3. Structure of Ecosystem

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Paper : 12 Principles of Ecology

Module : 21 Types of Ecosystems: Aquatic Ecosystem - Fresh Water Ecosystem: Part 1

Development Team

Paper Coordinator : Prof. D.K. Singh

Department of Zoology, University of Delhi Principal Investigator : Prof. Neeta Sehgal

Department of Zoology, University of Delhi

Content Writer : Dr. Sushma Bhardwaj , Deshbandhu College, DU Ms. Harshita Mishra, Research Scholar, DU

Content Reviewer : Prof. K.S. Rao

Department of Botany, University of Delhi Co-Principal Investigator : Prof. D.K. Singh

Department of Zoology, University of Delhi

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Description of Module

Subject Name ZOOLOGY

Paper Name Principles of Ecology Module Name/Title Types of Ecosystems

Module Id M21: Aquatic Ecosystem: Freshwater Ecosystem: Part 1 Keywords Ecosystem, Freshwater ecosystem, limnology, lake, river,

stream

1. Learning Objectives

After the end of this module you will be able to

 Understand the structure, function and composition of an ecosystem

 Differentiate between ecosystem, ecology and environment

 List the different type of ecosystems and their characteristics.

 Know the characteristics of Freshwater Ecosystem

 Explain the limiting factors of Freshwater Ecosystem

2. Introduction

Any assemblage of plants and animals able to exist within an area will in time, form a biotic community. In this community, the different species tend to interact with one another and to modify the conditions of life within which each exists. They therefore develop the interrelationships and inter-dependences which constitutes an ecosystem. All ecosystems of the earth, together, form the biosphere. The biosphere is that portion of the earth within which life exists. It includes all oceans and freshwater, lower layers of the atmosphere and the outer skin of the earth‘s crust- the rocks and soil of the earth‘s surface.

The term ecosystem was proposed by A.G. Tansley in 1935, who defined it as ‗the system resulting from integration of all the living and non-living factors of environment‘. An ecosystem is an overall integration of whole mosaic of interacting organisms and their environment. It is normally an open system with a continuous, but variable, influx and loss of materials and energy. It is a basic, functional unit with no limits of boundaries, consisting of both biotic and abiotic components interacting with each other, both necessary for maintenance of life upon earth. Therefore, it represents the highest level of energy based ecological interaction.

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Fig.1: Schematic Representation of an Ecosystem

3. Structure of Ecosystem

3.1. Trophic Structure of the Ecosystem

Based on the trophic structure (trophe = ‘nourishment‘), ecosystem is two layered.

1. Autotrophic (―self-nourishing‖) Stratum is the upper layer, also called the ‗green belt‘ of chlorophyll containing plants. This stratum is predominated by fixation of light energy, use of simple inorganic substances, and the buildup of complex substances.

2. Heterotrophic (―other- nourished‖) Stratum is the second layer, also known as the

―brown belt‖ of soil and sediments, decaying matter, roots, etc. This level is dominated by the use, rearrangement and decomposition of complex materials.

3.2. Components of Ecosystem

Living organisms and their non-living environment are in-separable and interact with each other in an ecosystem (Fig. 2). Therefore, they constitute the two major components of an ecosystem:

a. Abiotic (Non- Living) component b. Biotic (Living) component

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Abiotic Component: It includes

 Inorganic compounds: Substances like, P, S, C, N, H, etc. which are involved in mineral cycling. The amount of these substances present in the ecosystem at any point of time is the indicator of its standing quality.

 Organic Substances: Substances like proteins, carbohydrates, lipids etc., present either in the biomass or in the environment i.e., biochemical structure. Amount and distribution of inorganic chemicals such as chlorophyll and organic substances, links the biotic and abiotic components of the ecosystem.

 Air, water and substrate environment, including the climate regime and other physical factors.

Biotic Components: It is the living component of an ecosystem and gives the trophic level (nutritional structure) to an ecosystem. The biotic components comprise of:

 Producers: They convert the sun‘s energy into chemical or food energy (usually green plants).

