17 Types of Aquatic Ecosystems-II : Marine Ecosystem

Sharda R. Gupta

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Module 31: Types of Aquatic Ecosystems-II

Objectives: To learn about marine ecosystems

Learning Objectives

  • Ø Coastal Marine Ecosystems
  • Ø Marine Pelagic Environment
  • Ø The Deep ocean, Seamounts and Hydrothermal Vents

 

In Module 30 (Types of aquatic ecosystems-I), you have learnt about the Hydrological Cycle Limiting Factors in Aquatic Ecosystems, Aquatic Organisms and Freshwater Aquatic Ecosystems.

In module 31 (Types of Aquatic Ecosystems-II: Marine Ecosystem) Marine ecosystems will be discussed in detail

31.1. MARINE ECOSYSTEMS

 

Marine ecosystems are home to a wide variety of species ranging from planktonic organisms that form the base of the marine food web to large marine mammals. Many species depend on marine ecosystems for both food and shelter from predators. Tiny phytoplankton provide 60% of the oxygen on earth and form the basis of the ocean food chain up to fish and marine mammals, and ultimately human consumption.

 

Horizontal zones of marine ecosystems (those extending from land out to sea) include the Coastal Zone and the vast open waters of the ocean (the pelagic Zone). Overview of oceanic life zones is shown in Figure 31.1.

 

Ocean has relatively uniform conditions, less adaptation required, less speciation. Marine species are mostly benthic (98%) compared to pelagic (2%). Oceans are the living environment for about 250,000 species of marine plants and animals (Trujillo and Thurman 2014), Figure 31.2.

 

31.1. Coastal Ecosystems

 

Coastal habitats are those above the spring high tide limit or above the mean water level in non-tidal waters. Coastal habitats account for about 30% of all marine biological productivity. Habitats such as mangroves and sea grasses protect the coastlines from wave action and erosion.

 

These include the littoral, marine sandy beach and marine mud flat systems. Estuarine, mangrove and coral reef communities are quite important ecologically as well as economically. Coral reefs, mangroves and sea grasses are some of the world’s most biodiverse marine ecosystems, home to a significant proportion of the world’s known marine species. These ecosystems not only have a high ecological value, but are also of critical socio-economic importance. These ecosystems provide essential services such as buffering against storm surge, stabilizing coastal and near-shore areas against erosion, serving as nursery habitats for commercially important fish species and supporting livelihoods and economic activities, such as tourism and fishing (http://www.cbd.int). Coastal systems

 

  • (i) The littoral ecosystems occupy coastal belts washed by waves and alternatively exposed and submerged by tides. Marine macroalgae, or seaweeds, are plant-like organisms that generally live attached to rock or other hard substrata in coastal areas. The biota is characterized by barnacles, oyster, limpet, kelp (e.g., Fucus laminaria) or red algae (e.g., Gigartina). Seagrasses are found in shallow salty and brackish waters in many parts of the world, from the tropics to the Arctic Circle except Seagrasses belong to a group of plants called monocotyledons, have leaves, roots and veins, and produce flowers and seeds. Seagrasses are capable of capturing and storing a large amount of carbon from the atmosphere. Dugongs in particular are very dependent on seagrass for their food, and an adult can eat up to 40 kg of seagrass a day! Healthy seagrass beds are critical to dugong survival In India, seagrass beds are found along the coasts of Tamil Nadu, Lakshadweep islands, Andaman and Nicobar Islands and the Sundarban. Eight hundred and forty four species of seaweeds are found in shallow waters all along the Indian coast particularly in Tamil Nadu, Gujarat, Goa, Maharashtra and Lakshadweep (Venkataraman et al. 2012)

 

  • (ii) Marine sandy beaches: occur around the world in the littoral belt. The tides keep on submerging and exposing sandy beaches. The surface sand keeps on moving under the influence of tides. Thus, the environment is highly unstable and difficult. The organisms are alternately exposed (low tide) and submerged (high tide). Beach animals are largely dependent on plankton and detritus or dead particles brought in by river waters, though non-diatoms and other algae grow on the beach itself. Crustaceans and worms burrow in the sand, and microscopic organisms like copepods, rotifers and protozoan live in the water that fills spaces between sand particles. Fish and shrimps keep on moving up and down with water flow, and clams and snails occur at low tide levels.

