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- Aquatic ecosystems refer to plant and animal communities occurring in water bodies.
- Aquatic ecosystems are classified into two subgroups: 1) Freshwater ecosystems, such as rivers, lakes and ponds; 2) Marine ecosystems, such as oceans, estuary and mangroves.
- Aquatic ecosystems are classified on the basis of salinity into the following types:
- Freshwater 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 freshwater ecosystems: 1) Static or still water (Lentic) ecosystems, e.g. pond, lake, bogs and swamps. 2) 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 seawater (i.e., 35 ppt or above). E.g. 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.
- An estuary is a place where a river or a stream opens into the sea (mouth of the river).
- It is a partially enclosed coastal area of brackish water (salinity varies between 0-35 ppt) with one or more rivers or streams flowing into it, and with a free connection to the open sea.
- At the estuaries, freshwater carrying fertile silt and runoff from the land mixes with the salty sea water.
- Estuaries form a transition zone (ecotone) between river environments and maritime environments.
- Examples of estuaries are river mouths, coastal bays, tidal marshes, lagoons and deltas.
- Estuaries are formed due to rise in sea level, movement of sand and sandbars, glacial processes and tectonic processes.
- All the plants and animals in the estuaries are subjected to variations in salinity to which they are adapted (osmoregulation).
- Estuaries are greatly influenced by tidal action. They are periodically washed by sea water once or twice a day based on the number of tides.
- In some narrow estuaries, tidal bores are significant. Tidal bores cause great damage to the estuarine ecology.
Importance of Estuaries
- They are the most productive (more productive than wetlands) water bodies in the world because of the mixing of freshwater and saline water zone where marine organisms of both the ecosystems meet.
- Ecotone regions (transitional zones) like mangroves, wetlands, estuaries, grasslands etc. have far greater productivity compared to natural ecosystems like a forest ecosystem, ocean ecosystem, pond ecosystem, riverine ecosystem, desert ecosystem etc. This is because of the wide-ranging species from the adjacent ecosystems being present in the ecotone.
- Also, an estuary has very little wave action, so it provides a calm refuge from the open sea and hence becomes ideal for the survival of numerous aquatic species.
- Estuaries are most heavily populated areas throughout the world, with about 60% of the world’s population living along estuaries and the coast.
- The vast mangrove forests on the seaward side of an estuary act as a barrier for the coastal habitat to check the wind speed during cyclones and high velocity landward winds.
- Mangroves act as a filter trapping suspended mud and sand carried by rivers which leads to delta formations around estuaries.
- Precipitation of clay and alluvium particles in the estuarine region is high because of the exposure to saline water (saline water precipitates fine alluvium).
- Estuaries store and recycle nutrients, traps sediment and forms a buffer between coastal catchments and the marine environment.
- They also absorb, trap and detoxify pollutants, acting as a natural water filter.
- Estuaries with their wetlands, creeks, lagoons, mangroves and sea-grass beds are rich in natural resources including fisheries.
- They are deep and well protected from marine transgressions, and hence they are ideal locations for the construction of ports and harbours.
- The banks of estuarine channels form a favoured location for human settlements, which use the estuaries for fishing and commerce but nowadays also for dumping civic and industrial waste.
Differences between Lagoon and Estuary
- A lagoon is a stretch of salt water separated from the sea by a low sandbank or coral reef.
- Backwaters in Kerala are mostly lagoons where seawater flows inwards through a small inlet that is open towards the sea.
- In estuaries, the water flows fast and strong, while in lagoons the water is shallower and flows sluggishly.
- Estuaries are usually deeper than lagoons. Also, lagoons mostly don’t have any fresh water source while the estuaries have at least one. Lagoons are more saline than estuaries.
- Lagoons are formed due to falling in sea levels (coastline of emergence. E.g. Kerala Coast) whereas estuaries are mostly formed due to rise in sea levels (coastline of submergence. E.g. Konkan coast)
- Only certain types of plants and animals adapted to the “brackish” estuarine waters flourish in the estuaries.
- Factors influencing the distribution of organism in an estuary are its salinity and the amount of flooding.
- Estuaries support diverse habitats, such as mangroves, salt marshes, sea-grass, mudflats etc.