 Phagotrophs (phago = ―to eat‖) or Consumers: They depend upon the producers for food/energy. They may be herbivorous, carnivorous, etc.

 Saprotrophs (sapro + ―to ecompose‖) or Decomposers: They breakdown the complex compounds of dead or living protoplasm into simpler forms and release inorganic nutrients in the environment, which can be reused by the autotrophs. They are also referred as microconsumers and chiefly include bacteria, actinomycetes and fungi.

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Fig. 2: Schematic diagram of Ecosystem Function

Ecosystem is a functional unit. It includes environment (biotic + abiotic) and the inter- relationship of living organisms with each other and the environment. Ecosystem can be as big as our Earth or as small as a drop of water (water + micro-organisms). Ecosystem is thus a subset of ecology which is a vast subject area.

4. Types of Ecosystem

There are four major habitats in the biosphere, i.e., terrestrial, fresh water, marine and estuarine. Based on these habitats, the ecosystem study is classified under different categories. The ecosystem study based on habitat approach helps in developing an understanding of the organisms and physical factors associated with that particular ecosystem. This helps mitigate the pitfalls that may follow excessive generalization (Odum, 1996). Such studies also provide insight into the methods, instrumentation, and technical difficulties applicable to specific situations.

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Fig. 3: Classification of Ecosystem

5. Freshwater Ecology

According to scientific postulates, life began in ocean. Therefore, it would be logical to start the ecosystem study with marine habitat. However, in actual practice, it is best to start the study with the fresh water ecosystems for several reasons. Primarily, examples for fresh water ecosystems are in close vicinity of man and therefore, it can be easily studied and identified with. Secondly, these are relatively small ecosystems, and hence easily accessible with the use of relatively simple instruments. Finally, the comparatively small biodiversity of the fresh water ecosystem makes it easier for the beginner to understand the dynamics of the natural system.

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5.1. Freshwater Habitat

Any waterbody having a low salt concentration — usually less than 1% is called a freshwater body. Animals as well as plants in these freshwaters are adapted to the low salt concentrations and are unable to survive high concentration of salt present in oceans.

The study of relationship between living organisms and the fresh water environment constitutes the freshwater ecology. The study of physical, chemical, geological and biological aspects of freshwaters is called limnology.

In the year 1989, F.A. Forel, a Swiss physician, a naturalist and a professor at the University of Lausanne, published a paper describing the bottom of the fauna of Lac Leman, now called Lake Geneva. This publication of Forel‘s paper is now considered as the beginning of the scientific study of lakes. Forel earned himself an ever-lasting regard as the founder of limnology.

The study includes many disciplines, such as geology, chemistry, physics and biology, which constituted the core for the study of the inland waters. By 1900 limnology had gained a strong ecological approach. A paper published by S.A. Forbes, a naturalist and entomologist, in 1997, titled ―The lake as a microcosm,‖ had described the lake as a ―small world‖ in which environmental features and living organisms are in complex interrelationships and bound together by a web of interdependence.

Fig. 4: Lake Geneva

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Although the term limnology originally applied only to the study of lakes, in its current use it also refers to the study of streams. As we will see, these two types of water systems differ in many ways. The term potamology is frequently used for studying the river systems. The word stream is used to indicate any mass of water with unidirectional flow; mountain brooks, spring brooks, creeks and rivers. In recent years, ecologists, hydrologists and engineers have been giving special attention to stream systems.

5.2. Types of Freshwater Habitat

Freshwater habitats are studied under three major categories for convenience:

a. Standing water or Lentic (lenis, calm) Habitat: e.g., Lake, pond, swamps etc.

b. Running water or Lotic (lotus, washed) Habitat: e.g., spring, streams, rivers etc.

c. Wet Lands: Marshes and swamps.

Fig. 5: Fresh Water Lake (Lentic Habitat) Fig. 6: River (Lotic Habitat)

There are no boundaries between these categories or within these categories. The Geological change tends to produce a gradient in the direction indicated, while biological processes often work to stabilize or slow down the process of lake filling and stream erosion. Thus lakes tend to fill up while streams tend to cut down to base level and thus change as a result of the action of water.

