General characteristics & faunal adaptations in: • Aquatic (freshwater ecosystem: lotic and lentic)

Aquatic Ecosystem 

Civilizations have depended on water bodies such as lakes, reservoirs, rivers and wetlands. Water is essential not only to sustain human life but also to support the activities that form the basis for thriving economics. Though the water resources are essential to human societies who could pollute and degrade and limiting their beneficial uses. Agriculture, mining, urban development and other activities can pose risks to freshwater bodies and hence steps have to be taken to reduce these risk factors.

Risk analysis requires knowledge of how human land use affects physical, chemical and biological characters of the aquatic systems. One of the critical areas required to understand how human actions and natural processes affect lakes, reservoirs, rivers and wetlands is the science called Limnology. It is a multidisciplinary science that integrates the basic sciences (Biology, Chemistry, Physics and Geology) in order to study inland waters as complex ecological systems.

Definition

The term Limnology is derived from Greek word; Limne means lake and logos means knowledge. Limnology is often regarded as a division of ecology or environmental science.

It is however, defined as “the study of inland waters” (running and standing waters fresh and some times saline; natural or man made). This includes the study of lakes, ponds, rivers, reservoirs, swamps, streams, wet lands, bogs, marshes etc. Hence, it is commonly defined as that branch of science which deals with biological productivity of inland waters and with all the causal influences which determine it (Welch, 1963).

Biological productivity, as used in this definition, includes its qualitative and quantitative features and its actual and potential aspects. Under the term inland waters are included all kinds or types of water – running or standing; fresh, salt or other physicochemical composition which are wholly or almost completely included within the land masses. Causal influences involve various factors – physical, chemical, biological, meteorological etc which determine the character and quantity of biological production.

 

History

The term Limnology was coined by Francois-Alphonse Forel (1841 – 1912) who established the field with his studies on Lake Geneva. Interest in the discipline rapidly expanded and in 1922 August Thienemann (a German Zoologist) and Einar Naumann (a Swedish Botanist) co-founded the International Society of Limnology (SIL, for originally Societas Internalis Limnologiae). Forel’s original definition of limnology, oceanography of lakes was expanded to encompass the study of all inland waters.

Welch (1935) conceived the problem of “Biological productivity” as the central theme of Limnology. He defined Limnology as that branch of science which deals with all causal influences which determine it. According to Schwoerbel (1987), Limnology is the science of inland waters viewed as ecosystems together with their structures, materials and energy balance.

Kiihnelt (1960) considered limnology as a sub set of ecology along with “Oceanography” (which is concerned with marine ecosystem) and “Epheirology” (which deals with terrestrial habitats).

In short, Limnology is the study of all aquatic systems including lakes, wetlands, marshes, bogs, ponds, reservoirs, streams, rivers etc. with regard to their physical chemical and biological characteristics.

In addition to the above, certain other terms, like Hydrobiology, Freshwater Biology, Aquatic Biology, Aquatic Ecology etc, are sometimes loosely used as synonymous to the word 'Limnology'. But, most of these terms are names under which a diverse variety of subject matter is included and only a part of it is limnological in nature.

General characteristics & faunal adaptations in:  • Aquatic (freshwater ecosystem: lotic and lentic)

 Fresh water Ecosystem

Freshwater Ecosystem Characteristics

·        The freshwater ecosystem is a habitat for various plant and animal species. One of the major reasons is that it is quite rich in terms of nutrition and minerals.

·        The freshwater ecosystem is less saline, unlike the marine ecosystem.

·        The temperature in this ecosystem varies depending on some factors like location, season, and depth from the water surface.

·        During summers, the temperature of the freshwater ecosystem generally ranges from 30-71 degrees Fahrenheit. Whereas during winters, the temperature ranges from 35-45 degrees Fahrenheit.

·        The size and shape of freshwater ecosystems vary depending on location, an area covered, and depth of water bodies.

·        The freshwater ecosystem contains sediments at the bottom. In gentle flowing freshwater bodies or still water bodies, the sediments remain in place.

·        The freshwater ecosystem provides a suitable environment for various species of flora and fauna. 

 

The fresh water on earth is broadly classified as

1.     Lotic water

2.     Lentic water.

 Lotic Freshwater Ecosystem or Running water ecosystem

The lotic water in simple terms is called running water.

