Limnology

Determination of physical characteristics of inland (lentic and lotic) waters

Practical No. : 2 & 3

1. Determination of physical characteristics of inland (lentic and lotic) waters
Physical properties of water in any aquatic system are largely regulated by the existing meteorological conditions and chemical properties. The effect of physical forces like light and heat is of great limnological significance as they are solely responsible for many of the phenomena like thermal stratification, chemical stratification, diurnal and seasonal variations in the number and distribution of plankton, spatial distribution of micro - and macroorganisms. In the light of above considerations it is essential to record the important physical parameters as much relevant information can be derived from it.
Lentic systems are diverse, ranging from a small, temporary rainwater pool a few inches deep to Lake. Examples include ponds, basin marshes, ditches, reservoirs, seeps, lakes, and vernal / ephemeral pools.
Lotic is nothing but the running water series are included all forms of inland waters in which the entire water moves continuously in a definite direction. Genetically it is stated as brook – creek – river series.

Lotic water bodies (River & Stream)

Lentic water bodies (Lake & Pond)

The most important features which are having practical implications are delineated here.
1. Water surface
Visual changes in the nature of water surface are mainly due to the wind action and to a certain extent are governed by the topography of the surrounding area. This phenomenon is responsible for the localization and dispersion of the algal bloom and other particulate organic matter. Therefore, observe the water surface and record its condition as mirror smooth, rippled, wavy and highly wavy and so on.

2. Water colour
Light coming from the lake surface yields an apparent colour which is the result of the true colour of water (due to materials in solution), seston (due to living and non-living particulate matter) and the reflections of surface and sub surface objects. The colour is best judged by observing through a water telescope or also by the standard empirical colour scale (or even by visual observations).

3. Water temperature
The temperature of surface and subsurface waters can be recorded by drawing water samples with the help of a sampler (preferably having an inbuilt thermometer) or by dipping the thermoprobe to the desired depth.
Water temperature is measured using a thermometer. Keep the bulb of a thermometer completely immersed in the surface layers of water for 2-3 minutes until it reaches a constant value. Measure the temperature of the water while keeping the thermometer in the water.
i. Surface temperature
Measurement of surface water temperature is very important. Any good grade, simple type of mercury thermometer would serve the purpose. Any common chemical thermometer graduated to 0.2°C can conveniently be used, but it should be secured well in a metal case to avoid loss in the field. Surface water is taken in a plastic container and its temperature is recorded immediately by dipping the thermometer for about one minute or more.

ii. Subsurface temperature
Measurement of temperatures at various depths below the water surface requires specially designed instruments. Temperature often differs with depth and positions at which temperature is to be measured are remote from the observer. Subsurface water temperature can be measured by different apparatus and are delineated below.
a. Thermo-flask sampler method
A thermos flask sampler along with in-built thermometer is lowered in water to desired depth and closed with the help of cord. Then it is pulled out of water. The temperature is read from the in-built thermometer.
b. Reversing thermometer method
This instrument has a main thermometer which registers the temperature and an ordinary thermometer which is used for correcting the change brought about by atmospheric temperature. It consists of a conventional bulb connected to a capillary in which a constriction is placed so that upon reversal the mercury column breaks off in a reproducible manner. The mercury runs down into a smaller bulb at the other end of the capillary, which is graduated to read temperature. A 360° turn in a locally widened portion of the capillary serves as a trap to prevent further addition of mercury if the thermometer is warmed and the mercury expands past the break-off point. The remote-reading potentialities of reversing thermometers make them particularly suitable for use in measuring subsurface temperature as a function of pressure. In this application, both protected thermometers and unprotected thermometers are used, each of which is provided with an auxiliary thermometer. They are generally used in pairs in Nansen reversing water bottles. They are usually read to 0.01°C, and after the proper corrections have been applied, their readings are considered reliable to 0.02°C.