 

  • (iii) Marine mud flat: These communities occur in bays and estuaries, where water is quieter, unlike on beaches where water keeps on moving up and down. However, like the beaches, animals and microbes of mud flats receive their food from outside as dead remains of salt marsh plants, and as plankton and dead particles from tides and river drainages that meet. However, primary production also occurs on mud flats, with microscopic algae in the upper surface and photosynthetic bacteria below that. Among the animals, snails occur on the surface of mud, and numerous forms of organisms, such a clams, distinctive worms, specialized crustaceans, and microscopic forms occur within the mud.

 

  • (iv) Estuaries: These are the systems where fresh water of rivers meets the salty water of the ocean, are complex ecosystems of high productivity, often combining salt marsh and mud flats with deeper water. The mixing of fresh water and seawater creates a brackish environment where salinity is highest closer to the sea and lowest in the river Estuaries harbour unique plant and animal communities because their waters are brackish, a mixture of fresh water draining from the land and salty seawater. Estuaries function on both planktonic-based and detritus-based food webs (see Smith 1996). About 80 per cent of commercial marine fisheries depend in one way or other on estuaries (as a source of food, as a spawning ground, etc.). An estuary is formed where a river or stream runs into the sea forming a transition zone between fresh water and saltwater.

New York City, with a population of over eight million people, is an example of a major urban area located on an estuary (http://oceanservice.noaa.gov/education/tutorial_estuaries).

 

Most of the India’s major estuaries occur on the east coast. The Chilika Lake is one of the most beautiful estuaries in India. Some rivers meet the sea without forming an estuary because, for example, the geography of the river prevents seawater entering it. In some cases, the river channel divides creating multiple, much smaller entry points to the sea. These are known as deltas. The world’s largest delta is at the mouth of the Ganga River, which lies 60% in Bangladesh and 40% in India (Fig.31.3). The deposition of nutrients here has led to the area being one of the most fertile regions in the world.

  • (v) 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.

 

Mangroves cover about 137 000 km2 of the Earth’s surface

Most of the world’s mangroves are in the tropical and subtropical regions.

They are found in 123 countries around the world.

75 percent of the world’s mangroves are found in just 15 countries.

 

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 mangrove trees show a variety of adaptations such as knee roots, prop roots, pneumatophores, vivipary, etc. 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 (Fig. 31.4), Bhitarkanika, Krishna and Godavari delta of Andhra Pradesh, Andaman and Nicobar Islands, Gulf of Kachchh, and the Pichavaram-Vedaranyam area of Tamil Nadu coast.

  • (vi) 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).

Coral reefs are usually categorized into: (i) barrier reefs (which parallel shorelines along continents; e.g. the Great Barrier Reef of Australia, Box 31.1), (ii) fringing reefs (around islands), and (iii) atolls (horseshoe-shaped reefs surrounding a lagoon).

 

Coral reefs provide some of the most biologically rich, productive and economically valuable ecosystems on Earth. Over 25 per cent of all marine species live in coral reefs, and yet they cover less than 0.1 per cent of the ocean (Spalding et al., 2001). Coral reefs support the highest marine biodiversity in the world. More than 500 million people worldwide depend on them for food, storm protection, jobs, and recreation. All the three major reef types occur in India (atoll, fringing and barrier) (Box 31. 2).

 

Box 31.1: Australia’s Great Barrier Reef

 

  • The Great Barrier Reef is located on the north-east coast of Australia. It contains the world’s largest collection of coral reefs, with 400 types of coral, 1,500 species of fish and 4,000 types of mollusc.
  • The Great Barrier Reef is home to a wide range of life, including fish, sea turtles, giant clam, seahorse, sea turtles, sharks and many more.
  • Climate change is the biggest threat to the Great Barrier Reef.It is the habitat of species such as the dugong (‘sea cow’) and the large green turtle, which are threatened with extinction. http://whc.unesco.org/en/list/154

Box 31.2. Coral Reefs in India

 

The mainland coast of India has two widely separated areas containing reefs, i.e., the Gulf of Kachchh in the northwest, and Palk Bay and Gulf of Mannar in the southeast. There are patches of reef growth on the West Coast, for example, coral reefs at Malvan. The Andaman and Nicobar Islands in the Bay of Bengal have fringing reefs around many islands, and a long barrier reef (329 km) on the west coast. The Lakshadweep islands in the Arabian Sea also have extensive reefs but these are also poorly explored. Venkataraman, K. 2011. Coral reefs in India. DOI: 10.1007/978-90-481-2639-2_64

 

31.2. Marine Pelagic Environment

 

Pelagic zone includes the entire ocean water column. The pelagic zone has the largest volume, and the greatest  vertical range  depth,  11,000  metres.  Pelagic  life  is  found  throughout  the  water  column, although the numbers of individuals and species decrease with increasing depth.

