Aquatic ecosystem - (Part - 1) | Environment for UPSC CSE PDF Download

An aquatic ecosystem is an ecosystem in a body of water. Communities of organisms that are dependent on each other and their environment live in aquatic ecosystems. The two main types of aquatic ecosystems are marine ecosystems and freshwater ecosystems.

• Freshwater ecosystems - The salt content of fresh bodies is shallow, always less than 5 ppt (parts per thousand).
  Example: lakes, ponds, pools, springs, streams, and rivers
• Marine ecosystems - the water bodies containing salt concentration equal to or above that of seawater (i.e., 35 ppt or above).
  Example: shallow seas and open ocean
• Brackish water ecosystems - these water bodies have salt content between 5 to 35 ppt.
  Example: estuaries, salt marshes, mangrove swamps and forests.

Aquatic Organisms

• The aquatic organisms are classified based on their zone of occurrence and their ability to cross these zones.
• The organisms (both flora and fauna) in the aquatic ecosystem are unevenly distributed but can be classified based on their life form or location into five groups

(i) Neuston

• These are unattached organisms which live at the air-water interface such as floating plants, etc. 
• 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 interface and obtain most of their food within the water.
• E.g., beetles and back-swimmers.

(ii) Periphyton

• These are organisms that remain attached to stems and leaves of rooted plants or substances emerging above the bottom mud such as sessile algae and their associated animals.

(iii) Plankton

• This group includes both microscopic plants like algae (phytoplankton) and animals like crustaceans and protozoans (zooplankton) found in all aquatic ecosystems, except certain swift-moving waters. 
• The planktons' locomotor power is limited so that their distribution is controlled largely by currents in the aquatic ecosystems.

(iv) Nekton

• This group contains animals which are swimmers. 
• The nektons are relatively large and powerful as they have to overcome the water currents. 
• The animals range in size from the swimming insects (about 2 mm long) to the largest animals, the blue whale.

(v) Benthos

• The benthic organisms are those found living in the bottom of the water mass. 
• Practically every aquatic ecosystem contains well-developed benthos.

Factors Limiting the Productivity of Aquatic Habitats

Sunlight and oxygen are the most important limiting factors of the aquatic ecosystems, whereas moisture and temperature are the main limiting factors of the terrestrial ecosystem.

➤ Sunlight

• 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.
• Based on light penetration and plant distribution, they are classified as photic and aphotic zones.

➤ Photic zone

• It is the upper layer of the aquatic ecosystems, up to which light penetrates and within which photosynthetic activity is confined. 
• The depth of this zone depends on the transparency of water. 
• Both photosynthesis and respiration activity takes place. 
• The photic (or "euphotic") zone is the lighted and usually well-mixed portion that extends from the lake surface down to where the light level is 1% of that at the surface.

➤ Aphotic zone

• The lower layers of the aquatic ecosystems, where light penetration and plant growth are restricted forms the aphotic zone. 
• Only respiration activity takes place. 
• The aphotic zone is positioned below the littoral and photic zones to bottom of the lake where light levels are too low for photosynthesis. Respiration occurs at all depths, so the aphotic zone is a region of oxygen consumption. This deep, unlit region is also known as the profundal zone.

Winterkill

Snow cover of ice on water body can effectively cut off the light, plunging the waters into darkness. Hence photosynthesis stops but respiration continues. 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.

➤ Dissolved oxygen

• In aquatic ecosystems, oxygen is dissolved in water, where its concentration varies constantly depending on factors that influence the input and output of oxygen in the water. 
• In freshwater, the average concentration of dissolved oxygen is 0.0010 per cent (also expressed as 10 parts per million or 10 ppm) by weight, which 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.
• Therefore, the quantity of dissolved oxygen present in an ecosystem depends on the rate at which the aforesaid two processes occur. 
• Dissolved oxygen escapes the water body through the air-water interface and 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 water body temperature increases the rate at which oxygen is depleted from water. 
• When the dissolved oxygen level falls below 3-5 ppm, many aquatic organisms are likely to die. 
• Other limiting factors which influence on aquatic productivity are

➤ Transparency

• 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 photosynthetic activity in a significant way.

➤ Temperature

• The water temperature changes less rapidly than the temperature of air because water has a considerably higher specific heat than air, i.e. larger amounts of heat energy must be added to or taken away from water to raise or lower its temperature. 
• 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 aquatic organisms' survival compared to the changes in air temperatures in the terrestrial organisms.

