Aquatic Ecosystem
Aquatic Ecosystems

Aquatic Ecosystem

Aquatic Ecosystems for UPSC

Aquatic ecosystems refer to plant and animal communities occurring in water bodies. Aquatic ecosystems are classified on the basis of salinity into following types:

Fresh water ecosystems — Water on land which is continuously cycling and has low salt content (always less than 5 ppt) is known as fresh water. There are two types of fresh water ecosystems: (i) Static or still water (Lentic) ecosystems e.g. pond, lake, bogs and swamps. (ii) Running water (Lotic) ecosystems e.g. springs, mountain brooks, streams and rivers.

Marine ecosystems — the water bodies containing salt concentration equal to or above that of sea water (i.e., 35 ppt or above). Eg: shallow seas and open ocean.

Brackish water ecosystems — these water bodies have salt content in between 5 to 35 ppt. e.g. estuaries, salt marshes, mangrove swamps and forests.

Aquatic Organisms

The aquatic organisms are classified on the basis of their zone of occurrence.

Neuston: These organisms live at the air-water interface e.g. floating plants. Some organisms spend most of their lives on top of the air-water interface such as water striders, while others spend most of their time just beneath the air-water interlace and obtain most of their food within the water.

E.g., beetles and back-swimmers.

Periphyton: These are organisms which remain attached to stems and leaves of rooted plants or substances emerging above the bottom mud such as sessile algae.

Plankton: Microscopic floating organisms such as algae, diatoms, protozoans and larval forms are called plankton. This group includes both microscopic plants like algae (phytoplankton) and animals like crustaceans and protozoans (zooplankton).

The locomotory power of the planktons is limited so that their distribution is controlled, largely, by currents in the aquatic ecosystems.

Nekton: This group contains powerful swimmers that can overcome the water currents. The animals range in size from the swimming insects to the largest blue whale.

Benthos: The benthic organisms are those found living at the bottom of the water mass.

Factors Limiting the Productivity of Aquatic Habitats

Sunlight and oxygen are the most important limiting factors of the aquatic ecosystems while temperature and humidity are limiting factors. Of the terrestrial ecosystem.


  • Sunlight penetration rapidly diminishes as it passes down the column of water.
  • The depth to which light penetrates a lake determines the extent of plant distribution.
  • Suspended particulate matters such as clay, silt, phytoplankton, etc. make the water turbid.
  • Turbidity limits the extent of light penetration and the photosynthetic activity in a significant way.
  • Based on light penetration and plant distribution they are classified as photic and aphotic zones.

Photic zone

  • Open water zone (or photic zone) – In this zone sunlight supports photosynthetic algae, and the species that feed upon them.
  • Photic (or “euphotic”) zone is the portion that extends from the lake surface down to where the light level is 1% of that at the surface. The depth of this zone depends on the transparency of water.
  • It is the upper layer of the aquatic ecosystems within which photosynthetic activity is confined. Both photosynthesis and respiration activity takes place.

Aphotic zone

  • The lower layers of the aquatic ecosystems, where light penetration and plant growth are restricted forms the aphotic zone (profundal zone). Only respiration activity takes place in this zone.
  • Aphotic zone extends from the end of the photic zones to bottom of the lake.

Dissolved oxygen

  • In fresh water the average concentration of dissolved oxygen is 10 parts per million or 10 ppm by weight. This is 150 times lower than the concentration of oxygen in an equivalent volume of air.
  • Oxygen enters the aquatic ecosystem through the air water interface and by the photosynthetic activities of aquatic plants.
  • Dissolved oxygen escapes the water body through air-water interface and through respiration of organisms (fish, decomposers, zooplanktons, etc.).
  • The amount of dissolved oxygen retained in water is also influenced by temperature. Oxygen is less soluble in warm water. Warm water also enhances decomposer activity. Therefore, increasing the temperature of a water body increases the rate at which oxygen is depleted from the water.
  • When the dissolved oxygen level falls below 3-5 ppm, many aquatic organisms are likely to die.