- Estuaries are very dynamic and productive ecosystems since the river flow, tidal range and sediment distribution is continuously changing in them.
- In general, the phytoplankton of estuaries are diatoms, dinoflagellates, green algae, blue-green algae.
- Towards the sea coast of the estuaries, there are large algae and seagrasses. Near the mouth of the rivers and deltas, there are mangrove forests.
- Estuaries are homes to all king of terrestrial or land-based plants and animals, such as wood storks, pelicans, coniferous and deciduous trees and butterflies.
- Estuaries are also home to unique aquatic plants and animals, such as sea turtles, sea lions, sea catfish, saltworts, eelgrass, salt grasses, cordgrasses, seagrass, sedge, bulrush etc.
- The Country has 14 major, 44 medium and 162 minor rivers drains into the sea through various estuaries.
- Major estuaries occur in the Bay of Bengal. Many estuaries are locations of some of the major seaports.
- Most of India’s major estuaries occur on the east coast. In contrast, the estuaries on the west coast are smaller (in environmental studies, deltas are considered as subsections of estuaries).
- Two typical examples of estuaries on the west coast are the Mandovi and Zuari estuaries.
Issues of Indian Estuarine Ecosystem
- Modifications of the estuarine catchments result in changes in water flow in various estuaries, either far in excess or much lower than required (E.g. Hooghly, Godavari, Pulicat etc.)
- Pollution through industries and combined city sewage discharge.
- Recreational boating and fishing.
- Navigation, dredging and shipping (e.g. Hooghly).
- Expansion of urban and rural settlements, mining & industries, agriculture and dumping of solid wastes.
- Overexploitation of target fish stock due to increased demand.
- Reclaiming the fringed areas for intensive aquaculture in pens.
- Obstructing the migratory routes of fish and prawn recruitment (e.g., Chilka, Pulicat).
- Polluting the environment through feeding of stocked fish and prawn in pens (Chilka).
- Destruction of biodiversity through prawn seed collection and operation of small-meshed nets (e.g., Hooghly, Chilka, Pulicat).
- Submergence of catchment areas due to rising in water level.
- Mangroves represent a characteristic littoral (near the seashore) forest ecosystem.
- These are mostly evergreen forests that grow in sheltered low lying coasts, estuaries, mudflats, tidal creeks backwaters (coastal waters held back on land), marshes and lagoons of tropical and subtropical regions.
- Mangroves grow below the high water level of spring tides.
- The best locations are where abundant silt is brought down by rivers or on the backshore of accreting sandy beaches.
- Mangroves are highly productive ecosystems, and the trees may vary in height from 8 to 20 m. They protect the shoreline from the effect of cyclones and tsunamis.
- They are breeding and spawning ground for many commercially important fishes.
- Since mangroves are located between the land and sea, they represent the best example of ecotone.
- Mangroves are shrubs or small trees that grow in coastal saline or brackish water.
- Mangroves are salt tolerant trees, also called halophytes, and are adapted harsh coastal conditions.
- Mangrove vegetation facilitates more water loss. Leaves are thick and contain salt-secreting glands. Some block absorption of salt at their roots itself.
- They contain a complex salt filtration system and complex root system to cope with salt water immersion and wave action.
- They are adapted to the low oxygen (anoxic) conditions of waterlogged mud.
- They produce pneumatophores (blind roots) to overcome the respiration problem in the anaerobic soil conditions.
- Mangroves occur worldwide in the tropics and subtropics, mainly between latitudes 25° N and 25° S.
- They require high solar radiation to filter saline water through their roots. This explains why mangroves are confined to only tropical and sub-tropical coastal waters.
- Mangroves occur in a variety of configurations. Some species (e.g. Rhizophora) send arching prop roots down into the water.
- While other (e.g. Avicennia) send vertical “Pneumatophores” or air roots up from the mud.
- Adventitious roots which emerged from the main trunk of a tree above ground level are called stilt roots.
Prop roots and pneumatophores
- Mangroves exhibit Viviparity mode of reproduction. i.e. seeds germinate in the tree itself (before falling to the ground).
- This is an adaptive mechanism to overcome the problem of germination in saline water.
Mangroves in India
- The mangroves of Sundarbans are the largest single block of tidal halophytic mangroves of the world.
