Estuarine Ecosystem

Estuarine Ecosystem

Estuarine Ecosystem

  • 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, fresh water 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 seawater once or twice a day based on the number of tides.
  • Estuaries are greatly influenced by tidal action. They are periodically washed by seawater 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.

  • 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.
  • 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 harbors.
  • The banks of estuarine channels form a favored 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 saltwater separated from the sea by a low sandbank or coral reef.
  • Example – Backwaters in Kerala are mostly lagoons where seawater flows inwards through a small inlet that is open towards the sea.
  • The main difference between lagoons and estuaries is in the flow dynamics of the waterbodies: in estuaries, the water flows fast and strong, while in lagoons the water is more shallow 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)

Estuarine Vegetation

  • Only certain types of plants and animals specially adapted to the “brackish” estuarine waters flourish in the estuaries.
  • Factors influencing the growth and distribution of organism in an estuary are – its salinity and the amount of flooding.
  • Estuaries support diverse habitats, such as mangroves, salt marshes, seagrass, mudflaps, 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’s 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 kind of terrestrial or land-based plants and animals, such as wood storks, pelicans, coniferous and deciduous trees and butterflies.
  • Estuaries are also homes to unique aquatic plants and animals, such as sea turtles, sea lions, sea catfish, saltworts, eelgrass, salt grasses, cordgrasses, seagrass, sedge, bulrush etc.

India Estuarine Ecosystem

  • The Country has 14 major, 44 mediums and 162 minor rivers drain 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.
  • Example – Hooghly, Narmada, Krishna, 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 sea shore) 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.
  • Since mangroves are located between the land and sea they represent the best example of the 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 to live in 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 saltwater 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 subtropical 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.
  • 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.

Importance of Mangroves

  • 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.
  • Mangroves act as a filter trapping suspended mud and sand carried by rivers which leads to delta formations around estuaries.
  • They are breeding and spawning ground for many commercially important fishes.
  • The moderate monsoon tidal flows.
  • Prevents coastal soil erosion.
  • Enhance the natural recycling of nutrients.
  • It supports numerous flora, avifauna, and wildlife.
  • Supplies wood, firewood, medicinal plants, and edible plants to local people.

Mangrove profile in India


  • The largest single block of tidal holophytic mangroves of the world.
  • Major species of Sunderbans – Herritiera frames, Rhizophora spp., Bruguiera spp., Ceriops decandra, Sonneratia spp. and Avicennia spp., Nypa fruticans are found along the creeks.
  • It is famous for the Royal Bengal Tiger and crocodiles.

Mangroves of Bhitarkanika

  • It is the second-largest mangrove forest in the Indian sub-continent
  • The 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 – degraded because of the construction of aquaculture ponds and salt pans.
  • West coast of India has mostly scrubby and degraded mangroves along the intertidal. region of estuaries and creeks in Maharashtra, Goa, and Karnataka.
  • Kerala coast – is very sparse and thin.
  • Gujarat (north-west coast) – mangroves Avicennia marina, Avicennia officinalis, and Rhizophora mucronata are found mainly in Gulf of Kachchh and the Kori creek.
  • Mangroves are of the 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.
  • In size, mangroves range from bushy stands of dwarf mangroves found in Gulf of Kuchchh to taller stands found in the Sunderbans.

Coral Reefs

Coral Reefs

  • Coral reefs are built by and made up of thousands of tiny animals—coral “polyps”— that are related to anemones and jellyfish.
  • Polyps are shallow-water organisms which have a soft body covered by a calcareous skeleton. The polyps extract calcium salts from seawater to form these hard skeletons.
  • The polyps live in colonies fastened to the rocky seafloor.
  • The tubular skeletons grow upwards and outwards as a cemented calcareous rocky mass collectively called corals.
  • When the coral polyps die, they shed their skeleton [coral] on which new polyps grow.
  • The cycle is repeated for over millions of years leading to accumulation of layers of corals [shallow rock created by these depositions is calledreef].
  • These layers at different stages give rise to various marine landforms. One such important landform is called a coral reef.
  • Coral reefs over a period of time transform or evolve into coral islands (Lakshadweep).
  • The corals occur in different forms and colors, depending upon the nature of salts or constituents they are made of.
  • Small marine plants (algae) also deposit calcium carbonate contributing to coral growth.

Coral Reef Relief Features

  • A fringing reef, barrier reef, and atoll (coral islands are formed on atolls) are the most important relief features.

