Limnology

Limnology is the word derived from the Greek word ‘Lime’ which means lake and the logos means study. Thus Limnology is the study of fresh water bodies. It studies the physical, chemical & biological properties of fresh water ecosystem. It includes the study of (natural as well as artificial) lakes, ponds, streams, rivers, springs, wetlands, groundwater, etc.
In other words, Limnology is the science of studies of the freshwater and its organisms as well as its physical & chemical properties.
Types of ecosystem:
 Terresterial ecosystem
 Aquatic ecosystem
Terresterial ecosystem are of three types as:
 Grassland ecosystem
 Forest ecosystem
 Desert ecosystem
Aquatic ecosystem are of two types:
 Fresh water ecosystem (0.5 gm of salt per litre)
 Marine ecosystem (30.37 gm of salt per litre)
Fresh water ecosystem are of two types:
 Lotic i.e. running water ecosystem like river, stream, etc.
 Lentic i.e. Stagnant water ecosystem like lake, pond, etc.
Scope of Limnology
As we know water resource are very useful for human beings for their daily life activities. More over the water is used in agriculture, industrial purposes, household purposes, irrigation, recreational activities as well as environmental purposes. The water we require is fresh water and is available only 25% whereas marine water is about 97.5% on the earth. However, fresh water is a renewable source of water yet the world’s supply of fresh water is constantly decreasing because demand exceeds supply in many parts of the world population at an unbelievable rate. Thus, many more countries are expected to experience the lack of fresh water in the near future.
Unfortunately, our rivers , ponds, lakes, streams have been a dumping site of the solid & liquid waste (sewage) which has led great impact to the aquatic organisms, aesthetic & recreational values of the aquatic ecosystem. Thus, the scope of limnology is very essential for human walfare for the environmental conservation of the aquatic organisms habitats and evaluating the water quality for not being to degrade the quality of it. It also describes the type of lakes and ponds according to the chemical & biological parameter analysis. For e.g. high BOD (10 mg/l) & COD (100mg/l) & Phosphorous>0.1 mg/l together with the dominance of BGA (10 genera) suggests lake or ponds as mesotrophic for e.g. Rani Pokhari. Likewise high ammonia (18.7 mg/l ) and Phosphorous(2.6 mg/l) suggests as eutrophic lake e.g. Kamal pokhari and fair oxygen content (DO>6mg/l) , BOD and COD < 12mg/l , low nutrients Nitrogen & Phosphorous <(0.1mg/l) suggests the lake or pond as Oligotrophic lake for e.g. Taudaha.
Types of fresh water
Less than one percentage of salt concentration in water bodies are called fresh water. There are three types of freshwater as:
 Ponds and lakes
 Streams and rivers
 Wetlands
Ponds and lakes
Ponds are smaller in size than lake and are not so deep, lakes are larger in size than a pond and are generally deep. According to the depth & its location lakes are of following types:
 Glacial lake-This is the lake situated at the high altitude formed by the melting of ice.
 Tectonic lake- This is the lake formed due to the upliftment of tectonic plate. Found in mid hills of Nepal. Phewa, Begnas Syapru Tal, etc. are the examples of Tectonic lake.
 Oxbow lake- Lakes found in the Terai region. Bish Hazari Tal, Gosain kunda,Ghoda ghodi Tal etc. are some examples of Oxbow lake. During the downstream flow of river the river forms turn which form a lake named Oxbow.
 Crater lake- Crater formed on the top volcanic mountain peak after volcano get dominant & precipitation get collected in this area forming a lake called Crater lake. For e.g. Mt. Mazma oregon of USA.
 Artificial lake-Lake created by the human beings due to excavation of a site or dammaging stream or river for hydropower processes. For e.g. Rani Pokhari, Strip Pits, Querry Pits, borrow Pits & reservoir deep.
Stream & Rivers
They, change their routes from head water to the mouth deep. Diversity high in middle part of river due to high DO value. The upper part water is cooler and high amount of oxygen. So, trout, salmon, etc. are present. The middle part of river/ stream has a rapid increasing in width along with its species diversity. Mostly aquatic green plants and algae are found.
At the down part of river becomes muddy, oxygen level decreases light penetration is lower than upstream, low fish and less diversity occurs because of lower oxygen level. Species like Catfish, Crab, etc are only found.
Wetlands
Wetlands is the marsh, swampy, fenny & the boggy area with richest diversity among all the ecosystem. Macrophytes like Nelumbo,Chestnut, Potamogetom, Eichhornia, etc were found.
Physico-Chemical & Biological water Quality index
Water Quality Index-It is the physical, chemical and biological characteristics of water that refers to the stability of the water to sustain living organisms & other uses such as drinking, bathing, irrigation, for industrial purposes, and also to know the health of the aquatic ecosystems. The water quality index measures dissolved oxygen, pH, Conductivity, total nitrogen & total phosphorous content. The WQI is determined by comparing the water samples according to the set of standards such as APHA standards and European Union drinking water standards, etc. It is an indicator of eutrophication, nutrient pollution, acidification & salinization, etc. Factors leading to the deterioration of water quality are due to domestic water waste, solid waste, industrial waste, man made natural disaster, increase in the use of agro chemicals and change in land use pattern. Some of the water quality index are as follows:
 Dissolved Oxygen(DO)- It is a measure of free oxygen dissolved in water. It is essential for the metabolism of all aerobic aquatic organisms. Reduced levels of DO shows harmful to aquatic lifes & even for the plants too and fish. Therefore, DO in water bodies must be 6.0 mg/l.