Or

The water bodies moving in one direction is known as a lotic freshwater ecosystem.

It includes

1.     Rivers

2.     streams

3.     Springs

are common examples of lotic ecosystems.

 

There are many rivers and stream that flows from their origin and ultimately meets with other water channels or oceans at its mouth. Lotic freshwater travel through different locations from its source to mouth.

 

Lentic Freshwater Ecosystem or Standing water ecosystem

An aquatic ecosystem within stagnant or standing  or still water like ponds and lakes is known as Lentic Freshwater Ecosystem.

 Lentic ecosystem found in various sizes ranging from a few square meters to thousands of square km.

It includes

1.     Lakes

2.     Ponds

3.     Ditches

4.     Marshes

5.     Bogs

Some ponds last just for a few months as these are seasonal like sessile pools. On the other hand, lakes may exist for many years. Lentic ecosystem, i.e. Ponds and lakes, support a very limited number of species.

Lentic ecosystem divided  into three zones based on their depth and distance from the shoreline. 

1.     Littoral Zone

2.     Limnetic Zone or Photic Zone

3.     Profundal Zone or Aphotic Zone

 

Littoral Zone

This is the topmost zone in the area near the shoreline of a pond or lake. The Littoral zone is characterized by a shallow and warm zone of a lentic ecosystem.

This zone provides shelter to various species of algae, few species of aquatic plants, clams, crustaceans, amphibians, snails, and various insects, etc.

Flora and fauna found in the littoral zone generally serve as food for other creatures like ducks, turtles, etc.

Limnetic Zone or Photic Zone

The open water zone where sunlight supports the photosynthesis process is known as the photic zone. The photic zone is also termed as a limnetic zone.

It is the zone of a lentic ecosystem that is generally dominated by planktons (both phytoplankton and zooplankton). As planktons are the primary producers, hence the limnetic zone plays an essential role in the food chain of a freshwater ecosystem.

 Profundal Zone or Aphotic Zone

The deepwater zone where sunlight hardly penetrates is known as a profundal or aphotic zone. Photosynthesis is not possible in this zone due to the scarcity of sunlight. The aphotic zone is cold as compared to the other two zones.

The aquatic animals found in the profundal zone are heterotrophs in nature as they consume dead organisms.

 

Lakes 

Size of lake

Lakes differ in area from those ranges from a pond to those of great size. Lake Superior, the largest body of freshwater flow has an area of more than 49,600 km2. The Caspian sea with an area of 2,72,000 km2 is sometimes considered as having the quality of lake.

Lake Chad in Africa has 64,000 km2 during wet season, but is reduced to 9,600 km2 in the dry season. Ten of the large lakes in America including Great lakes have an combined area of about 2,03,200 km2. However the number of lakes whose area exceeds more than 8,000 km2 is insignificant when compared to many thousands of lakes of lesser magnitude of 11,000 or more lakes or ponds in Michigan.

Depth of lake

Lakes vary in depth but even the deepest lake will never approaches the depth of ocean. It is important to note that the lake Baikal has a greatest depth contains about 20% of the total volume of freshwater and it is also the deepest known lake with a maximum depth of 1620 m. In North America, Crater lake in Oregon is about 608m. Lake Tahoe is 487m, Lake Chelan (Washington) 457m. Seneka lake 188m, Lake Superior 393m, Lake Michigan 281m, Lake Huron 228m, Lake Ontario 273m, Lake Erie 64m, and the last 5 lakes constitute Great lakes of America.

Thermal Stratification of a Lake

Lakes in temperate latitudes exhibit marked seasonal temperature changes which may be described as follows:

Winter:

During winter the coldest water forms ice at 0°C (32°F) and floats at the surface.The water at increasing depth below the ice is progressively warmer and denser. The heaviest water, at the bottom of the lake, has a winter temperature of 4°C and throughout winter the water remains relatively stable.

Spring:

Following the ice melt, the surface water gradually warms to 4°C. At this point the water column is nearly isothermal, i.e., all the water is of uniform temperature and density. Hence, the strong spring winds cause considerable stirring, which results in a complete mixing of water, dissolved oxygen, and nutrients from the lake surface to the lake bottom, a phenomenon known as the spring overturn or spring turnover As the spring progresses, however, the surface waters naturally become warmer and lighter than the water at lower levels, as a result, the lake becomes thermally stratified into the following three zones

Typical Thermal Stratification of a lake into three strata

(a)   Epilimnion

(b)   Hypolimnion:

(c)    Metalimnion:

 a.      Epilimnion: The upper stratum, which usually has the highest dissolved oxygen concentration and is characterized by a temperature gradient of less that 1°C per metre of depth, is the epilimnion.