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Freshwater habitats occupy a relatively small portion of earth‘s surface as compared to marine and terrestrial habitats. However, they play a far more important role for Humans because of the following reasons:

1. They are the cheapest and the most convenient source of water for industrial as well as domestic needs.

2. They are the ―bottle-neck‖ of the hydrological cycle.

3. They are a convenient and cheapest sinks for waste disposal systems.

5.3. Limiting Factors ofFreshwater Ecosystems 5.3.1. Temperature

The unique properties of water maintain the temperature fluctuation in the water bodies. The variation is small, decreasing gradually from the surface to the bottom of the water body.

Some of these thermal properties are:

 High specific heat: To change the temperature of water, a relatively very large amount of heat is required compared to other substance of same mass. One gram-calorie (gcal) of heat is required to raise one milliliter (ml) of water by one degree centigrade (between 15⁰C to 16⁰C).

 High latent heat of fusion: 80 calories are required to change one gram of ice into water without changing its temperature.

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 Highest latent heat of evaporation: 536 calories per gram are absorbed during evaporation which occurs more or less continually from vegetation, water, and ice surfaces. Major portion of the solar energy dissipates in the evaporation of water from the ecosystems of the world. This flow of energy moderates the climate which makes life possible on earth.

 Highest density at 4⁰ C helps in expanding water from both above and below the surface and making it lighter. This property prevents the water bodies like lake from freezing.

However, temperature is a major limiting factor for the aquatic biota because aquatic organisms often have a narrow tolerance (Stenothermy). Therefore, even a small change in temperature can have widespread effects. Temperature change produces characteristic patterns of circulation and stratification with the water body which in turn influences water bodies as well. Waterbodies like rives also play role in controlling the climate of the land and its surroundings.

5.3.2. Transparency

Transparency is an important parameter to measure the penetration of light. Penetration of light is limited by suspended materials, restricting the photosynthetic zone wherever aquatic habitats have appreciable depth. Turbidity is caused either by clay and split particles or by the excretory products of the living organisms. Turbidity caused due to suspended clay and split particles is considered as an important limiting factor. On the other hand turbidity caused by living organism is the measurement of productivity of the water body.

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Turbidity has a direct impact on the penetration of light. Turbid water restricts the amount of light reaching the bed. Large shallow lakes with large area tend to be more productive than those with deep water because more light reaches the lake bed. In river systems, heavily shaded streams rely on the degraded organic matter and leaves from the surrounding catchment to fuel the ecosystem. In larger rivers the canopy is open which allows the algal production.

Transparency can be measured with an instrument called Secchi disk. It consists of a white disc about 20 cm in diameter that is lowered from the surface until it just disappears from the view. Secchi disk transparency of unproductive clear lake is about 40 meters whereas for a heavily turbid waterbody it is as low as a few centimeters.

Fig. 8: Use of Secchi Disk to measure the transparency of lake

In the most widely studied Wisconsin lakes, Secchi disk transparency represents the zone of light penetration down to about 5% of the solar radiation reaching the surface. The 5% level marks the lowest limit of major photosynthetic zone.

5.3.3. Current

Current plays important role in estimating the distribution of salts, vital gases and small organisms. It serves as an important limiting factor for the lotic systems especially for streams.

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5.3.4. Concentration of respiratory gases

Oxygen and the carbon dioxide concentration are often limiting in freshwater environment.

Dissolved oxygen content (DO) and Biological oxygen demand (BOD) are important determinants for measuring the biological and physical health of a waterbody. These are one of the most intensively studied physical factors. Dissolved oxygen reflects the productivity.

Biochemical Oxygen Demand or Biological Oxygen Demand (BOD), measures the amount of dissolved oxygen (DO) which is utilized by the aerobic microorganisms while decomposing the organic matter in waterbody, at certain temperature and over a specific period of time.

BOD affects the amount of dissolved oxygen (DO) in streams and rivers. The oxygen consumption rate is affected by many variables: viz. temperature, pH, the presence of certain kinds of microorganisms, and the type of organic and inorganic material in the water.