Fringing Reefs (Shore Reefs)

Fringing Reefs

  • Fringing reefs are reefs that grow directly from ashore. They are located very close to land, and often form a shallow lagoon between the beach and the main body of the reef.
  • A fringing reef runs as a narrow belt [1-2 km wide]. This type of reef grows from the deep sea bottom with the seaward side sloping steeply into the deep sea. Coral polyps do not extend outwards because of a sudden and large increase in depth.
  • The fringing reef is by far the most common of the three major types of coral reefs, with numerous examples in all major regions of coral reef development.
  • Fringing reefs can be seen at the New Hebrides Society islands off Australia and off the southern coast of Florida.

Barrier Reefs

Barrier Reefs

  • Barrier reefs are extensive linear reef complexes that parallel ashore and are separated from it by the lagoon.
  • This is the largest (in size, not distribution) of the three reefs, runs for hundreds of kilometers and is several kilometers wide. It extends as a broken, irregular ring around the coast of an island, running almost parallel to it.
  • Barrier reefs are far less common than fringing reefs or atolls, although examples can be found in the tropical Atlantic as well as the Pacific.
  • The 1200-mile long Great Barrier Reef off the NE coast of Australia is the world’s largest example of this reef type.
  • The GBR is not actually a single reef as the name implies, but rather a very large complex consisting of many reefs.



  • An atoll is a roughly circular (annular) oceanic reef system surrounding a large (and often deep) central lagoon.
  • The lagoon has a depth 80-150 meters and may be joined with seawater through a number of channels cutting across the reef.
  • Atolls are located at great distances from deep see platforms, where the submarine features may help in the formation of atolls, such as a submerged island or a volcanic cone which may reach a level suitable for coral growth.
  • An atoll may have any one of the following three forms-
    • true atoll—a circular reef enclosing a lagoon with no island;
    • an atoll surrounding a lagoon with an island;
    • a coral island or an atoll island which is, in fact, an atoll reef, built by the process of erosion and deposition of waves with island crowns formed on them.
  • Atolls are far more common in the Pacific than any other ocean.
  • The Fiji atoll and the Funafuti atoll in the Ellice/Island are well-known examples of atolls.
  • A large number of atolls also occur in the Lakshadweep Islands.
  • In the South Pacific, most atolls occur in mid-ocean. Examples of this reef type are common in French Polynesia, the Caroline and Marshall Islands, Micronesia, and the Cook Islands.
  • The Indian Ocean also contains numerous atoll formations. Examples are found in the Maldives and Chagos island groups, Seychelles, and in the Cocos Island group.

Functions of Coral Reefs

  • Corals are called the rainforests of the ocean because of their high biodiversity and productivity.
  • Coral reefs are natural protective barriers against erosion and storm surge.
  • The coral animals are highly adapted for capturing plankton from the water, thereby capturing nutrients.
  • Largest biogenic calcium carbonate producer on earth.
  • They provide a substrate for mangroves.
  • Coral reefs provide habitat for a large variety of animals and plants.

Ideal Conditions for Coral Growth

  • Stable climatic conditions: Corals are highly susceptible to quick changes. They grow in regions where the climate is significantly stable for a long period of time.
  • Perpetually warm waters: Corals thrive in tropical waters [between 30°N and 30°S latitudes, the temperature of the water is around 20°C] where diurnal and annual temperature ranges are very narrow.
  • Shallow water: Coral require a fairly good amount of sunlight to survive. The ideal depths for coral growth are 45 m to 55 m below the sea surface, where there is abundant sunlight available.
  • Clear saltwater: Clear saltwater is suitable for coral growth, while both freshwater and highly saline water are harmful.
  • Abundant Plankton: Adequate supply of oxygen and microscopic marine food, called plankton [phytoplankton], is essential for growth. As the plankton is more abundant on the seaward side, corals grow rapidly on the seaward side.
  • Little or no pollution: Corals are highly fragile and are vulnerable to climate change and pollution and even a minute increase in marine pollution can be catastrophic.

Distribution of Coral Reefs

Corals and Zooxanthellae

  • Many invertebrates, vertebrates, and plants live in close association with corals, with tight resource coupling and recycling, allowing coral reefs to have extremely high productivity and biodiversity, such that they are referred to as the Tropical Rainforests of the Oceans’.
  • Scleractinian corals build skeletons of calcium carbonate sequestered from the water.
  • Scleractinian corals come under Phylum Cnidaria, and they receive their nutrient and energy resources in two ways.
    1. They use the traditional cnidarian strategy of capturing tiny planktonic organisms with their tentacles (All about Phylum Cnidaria is given in NCERT).
    2. Having a symbiotic relationship with a single cell algae known as ZOOXANTHELLAE.
  • Zooxanthellae are autotrophic [prepare their own food] microalgae belonging to various taxa in the phylum Dinoflagellata.
  • Coral = Phylum Cnidaria.
    Zooxanthellae = Phylum Dinoflagellata.