The classification of organisms and limiting factors discussed here apply in general to all aquatic ecosystems - lakes, Ponds, Rivers, Streams, Estuaries, oceans and seas.

Lake Ecology

Anybody of standing water, generally large enough in area and depth, irrespective of its hydrology, ecology, and other characteristics is generally known as a lake.

Ageing of Lakes

• Like any organism, lakes are born as they originate by various geological and geomorphic events, and 'grow' with time to change their various morphological and functional characteristics and eventually die. 
• They 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'.

➤ In India

• In India, natural lakes (relatively few) mostly lie in the Himalayan region, Indus, Ganga, and Brahmaputra's floodplains. 
• 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 humanmade lake in India, dating back to 300 BC.
• Lakes are also classified based on their water chemistry. Based on salinity levels, they are known as Freshwater, Brackish or Saline lakes (similar to that of the aquatic ecosystem). 
• Based on their nutrient content, they are categorized as Oligotrophic (deficient 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.

➤ General Characteristics of Oligotrophic and eutrophic Lakes 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-adsorbers. 
• On-site algae skimming and separator thickening. 
• Artificial mixing / Destratification (permanent or intermittent). 
• Harvest of fishes and macrophytes. 
• Sludge removal.

Ameenpur Lake - First Biodiversity Heritage Site

Ameenpur Lake gets the first Biodiversity Heritage Site status in the country under the biodiversity act, 2002. it is an ancient humanmade lake in a western part of Telangana. Biodiversity Heritage Site is areas of biodiversity importance, which harbour rich biodiversity, wild relatives of crops, or areas, which lie outside the protected area network. Biodiversity Heritage sites are managed by a locally constituted Biodiversity Management Committee and get funding for its protection.

Eutrophication

➤ What is it?

• Greek word – Eutrophia means adequate & healthy nutrition. 
• Eutrophication is a syndrome of the ecosystem, response to the addition of artificial or natural nutrients such as nitrates and phosphates through fertilizer, sewage, etc. that fertilize the aquatic ecosystem. 
• It is primarily caused by phosphate and - or nitrate leaching containing fertilizers from agricultural lands into lakes or rivers.
• The growth of green algae which we see in the lake surface layer is the physical identification of a Eutrophication. 
• Some algae and blue-green bacteria thrive on the excess ions, and a population explosion covers almost entire surface layer is known as an algal bloom. This growth is unsustainable, however. 
• As Algal Bloom covers the surface layer, it restricts the penetration of sunlight. Diffusion of gas from atom 
• All respiring animals require oxygen in the water, and it is replenished by diffusion and photosynthesis of green plants. 
• The oxygen level is already low because of the population explosion, and further oxygen is taken up by microorganisms which feed off the dead algae during the decomposition process. 
• Due to reduced oxygen level, fishes and other aquatic organism suffocate, and they die. 
• The anaerobic conditions can promote bacteria growth, which produces toxins deadly to aquatic organisms, birds, and mammals. 
• All this eventually leads to the degradation of the aquatic ecosystem and the death of its organisms. 
• It often leads to change in animal and plant population & degradation of water & habitat quality.

Types

(i) Natural

• Deposition of nutrients [such as depositional environments. When the nutrients flow into the system on temporal basics. 
• It Occurs over centuries. 
• Example: Seasonally inundated tropical flood plains.

(ii) Manmade

• Occurs in decades 
• These inputs may come from untreated sewage discharges, fertilizer runoff from farm fields, golf courses, park, etc. & from animal wastes. 
• Combustion of fossil fuel [produces gases – nitrogen oxides 
• Growing urban population in the coastal areas

Sources

(i) Point sources

• Directly attributable to one influence 
• In point source, nutrient waste travels directly from source to water. 
• Point sources are easy to regulate.

(ii) Non-point source

• Is from various ill-defined and diffuse sources 
• Vary spatially and temporarily and are difficult to regulate.

Effects

➤ Change in the ecosystem

• Eutrophication eventually creates detritus layer in the ponds & lakes and produces a successively shallower depth of surface water. 
• Eventually, the water body is reduced into marsh whose plant community is transformed from an aquatic environment to recognizable terrestrial ecosystem.

➤ Decreased biodiversity

• Algal blooms restrict the sunlight to penetrate & affects the photosynthesizing plants. It causes the death of plants. 
• Bacteria consumes all the oxygen on decomposition & results in devoid of oxygen. Eventually, it leads to the death of all living organism in the aquatic ecosystem.