The greater the BOD, the more rapidly oxygen is depleted in the stream. Less BOD indicates less availability of oxygen to higher forms of aquatic life. A high BOD mean low dissolved oxygen (DO). As a result of such conditions aquatic organisms become stressed, suffocate, and die.

5.3.5. Concentration of biogenic salts

Concentration of salts like phosphates and nitrates are the limiting factors of freshwater ecosystems. Calcium and some other salts may also be the limiting factors in some lakes. The algal productivity of most of the fresh water lake is limited by availability of the inorganic form of phosphorus as ion phosphate. When lakes are artificially fertilized with phosphorus in the form of phosphates, there is a remarkable increase in productivity. Phosphate received from runoff from agricultural fields and the sewage supply to the lakes also results in enhanced productivity. However, presence of other salts like potassium, ammonium or inorganic carbon has no effect on the productivity. Therefore, these nutrients are not the considered as primary limiting factors. Studies have revealed that if a freshwater lake is first

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5.3.6. Isolation due to land barriers

Distribution and number of organisms in freshwater habitats is also influenced by the ability of the organisms to disperse in their surroundings. Isolation due to the presence of land barriers does not allow free movement of the organism to become established in places otherwise favorable.

Example: Fishes of the streams, which are separated only by a few miles from land but isolated by water, may have their niches occupied by different species. On the other hand, most small organisms-algae, crustacea, protozoa and bacteria, etc. have a wide range of dispersal. Thus, same kinds of water flea are observed (Daphnia, for example) in a pond in the United States as in England.

5.3.7. Osmoregulation

The freshwater organisms face a problem of osmoregulation. The concentration of salts is greater in the internal fluids of the body or cells of the freshwater organisms than in the freshwater surrounding environment. Therefore, their body fluid is hypertonic and water tends to enter the body by osmosis if membranes are readily permeable to water or salts must be concentrated if membranes are relatively impermeable.

Fig. 9: Osmoregulation in fresh water fishes

Contractile vacuoles in the cells of the freshwater protozoans and the kidneys of fishes have developed efficient means of excreting water. The difficulty in osmoregulation is one of the

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Ecological classification of fresh water organisms

All freshwater ecosystems (rivers, streams, lakes, ponds, wetlands etc.) are home to diverse life forms, which are often interpreted in the form of food chain or food web. The variety and number of the biotic community in a freshwater food web depends on the productivity of the water body. The productivity of the water body depends on the availability of energy, mostly in the form of solar energy; raw materials such as nutrients and minerals available in water and the dissolved oxygen. The available energy constantly changes daily and due to seasonal cycles, and the raw materials is continuously circulated via water cycle, carbon cycle, nitrogen cycle, phosphorus cycle, etc., through and within the ecosystem. These cyclic ﬂuctuations helps in determining the short-term productivity of the ecosystem.

The freshwater communities are the primary determining factors of the productivity of their respective ecosystems. These may be classified on the basis of their position in the energy or food chain as:

Autotrophs (Producers): This includes the green plants and the chemosynthetic microorganisms.

Phagotrophs (Macroconsumers): Includes primary, secondary and tertiary consumers, including herbivores, predators, parasites, etc.

Saprotrophs (Microconsumers or decomposers): These can be further sub-classified according to nature of the organic substrate decomposed.

Aquatic organisms may also be classified according to their habitat in water, based on their mode of life. They can be classified as:

Benthos: These are the organisms which remain attached or rest on the bottom or live in the bottom sediments. Benthos may be divided according to mode of feeding as filter feeders, e.g. Clams and deposit feeders, e.g. snails.

Periphytons or Aufwuchs: These are the plants and animals that attach or cling to stems and

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Plankton: These are the floating organisms whose movements are more or less dependent on current. While some of the zooplanktons exhibit active swimming movements that aid in maintaining vertical position, plankton as a whole is unable to move against appreciable currents. In practice, net plankton is that which is caught in a fine-meshed net which is towed slowly through water; nanoplankton is too small to be caught in a net and must be extracted from water collected in the bottle or by means of a pump.