- This mangrove forest is famous for the Royal Bengal Tiger and crocodiles.
- Mangrove areas here are being cleared for agricultural use.
- The mangroves of Bhitarkanika (Orissa), which is the second largest in the Indian sub-continent, harbour high concentration of typical mangrove species and high genetic diversity.
- Mangrove swamps occur in profusion in the intertidal mudflats on both side of the creeks in the Godavari-Krishna deltaic regions of Andhra Pradesh.
- Mangroves of Pichavaram and Vedaranyam are degraded mainly due to the construction of aquaculture ponds and salt pans.
- On the west coast of India, mangroves, mostly scrubby and degraded occur along the intertidal region of estuaries and creeks in Maharashtra, Goa and Karnataka.
- The mangrove vegetation in the coastal zone of Kerala is very sparse and thin.
- In Gujarat (north-west coast) mangroves Avicennia marine, Avicennia officinalis and Rhizophora mucronata are found mainly in Gulf of Kutch and the Kori creek.
- Mangroves are of scrubby type with stunted growth, forming narrow, discontinuous patches on soft clayey mud.
- The condition of the mangroves is improving especially in the Kori creek region, which is a paleodelta of the Indus river (once upon a time it was part of Indus delta).
- In size, mangroves range from bushy stands of dwarf mangroves found in Gulf of Kutch, to taller stands found in the Sunderbans.
- On the Andaman & Nicobar Islands, the small tidal estuaries and the lagoons support a dense and diverse undisturbed mangrove flora.
Importance of Mangroves
- Mangrove plants have (additional) special roots such as prop roots, pneumatophores which help to impede water flow and thereby enhance the deposition of sediment in areas (where it is already occurring), stabilise the coastal shores, provide a breeding ground for fishes.
- Mangroves moderate monsoonal tidal floods and reduce inundation of coastal lowlands.
- They prevent coastal soil erosion.
- They protect coastal lands from tsunami, hurricanes and floods.
- Mangroves enhance the natural recycling of nutrients.
- Mangrove supports numerous florae, avifauna and wildlife.
- Provide a safe and favourable environment for breeding, spawning, rearing of several fishes.
- They supply woods, firewood, medicinal plants and edible plants to local people.
- They provide numerous employment opportunities to local communities and augments their livelihood.
- They are destroyed for conversion of the area for agricultural purpose, fuel, fodder and, salinisation, mining, oil spills, aquacultural (shrimp farming), use of chemical pesticides & fertilisers, industrial purposes.
Q. Which one of the following is the correct sequence of ecosystems in the order of decreasing productivity?
- Oceans, lakes, grasslands, mangroves
- Mangroves, oceans, grasslands, lakes
- Mangroves, grasslands, lakes, oceans
- Oceans, mangroves, lakes, grasslands
- Productivity = production/unit area/unit time.
- Production/unit area depends on the number and diversity of producers.
- Ecotones have greater productivity compared to the surrounding ecosystems. Mangroves and grasslands are ecotones. (Tropical Rainforests is an exception as it has productivity comparable to wetlands because of its rich diversity of primary producers).
- Note: Grasslands are not transitional all the time. E.g. Steppe. Non-transitional grasslands have very low productivity because of very limited diversity of primary producers. (Grasslands become transitional only when they are narrow).
- So, the order of decreasing productivity will be like
- Mangroves ,…… , …… , ……..
- Oceans = very deep and hence productivity is limited to the surface only (Below in the aphotic zone productivity is negligible. Aphotic zone in oceans is few kilometres).
- Also, surface water in oceans is very poor in nutrients. Nutrient-rich cold water flows as a sub-surface flow lying in the aphotic zone. Sunlight and nutrients are far apart, and hence primary productivity is very low except in regions where there is an upwelling of nutrient-rich cold water (Cold and Warm current mixing zones).
- So open ocean ecosystem has the least productivity. (Desert ecosystem also has very low productivity, lesser than oceans).
- So, the answer will look like Mangroves, ………., ………, Oceans.
- The only such option is c)
Answer: c) Mangroves, grasslands, lakes, oceans.
- Lakes just like oceans have low productivity. But due to some plants in the photic zone, lakes have productivity slightly greater than that of oceans.