Chapters :
  • Ecology & Ecosystem – 02
  • What is coral bleaching?
  • Mangrove

Ecology & Ecosystem – 02

Pelagic Marine Ecosystem:

The pelagic is divided into two provinces: the neritic province, water that overlies the continental shelf, and the oceanic province. Because conditions change with depth, the pelagic is divided into several distinct vertical layers or zones

Pelagic zone can be further subdivided into ecological zones based on depth: in the epipelagic or photic zone(0-200m) , there are sharp gradients in illumination, temperature, and salinity; the mesopelagic (200-1000m), has little light to penetrate and the temperature gradient is more even and gradual, without much seasonal variation and this zone contains an oxygen-minimum layer and often the maximum concentration of nutrients (nitrate and phosphate);in the bathypelagic (1000-4000m), darkness is virtually complete, except for bioluminescent organisms; temperature is low; and water pressure is great; and the abyssopelagic region (4000 to 6000m). 

Areas deeper than 6000m are called hadal pelagicregions – which includes areas found in deep-sea trenches and canyons, while they account for the deepest regions on the planet’s oceans, they compose a very small fraction of the total oceanic environment (for example, the Mariana trench is a hadal environment).  

The pelagic environment is home to two basic groups of marine organisms. The first group consists of the plankton (Greek for “wandering”) which is appropriate because they possess little power to “swim” any significant distance, and are thus passively transported by ocean currents. Planktonic plants are known as phytoplankton, and planktonic animals are known as zooplankton (e.g. jellyfish, small crustaceans, pelagic snails, etc.). 

The second group  of pelagic marine organisms are the free swimming nekton, including marine mammals, fish, squid, and some larger crustaceans. 

Coral reef ecosystem and mangrove swamp ecosystem fall in the neritic province. Coral reefs are among the best-known marine systems, because of their extraordinary biological productivity and their diverse and beautiful organisms. Reefs are colonies of minute, colonial animals (coral polyps) that live symbiotically with photosynthetic algae. Calcium-rich coral skeletons shelter the algae, and algae nourish the coral animals. Although various types of corals can be found from the water’s surface to depths of 6000 m, reef-building corals are generally found at depths of less than 45 m.

Mangroves are the dominant ecosystems that line the coasts of subtropical and tropical coastlines around the world. Mangroves are a diverse group of salt-tolerant trees that grow along warm, calm marine coasts around the world. Growing in shallow, tidal mudflats, mangroves help stabilize shorelines, blunt the force of storms, and build land by trapping sediment and organic material. Detritus, including fallen leaves, collects below mangroves and provides nutrients for a diverse community of animals and plants. Both marine species (such as crabs and fish) and terrestrial species (such as birds and bats) rely on mangroves for shelter and food. Mangroves are found in south Florida, the Caribbean, Southeast Asia, Africa, and other parts of the world where tropical mudflats occur

Coastal ecosystems 

I.Coral reef ecosystem: 

Coral reefs are built around coral, a coelenterate animal, related to the jellyfish and sea anemones in the warm waters of tropical and subtropical oceans. The animal forms a colonial structure embedded in a skeleton of calcium carbonate or it’s mass. Within the cells of the coral there occur numerous cells of symbiotic endozoic algae, dinoflagellates. Thus, the coral represents a partnership, with the algae providing food, and the coral giving structural support and nutrients to algae. Most of a coral’s energy is produced by microscopic plants called zooxanthellae that live within the coral. These zooxanthellae actually give corals their beautiful colours. The coral also feeds on plankton of water, and by doing so procures nutrients to be used by its partner, the dinoflagellates. By maintaining a highly efficient nutrient cycling and high level of nutrient conservation, the coral reefs attain high productivity. Like tropical rainforests, their species diversity and richness in form and colour are tremendous. The coral reef communities include calcareous bryozoans, polychaete worms, molluscs (all occurring in coral mass); sea anemones, sponges, crabs, snails and echinoderms (occurring in abundance in association with the ecosystem).

Ideal conditions for coral growth:

  • Stable climatic conditions: Corals are highly susceptible to quick changes. They grow in regions where climate is significantly stable for a long period of time.
  • Perpetually warm waters: Corals thrive in tropical waters [30°N and 30°S latitudes, The temperature of water is around 20°C] where diurnal and annual temperature ranges are very narrow.
  • Shallow water: Coral require fairly good amount of sunlight to survive. The ideal depths for coral growth are 45 m to 55 m below sea surface, where there is abundant sunlight available.
  • Clear salt water: Clear salt water is suitable for coral growth, while both fresh water 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.

What is coral bleaching?

The stunning colors in corals come from a marine algae called zooxanthellae, which live inside their tissues. This algae provides the corals with an easy food supply thanks to photosynthesis, which gives the corals energy, allowing them to grow and reproduce.