  • An ice layer on the top of a water body can effectively cut off light. Photosynthesis stops but respiration continues in such water body. Thus in shallow lakes, the oxygen gets depleted. Fish die, but we won’t know it until the ice melts and we find floating fish. This condition is known as winterkill.


  • Transparency affects the extent of light penetration.
  • Suspended particulate matters such as clay, silt, phytoplankton, etc make the water turbid.
  • Consequently, it limits the extent of light penetration and the photosynthetic activity in a significant way.


  • Water temperature changes slowly than air temperature because of higher specific heat.
  • Since water temperatures are less subject to change, the aquatic organisms have narrow temperature tolerance limit.
  • As a result, even small changes in water temperature are a great threat to the survival of aquatic organisms when compared to the changes in air temperatures in the terrestrial organisms.

Lake Ecology

  • Anybody of standing water, generally large enough in area and depth is known as a lake.
  • The largest lake in the world is Lake Superior in North America. Lake Baikal in Siberia is the deepest. Chilka lake of Orissa is the largest lake in India.
  • Three main zones can be differentiated in a lake –
    1. Peripheral zone (littoral zone) with shallow water.
    2. Open water beyond the littoral zone where water is quite deep.
    3. Benthic zone (bottom) or the floor of the lake.

Ageing of Lakes

  • Lakes receive their water from surface runoff (sometimes also groundwater discharge) and along with it various chemical substances and mineral matter eroded from the land.
  • Over periods spanning millennia, ageing occurs as the lakes accumulate mineral and organic matter and gradually, get filled up.
  • The nutrient-enrichment of the lakes promotes the growth of algae, aquatic plants and various fauna. This process is known as natural‘eutrophication’.
  • Similar nutrient enrichment of lakes at an accelerated rate is caused by human activities (discharge of wastewaters or agricultural runoff) and the consequent ageing phenomenon is known as cultural eutrophication’.
  • On the basis of their nutrient content, lakes are categorized as Oligotrophic (very low nutrients), Mesotrophic (moderate nutrients) and Eutrophic (highly nutrient rich).
  • The vast majority of lakes in India are either eutrophic or mesotrophic because of the nutrients derived from their surroundings or organic wastes entering them.

Lakes in India

  • In India, natural lakes (relatively few) mostly lie in the Himalayan region, the floodplains of Indus, Ganga and Brahmaputra.
  • In the semi-arid and arid regions of western and peninsular India, tens of thousands of water bodies have been constructed over millennia.
  • Lake ‘Sudarshan’ in Gujarat’s Girnar area was perhaps the oldest man-made lake in India, dating back to 300 BC.
  • Lakes are also classified on the basis of their water chemistry. Based on the levels of salinity, they are known as Freshwater, Brackish or Saline lakes (similar to that of classification of the aquatic ecosystem).
  • On the basis of their nutrient content, they are categorized as Oligotrophic (very low nutrients), Mesotrophic (moderate nutrients) and Eutrophic (highly nutrient rich).
  • The vast majority of lakes in India are either eutrophic or mesotrophic because of the nutrients derived from their surroundings or organic wastes entering them.

Removal of the nutrients from a lake

  • Flushing with nutrient-poor waters.
  • Deepwater abstraction.
  • On-site P-elimination by flocculation/flotation with water backflow, or floating Plant NESSIE with adsorbents.
  • On-site algae removal by filters and P-absorbers.
  • On-site algae skimming and separator thickening.
  • Artificial mixing / Destratification (permanent or intermittent). ·
  • Harvest of fishes and macrophytes.
  • Sludge removal.