Symbiotic Relationship Between Corals and ZOOXANTHELLAE


  • Zooxanthellae live symbiotically within the coral polyp tissues and assist the coral in nutrient production through its photosynthetic activities.
  • These activities provide the coral with fixed carbon compounds for energy, enhance calcification, and mediate elemental nutrient flux.
  • The host coral polyp in return provides its zooxanthellae with a protected environment to live within, and a steady supply of carbon dioxide for its photosynthetic processes.
  • The symbiotic relationship allows the slow-growing corals to compete with the faster growing multicellular algae. The corals can feed by day through photosynthesis and by night through predation.
  • The tissues of corals themselves are actually not the beautiful colors of the coral reef but are instead clear. The corals receive their coloration from the ZOOXANTHELLAE living within their tissues.

Coral Bleaching or Coral Reef Bleaching

Coral Bleaching or Coral Reef Bleaching
  • Disturbances affecting coral reefs include anthropogenic and natural events.
  • Recent accelerated coral reef decline is related mostly to anthropogenic impacts (overexploitation, overfishing, increased sedimentation, and nutrient overloading).
  • Natural disturbances which cause damage to coral reefs include violent storms, flooding, high and low-temperature extremes, El Nino Southern Oscillation (ENSO) events, subaerial exposures, predatory outbreaks, and epizootics.
  • Coral reef bleaching is a common stress response of corals to many of the various disturbances mentioned above.
  • Bleaching occurs when
    the densities of zooxanthellae decline and/or
    2. the concentration of photosynthetic pigments within the zooxanthellae fall. [it is no more use for the coral and the coral will bleach it]
  • When corals bleach they commonly lose 60-90% of their zooxanthellae and each zooxanthella may lose 50-80% of its photosynthetic pigments.
  • If the stress-causing bleaching is not too severe and if it decreases in time, the affected corals usually regain their symbiotic algae within several weeks or a few months.
  • If zooxanthellae loss is prolonged, i.e. if the stress continues and depleted zooxanthellae populations do not recover, the coral host eventually dies.
Coral Bleaching may also be Beneficial
  • Recent research has revealed that corals that are consistently exposed to low levels of stress may develop some kind of resistance to bleaching.

Ecological Causes of Coral Bleaching


  • Coral species live within a relatively narrow temperature margin, and anomalously low and high sea temperatures [corals are absent on the west coast of tropical temperate continents because of the cold currents]can induce coral bleaching.
  • Bleaching events occur during sudden temperature drops accompanying intense upwelling episodes [El-Nino], seasonal cold-air outbreaks.
  • Most reefs recovered, with low levels of coral deaths, but the damage has been severe at places.
  • This is an instance of coral reefs’ susceptibility to increased water temperatures combined with ocean ACIDIFICATION.
  • While the rising temperatures have increased the frequency and intensity of bleaching, acidification has reduced corals calcifying ability.
  • Small temperature increase over many weeks or large increase (3-4 °C) over a few days will result in coral dysfunction.
  • Coral bleaching has occurred mostly during the summer seasons or near the end of a protracted warming period.
  • They are reported to have taken place during times of low wind velocity, clear skies, calm seas, and low turbidity. The conditions favor localized heating and high ultraviolet (UV) radiation.
  • UV radiation readily penetrates clear sea waters. The corals actually contain UV absorbing compounds which can block potentially damaging UV radiation. But rising temperatures mean reduction in the concentration of this UV absorbing compounds in corals.

Subaerial Exposure

  • Sudden exposure of reef flat corals to the atmosphere during events such as extremely low tides, ENSO-related sea level drops or tectonic uplift can potentially induce bleaching.
  • The consequent exposure to high or low temperatures increased solar radiation, desiccation and seawater dilution by heavy rains could all play a role in zooxanthellae loss, but could also very well lead to coral death.

Fresh Water Dilution

  • Rapid dilution of reef waters from storm-generated precipitation and runoff has been demonstrated to cause coral reef bleaching.
  • Generally, such bleaching events are rare and confined to relatively small, nearshore areas.