Nektons: They are free swimming organisms able to navigate at will (and hence capable of avoiding plankton nets, water bottles, etc). E.g. Fish, amphibians, swimming large insects, etc.

Neustons: These are the resting or swimming organisms on the surface of water.

Fig. 10: Classification of freshwater communities

The Lotic Aquatic System

Lotic aquatic systems are the fresh water bodies with flowing waters. Water in lotic system is in a state of constant motion. Rivers and streams are the most common examples of such systems. The basic function of lotic water bodies is assumed to carry the excess of rain water back to the sea. It is estimated that the amount water that these lotic bodies carry to the oceans amounts to approximately 25 cm of rains that can be distributed evenly on the land surface.

Typically, the lotic aquatic systems have the following properties:

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 The flow of water in continuous and unidirectional in a lotic system. Water mostly flows in narrow channels that are less deep than the lakes. Water from diverse sources can pool into a lotic body. The bottoms of such water bodies continuously receive materials from the biomass of surface water.

 The volume of water changes rapidly which results in change in the velocity of water currents. There is also a wide range of fluctuation in the water level in a lotic system.

 Water in these systems act as effective agent of transportation, transfer, and dilution.

They continuously erode materials along their channels and deposit them to other places.

 There is no vertical stratification and is a thorough mixing of contents.

 The physico-chemical statistics of water quality is in a state of perpetual change. The environment of a lotic aquatic system is relatively unstable as compared to the lentic system.

 There is a rich supply of oxygen which is derived from air above and also the photosynthetic outputs of the autotrophs. Oxygen is evenly distributed across the water body and oxygen depletion is very rare in unpolluted lotic systems.

 Turbidity limits the light penetration to the deeper zones. Very less amount of carbon dioxide present in these waters. Such conditions limit productivity. Therefore, algae bloom and growth of other organisms is rarely observed in moving waters.

 Area and depth have little correlation with the productivity.

 Water current is the dominant and a determining feature of a lotic system. Organisms having effective mechanism to anchor at one place in the water current are generally observed in these systems. Productivity is less in rapidly flowing waters. Productivity is inversely proportional to the current velocity. Living organisms disseminate their spores,

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Stream Creek River

Fig. 11: Lotic water bodies

Characteristics of the Lotic Aquatic System:

The properties of rivers and streams revolve around the three basic conditions prevailing in these water bodies:

1. Current: It is the major controlling and the limiting factor in streams.

2. Land-water interchange: It is relatively more extensive in streams, resulting in more open ecosystem and a heterotypic community metabolism.

3. Oxygen: Distribution of oxygen is generally more uniform in streams and there is little or no thermal or chemical stratification.

These conditions and their influence on the properties of the lotic system and its community have been discussed in detail in this section.

Current

Presence of a definite and continuous current is the primary feature of all lotic systems.

However, the velocity of the current varies greatly in different parts of the same stream, both longitudinally and transversely to the axis of flow, from one time to another.

In large streams or rivers, the current may be reduced to a virtually standing water conditions.

On the other hand, wave action along the rocky or sandy shores of lakes, in absence of rooted vegetation, may virtually duplicate the stream conditions. Nevertheless, current is the most important primary factor which makes a stream different from a pond or a lake and also

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The velocity of the current is determined by the steepness of the surface gradient, the roughness of the stream bed and the depth and the width of the stream bed. At any point there is a ‗micro-stratification‖ of current. The surface gradient alone can give a good index to average current conditions. The gradient in feet per mile, for example, can easily be determined from topographic maps or measured with simple surveying instruments in the field. For example, in Ohio, Trautman (1942) found that distribution of bass and other fish was well correlated with gradient of flow. Small-mouth bass were found largely in stream sections having gradient of 7 to 20 feet per mile; they were almost never collected when the gradient was below 3 or above 25 feet per mile.

Land-water Interchange

The depth of water and the cross-section area is much smaller in streams than the lakes.

Therefore, the land-surface junction is relatively great in proportion to the size of the stream habitat. Thus the streams are more intimately associated with the surrounding land than the lakes and ponds. Most streams depend upon the land areas, back waters, connected ponds and lakes for their energy supply. Though streams have their own community of producers, these are usually inadequate to support the large consumer community. Many primary consumers of the stream are detritus feeders which depend upon the organic material that are swept or fall from the terrestrial vegetation.