➤ 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

• When die or eaten, some algal blooms release neuro & hepatotoxins, killing aquatic organisms & pose a threat to humans. (e.g.) Shellfish poisoning. 
• Depletion of dissolved oxygen level. 
• Increased incidences of fish kills & loss of desirable fish species & reduction in harvesting 
• Loss of coral reefs. 
• The decrease in water transparency and increased turbidity. 
• Affects navigation due to increased turbidity. 
• Colour (yellow, green, red), smell and water treatment problems. 
• Increased biomass of inedible toxic phytoplankton 
• Increase in the bloom of gelatinous zooplankton 
• Increased biomass of benthic and epiphytic algae 
• Unsuitable for aesthetic recreation, and reduction in the value of water body

➤ Mitigation

• 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 nutrient are deposited in the buffer zones instead of deposition in water. 
• Minimizing the non-point pollution. 
• 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. 
• Treatment of Industrial effluents 
• Organic farming & Integrated Farming System. 
• Reduction in livestock densities 
• Improving the efficiency of use of fertilizer
• Treatment of runoff from street & storm drains. 
• Reduction in nitrogen emission from vehicles and power plants 
• Increase in efficiency of nitrogen & phosphorous removal from municipal wastewater

➤ Policies

• Multi dimensional in Nature should include 
• Education & awareness 
• Research, monitoring & evaluation 
• Regulations. 
• Fiscal & economic incentives 
• Ecosystem preservation & restoration

Harmful Algal Blooms

➤ What are 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 favourable for their development, these cells may multiply rapidly and form high numbers of cells called an algal bloom. 
• A bloom often results in a colour change in the water. Algal blooms can be any colour, 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 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).

➤ Why Red Tide is a misnomer?

• "Red Tide" is a common name for such a phenomenon where certain phytoplankton species contain pigments and "bloom" such that the human eye perceives the water to be discoloured. 
• Blooms can appear greenish, brown, and even reddish-orange depending upon the type of organism, the type of water, and the organisms' concentration. 
• The term "red tide" is thus 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 not discolour the water. 
• They are scientifically referred to as Harmful Algal Blooms (HABs).

➤ What are the causes of these blooms?

• Blooms occur when several colonies start combining rapidly when conditions such as nutrient concentrations, salinity and temperature are optimal. 
• Blooms can be due to several reasons. Two common causes are nutrient enrichment and warm waters. 
• Nutrient enrichment of water, especially phosphates and nitrogen, is often the result of pollution and can cause algal blooms. 
• Water temperature has also been related to the occurrence of algal blooms, with hot water being conducive to blooms.

➤ How are HABs dangerous to fish and humans?

• HABs can deplete 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 low in oxygen. 
• When oxygen levels become too low, fish suffocate and die. 
• 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.

[Question: 513826]

➤ Is HAB's an environmental hazard?

• Yes, these events can make people sick when contaminated shellfish are eaten, or when people breathe aerosolized HAB toxins near the beach. 
• Besides, HAB events can result in the closure of shellfish beds, massive fish kills, death of marine mammals and seabirds, and marine habitats' alteration. 
• Consequently, 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.

➤ How do we get exposed to HAB toxins?

• Most illness associated with HAB exposure results from consuming toxins that are present in shellfish or finfish. 
• Some HAB toxins can become airborne during bloom, and people can become ill by inhaling toxins.

➤ Is it safe to eat seafood?

• In general, it is safe to eat seafood. 
• However, consuming shellfish that have been harvested from waters with high levels of harmful algae and consuming fish that have lesions or that were caught in an area during an algal bloom can result in illness.

➤ HABs and Climate Change

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

Wetland Ecosystem

• Wetlands are intermediate in character between deep-water and terrestrial habitats, also transitional in nature, and often located between them. 
• These habitats experience periodic flooding from adjacent deep-water habitats and therefore supports plants and animals specifically adapted to such shallow flooding or waterlogging of the substrate, were designated as wetlands. 
• They included 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 where water gets stagnated due to poor drainage or relatively impervious substrata & Bogs, fens and mangroves due to similar ecological characteristics

➤ Characteristics

• Covered by water (or) has waterlogged soil for at least seven days during the growing season. 
• Adopted plant life (hydrophytes) 
• Hydric soils (not enough O₂ available for some plants)

Wetlands Classification

➤ Functions of Wetlands

• Habitat to aquatic flora and fauna, as well as 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 
• Stabilization of local climate 
• Source of livelihood to local people 
• Genetic reservoir for various species of plants (especially rice) 
• Supporting specific diversity

➤ 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

➤ Mitigation

• 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

• Although the Ministry of Environment and Forests has not adopted a clear distinction 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). 
• These water bodies are used primarily for drinking water supplies, irrigation and/or recreation. 
• Excessive growth of macrophytes (both submerged and free-floating) generally present in wetland, affects the water quality adversely and interfere with the utilization of the water body. 
• However, marginal aquatic vegetation (wetlands), particularly comprising of emergent plants and those inhabiting waterlogged soils, is not only desirable but is to be promoted because it checks erosion, serves habitat for wildlife and helps improve water quality. 
• 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 the wetland from the viewpoint of ecosystem and biodiversity conservation.