 

Neretic Zone

 

The neritic zone is the portion of the world’s oceans stretching from the edge of the intertidal zone to approximately the edge of the continental shelf. It forms part of the epipelagic zone, the 200 meters closest to the surface, which is also known as the sunlight zone. Sunlight penetrates the entire water column. The neritic zone is permanently covered with generally well-oxygenated water, receives plenty of sunlight and has low water pressure; moreover, it has relatively stable temperature, pressure, light and salinity levels, making it suitable for photosynthetic life.

 

Availability of light leads to the development of extensive algal communities, ranging from giant kelp “forests” through smaller communities of green, brown or red algae, to a film of unicellular algae (Phytoplanktons).Rich fauna of fish, crustaceans and others live and feed there. The animals common in the neritic zone are the eel, sea turtle, potato grouper, dolphins, and sea stars. World’s major fish catch and invertebrates catches are from neritic zone. Most of fishing activities occur in this zone. Fishing is essential to the livelihoods of millions of people around the world, but our growing demand for fish is changing marine ecosystems.

 

The depletion of light leads to development of different flora and fauna complements on the deeper bottoms. In particular, the benthic zone (shallow ocean floor) in the neritic is much more stable than in the intertidal zone.

 

Oceanic Province

 

The vertical zonation in the oceanic province (Fig.31.5) is as follows:

 

  • The epipelagic euphotic zone: It is the sunlit upper layer of the ocean, also known as the photic or euphotic zone. It reaches from the surface to about 200 meters deep.
  • The mesopelagic zone stretching from 200 to about 1,000m deep. This zone is also known as the twilight zone.
  • The bathypelagic zone: this zone reaches to about 4,000m deep. The bathypelagic zone is also known as the midnight zone because no light reaches it. Many of the animals in this zone, including squid and fish, are bioluminescent. Bioluminescent organisms produce light through chemical reactions in their bodies.
  • The abyssopelagic zone: The water at the bottom of the ocean is very salty and cold (2oC). It extends from 4000 m deep to the sea floor.
  • The hadalpelagic zone: The deepest ocean zone, found in trenches and canyons. Hadal trenches, the deepest places on Earth, plunge to as deep as 11,000m below the surface of the sea.

 

  1. i) The Epipelagic Euphotic zone

 

These waters usually extend to a depth of 200m. Most life in the ocean is found in a very thin layer at the surface. This is because light from the Sun can only penetrate seawater to a maximum of about 200 metres. Despite the fact that oceanic phytoplankton make up only about one percent of all biomass on Earth, they perform nearly 50 per cent of all photosynthesis. Pelagic life consists of three categories, i.e. The phytoplankton, Zooplankton and Nekton. In the surface pelagic zone, light supports photosynthesis by the phytoplanktons such as diatoms and dinoflagellates. Zooplanktons are mostly copepods that feed on the phytoplankton. Marine zooplankton includes a diverse mixture of other animals, such as shrimp like euphuists (krill), arrow worms, comb jellies, tunicates etc.

 

(ii) The Deep ocean, Seamounts and Hydrothermal Vents

 

The deep ocean constitutes most of the ocean’s volume as well as the largest living space on Earth. It is

 

comprised of the mesopelagic zone, the bathypelagic zone, and the Abyssal-benthic zone. The marine deep pelagic communities occur below the lighted surface waters. Many of the fish and crustaceans which swim between the surface and deeper waters egest and excrete organic matter to be used by communities of deeper waters. In this dark pelagic zone, carnivorous copepods and other crustaceans occur abundantly, along with colonial bacteria. In the deepest part of the oceans, animals may be without functional eyes and may have light-producing organs or luminescence. Deep-sea environments (the waters and sediments of the ocean >200 m deep) are the largest biome of the world, representing more than 65% of the Earth’s surface and more than 95% of the global biosphere (Herring, 2001), with a key regulatory role in global biogeochemical cycles. They play a fundamental role in global biogeochemical cycles and their functions allow existence of life on our planet.