When corals get stressed, from things such as heat or pollution, they react by expelling this algae, leaving a ghostly, transparent skeleton behind. This is known as ‘coral bleaching’. Some corals can feed themselves, but without the zooxanthellae most corals starve.

Causes of Coral Bleaching:

  • Rise in Sea Temperature: Most coral species live in waters close to the warmest temperature they can tolerate i.e., a slight increase in ocean temperature can harm corals. El Nino elevates the sea temperature and destroys coral reefs.
  • Ocean Acidification: Due to rise in carbon dioxide levels, oceans absorb more carbon dioxide. This increases the acidity of ocean water and inhibits the corals ability to create calcareous skeletons, which is essential for their survival.
  • Solar radiation and ultraviolet radiation: Changes in tropical weather patterns result in less cloud cover and more radiations which induce coral bleaching.
  • Infectious Diseases: Penetration of bacterium like vibrio shiloi inhibits photosynthesis of zooxanthellae. These bacteria become more potent with elevated sea temperatures.
  • Chemical Pollution: Increased nutrient concentrations affect corals by promoting phytoplankton growth, which in turn supports increased numbers of organisms that compete with coral for space.
  • Increased Sedimentation: Land clearing and coastal construction result in high rates of erosion and a higher density of suspended silt particles which can smother corals when particles settle out (sedimentation), reducing light availability (turbidity) and potentially reducing coral photosynthesis and growth.
  • Human Induced Threats: Over-fishing, pollution from agricultural and industrial runoff, coral mining, development of industrial areas near coral ecosystems also adversely impact corals.
  • Invasive Species:  Lionfish are native to coral reefs in the tropical waters of the South Pacific and Indian Oceans. This is an invasive species that threatens the well-being of coral reefs and other marine ecosystems


Mangrove refers to an ecological group of halophytic plant species as well as to a variety of complex plant communities dominated by these species. The mangrove ecosystems are found in the inter-tidal zones of sheltered shores, estuaries, creeks, backwaters, lagoons, marshes and mud-flats, along the tropical coastlines of Asia, Australia and Americas.

The dominant salt-tolerant, sclerophyllous broadleaved trees form a unique ecosystem with associated plants, including epiphytic and terrestrial ferns, orchids, lichens, non-mangrove halophytes, sea grasses and seaweeds, and fauna such as fish, shrimp, shellfish, crabs, lobsters, reptiles and birds. 

These provide habitats, spawning grounds, nurseries and nutrients to a number of animals and harbour several endangered species ranging from reptiles (e.g. crocodiles, iguanas and snakes) and amphibians, to mammals (tigers, deer, otters and dolphins) and birds (herons, egrets, pelicans and eagles). The tree trunks, leaves and roots provide habitats for diverse fauna. Many organisms colonise mangrove roots such as sponges, clams, algae and oysters. 

The coastal zone of India and that of Andaman and Nicobar Islands have the presence of extensive and diverse mangroves. These ecosystems are found along both the east and west coasts of India. In India, important mangrove areas are in the Sundarban, Bhitarkanika, Krishna and Godavari delta of Andhra Pradesh, Andaman and Nicobar Islands, Gulf of Kachchh, and the Pichavaram-Vedaranyam area of Tamil Nadu coast.

Adaptation in mangrove ecosystem:

Mangrove environment is highly dynamic and harsh and mangrove species are variously adapted to cope with these environmental conditions.

Breathing roots: 

Underground tissue of any plant requires oxygen for respiration and in mangrove environment, oxygen in soil is very limited or nil. This necessitates mangrove root system to take up oxygen from the atmosphere. For this purpose, mangrove species have specialized above ground roots called breathing roots or pneumatophores. In some species, these roots are pencil sized and peg like whereas in some other species they look like a knee. These roots have numerous pores through which oxygen enters into the underground tissues. In some plants buttress roots function as breathing roots and also provide mechanical support to the tree.

Silt roots: 

In some mangrove species, roots diverge from stems and branches and penetrate the soil some distance away from the main stem as in the case of banyan trees. Because of their appearance and because they provide the main physical support to these they are called as stilt roots. These roots also have many pores through which atmospheric oxygen enters into the roots.


Saline water, unconsolidated saline soil with little or no oxygen is not a conducive environment for seeds to germinate and establish. To overcome this, mangrove species have unique way of reproduction, which is generally known as vivipary. In this method of reproduction, seeds germinate and develop into seedlings while the seeds are still attached to the parent tree. These seedlings are normally called as propagules and they photosynthesize while still attached to the mother tree. The parent tree supplies water and necessary nutrients. They are buoyant and float in the water for sometime before rooting themselves on suitable soil.

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