 Biological Oxygen Demand(BOD)- It is the amount of oxygen utilized by micro-organisms in decomposing the organic matter. The complete degradation of organic matter may take as long as 20 to 30 days. In practice BOD test has been developed for five days at 20̊c.
 Chlorine-It occurs naturally in all typpes of water. In natural fresh water its concentration is quite low. Chlorine in water is the discharge of domestic wastes or sewage or from industrial effluents. It serves as an indicator of pollution by sewage. For drinking purposes chloride should be 250 mg/l.
 Turbidity-The turbidity in water bodies is caused by the amount of suspended particles of clay, silt, organic matter, etc. Turbidity affect the penetration of light in the water bodies & affects the photosynthesis process as well as for drinking, bathing & for induatrial purposes. For drinking water purposes, the suspended particles must be zero.
 pH- It is the measure of the amount of acidity or alkalinity of a water body. It can affect aquatic organisms by imparting/harming in the process of respiration, growth & development of fish. European Union of drinking water directive parameters showed that the pH of water must be 6.5-8.5.
 Bacteriological quality of drinking water-Organisms such as Escherchia coli must not be detectable in any 100ml sample in the water intended for drinking of an treated water entering the distribution system. Otherwise, it may lead to water borne diseases.
 Conductivity-It is a measure of the ability of water to carry an electric current which depends on the presence of ions. Increase in conductivity leads to change or reduce in aquatic bio-diversity. European Union of drinking water directive parameters shows that the conductivity of water must be 400 micro siemens per centimeter.
 Total Nitrogen & Total Phosphorous- These are the naturally occuring elements essential for all living organisms. Increase in nitrogen or phosphorous in natural water as a result of human activities in the drainage basin e.g. from the agricultural runoff from manure and chemical fertilizers & from municipal and industrial waste discharge, can overstimulate the plant growth and choke off oxygen supplies. If the concentration of total phosphorous in greater than 0.3 mg/l, it can cause algal blooms in lake and ponds. European Union for drinking water directive parameters shows that the total nitrogen should be nitrates 50mg/l and nitrites 0.1 mg/l in aggregate.
The health and the well being of the human being and the ecosystem depend heavily on the quality of the available water resources. Water is necessary for all the biological life. It also supports global food production by providing the fundamental resource upon with agriculture livestock production, fisheries and aquaculture production. Water is crucial for industrial activity & municipal services.
Water quality is at risk from industrial effluents, fertilizer runoff from agricultural land and poorly treated minicipal waste. Most of the nitrogen & phosphorous released into the environment comes from these three sources. Phosphorous & nitrogen are the important nutrients in fresh water but higher concentration of nitrogen & phosporous leads to the eutrophication ( causing algal blooms in water bodies) as a result it causes decrease in oxygen concentration in the water bodies for the aquatic life.
In particular mayflies, stoneflies, caddisfly larvae, trout, salmon, etc. are sensitive to water quality degradation. When water quality become worse the populations of these invertebrates & vertebrates aquatic animals can be expected to decrease or disappear from the water bodies. So, that special attention should be given to the percentage of Mayflies, Stoneflies & Caddisflies in freshwater invertebrate samples from aquatic ecosystem. This percentage is referred to as ‘EPT’an acronym referring to the taxonomic orders Ephemeroptera(mayflies), Plecoptera(Stoneflies) & Tricoptera(Caddisflies).
There is a vital connection between water quality & health. The rivers are the main places for the disposal of urban solid waste, domestic effluents & industrial effluents which are responsible for polluting the water and causing water borne diseases. Poor sanitation/cleanliness, poor water quality has brought about the most prevalent water borne diseases in Nepal such as Diarrhoea(Bacillus coli), Cholera(Vibrio cholerae), Typhoid(Salmonella typhi), etc. by consuming contaminated water & contaminated food as well.
Recommendations
 Effects for managing waste disposal, maintaining drinking water & sewage pipelines, controling open defecation are urgently required.
 Conduct effective awarness programmes for water quality to be needed and must amend & execute water act, rules & regulations if necessary for maintaining the water quality condition.
 Carryout research on water source protection and management.
 Adopt and enforce industrial effluents standards.
Different Zones of Lakes & ponds
There are three zones of lakes on the basis of depth and the penetration of the light as follows:
 Littoral Zone
 Limnetic Zone
 Profundal Zone

Littoral Zone-It is the shalllow zone at the surface of the lakes or ponds. It is the warmest zone wit hdiverse species, several species of algae, rooted & floating aquatic plants, snouts, insects, fishes & amphibians were found and this region has maximum light penetration.
