Introduction, types, characteristics features structure and function of an aquatic ecosystem: Ponds, streams, lakes rivers, oceans and estuaries

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Introduction, types, characteristics features structure and function of an aquatic ecosystem: Ponds, streams, lakes rivers, oceans and estuaries

AQUATIC BIOMES

Water covers nearly 70% of the earth‘s surface, providing abode to numerous species of organisms. In contrast to terrestrial biomes, which are identified and classified on the dominant vegetation, aquatic biomes are distinguished on the basis of physical attributes such as salinity, strength of current, availability of light and oxygen content. Global aquatic ecosystems fall into two broad categories– the freshwater biomes and marine biomes.

FRESHWATER BIOMES

Freshwater regions occupy a relatively small portion of the Earth‘s surface (less than 5%) as compared to marine and terrestrial habitats but their importance to humans is greater than their relative area because they are the most convenient and cheapest source of water for domestic and industrial use and provide a convenient and economical waste disposal system. Freshwater habitats are divided into two groups – lentic or standing-water ecosystems such as lakes and ponds and lotic or running-water ecosystems such as springs, streams and rivers. Associated with lentic and lotic ecosystems are the wetlands, where water is at or near the surface or the land is covered by shallow water such as swamps, marshes and bogs.

Lentic ecosystems (Lakes and ponds).

 

Lakes and ponds are inland depressions containing standing water. They are geographically isolated from one another and scattered throughout the earth‘s surface. They vary in size from just a few square meters to thousands of square kilometers and range in depth from one meter to over 1000 meters. The life span of ponds ranges from a few weeks or months for small seasonal or temporary ponds to several hundred years for larger ponds. Although a few lakes, such as lake Baikal, are ancient, most large lakes dates back to Pleistocene glaciation.

 

In summer, the high atmospheric temperature warm the upper layer of water and the wind mixes this water well. This upper, warm, circulating layer of water is called epilimnion. Below this lies denser, cold, non-circulating water layer called the hypolimnion. Between these two layers is a zone of steep temperature gradient called thermocline, that acts as a barrier to the exchange of materials.

Typically lakes and ponds can be subdivided into following zones based on the depth and distance from the shoreline

 

(a)   Littoral zone – It is the shallow water zone near the shore of a lake or pond in which light penetrates to the bottom. It is typically occupied by rooted and floating aquatic plants.

(b)   Limnetic zone – It is the near-surface open water zone that is surrounded by littoral zone and extends upto the depth of effective light penetration. This zone is dominated by plankton, both phytoplankton and zooplankton. Both littoral and limnetic zones constitute the euphotic zone where production is greater than respiration (P/R >1).

(c)    Profundal zone – The deep water zone which is beyond the depth of effective light penetration is the profundal zone where respiration is greater than production (P/R< 1). Between the limnetic and profundal zone is a level at which P/R=1 and is known as compensation level. In ponds the littoral zone is relatively large and the limnetic and profundal zones are small or even absent.

(d)   Benthic zone – The bottom region of both littoral and profundal zones is the benthic zone which is the zone of decomposition.

 

 

Based on primary productivity lakes may be classified into two broad categories Oligotrophic lakes

These are deep lakes having low surface to volume ratio with the hypolimnion larger than the epilimnion. Water is clear and appear blue to blue-green in sunlight. Bottom sediments are largely inorganic. The nutrient content of the water is low. Low nutrient availability results from a low input of nutrients from external sources and this causes low primary productivity. Littoral plants are scarce and plankton density is low.

 

Eutrophic lakes

These are shallow lakes with high surface to volume ratio. They are rich in nutrients, especially nitrogen and phosphorus, that stimulate a heavy growth of algae and other aquatic plants and result in high primary productivity. Littoral vegetation is more abundant and plankton density is high and such lakes exhibit characteristic plankton blooms.

Phytoplankton become concentrated in the upper layer of the water, giving it a green colour. The turbidity reduces the light penetration and restricts biological productivity to a narrow zone of surface water. Dead organic matter accumulates on the lake bottom where bacteria convert the dead matter into inorganic substances. The activities of decomposers deplete the oxygen level to a point where the aerobic life cannot exist. The number of species declines in such lakes although  the  number and biomass  of  organisms may remain  high. The high rate of decomposition speeds up cycling of nutrients that further increases plant production. This natural filling in of lake basin with organic matter and nutrient enrichment is termed as eutrophication.