This stratum contains more or less uniformly warm, circulating, and fairly turbulent water.

(b) Hypolimnion:

The lower stratum of water characterized by a temperature gradient of less than 1°C per metre of depth is the hypolimnion (literally the “lake below” or “Bottom Lake”). This part contains more dense, cooler, and relatively quiet water.

(c) Metalimnion:

It is the transitional stratum of marked thermal change between the epilimnion and hypolimnion. The middle layer of the lake, characterized by a temperature gradient of more than 1°C per metre of depth is the thermocline.

This zone effectively divides the lake into two layers, the upper epilimnion, and the lower hypolimnion. This division is not merely an interesting physical phenomenon but it has consequential effects on the ecology of the lake.

The epilimnion is well lit and oxygenated with sufficiently high temperatures to promote algal productivity and hence to support zooplankton and fish. When nutrients are in ample supply, algal growth is accelerated and blooms may occur. By contrast the hypolimnion is cold, dark and becomes progressively deoxygenated as the decaying remains of organisms sink down from the epilimnion.

epilimnetic material provides an energy source for benthic invertebrates. The sinking of dead algae and zooplankton from the epilimnion not only contributes to the potential deoxygenation of the hypolimnion but also prevents immediate recycling of nutrients. Nutrient depletion may become so high that algal growth is limited.

Lakes undergoing complete circulation in spring and autumn separated by thermal summer stratification and winter inverse stratification are called dimictic lakes . Such lakes are quite common among temperate lakes of moderate size.

Classification of the lake on the basis of productivity

Lakes are classified based on productivity as follows:

Oligotrophic lakes: These have low primary productivity, and low biomass associated with low concentrations of nitrogen and phosphorous (nutrients). They tend to be saturated with oxygen.

Mesotrophic lakes: These are lakes in transition from oligotrophic to eutrophic conditions. Some depression of oxygen concentration occurs in hypolimnion during summer stratification.

Eutrophic lakes: These display high concentration of nutrients, high biomass productivity and low transparency. Oxygen concentrations can get very low (as low as 1 mg/L) in the hypolimnion during summer.

Hypereutrophic lakes: These are lakes at the extreme end of eutrophication with very high concentration of nutrients and associated biomass production. Anoxia or complete loss of oxygen takes place in the hypolimnion during summer.

Dystrophic lakes: These are organic rich lakes (humic and fulvic acids) fed by external inputs of the lake (watershed).

Changes, which occur in the lake due to addition of nutrients (on account of human activities), are called as eutrophication. The additional nutrients (primarily nitrogen and phosphorous) most usually come from sewage, industrial effluents or agricultural fertilisers, or all the three.

Bacteria that use up the oxygen in the water first decompose raw sewage. Sewage always contains a higher concentration of phosphates than nitrogen due to increased use of phosphate containing detergents. In unpolluted waters, growth of algae is inhibited by the lack of phosphorous but the addition of these nutrients (through domestic sewage, other effluents etc.) increases algal growth in the receiving waters. While algae produce oxygen during the day due to photosynthesis, they use it up in the night. Thus, dense algal blooms cause suffocation of fish and other aquatic organisms.

Increased algal production due to eutrophication initiates other changes in the aquatic ecosystem.

Thermal Classification of lakes

According to Hutchinson (1957), following are the classification of lakes based on changes in temperature of surface water.

a. Amictic: No mixing of bottom and top water; lakes insulated or protected by ice-corer, there is no effect of weather or external factors.

b. Monomictic: One mixing of the two waters during the year (most deep lakes of the world).

c. Cold monomictic: Water here at any depth never exceeds 4°C; they are ice-bound or ice-covered only in winter; there are inverse thermal stratification top waters 0°C and bottom waters 4°C (since water at 4°C is heaviest); only one mixing at temperatures not more than 4°C in spring / summer eg, Polar lakes.

d. Warm monomictic: Temperature of water never falls below 4°C at any depth. Direct thermal stratification top waters 10 - 20°C and bottom waters 8 - 4°C; only one mixing in a year in a winter eg, Most subtropical deep lakes.

e. Ploymictic : Mixing is continuous, but occurs only at low temperatures.