Limnetic Zone-It is the middle part of the lake & ponds where there is warm with light penetration. At this zone maximum species of phytoplanktons & zooplanktons are found.
Profundal Zone-It is the lower part of the lakes or ponds where there is no light penetration due to which this region is colder & darker. Founa such as a toxic organisms are found such as tubificids which feed on dead aquatic organisms or planktons fallen from limnetic zone.
Variation of Temperature in Lakes & Ponds
Lakes & ponds can be divided into two parts from the temperature point of view. The upper part which is below the surface and get direct sunlight is called Epilimniom which has higher teamperature. And the bottom level which receive no sun rays is called Hypolimnion.In between these two region there is one narrow zone called Thermocline where water temperature changes rapidly. During summer temperature can range from cold to 22̊c from bottom to surface. During winter temperature at the bottom is 4̊c and 0̊c i.e.ice form at the surface.

During Spring the fall season there is a mixing of top & bottom water due to the wind which results uniform temperature from top to the bottom around 4̊c. Oxygen is uniform in some temperature. This mixing results uniform oxygen.
Freshwater Bio-Diversity
Some of the freshwater biodiversity are described as follows:
 Macrophytes-These are aquatic plants growing in or near the shore of water bodies. Macrophytes are beneficial to lakes because they provide cover for fish & substrate for aquatic invertebrates. They also produce oxygen which assists well functioning of the lake as a result diversity in organisms increase which may result & provide food for some fish & other aquatic animals. Thus, lack of macrophytes in water body is expected to reduce in population of Sport & Forage fish and waterfowl. In addition, the absence of macrophytes may also indicate water quality problems as a result of excess turbidity, herbicides or salinization and may indicate water quality is good if the analysis of physical, chemical & the biological parameters of the water samples are analysed and found nothing in the water sample. However, an overabundance of macrophytes can also result due to high nutrients from various sources like agriculture runoff, detergents that washed out in the lakes and that may interface lake functioning preventing recreational activities such as swimming, fishin & bathing. Thus, Macrophytes are the excellent indicators of watershed health because they provide information about nutrients, light condition at the bottom of the water bodies (limnetic & profundal zone), toxic contaminants, turbidity, water level change & salt concentration. Macrophytes found on the water bodies are:-Submerged-Hydrilla, Potamogetom, Utricularia, Emergent-Typha, Scirpus, Nelumbo, Floating-Lemma, Wolfia, Water Hyacinth(Eichhornia Crassipes).
 Periphytes or Aufwuchs- These are the organisms both plant & animals attached to the stems and leaves of rooted plants or other surfaces such as rocks projecting above the bottom for example:- Pond snails, Nymphs of Dragonflies, Mayflies, Stoneflies, Flatworms, etc. These are the producers and secondary consumers. These are also sensitive indicators of environmental change in Lotic water because they are attached to the substrate, rocks,l etc. For example, diatoms in particular are useful indiactors of biological condition of water & they are Ubiquitous & found in all lotic systems and represent good water quality.
 Benthic Organisms- Aquatic invertebrates that live in the bottom parts of the water are called Benthic micro invertebrates or benthos (benthic-bottom, macro-large, invertebrate-without backbone) and make good indicators of water bodies health because they live in the bottom of the water for all or most of their life, stay in areas suitable for their survival, are easy to collect and are helpful to know the water quality according to the types of macroinvertebrates found on water bodies. For example, Stoneflies, Dragonflies are considered as clean water benthos whereas presence of tubificids indicates the water bodies as polluted due to excess organic wastes.


 Planktons
The word plankton comes from the Greek word ‘planktons’ which means ‘drifting’. Planktons are microscopic organisms that float freely in water bodies. Plankton is made up of tiny plants(phytoplankton) and tiny animals(zooplankton) and bacteria.
 Phytoplanktons-They are primary producers(autotrophs). They are the base of the food chain. They use Chrolophyll to convert energy(sunlight), inorganic chemicals(nitrogen & dissolved carbondioxide) to carbohydrates or chemical energy. Phytoplanktons are the food for the zooplankton and fish and other secondary consumers. For e.g. diatoms, Cyanobacteria, dinoflagellates, spirogyra, euglena, etc.
 Zooplanktons-These are microscopic animals that eat other planktons. Abundance of zooplankton will help to decline the phytoplanktons populations by preying on them and increases predation by juvenile fishes reduces the zooplankton populations. Some zooplanktons are larval or immature stages of larger including molluscs like (Snails & Squad), crustaceans(daphnia, Krill,copepods), fish, etc.
 Fish-Fish are useful indicators to assess the water quality either it is clean or polluted. Fish living in water bodies is not enough for the understanding of the people but also need to know what kinds of fish are there how many and their indicators of watershed condition because they live in water all of their life and indicate water resources quality providing information about the physical, chemical and biological condition of water bodies. For example, trout and salmon represents in water bodies indicate good water quality whereas catfish & carpfish represents water is being dirty or becoming polluted.