Coral reef distribution in India

Coral reef distribution in India

1. Andaman and Nicobar Islands

  • Situated in the Bay of Bengal, exclusively fringing reefs of about 500 islands, most of these islands have healthy biodiversity.
  • Andaman and Nicobar Islands, Gulf of Kutch and Gulf of Mannar are regions where there is not a significant freshwater intrusion. But Sunderbans are the mouth of Ganga-Brahmaputra river system and hence there is no coral reef formation there.

2. Gulf of Kutch

  • Exclusively consists of fringing reefs. The reefs are relatively less developed due to the large range of temperature and high salinity. The harbors have less biodiversity. The entire Gulf of Kutch is also known as a marine national park.

3. Gulf of Mannar

  • Fringing reefs with a chain of 21 islands from Rameswaram in the north to Thoothukudi (Tuticorin) in the south. This part of the gulf forms part of the Gulf of Mannar biosphere reserve.

4. Lakshadweep

  • Exclusively coral atolls with 36 islands, of which 10 are inhabited. The islands range from less than 1 km (0.62 mi) to about 9 km (5.6 mi) in length and do not exceed 2 km (1.2 mi) in width.

5. Others

  • Tarkarli in Malwan, Maharashtra is a smaller reef. Angria Bank is a coral reef off Vijaydurg in Maharashtra. There is a coral reef in Netrani Island in Karnataka.
  • Coral reefs don’t form in areas where there is a significant intrusion of freshwater and cold water.
  • Most of the mighty Indian Rivers flow into the Bay of Bengal and hence coral reefs are absent on the east coast of India.
  • Coral reefs are also absent on the west coast of Africa (Benguela Current), South America (Peruvian or Humboldt Current), Europe (Canaries Current) and other continents due to cold currents.


When corals are exposed to high concentrations of chemical contaminants like copper, herbicides and oil, coral bleaching happens.



  • Pathogen-induced bleaching is different from other sorts of bleaching.
  • Most coral diseases cause patchy or whole colony death and sloughing of soft tissues, resulting in a white skeleton (not to be confused with bleached corals).

Spatial and temporal range of coral reef bleaching

  •  Nearly all of the world’s major coral reef regions (Caribbean/ western Atlantic, eastern Pacific, central and western Pacific, Indian Ocean, Arabian Gulf, Red Sea) experienced some degree of coral bleaching and mortality during the 1980s.
  • Prior to the 1980s, most mass coral moralities were related to non-thermal disturbances such as storms, aerial exposures during extreme low tides, and Acanthaster outbreaks.
  • Coral bleaching accompanied some of the mortality events prior to the 1980s during periods of elevated seawater temperature, but these disturbances were geographically isolated and restricted to particular reefs zones.
  • In contrast, many of the coral bleaching events observed in the 1980s occurred over large geographic regions and at all depths.

Cold Water Corals

  • Cold-water corals inhabit deep, cold water.
  • The United Nations Environment Programme (UNEP) reports that there are more cold-water coral reefs worldwide than tropical reefs.
  • There are only about 6 different coral species associated in building with these reefs. The largest cold-water coral reef is the Rost Reef off Norway.

Initiatives to Protect Marine and Coastal Environments

1. Coastal Ocean Monitoring and Prediction System (COMAPS)

  • Being implemented from 1991.
  • Assesses the health of coastal waters and facilitates the management of pollution-related issues.

2. Land-Ocean Interactions in the Coastal Zone (LOICZ)

  • Launched in 1995.
  • Investigates the effects of global change on the coastal zone.
  • Aims to develop, on a scientific basis, the integrated management of coastal environments.

3. Integrated Coastal and Marine Area Management (ICMAM)

  • Launched in 1998.
  • Aims at integrated management of coastal and marine areas.
  • Model plans for Chennai, Goa, and Gulf of Kutch being prepared.

4. Society of Integrated Coastal Management (SICOM)

  • Launched in 2010.
  • A major national initiative to protect coastal ecosystems.
  • A professional body with experts in various aspects of coastal science and management was created.

5. Institutions for Coastal Management

  • The Notification on Coastal Regulation Zone (CRZ), 1991 (as amended from time to time) aims at protecting coastal stretches in India.
  • India has created institutional mechanisms such as the National Coastal Zone Management Authority (NCZMA) and State Coastal Zone Management Authority (SCZMA) for enforcement and monitoring of the CRZ Notification.
  • These authorities have been delegated powers under Section 5 of the Environmental (Protection) Act, 1986 to take various measures for protecting and improving the quality of the coastal environment and preventing, abating and controlling environmental pollution in coastal areas.
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