Limnology (2+1)

Practical No. : 8 & 9

Enumeration and biomass estimation of freshwater plankton
Qualitative estimation of plankton :
Qualitative analysis of plankton involves three steps, they are Splitting, Sorting, Counting
a. Splitting:
• Transfer well mixed sample in to the Folsom’s plankton splitter.
• Circular drum is moved back and forth to homogenize and to divide the sample into equal parts
• The well mixed representative subsamples are poured in to container or boats.
• The aliquot of each boat / container is the representative of the total catch
• The aliquot obtained from each container may further split depending on the concentration of the plankton.

b. Sorting:
• Well mixed representative sample is poured into a clean petridish.
• Place the petridish on the dark background
• Use hand lenses or eye pieces (10 x) to identify the bigger organisms.
• Individual specimens belonging to same genera or species are grouped together and preserved separately.
• Identify the specimens to generic or species level.
c. Counting:
• Remove all macro zooplankton in the sample.
• Measure the total volume of water and mix well
• Transfer 1 ml of water sample into Sedgwick rafter counting cell by using stempel pipette or wide mouthed pipette.
• Place the cover slip diagonally across the counting cell and introduce the sample from the corner. The cover slip moves into its proper position by capillary action.
• The prepared cell is placed under microscope and allowed for sedimentation.
• The cell is moved horizontally or vertically along the first row of squares and the organisms in each square are counted.
• Organisms are identified to genera/species and counted accordingly.
• If one lack sufficient time to count all the squares in the counting cell, count just a few transects considering the homogenous settlement of the plankton.
• The total number of organisms are then computed by multiplying the number of individuals counted in these transects with the ratio of the whole chamber area to the area of intercepted transects.
• For error free results take replicate counts.

Calculation:
The total number of zooplankton present in a litre is given by

N= n × v / V

Where : N = total number of zooplankton per litre

n = average number of zooplankton in 1 ml

v = volume of plankton concentrate (ml)

V = volume of total water filtered

Quantitative estimation of plankton
i. Volumetric analysis :
a. Settlement volume method:
• The plankton is allowed to settle by gravity and the space occupied by the settled material is taken as settled volume.
• Concentrate the sample using a net. A known volume of sample is transferred into a graduated cylinder or sedimentation / centrifuge tube.
• Mix well and allow the sample to settle for 1-2 days.
• The settled volume of the sample is recorded (cc).
• The volume of the sample s then calculated in m³
b. Displacement method:
• The space occupied by the plankton is measured in terms of the equivalent volume of liquid displaces.
• The sample is filtered through plankton net of known mesh size.
• The concentrated plankton is carefully removed and transferred to 25ml measuring jar.
• Pour the water from 25ml burette into the measuring jar till the 25ml marks is reached.
• Record the volume remained in the burette (cc).
• Displaced volume is equivalent to the plankton present in the sample.
ii. Gravimetric method:
It is the total weight of planktonic organisms concentrated from the known volume of water.
a. Wet weight
It is the raw weight of the planktonic organism with their natural body fluids.
• Plankton sample is screened through filters.
• Adhering water is blotter off with filter paper.
• Transfer the plankton matter to a pre-weighed aluminium foil.
• Weigh the sample near to a milligram.
b. Dry weight
It is the raw weight of the planktonic organism without water content.
• The plankton concentrate for which the wet weight is already known is placed in pre-weighed platinum dish or silicon crucible.
• Dry in a electric oven at 60^oC until all the water is evaporated
• The weight of the dish and contents are then weighed.
• The difference between the final weight and dish weight gives the dry weight.
Note : The best expression of plankton biomass is dry weight preferably ash free dry weight or carbon weight. It is important to indicate the method of measurement along with results.