Lotic ecosystems (Rivers)

Streams and rivers are the bodies of continuously flowing water moving in one direction. They get their start at headwaters, which may be springs, snowmelt or even lakes, and then  travel  all the way  to their mouths,  usually another water channel  or the ocean. The direction of flow is dictated by the lay of land, the underlying rock condition and the slope. Fast flowing water moving downstream cause extensive erosion that cuts the channel wider and deeper, and carries lot of debris that sooner or later are deposited within or along the stream where flow of water is less. Lotic system is an open ecosystem in which land-water interchange is relatively more extensive than other aquatic ecosystems. The major source of energy and nutrients is in the form of detritus carried into the streams from the surrounding land. Sometimes they also receive plankton and detritus from the connected lakes and ponds.

The lotic ecosystem is essentially a continuum of physical and biological conditions from the headwaters to the river‘s mouth. These changing conditions in the gradient from small to large streams have been termed the river continuum concept.

 

Wetlands

Wetlands are the lands transitional between terrestrial and aquatic systems and thus exhibit some of the characteristics of each. There are many kinds of wetlands which can be grouped into three major categories: swamps, marshes and peatlands.

Swamps are the wetlands with wooded vegetation (trees and shrubs). They may be deepwater swamps dominated by swamp oaks and cypress trees; or they may be shrub swamps dominated by alder and willows; or they may be forested woodlands along large river systems that are occasionally flooded by river waters.

Marshes are the wetlands dominated by emergent vegetation, plants with roots in soil (which is covered part or all of the time by water) and leaves held above the water. There are no trees in marshes and dominant vegetation are reeds, sedges, grasses and cattails.

Peatlands are the waterlogged areas saturated by groundwater or rainwater in which there is an accumulation of partially decayed organic matter. Peatlands fed by water moving through mineral soil dominated by sedges and slightly acidic are called fens. Peatlands dominated by Sphagnum mosses, receiving their water supply largely from precipitatin and highly acidic are called Bogs.

 

MARINE/OCEAN ECOSYSTEMS

Marine regions cover about 71% of the earth‘s surface and include oceans and associated ecosystems like estuaries, tidal marshes and swamps, sea shores and coral reefs. Oceans are the largest of all ecosystems and play a key role in controlling the world‘s climates, the atmosphere, and the functioning of major mineral cycles.

The important physical factors that influence life in marine environments are listed as follows:-

Salinity – The average salinity of sea water is 35‰ of which about 80% is contributed by sodium chloride and the rest by calcium, magnesium and potassium salts. The major anions in sea water are chloride, sulfate, bicarbonate, carbonate and bromide.

 

Light – Light is an important factor in the life of marine organisms due to its influence on photosynthesis, heating, radiations and vision. The volume of surface area lighted by sun is small compared to the volume of water in ocean.

 

Temperature – Ocean is the largest store house of sun‘s heat and thus regulates the temperature of the world. The range of temperature in sea is far less than that on the land; the extremes of temperature ranges from –3° C to 40° C.

 

Pressure – Pressure in the ocean varies from 1 atmosphere at the surface to 1000 atmosphere at its greatest depth and this has a pronounced effect on the distribution of life.

 

Currents – The sea is in continuous circulation and this is brought about by the currents. Air temperature difference between poles and equator set up strong winds, which together with rotation of earth create definite currents.

Sea shores

Sea shore is the place where sea meets the land. It is the littoral or intertidal zone roughly bounded by the extreme high tide mark (supra littoral fringe) and the extreme low tide mark (infra littoral fringe).

 

1.   Rocky shores

The rocky shores present solid substratum for growth of wide variety of algae and for attachment of many sessile animals. Sessile animals abound to the rocks are many sponges, colonial hydrozoans, anthozoans (e.g sea anemones) and bryozoans. The sedentary animals inhabiting the rocky shores are a variety of mollusks like limpets, mussels and oysters, barnacles and tunicates.

 

All rocky shores have the following three basic zones, characterized by the dominant organisms occupying them.

 

Supralittoral fringe – At the uppermost end is a zone of bare rock marking the transition between land and sea.

 

Littoral zone – It is the region covered and uncovered daily by tides and tends to be divided into subzones.

 

Infralittoral fringe – It is the zone uncovered only at the spring tides for short periods of time. This zone consists of forests of large brown alga, Laminaria (one of the kelps) with a rich undergrowth of other brown (Fucus) and red algae.

 

2.  Sandy shores and Mudflats

In contrast to rocky shores which appear studded with life, both sandy shores and mudflats appear barren of life. They are also subjected to extremes of temperature, salinity, wave action etc.

Like rocky shores, sandy and muddy shores also exhibit zonation of life related to tidal influences. These zones are: supralittoral zone (dominant organisms are ghost crabs and beach hoppers), littoral or intertidal zone (abundance burrowing animals of varied groups) and infralittoral zone (predominant organisms are starfishes, sand dollars, predatory crabs and fishes).