Also, streams export energy in the form of emerging insects and stream life removed by air- breathing predators. Thus stream form an open ecosystem that is interdigitated with terrestrial and lentic systems. Therefore, measurement of the productivity of a lotic system must include the adjacent land and standing water systems. According to Mann (1969), Thames River, which is very productive and densely populated with fish and molluscs, depends for about half of its energy flow on organic matter from outside the stream such as leaves and sewage solids (Allochthonous materials), i.e., on detritus food chain rather than grazing.

Oxygen

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conditions in streams. Due to the small depths, large surface exposed to the air, and constant motion, streams generally contain an abundant supply of oxygen, even when there are no green plants. For this reason stream animals generally have a narrow tolerance and are especially sensitive to reduced oxygen. Therefore, stream communities are especially susceptible to and quickly modified by any type of organic pollution which reduces the oxygen supply. Therefore, streams are the first victims of urbanization.

Zonation in Lotic System

Rivers and streams have two major zones:

Rapid Zone: This is the zone of shallow water where velocity of current is great enough to keep the bottom clear of silt and other loose materials, thus providing a firm substrate. This zone is occupied largely by specialized benthic or periphytic organisms which become firmly attached or cling to a firm substrate and also by strong swimmers such as darter fish.

Pool Zone: The is the zone of deeper water where velocity of current is reduced and slit and other loose materials tend to settle to the bottom, thus providing a soft bottom , unfavorable for surface benthos but favorable for burrowing forms, nektons and in some cases, planktons.

Fig. 12: a) Rapid Zone b) Pool Zone

The Lotic Community

The lotic community is studied under two broad groups: the rapid community and the pool community.

Rapid Community: The organisms of this group show various adaptations for living in swift

a b

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and its pupa in a cocoon. The larva has sucker at its posterior end and has a ‗head net‘ used for straining food from the water. Other examples include, Bibiocephala larva; the larva of riffle beetle, Psephenus; mayfly nymph, stonefly nymph, etc.

Current is the major limiting factor in the rapid zone. The type of the bottom like sand, pebbles, clay, bedrock or rubble rock, is very important in determining the nature of the communities and population density of the community dominants. Flat or rubble rocks generally produce the largest variety and highest density of bottom organisms.

The biota of the rapid zone in the rivers is highly clumped due to the absence of the firm substrate. Benthic forms have highest density in this zone. Planktons are generally missing in the streams due to strong current. A solid bottom like stones, pebbles of the rapid zone offer favorable surface for the growth of the flora and fauna, to attach or cling.

Pool community: The bottom dwellers of the pool community include burrowing forms like mayfly nymph, Hexagenia; dargon fly nymph, Progomphus etc. The species composition of pool zone consists of some organisms that are present in the ponds. For example, the gyrinid beetles ‗gyrate‘ as well on the surface of a quiet pool as on the surface of the littoral zones of ponds, while bluegills, typical pond fish, also reside in deeper pools of streams.

The soft, continually shifting bottom of the pool areas generally limits smaller benthic organisms to burrowing forms, but the deeper, more slowly moving water is favorable for nektons, neustons and the planktons. Sand or soft silt is the least favorable type of bottom and supports the smallest number of species and individuals of benthic forms. Clay bottom is generally more favorable than sand.

Nektons and burrowing forms, such as clams, burrowing Odonata and Ephemeroptera are common in pools. Stream fish find refuge in the pool zone and feed in or at the base of the rapids, thus linking the two zones.

Adaptations in the organisms of lotic habitat

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a. Permanent attachment to a firm substrate:

Attachment to substrates such as a stone, log, or leaf mass is an important adaptation of these organisms. This category includes:

Producers: (i) Attached green algae has long trailing filaments to attach (ii) Encrusting diatoms which cover various surfaces

(iii) Aquatic mosses of genus Fontinalis and others which covers stones Consumers: Fresh water sponges and caddis larvae, which cement their cases to stones.