4. Turbidity
Suspension of particles in water interfering with passage of light is called turbidity. The suspended particles may be clay, silt, finely divided organic and inorganic matter, plankton and other microscopic organisms. The following methods are generally used for determining the turbidity of water sample.
a. By Jackson’s candle Turbidity meter (JCT) and
b. Electrical / Electronic turbidity meter commonly called as Naphlometer
a) By Jackson’s candle Turbidity meter (JCT)
It is based on the transmittance of light from a frame of a ‘standard candle’ through the sample column of certain path length such that the flame becomes indistinguishable against background illuminations. Turbidity is inversely proportional to the path length. In brief, it is based on comparison of intensity of light scattered by the sample and a standard reference under comparable conditions. Higher the intensity of scattered light, higher is the turbidity.
The lowest turbidity which could be measured with JCT is 25 units. As such indirect secondary methods are required for measuring turbidities in the range of 0-5 units. However, the results obtained with different types of secondary instrument do not match with one another because of fundamental difference in the optical systems even though the instrument are all pre-calibrated against JCT. This method is not in much use today mainly because of difficulties in getting the standard candle.

b) Nephlometric method
Principle : Formalin polymer is the turbidity standard preference suspension for water. It is easy to prepare and is more reproducible in its light scattering properties. A given concentration of formalin suspension having 40 NTU has approximately a turbidity of 40 JTU. Therefore, turbidity based on formalin will approximate units desired from JCT but will not an identical.
Materials : i) Turbidity meter – It consists of a light source for illumination of the sample and one or more photoelectric detectors with readout device to indicate the intensity of light scattered at 90o incident light. The sensitivity of the instruments permits detection of turbidity difference of 0.02 NTU or less in water having turbidity <1.0 NTU. The instrument measures generally from 0-40 NTU. Various ranges are necessary to obtain both adequate coverage and sensitivity. However, meters having wider ranges are also available today.
ii) Sampler tube – Clear colourless glass scrupulously cleaned both inside and outside without scratch the tube be sufficiently long so that they are not required to be touched where light strikes.
Reagents :
i) Turbidity free water – It is obtained by passing distilled water through membrane filter having pore size < 100 mm. If filtration does not reduce turbidity, distilled water itself is used.
ii) Stock turbidity suspension
a) Solution I – It is prepared by dissolving 1.0 g of Hydrazine sulphate in distilled water and diluted to 100 ml in a volumetric flask.
b) Solution II – It is prepared by dissolving in 10.0 g of Hexamethylene Tetramine in distilled water and diluted to 100 ml volumetric flask.
c) In a 100 ml volumetric flask, 5.0 ml of solution I is mixed with 5.0 ml of solution II. Allowed to stand for 24 hours at 25+3oC. This be diluted to 100 ml mark of the flask and shake properly. Turbidity of this suspension is 400 NTU.
iii) Standard turbidity suspension – 10 ml of stock suspension be diluted to 100 ml by turbidity free water. The turbidity of this suspension is defined as 40 NTU. Such suspension is prepared weekly.
Procedure :
i) Turbidity meter calibration – Generally these meters are kept calibrated by the manufacturers. Further, manufacturers operation manual could be useful in calibration if required.
ii) Measurement of turbidity of < 40 NTU – The sample is thoroughly shaken. The air bubbles are allowed to escape. The sample is poured into the turbidity meter tube and the turbidity is read directly from the scale or from the calibration curve.
iii) Measurement of turbidity of . 40 NTU – The sample is diluted with turbidity-free water until its turbidity falls between 30.0 and 40.0 NTU. Now the turbidity of the original sample is computed from the turbidity of the diluted sample and dilution factor. The stock turbidity suspension of 400 NTU be used for continuous monitoring. High turbidities determined by direct measurement are likely to differ appreciably from those determined by dilution technique.
Calculation :

NTU = A (B+C)/C

Where, A = NTU found in the diluted sample

B = Volume of dilution water

C = Sample volume taken for dilution.

Interpretation of Result :
The turbidity readings be reported as follows

5. Transparency
The turbidity of water is directly related to light penetration and visibility or transparency which can be measured by Secchi disc. This disc was devised by an Italian scientist, Secchi (1865) for studying the transparency of aquatic bodies. The Secchi disc is a metallic plate of 20 cm diameter with four (alternate black and white) quadrants on the upper surface and a hook in the centre to tie a graduated rope.
Principle : The transparency is inversely proportional to the turbidity of water, which in turn is directly proportional to the amount of suspended organic and inorganic matters. When the disc is gradually lowered in water it remains visible in the euphotic zone, only to that lower level where light is about 15% of the radiation at the surface.
Requirement : Secchi disc, measuring tape, graduated nylon rope etc.
Method :
a. Lower the disc in water and note the depth (in cm) at which it disappeared
b. Now slowly raise the disc upward and note the depth at which it reappears
c. Take the average value of Secchi disc depth (SDD) or transparency
Calculations : Calculate the euphotic limit and vertical attenuation coefficient as follows
Euphotic limit (cm) = 2.5 x SDD
Vertical attenuation coefficient (Extinction coefficient) = 1.7/SDD