 

 

Ponds

Ponds are defined as small, shallow, inland standing water bodies, where rooted plants can grow over most of the bottom. Ponds are mainly of three general classes, they are :

i. Those which represent the pond stage in the extinction of previously existing lakes

ii. Those whose basins have never been large or deep (not preceded by a lake) but or for some special reason, have persisted in the pond stage and

iii. Those whose basins are the results of man’s activities (excavations, quarries, impoundments, etc.)

Natural process alone are constantly forming new pond basins (cut-offs from streams solution basins, beach ponds, and many others), some of which are never more than temporary ponds from the beginning; others qualifying as permanent ponds at least for a period in their existence.

 Classification of ponds

With respect to seasonal duration, ponds are divided into two general classes

a. Permanent – those which contain some water the year round and

b. Temporary – those in which the basin contains water at certain times or seasons and becomes dry at others.

Those which occur for a limited period in spring are called Vernal ponds

Those which contain water in spring, dry up during summer, and again contain water in the autumn are called Vernal autumnal ponds and

Those which contain some water throughout the open season but freeze to the bottom in winter have been called Aestival ponds.

Other classifications of ponds are as follows

Natural ponds

These are perennial shallow water bodies. When a stream shifts its position it leaves behind an isolated body of standing water which forms the "Ox-Bow" pond. In limestone regions where depressions are formed due to the solution of the underlying strata, the water gets accumulated either by flood water or rainfall and natural ponds are formed. Sometimes the last remnant of a lake whose basin has become filled progressively by sedimentation in course of time is transformed into a pond.

Artificial pond

Most of the fish ponds are semi artificial ponds. Some are constructed by erecting dams across a stream or basin and their water level can be regulated by inflow and drainage where pisciculture is practiced. Fish pond is a shallow body of water that can be drained completely. It is often supplied by running water, but also by spring, ground or rain water.

Pools or Temporary Ponds

They occur in depressions in the ground either at the margin of glaciers where they fill with melt water or in the vicinity of river bed, after the floods have receded. The water thus collected usually is very shallow and measures maximum to a few feet only. Also prolonged rainfall may form temporary small pools. All these pools dry up in some part of the year, and as such organisms in these habitats must be able to survive in a dormant stage during dry periods and be able to move in and out of the pools.

General Characteristics of ponds

• Ponds are small, shallow standing water bodies.

• They have calm water

• Have more vegetation

• Growth of plants can also found at the bottom

• They have outlet streams

• The movement of water is minimum

• They have slight wave action

• The average depth of water is 8 – 10 feet

• The temperature of the pond more or less changes with that of atmosphere

• Light penetrates up to the bottom

Running water or Lotic Ecosystem

Water  Streams and Rivers

STREAMS

Streams are zones where a rapid flow of shallow water produces a shearing stress on the stream bed, resulting in a rocky or gravel substratum covered by fully oxygenated water. Streams may vary in size from tiny rivulet to rivers. As time goes the stream may develop into river or increase its size, whereas the size of reservoirs decreases as time passes. They are more numerous in regions of abundant rain fall. They are temporary or permanent. Streams are closely linked to their watersheds. The productivity of streams is often dependent on terrestrial bases, grasses and other debris. The allocthonous materials contribute most of the food and energy to the organisms in the stream. Benthic invertebrates like insect larvae constitute the invertebrate fauna. True plankton are almost absent in streams, and are common only in deep slow moving stretches of rivers. All biota in streams are influenced by the unidirectional current.

Physical conditions

The annual change in stream temperature is 10 to 20°C. Although large rivers do not change in temperature very much on a daily basis, a small unshaded stream may heat up to 10°C in a few hours on a hot summer’s day and cool by the same amount at night. The temperature of most streams is lowest in the upland and becomes gradually warmer in the lower reaches.

The velocity of stream water varies with the landforms. In plains, streams are slow and sluggish throughout their length. In mountain stretches the speed of water may be rapid.

Stream water has uniform temperature and the difference between the surface and bottom is virtually negligible. The stream follows air temperatures more closely than lake waters and the factors responsible are depth of water, current velocity, bottom material, temperature of entering water, exposure to direct sunlight and degree of shading etc.