Fig.13: Community of lotic water bodies

b. Hooks and suckers:

Animals of rapid zones have hooks or suckers that enable them to grip even smooth surfaces.

The Dipteran larvae, Simulium, Blepharocera, Hydropysche, and the caddis are remarkable in the structure of their suckers and are the only animals to withstand the pounding of swift rapids and waterfalls. Simulium, not only has a sucker at its posterior end but also attaches itself by means of a silken thread. If dislodged, the legless larvae are kept from being swept away, with the help of the thread.

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Fig. 14: Suckers of Blepharocera

c. Sticky undersurface:

Many animals have sticky undersurface which helps them in adhering to surfaces.

For e.g. snails and flatworms.

d. Streamlined bodies:

All the stream animals have a streamlined body, that is, their body is more or less the shape of an egg, broadly rounded in front and tapering posteriorly, which offers minimum resistance to water flowing over it.

e. Flattened bodies:

Many rapid animals exhibit extremely flattened bodies which enable them to find refuge under stones and in crevices. Thus, the body of stonefly and mayfly nymphs living in swift water is much flatter than the body of nymphs related species living in ponds.

f. Positive rheotaxis:

Some animals orient themselves upstream and are able to swim against the current. This is an inherent behaviors pattern known as positive rheotaxis and is an important adaptation.

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Many stream animals have an inherent behavior pattern to cling close to a surface or to keep the body in close contact with the surface. Thus, when a group of stream stonefly nymphs are placed in a dish, they make contact with the underside of sticks, debris or whatever is available, even clinging to each other if no other surface is available.

Representatives of Lotic Aquatic System River

Most of the rivers originate from glaciers when snow melts in the mountains. Therefore, at the headwaters, the rivers are usually cold and full of oxygen; they run swiftly through shallow riverbeds. Along its path down the mountains, they become broad, warmer, slower, wider and less in oxygen content. A river changes with the change in the properties of the land and the climate in which it runs through.

Characteristics of a river system Water flow/current

As the other lotic systems, water flow or current is one of the main factors in determining the characteristic of a river system. The strength of water current varies from torrential rapids to slow backwaters. The speed of water is also subjected to varying turbulence. The flow and volume of water can also be influenced by sudden water input coming from snow melt, rain and also the ground water. It alters the shape of the riverbeds via, sedimentation and erosion, and creates various types of changing habitats.

Light

Light is the source of primary productivity of the rivers. Also, the shadows casted by the light provide refuge for the prey species. The amount of light received by a river in various regions varies greatly. For example, the amount of light received by a river in a shady forest region is far less than that of a river flowing in a region where sun shines directly over the surface of the river. Deeper parts of rivers and the ones with high sediments have weak light penetration.

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Temperature

Temperature of the water varies with environment. Heating of water takes place through radiation at the water surface and the conduction via air and the surrounding substrate.

Temperature gradient is generally observed across the different depth of the river bodies.

Change in the climate, elevation and shading affects the water temperature and the organisms residing these environment, change their internal temperature to suite the external environment and are termed as poikilotherms.

Substrate

The substrate where the river organisms live, may be organic, including leaves, fine particles moss, wood and plant, or it may be inorganic consisting substances from the catchment areas such as pebbles, boulders, sand, gravel, silt or sand. These surfaces are not permanent and change significantly during floods.

Chemical properties of river water

The chemical nature of river water is determined by the inputs coming from the surrounding environment, i.e., the neighboring catchment area, rain and human interventions like release of pollutants. Oxygen is the important chemical that determines the productivity and the biotic density of the river body. It dissociated in the river body through water surface and is mixed via air current. Oxygen is limited in water if there is poor water circulation, large organic decay in the waterway or high animal activity.