Eutrophication – Algal Bloom

  • Eutrophic water body: it is a body of water rich in nutrients and so supporting a dense  plant population, the decomposition of which kills animal life by depriving it of oxygen.
  • Eutrophication is the response to the addition of nutrients such as nitrates and phosphates naturally or artificially, fertilizing the aquatic ecosystem.
  • Algal blooms are the consequence of Eutrophication.
  • The growth of green algae which we see in the lake surface layer is the physical identification of a Eutrophication.
  • Eutrophication occurs naturally due to deposition of nutrients [such as in depositional environments] carried by flood waters. It takes over centuries for eutrophication to occur naturally.
  • Similar nutrient enrichment of lakes at an accelerated rate is caused by human activities [discharge of wastewaters or agricultural runoff, Combustion of fossil fuel (produces gases —nitrogen oxides), growing urban population in the coastal areas) and the consequent phenomenon is known as cultural eutrophication’. It takes only
  • Phytoplankton (algae and blue-green bacteria) thrive on the excess nutrients and their population explosion covers almost entire surface layer. This condition is known as algal bloom.
  • Oxygen in the aquatic ecosystem is replenished by photosynthetic aquatic plants. Algal Blooms restrict the penetration of sunlight resulting in death of aquatic plants and hence restricts the replenishment of oxygen.
  • The oxygen level is already depleted due to the population explosion of phytoplankton.
  • Phytoplankton is photosynthetic during day time adding oxygen to the aquatic ecosystem. But during nights, they consume far more oxygen as they respire aggressively. i.e. Algal blooms accentuate the rate of oxygen depletion as the population of phytoplankton is very high.
  • The primary consumers like small fish are killed due to oxygen deprivation caused by algal blooms.
  • Death of primary consumers adversely affects the food chain and leads to the destruction of higher life forms.
  • Further, more oxygen is taken up by microorganisms during the decomposition process of dead algae, plants and fishes. Due to the reduced oxygen level, the remaining fishes and other aquatic organisms also die. All this eventually leads to degradation of
    the aquatic ecosystem.
  • The new anaerobic conditions [absence of oxygen] created to promote the growth of bacteria such as clostridium botulinum which produces toxins deadly to aquatic organisms, birds and mammals.

Effects of Eutrophication

  • Loss of freshwater lakes: Eutrophication eventually creates detritus layer in lakes and produces successively shallower depth of surface water. Eventually, the water body is  reduced into marsh whose plant community is transformed from an aquatic environment to a recognizable terrestrial ecosystem. [Lakes are one of the major sources of fresh water]
  • New species invasion: Eutrophication may cause the ecosystem competitive by transforming the normal limiting nutrient to abundant level. This cause shifting in species composition of the ecosystem.
  • Toxicity: Some algal blooms when died or eaten, release neuro & hepatotoxins which can kill aquatic organism & pose threat to humans. E.g.Shellfish poisoning.
  • Loss of coral reefs: Occurs due to decrease in water transparency (increased turbidity).
  • Affects navigation due to increased turbidity; creates colour (yellow, green, red), smell and water treatment problems; increases biomass of inedible toxic phytoplankton, benthic and epiphytic algae and bloom of gelatinous zooplankton.

Mitigation of Eutrophication

Checking water pollution is the ultimate solution to eutrophication.

  • Treating Industrial effluents domestic sewage to remove nutrient-rich sludge through wastewater processing.
  • Riparian buffer: Interfaces between a flowing body of water and land created near the waterways, farms, roads, etc. in an attempt to filter pollution. Sediments and nutrients are deposited in the buffer zones instead of deposition in water [Wetlands, estuaries are natural riparian buffers].
  • Increase in efficiency of nitrogen & phosphorous fertilizers and using them in adequate levels.
  • Nitrogen testing & modeling: N-Testing is a technique to find the optimum amount of fertilizer required for crop plants. It will reduce the amount of nitrogen lost to the surrounding area.
  • Encouraging organic farming.
  • Reduction in nitrogen emission from vehicles and power plants.

Harmful Algal Blooms

  • Algae or phytoplankton are microscopic organisms that can be found naturally in coastal waters.
  • They are major producers of oxygen and food for many of the animals that live in these waters.
  • When environmental conditions are favorable for their development, these cells may multiply rapidly and form high numbers of cells and this is called an algal bloom.
  • Water temperature has also been related to the occurrence of algal blooms, with unusuallywarm water being conducive to blooms.
  • A bloom often results in a color change in the water. Algal blooms can be any color, but the most common ones are red or brown. These blooms are commonly referred to as red or brown tides.
  • Most algal blooms are not harmful but some produce toxins and do affect fish, birds, marine mammals and humans. The toxins may also make the surrounding air difficult to breathe. These are known as Harmful Algal Blooms (HABs).

