may become land through succession
Indian lakes: Dal, Wular (largest freshwater lake in India, Jammu and Kashmir), Loktak (largest freshwater lake in the north-east, with floating phumdis), Chilika (brackish lagoon), Pulicat, Vembanad, Sambhar (saline)
Indian rivers: Ganga, Brahmaputra, Indus, Godavari, with the Ganga river dolphin and gharial as indicator species

What Else Should You Remember?

Phumdis: floating mats of vegetation in Loktak Lake
Keibul Lamjao: the only floating national park, home of the sangai deer
Ramsar sites: many Indian lakes are Ramsar wetlands
Algal bloom: dense growth of algae that can release toxins
Water hyacinth: invasive plant of lakes, as in Endemic Species, Keystone Species, Indicator Species, Invasive Species
Dissolved oxygen: higher in cold, flowing water
BOD and COD: measures of pollution
Biological indicators: stonefly and mayfly larvae show clean water, tubifex worms show organic pollution
River continuum concept: describes gradual change in the river from headwaters to mouth
Floodplain: rich seasonal ecosystem
Riparian vegetation: banks protect water quality
Dams: change flow, trap silt and affect fish migration
Hilsa: anadromous fish that moves from sea to river to breed
Eels: catadromous fish, moving from river to sea
Groundwater ecosystems: aquifers hold unique small animals
Lake restoration: desilting, sewage treatment and weed removal
National Lake Conservation Plan: programme for lake restoration
Namami Gange: programme for cleaning the Ganga
Link to wetlands: see Coastal and Wetland Ecosystems macro
Link to pollution: see Water Resources and Water Pollution macro
Caution: facts on lakes’ size can change with seasons
Oxygen sag: below a sewage outfall, a river loses oxygen and recovers slowly downstream
Fish zones: trout in upper courses and carp in lower slow waters
Floodplain fisheries: seasonal flooding feeds fish breeding in the Gangetic and Brahmaputra plains
Ponds in villages: traditional tanks recharge groundwater and support wildlife
Wetland law: the Wetlands (Conservation and Management) Rules, 2017 regulate wetlands
Thermal pollution: warm discharge from power plants lowers oxygen in water
Siltation: soil erosion in catchments fills lakes and reduces depth
Indicator fish: Ganga river dolphin shows river health and is the national aquatic animal
Lake cleaning schemes: National Lake Conservation Plan began in 2001 and later merged into the National Plan for Conservation of Aquatic Ecosystems

Why Does It Matter for the Exam?

Types: lentic, lotic
Lake zones: littoral, limnetic, profundal
Trophic types: oligo, meso, eutrophic
Current affairs link: Ramsar lakes and lake restoration
Common errors: saying oligotrophic lakes are nutrient-rich or mixing lake zones

As an analytical point, freshwater ecosystems are small in area but vital for drinking water, fisheries and biodiversity, and are among the most threatened. Controlling nutrient inflow from the catchment is the key step in protecting them.

Why Is This Topic Important?

Types Lentic, lotic
Zones Lake
Threat Eutrophication

What Came Next?

The next topic covers the marine ecosystem and marine organisms. Next topic: Marine Ecosystem and Marine Organisms.

Key People and Terms

Lentic Standing water
Lotic Flowing water
Littoral zone Shore zone
Epilimnion Warm surface layer
Eutrophication Nutrient enrichment
Oligotrophic Nutrient-poor

Exam Corner

Points to Remember

Freshwater salinity below 0.5 ppt.
Wular: largest freshwater lake in India.
Loktak has phumdis.
Eutrophication caused by N and P.
Upper river: cold, fast, oxygen-rich.

Do Not Confuse With

Oligotrophic and eutrophic Nutrient-poor versus nutrient-rich
Littoral and profundal Shallow shore versus deep dark zone
Anadromous and catadromous Sea to river versus river to sea

Memory Hook

Shore, open, deep: littoral, limnetic, profundal; N and P bloom the lake.

Mains Angle

Questions ask for zones, lake types and Indian lakes. These points help.

Types Lentic, lotic
Threats Eutrophication
Plans Restoration

A Question You May Face

An original practice question, not a past paper question.

“Describe the zones of a lake and the process of eutrophication, and suggest measures for restoring freshwater ecosystems.”

How to Answer

Types Lentic, lotic.
Zones Three.
Eutrophication Causes.
Measures Restoration.

GS Relevance

Prelims: lentic and lotic, lake zones, trophic status, eutrophication, Indian lakes and rivers. GS Paper 3: ecology and water.

Frequently Asked Questions

What are the zones of a lake?

A lake has three main zones: the littoral zone near the shore, which is shallow with rooted plants and many animals; the limnetic zone, the open water where light penetrates and phytoplankton and fish live; and the profundal zone, the deep area below the reach of light, where decomposers and bottom feeders live.

What is eutrophication?

Eutrophication is the enrichment of a water body with nutrients, mainly nitrogen and phosphorus, from fertilisers, sewage and detergents. This causes rapid growth of algae, and when they die their decomposition uses up oxygen, causing fish kills and loss of biodiversity. Human-caused eutrophication is called cultural eutrophication.

What is the difference between oligotrophic and eutrophic lakes?

An oligotrophic lake has few nutrients, clear water, high dissolved oxygen and low productivity. A eutrophic lake is rich in nutrients, has dense algal growth, low transparency and often low oxygen in deep water. Mesotrophic lakes lie in between, and many lakes shift towards eutrophic as they age or are polluted.

What are lentic and lotic ecosystems?

Lentic ecosystems are standing freshwater bodies such as ponds, lakes and swamps. Lotic ecosystems are flowing freshwater bodies such as rivers and streams. The two differ in oxygen content, temperature variation and the adaptations of organisms, which in streams are often streamlined or attached to stones.

What are phumdis?

Phumdis are floating islands of soil, vegetation and organic matter found in Loktak Lake in Manipur. They support the Keibul Lamjao National Park, the world's only floating national park, which is the last natural habitat of the sangai or brow-antlered deer.

PYQ Practice — Statement Analysis

1 Eutrophication is caused by excess nitrogen and phosphorus.

True

Leading to algal blooms.

2 The littoral zone is the deepest zone of a lake.

False

It is the shore zone.

3 Loktak Lake has floating phumdis.