 

Open Sea

The two major regions of sea are pelagic or whole body of water lying above the ocean floor, and benthic or the entire sea bottom. The pelagic region is further divided into two zones: neritic zone which is the shallow water zone on continental shelf and oceanic zone which is the region of open ocean beyond the continental shelf. The continental shelf is the underwater extension of continent. The edge of continental shelf may be within a few kilometers of the shore, or it may be several hundred kilometers from the shore. On the basis of light penetration the pelagic region is broadly divided into two vertical zones – euphotic zone (photosynthetic zone) that extends from surface to about 200m in which there are seasonal fluctuations and sharp gradients in light, temperature and salinity and aphotic zone of very little or no light penetration.

Coral reefs

Coral reefs are one of the most beautiful and specialized ecosystems, widely distributed in warm shallow waters of tropical and subtropical seas. They are built by carbonate-secreting organisms of which corals (coelentrata, Anthozoa) are the most conspicuous (hence the name coral reef) although other organisms like coralline red algae (Rhodaphyta, corallinaceae), foraminifera and mollusks also contribute equally to their structure.

Coral reefs are built only under water at shallow depths and need a stable foundation upon  which  to  grow.  Such  a  foundation  is   provided   by   shallow   continentalshelves. Reef building corals grow best at waters having average annual temperature of 240 C, a depth of 40-50 metres and average salinity of 35‰. Reefs are built upto the sea level.

 

Coral reefs are of four types:

v  Fringing reefs that grow along the rocky shores of islands and continents.

v  Barrier reefs parallel shores lines along continents.

v  Atolls are coral islands that begin as horseshoe shaped reefs surrounding a lagoon.

v  Outer reefs are isolated from the land

 

Coral reefs are complex ecosystems involving a close relationship between coral and algae. One kind of algae, the so called zooxanthellae (a symbiotic endozoic dinoflagellate algae) live in the tissues of coral polyp while other kinds of red and green coralline algae and filamentous algae live in the skeleton around or below the animal bodies.

Coral reefs are among the most highly productive ecosystems of the world; net productivity ranges from 1500 to 5000 gm C/m2 /yr. In addition to abundance of light and oxygen the two major factors responsible for high productivity are (1) Efficient biological recycling of nutrients by coralline community retain nutrients within the system and (2) Coral act as trap for nutrients that come with the flowing water.

Estuaries

Estuaries are the areas where the freshwaters of streams and rivers meet the saltwater of ocean. It is a semi enclosed coastal body of water such as a river mouth or a coastal bay, which has a free connection with the open sea and which is strongly influenced by tidal action. The salinity of estuary is intermediate between that of seawater (salinity is about 35‰) and freshwater ( salinity ranges from 0.065% to 0.3% ). Estuary is, thus, a transition zone or ecotone between freshwater and marine ecosystems. Estauaries differ in size, shape and volume of water flow, all influenced by the geology of the region in which they occur. They normally have a high silt content.

Current & Salinity . Current and salinity are two important factors that influence life in the estuary. Estuarine currents are complex and highly variable in different estuaries and mainly result from the interaction of one-direction stream flow, which varies with season and rainfall, with oscillating ocean tides and with the wind. Salinity of estuaries varies vertically and horizontally and fluctuates between 0.5% to 35%.

 

Biota and adaptation to estuarine environment

Estuary, being a transitional zone between freshwater and sea derives its biota from the adjacent marine and riverine habitats as well as typical estuarine organisms. The major components are the species restricted to estuarine situations, such as oysters and crabs, and those that come from the sea, such as shrimps. Very few species are derived from freshwater and only those that are capable of osmoregulation in saltier water.

Organisms inhabiting the estuary are faced with two problems- maintenance of postion and adjustment to changing salinity. The majority of estuarine organisms are benthic and are securely attached to the bottom, are buried in the mud or occupy crevices and crannies about sessile organisms. The dominant benthic organisms are the oysters which may be attached to hard objects or may form reefs.

Productivity

Estuaries are one of the highly productive ecosystems, second only to coral reefs and tropical rain forests.

An estuary is a ―nutrient trap‖ that is partly physical and partly biological. Nutrients and oxygen are carried into the estuary by tides. If vertical mixing takes place, these nutrients are not soon swept back out to sea, but circulate up and down among organisms, water and bottom sediments. Organisms also aid in trapping and cycling of the nutrients. The retention and rapid recycling of nutrients by benthos, the recovery of nutrients from deep sediments by microbial activity and deeply penetrating plant roots or burrowing animals create a sort of self enriching system in the estuary.