Use of Algae

Most species of algae or phytoplankton serve as the energy producers at the base of the food web, without which higher life on this planet would not exist.

Red Tide

  • Red tide is a common name for a phenomenon known as an algal bloom (large concentrations of aquatic microorganisms) when it is caused by a few species of dinoflagellates and the bloom take on a red or brown colour.
  • Blooms can appear greenish, brown, and even reddish orange depending upon the type of an organism, the type of water, and the concentration of the organisms.
  • The term “red tide” is a misnomer because blooms are not always red, they are not associated with tides, they are usually not harmful, and some species can be harmful or dangerous at low cell concentrations that do not discolor the water.

Causes of these blooms

  • When several colonies start combining rapidly when conditions such as nutrient concentrations, salinity and temperature are optimal.
  • Nutrient enrichment and warm waters.
  • Nutrient enrichment of water – especially phosphates and nitrogen, is often the result of pollution from nonpoint sources and can cause algal blooms.
  • Water temperature – unusually warm water is conducive to blooms.

HABs danger to fish and humans

  • Depletes oxygen in the water and lead to low dissolved oxygen levels.
    How it depletes oxygen?
  • When masses of algae die and decompose, the decaying process can deplete oxygen in the water, causing the water to become so low in oxygen.
  • When oxygen levels become too low, fish suffocate and die.the death of fishes further deplete oxygen.
  • Some algae species in blooms produce potent neurotoxins that can be transferred through the food web where they affect and even kill the higher forms of life such as zooplankton, shellfish, fish, birds, marine mammals, and even humans that feed either directly or indirectly on them.

HAB’s an environmental hazard?

  • Harmful Algal Blooms are considered an environmental hazard because of these events can make people sick when contaminated shellfish or finfish are eaten, or when people breathe aerosolized HAB toxins near the beach.
  • HAB events adversely affect commercial and recreational fishing, tourism, and valued habitats, creating a significant impact on local economies and the livelihood of coastal residents.

HABs and Climate Change

  • Because the growth, toxicity, and distribution of harmful algal bloom (HAB) species are all tied to the environment, changes in climate can change the occurrence, severity, and impacts of HAB events.

Wetland Ecosystem

  • A wetland is a land area that is saturated with water, either permanently or seasonally, such that it takes on the characteristics of a distinct ecosystem.
  • Wetlands are transition zones between terrestrial and aquatic ecosystems. E.g. Mangroves, lake littorals (marginal areas between highest and lowest water level of the lakes), floodplains (areas lying adjacent to the river channels beyond the natural levees and periodically flooded during high discharge in the river) and other marshy or swampy areas.
  • These habitats experience periodic flooding from adjacent deepwater habitats and therefore supports plants and animals specifically adapted to such shallow flooding or waterlogging.

India’s wetland

  • Waterlogged soil, adapted plant life (hydrophytes) and hydric soils (not enough O2) are the chief characteristics of wetlands.
  • India has totally 27,403 wetlands, of which 23,444 are inland wetlands and 3,959 are coastal wetlands.
  • Wetlands occupy 18.4% of the country’s area of which 70% are under paddy cultivation.
  • Natural wetlands in India range from high altitude wetlands in the Himalayas; floodplains of the major river systems; saline and temporary wetlands of the arid and semi-arid regions; coastal wetlands such as lagoons, backwaters, estuaries, mangroves, swamps
    and coral reefs, and so on.