6. Summary

Ecosystem, term proposed by A.G. Tansley, is a biological community consisting of organism interacting with each other and with their physical environment. Group of organisms interacting and interdependent on each other inhabiting the same area constitute biotic community. Ecosystem is composed of autotrophic chlorophyll containing plants and heterotrophic. Two major components of ecosystem are biotic (producers, consumers,

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includes Forest Ecosystem, Desert Ecosystem, and Grassland Ecosystem. Aquatic Ecosystem includes Fresh water and Marine ecosystem. Fresh water ecology gives the idea of Aquatic Ecosystem in the vicinity which is easily accessible affects out lifestyle. Fresh water bodies contain salt concentration less than 1%. The study of physical, chemical, geological and biological features of freshwater is called limnology. Fresh water habitat consists of Lentic or standing water habitat, Lotic or running water habitat and wetlands which include marshes and swamps. Temperature, transparency, water current, concentration of respiratory gases and biogenic salts act as limiting factors of freshwater ecosystem. Water has some unique properties like high specific heat, high latent heat of fusion and evaporation and highest density at 40⁰C which help in maintaining properties of Freshwater Ecosystem. Concentration of dissolved oxygen also plays a crucial role in Freshwater Ecosystem. Biological Oxygen demand (BOD) is important determinant of biological and physical well being of water body.

Less BOD i.e. high dissolved oxygen indicates less availability of oxygen to aquatic lives.

Salts of phosphates and nitrates are limiting factors of Freshwater Ecosystem. Distribution of organisms is also influenced by their ability to disperse in the surroundings. Due to difference in concentration of salts in internal fluid and in surroundings, freshwater organisms face a problem of osmoregulation. In protozoan, contractile vacuole is present for this purpose and kidneys in fishes are efficient organ for excreting water. In freshwater community, there are autotrophs (green plants, chemosynthetic organisms), phagotrophs (primary, secondary and tertiary consumers) and saprothophs (microconsumers or decomposers) are found. Aquatic organisms can also be classified as benthos (rest at the bottom, e.g., snails), periphytons (clinging to leaves), planktons (floating with the current), nektons (floating at their will, e.g., fishes, amphibians) and neustons (resting or swimming at the surface). Flowing fresh water bodies are called lotic water bodies. Vertical stratification is not observed and environment is comparatively unstable as compared to lentic water body. Flow of water in these lotic bodies is unidirectional and water level keeps on fluctuating. Lotic water bodies are efficient transporter; they erode materials along their channels and deposit them elsewhere. Oxygen is evenly distributed and amount of Carbon dioxide is very less. Due to all these factors algal blooms are rarely observed. The properties of rivers and stream have three basic conditions- water current, land-water discharge and oxygen distribution. Current is primary factor which

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and standing water system. Due to more exposure of oxygen to the streams, organisms inhabiting such water bodies have narrow tolerance towards limited supply of oxygen.

Streams get affected most easily by pollution and urbanization. Rivers and streams have two basic zones – rapid zone and pool zone. Velocity of current is strong in rapid zone such that bottom is clear of silt and loose material. Benthic and epiphytic organisms reside here. In pool zone, velocity of current is low and silt and other loose materials get settled to the bottom. Burrowing animals, nektons and planktons reside here. Lotic community can be divided into two groups – Rapid community and Pool community. Rapid community lives in swift current and is known as torrential fauna like black fly larva or riffle beetle larva.

Benthic community is highly observed and planktons are usually missing in this zone. Pool community includes burrowing animals like mayfly and dragonfly nymphs. Deeper, slow moving water is favorable for nektons, neustons and planktons. Organisms in lotic habitat are adapted for surviving in swift water. They are able to permanently attach to a firm substrate (attached green algae or aquatic mosses), they are often provided with hooks and suckers for attachment (dipteran larvae) or they have sticky surface. They have flattened bottom (snails and flatworms) and exhibit positive thigmotaxis and rheotaxis. River, a lotic aquatic water ecosystem has few characteristics like water flow or current which has capacity to alter riverbeds, sedimentation and erosion. Light also plays a significant role as it is source of primary productivity and the amount of light a river receiving determines its productivity and growth. Temperature, change in climate and type of substrate also plays important role in defining the characteristics of a river.

3. Structure of Ecosystem

Development Team

Contents

1. Learning Objectives

2. Introduction

3. Structure of Ecosystem

4. Types of Ecosystem

5. Freshwater Ecology

6. Summary