Extreme of turbidity occur in running water series and streams with rock beds the turbidity is minimal.

Stream systems increase their length, width and depth with increasing age. This is in distinct contrast to the reduction processes characteristic of all standing water units.

At any position along the course of a running water system, materials eroded at that point and all materials suspended or dissolved at the level are transported downstream with no opportunity to return. Interchanges of materials are more and have less depth than lakes.

Chemical conditions

The dissolved oxygen supply in uncontaminated stream is high at all levels often near saturation. The polluted streams show low dissolved oxygen due to accumulation of organic wastes. Stream which support more plants show diurnal variation of dissolved oxygen. The level of dissolved oxygen is controlled by the slope of channel and mode of flow.

Current in streams tends to keep the pH in uniform over considerable distances. It keeps any acidity due to accumulating free CO2 reduced. Streams waters do not develop the more intense acidities.

The dissolved solids of streams are affected by their irregular discharges. Most streams and rivers have maximum discharge during winter rains, particulate matters, nutrients like phosphate, iron and nitrate are transported to different parts by the flow of the streams. Streams fed by springs have more constant nutrients.

RIVER

River is said to be a natural stream of water usually fresh water flowing towards an ocean. In some cases river flows into the ground or dries up completely before reaching another body of water. Usually larger streams are called rivers while smaller streams are called creeks, brooks, rivulets, rills, and many other terms.

A river is a component of the hydrological cycle. The water within a river is generally collected from precipitation, through surface run off, ground water recharge and release of stored water in natural reservoirs such as glacier.

Topography

The water in a river is usually confined to a channel, made up of stream bed between banks. In larger rivers there is also a wider floodplain shaped by flood waters over-topping the channel. Flood plains may be very wide in relation to the length of river channel. This distinction between river channel and floodplain can be indistinct especially in urban areas where the floodplain of a river channel can become greatly developed by housing and industry.

Ecology

The flora and fauna of rivers use the aquatic habitats available, from torrential waterfalls through to lowland mires although many organisms are restricted to the freshwaters in rivers eg salmon and Hilsa.

Flooding

Flooding is a natural part of river cycle. The majority of the erosion of the river channels and the erosion and deposition on the associated flood plain occur during flood stage. Human activity, however has upset the natural way flooding occurs by walling of rivers set straight their courses and by draining of natural wetland.

Fauna of Freshwater Ecosystem

The freshwater ecosystem provides a perfect environment for various animal species. Some animals of freshwater prefer moving water bodies like rivers, whereas some others prefer to live in stagnant water like ponds, lakes, wetlands, etc.

 •Freshwater ecosystems contain several types of organisms that are grouped by their location and by their adaptation.•

·        Three groups of aquatic organisms include plankton, nekton, and benthos.•

·        Plankton are the mass of mostly microscopic organisms that float or drift freely in the water, and can be microscopic animals called zooplankton or microscopic plants called phytoplankton.•

·        Nekton are all organisms that swim actively in open water, independent of currents.•

·        Benthos are bottom-dwelling organisms are often attached to hard surfaces.

·        Decomposers are also aquatic organisms.

Fishes

Fishes are the most common species of the freshwater ecosystem. Some fishes like salmon, trout, etc. prefer to live in moving clean water with a high level of oxygen. On the other hand, small muddy ponds provide an ideal environment for fishes like catfish, carp, etc.

Some freshwater fishes like pike and sturgeon require a large area to live as they grow large. Big lakes are a perfect place for these fishes.

Mammals, amphibians, and Reptile

Various species of mammals are also living in a freshwater ecosystem such as beavers, otters, etc. Most of the mammals live in small water bodies like lakes. This type of ecosystem is preferable for these mammals because they come to shores to reproduce, feed and breathe.

Some animals come to water streams and rivers to feed like bears, whereas some other animals like muskrat spend their whole lifespan in ponds.

Amphibians like frogs, salamanders belong to wetlands. The freshwater ecosystem is also a home for some species of reptiles like an alligator, turtle, snakes, etc. These reptiles live in a stagnant freshwater ecosystem. 

Birds

Some birds are also a part of the freshwater ecosystem. Ducks, geese, etc. are commonly found in lakes, rivers, etc. On the other hand, some birds such as swallows belong to swamps and ponds. The birds like swallows are insect-eating, and swamps are an ideal shelter for these birds. It provides a good source of food.