 

The aquatic ecosystems constitute the marine environments of the seas and the fresh water systems in lakes, rivers, ponds and wetlands. These ecosystems provide human beings with a wealth of natural resources. They provide goods that people collect for food such as fish and crustaceans. Natural aquatic systems such as rivers and seas break down chemical and organic wastes created by man. However, this function has limitations, as the aquatic ecosystem cannot handle great quantities of waste. Beyond a certain limit, pollution destroys this natural function.

Very big ecosystem in the world. Marine ecosystem includes marshes, tidal zones, estuaries, the mangrove forest, lagoons, sea grass beds, the sea floor, the coral reefs

If aquatic ecosystems are misused or over utilized, their ability to provide resources suffers in the long term. Over-fishing leads to a fall in the fish catch. River courses that are changed by dams to provide electricity affect thousands of people who do not get a continuous supply of water downstream for their daily use. When wetlands are drained, their connected rivers tend to cause floods. These are all examples of unsustainable changes in the use of natural resources and nature’s ecosystems that are dependent on hydrological regimes.

Water is an important factor in all our ecosystems. Several ecosystems exist in freshwater and marine salt water. There is very little fresh water on earth, which is a key resource for people all over the world.

How are aquatic ecosystems used?

Man uses aquatic ecosystems for the clean freshwater on which his life is completely dependent. We need clean water to drink and for other domestic uses. Water is essential for agriculture. Fisherfolk use the aquatic ecosystems to earn a livelihood. People catch fish and crabs. They also collect edible plants. This is used locally as food or for sale in the market. Over fishing leads to a serious decline in the catch and a long-term loss of income for fisherfolk.

Marshes and wetlands are of great economic importance for people who live on their fish, crustacea, reeds, grasses and other produce.

Modern man impounds water in dams to be able to store it throughout the year. Agriculture and industry are highly dependent on large quantities of water. However this leads to problems for tribal people who have lived there before the dams were built as they are displaced to build large dams. These dams make rich people richer in the farmland and supports people in large urban centres that use enormous quantities of water. The poor tribal folk become even poorer as the natural resources they depend on are taken away as their lands are submerged under the water of the dam.

Dams are built across rivers to generate electricity. A large proportion of this energy is used by urban people, by agriculturists in irrigated farmlands and in enormous quantities for industry. Large dams have serious ill effects on natural river ecosystems. While water from dams used for irrigation has lead to economic prosperity in some areas, in semiarid areas that are artificially irrigated the high level of evaporation leads to severe salinisation as salts are brought up into the surface layers of the soil. This makes such lands gradually more and more saline and unproductive.

What are the threats to aquatic ecosystems?

Water pollution occurs from sewage and poorly managed solid waste in urban areas when it enters the aquatic ecosystem of lakes and rivers. Sewage leads to a process called eutrophication, which destroys life in the water as the oxygen content is severely reduced. Fish and crustacea cannot breathe and are killed. A foul odour is produced. Gradually the natural flora and fauna of the aquatic ecosystem is destroyed.

In rural areas the excessive use of fertilisers causes an increase in nutrients, which leads to eutrophication. Pesticides used in adjacent fields pollute water and kills off its aquatic animals. Chemical pollution from industry kills a large number of life forms in adjacent aquatic ecosystems. Contamination by heavy metals and other toxic chemicals affects the health of people who live near these areas as they depend on this water.

Case study

Threats to wetlands in Assam

Almost 40% of all wetlands in Assam are under threat. A survey conducted by the Assam Remote Sensing Application Center (ARSAC), Guwahati, and the Space Research Center, Ahemadabad, has revealed that 1367 out of 3513 wetlands in Assam are under severe threat due to invasion of aquatic weeds and several developmental activities. The wetlands of Assam form the greatest potential source of income for the State in terms of fisheries and tourism. Though the wetlands of Assam have the capacity of producing 5,000 tones of fish per hectare per year, around 20,000 tones of fish have to be imported to meet local demands. This is primarily due to poor wetland management

How can aquatic ecosystems be conserved?

For sustainable use of an aquatic ecosystem, water pollution must be prevented. It does not make sense to allow water to be polluted and then try to clean it up.

Changing the nature of the aquatic ecosystem from a flowing water ecosystem to a static ecosystem destroys its natural biological diversity. Thus dams across rivers decrease the population of species that require running water, while favouring those that need standing water.

Aquatic ecosystems, especially wetlands, need protection by including them in Sanctuaries or National Parks in the same way in which we protect natural forests. These sanctuaries in aquatic ecosystems protect a variety of forms of life as well as rare fish which are now highly endangered such as the Mahseer. Wetland Sanctuaries and National Parks are of greatest importance as this is one of the most threatened of our ecosystems. As the proportion of the earth’s surface that is naturally covered by wetlands is very small compared to forests or grasslands, the wetland ecosystems are very highly threatened