Wetland’s classification

Inland wetland

  • Natural
  • Man-made

Coastal wetland

  • Natural
  • Manmade

Functions of Wetlands

  • Habitat to aquatic plants and animals and also to numerous species of birds, including migratory species.
  • Filtration of sediments and nutrients from surface water.
  • Nutrients recycling, water purification, floods mitigation
  • Maintenance of streamflow
  • Groundwater recharging
  • Provide drinking water, fish, fodder, fuel, etc.
  • Control rate of runoff in urban areas
  • Buffer shorelines against erosion
  • Comprise an important resource for sustainable
  • Tourism, recreation and cultural heritage
Reasons for depletion
  • Conversion of lands for agriculture
  • Overgrazing
  • Removal of sand from beds
  • Aquaculture, Habitat Destruction and Deforestation
  • Pollution – Domestic waste, Agricultural runoff, Industrial effluents
  • Climate change
  • It is the replacement of unavoidably lost wetland resources with created or restored wetlands, with the goal of replacing as fully as possible the functions and public benefits of the lost wetland.
  • Survey and demarcation
  • Protection of natural regeneration
  • Artificial regeneration
  • Protective measures, Afforestation (percentage survival to be indicated)
  • Weed control
  • Soil conservation measures & afforestation
  • Wildlife conservation
  • Removal of encroachments
  • Eutrophication abatement
  • Environmental awareness
Distinction from Lakes
  • Ministry of Environment, forest and climate change doesn’t differentiate between lakes and wetlands.
  • The National Lake Conservation Programme (NLCP) considers lakes as standing water bodies which have a minimum water depth of 3 m, generally cover a water spread of more than ten hectares, and have no or very little aquatic vegetation (macrophytes).
  • Wetland has excessive growth of macrophytes (both submerged and free-floating) which affects the water quality adversely and interfere with the utilization of the water body.
  • Marginal aquatic vegetation (wetlands), inhabiting waterlogged soils, checks erosion.
  • Wetlands (generally less than 3 m deep over most of their area) are usually rich in nutrients (derived from surroundings and their sediments) and have abundant growth of aquatic macrophytes.
  • They support high densities and diversity of fauna, particularly birds, fish and macroinvertebrates, and therefore, have high value for biodiversity conservation. These shallow lakes are rightfully categorized as wetlands.
  • Lakes are generally less important when compared to wetland from the viewpoint of ecosystem and biodiversity conservation.

National Wetlands Conservation Programme (NWCP)

  • NWCP was implemented in the year 1985-86.
  • Under the programme, 115 wetlands (Annexure) have been identified by the Ministry which requires urgent conservation and management interventions.


  • Conservation of wetlands to prevent –
    •  further degradation
    • ensuring of wise use of wetlands for the benefit of local communities and overall conservation of biodiversity.


  • To lay down policy guidelines for conservation and management of wetlands in the country.
  • to undertake intensive conservation measures in priority wetlands
  • To monitor the implementation of the programme.
  • To prepare an inventory of Indian wetlands.

Criteria for Identification of Wetlands of National Importance

  • Same as those prescribed under the ‘Ramsar Convention on Wetlands’ and are as given below:
  • Sites containing representative, rare or unique wetland types
    (i) If it contains a representative, rare, or unique example of a natural or nearnatural wetland type found within the appropriate biogeographic region
  • Criteria based on species and ecological communities
    (ii) If it supports vulnerable, endangered, or critically endangered species; or threatened ecological communities.
    (iii) If it supports populations of plant and/or animal species important for maintaining the biological diversity of a particular biogeographic region.
    (iv) If it supports plant and/or animal species at a critical stage in their life cycles, or provides refuge during adverse conditions.
  • Specific criteria based on water birds
    (v) If it regularly supports 20,000 or more water birds.
    (vi) If it regularly supports 1% of the individuals in a population of one species or subspecies of water birds.
  • Specific criteria based on fish
    (vii) If it supports a significant proportion of indigenous fish subspecies, species or families, life-history stages, species interactions and/or populations that are representative of wetland benefits and/or values and thereby contributes to global biological diversity.
    (viii) If it is an important source of food for fishes, spawning ground, nursery and/or migration path on which fish stocks, either within the wetland or elsewhere, depend. Specific criteria based on water/life and culture
    (ix) If it is an important source of food and water resource, increased possibilities for recreation and eco-tourism, improved scenic values, educational opportunities, conservation of cultural heritage (historical or religious sites).
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