[Question: 513827]

➤ India's Wetlands

• Wetlands are areas of critical ecological significance: as they support biodiversity, support millions of people, directly and indirectly, protect from storms, flood control, improve water quality, supply food, fibre and raw materials. 
• India has 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; flood plains of the major river systems; saline and temporary wetlands of the arid and semi-arid regions; coastal wetlands lagoons, backwaters, estuaries, mangroves, swamps and coral reefs, and so on.

[Question: 513828]

➤ 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, requiring urgent conservation and management interventions.

Aim

Conservation of wetlands to prevent their further degradation and ensure their wise use benefits local communities and overall biodiversity conservation.

➤ Objectives

• To lay down policy guidelines for the conservation and management of wetlands in the country. 
• To provide financial assistance for undertaking intensive conservation measures in the identified wetlands.
• To monitor implementation of the programme, and to prepare an inventory of Indian wetlands. 
• The Central Government is responsible for overall coordination of wetland conservation programmes and initiatives at the international and national levels. It also provides guidelines, financial & technical assistance to state govt. 
• Since the land resources belong to them, the State Governments/UT Administration are responsible for managing the wetlands and implementing the NWCP to ensure their wise use.

➤ Criteria for Identification of Wetlands of National Importance

• Criteria for identification of wetlands of national importance under NWCP are the same as those prescribed under the 'Ramsar Convention on Wetlands' and are as given below: 
• Sites containing representative, rare or unique wetland types
• If it contains a representative, rare, or unique example of a natural or near-natural wetland type found within the appropriate biogeographic region. 
• Criteria based on species and ecological communities
• If it supports vulnerable, endangered, or critically endangered species; or threatened ecological communities.
• If it supports populations of plant and/or animal species important for maintaining a particular biogeographic region's biological diversity.
• 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 waterbirds 
• If it regularly supports 20,000 or more water birds.
• If it regularly supports 1% of the individuals in a population of one species or subspecies of waterbirds. 
• Specific criteria based on fish 
• 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.
• 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
• 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).

Montreux Record

Montreux Record is the principal tool under the Ramsar Convention, is a register of wetland sites on the List of Wetlands of International Importance It highlights those sites where adverse changes in ecological character have occurred, are occurring, or are likely to occur as a result of technological developments, pollution or other human interference and which are therefore in need of priority conservation attention.
It is maintained as part of the Ramsar List. Montreux Record is employed to identify priority sites for positive national and international conservation attention. Sites may be added to and removed from the Record only with the Contracting Parties' approval in which they lie.

➤ Montreux Record sites in India

• Chilika lake, Orissa was placed on the Montreux Record in 1993 due to siltation, which was choking the lake's mouth. Following the rehabilitation efforts of the government, it was removed from the Record in 2002. 
• For this achievement, the Chilika Development Authority received the Ramsar Wetland Conservation Award for 2002. Loktak lake, Manipur was included on the Montreux Record in 1993 (signifying habitat degradation) due to ecological problems such as deforestation in the catchment area, an infestation of water hyacinth and pollution. 
• The construction of a hydroelectric power plant has caused the local extinction of several native fish species. 
• The Keoladeo national park, Rajasthan was placed on the Montreux Record in 1990 due to water shortage and the unbalanced grazing regime. 
• The invasive growth of the grass Paspalum distichum has changed the ecological character of large areas of the site, reducing its suitability for certain water bird species, notably the Siberian crane. Loktak lake and Keoladeo national park are the two Montreux Record sites in India.

Neknampur Lake - First FTW Lake

The Neknampur Lake, Hyderabad is the largest floating treatment wetland in the country. Floating treatment wetland works based on hydroponics technique. Hydroponics permits plants to grow on the water with sunlight's help (no need of soil). A floating island unit is essentially a platform designed using styrofoam, bamboo, gunny bags and coir and it performs the function of a wetland.