 

The deep ocean benthos are rich in biodiversity though strange, occupying permanent cold and dark water surviving under intense pressure and scarcity of food that moves down from the distant surface waters.

 

Seamounts These are undersea mountains formed by volcanic activity rising at least 3,500 ft above the seafloor most occur in chains at hot spots or along plate boundaries. Some seamounts reach into surface, sunlit waters, may even break above the water to form islands, while others have their peaks a kilometre below the ocean surface. Seamounts harbor, and sustain diverse communities of animals

including the sponges, corals, crustaceans, gastropods, vertebrates, and other creatures that live on or around undersea peaks, with nutrients rich waters (http://ngm.nationalgeographic.com/).

 

Hydrothermal Vents: In 1977, oceanographers first discovered high-temperature, deep-sea hydrothermal vents along volcanic ridges in the ocean floor of the Pacific near the Galápagos Islands. Hydrothermal vents occur in volcanically active areas of the seafloor like mid-ocean ridges, where tectonic plates are pushing and pulling above magma hotspots in Earth’s crust.

 

Hot springs on the ocean floor are called hydrothermal vents. The heat source for these springs is the magma (molten rock) beneath submarine volcanoes (https://oceanservice.noaa.gov/facts/vents.html) . In these areas super-heated gases and chemically rich water erupt from the ground at temperatures of up to 400°C. The hot hydrothermal fluids mix with near-freezing seawater to form very fine-grained sulfide minerals. “Black smokers” are chimneys formed from deposits of iron sulfide, which is black. “White smokers” are chimneys formed from deposits of barium, calcium, and silicon, which are white. Hot seawater in hydrothermal vents does not boil because of the extreme pressure at the depths where the vents are formed (https://oceanservice.noaa.gov/facts/vents.html). The temperature of the plume of white smokers is usually lower than that of black smokers. Vents are temporary features on the seafloor. They become inactive when seafloor-spreading moves them away from the rising magma or when they become clogged.

 

The vent organisms can survive and thrive in extreme pressures and temperatures and in the presence of toxic mineral plumes. The conversion of mineral-rich hydrothermal fluid into energy is a key aspect of these unique ecosystems. Through the process of chemosynthesis, bacteria provide energy and nutrients to vent species without the need for sunlight. The study of hydrothermal vent ecosystems is contributing to our understanding of the requirements for life (https://www.nationalgeographic.org/media/deep-sea-hydrothermal-vents/).

 

Microbial organisms are able to withstand these extreme temperatures to create energy from the chemical compounds via a process called chemosynthesis. Some of these microbes live symbiotically inside tubeworms, while others form large mats, which attract progressively larger organisms that graze on them. Primary consumers include giant clams, mussels, and polychaete worms that filter bacteria from water and graze on bacterial film on rocks Top carnivores at vents include the eel-like zoarchid fish, which may feeds on snails, limpets, and amphipods.     So far, over 500 species that live only at hydrothermal vents have been discovered; it is possible that these communities are the oldest ecosystems on Earth and the place where life began (SCBD 2012).

 

SUMMARY

 

  • Marine ecosystems are home to a wide variety of species ranging from planktonic organisms that form the base of the marine food web to large marine mammals. Tiny phytoplankton provide 60% of the oxygen on earth.
  • Coastal habitats are those above the spring high tide limit or above the mean water level in non-tidal waters. Examples of Coastal habitats are such as mangroves and seagrasses.
  • Estuaries, where fresh water of rivers meets the salty water of the ocean, are complex ecosystems of high productivity, often combining salt marsh and mud flats with deeper water.
  • The mangroves are found in the inter-tidal zones along the tropical coastlines. Coral reefs are mainly found in the warm waters of tropical and subtropical oceans.
  • Pelagic zone includes the entire ocean water column. The neritic zone is the portion of the world’s oceans stretching from the edge of the intertidal zone to the edge of the continental shelf.
  • The oceanic province of pelagic zone has the largest volume, and shows vertical zones.
  • Seamounts are undersea mountains formed by volcanic activity rising at least 3,500 ft above the seafloor, most occur in chains at hot spots or along plate boundaries. Hot springs on the ocean floor are called hydrothermal vents.
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References

 

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