Aquatic ecology and Biodiversity

Unit 12- Biodiversity and conservation

Practical 4- Determination of mesh size of plankton net and size of plankton
Experiment No. 4
Micrometry is an optical method whereby the length, breadth and other details of an organism are measured under microscope.
Requisites
Microscope, Sedgewick rafter cell, Occular micrometer, Stage micrometer, Plankton sample, etc.
Procedure
Fix the occular in the eye piece of the microscope. A good individual specimen / a small piece of plankton cloth is placed on the stage of the microscope. Measure the number of occular division by the specimen (standard length/caudle length). The number of occular divisions multiplied by the corresponding calibrated values gives the length of the animals.

Calibration of occular
To ascertain the exact value of each division in micrometers (µm) use stage micrometer. The stage micrometer which is 1.0mm long is divided into 1000µm and placed on the stage of the microscope. Focus and align with the occular scale in the eye piece. The stage micrometer is then moved carefully until its zero line is exactly coincided with that of the occular. Record how many division on the occular corresponded with number of divisions on the stage.
Example - 60division of ocular correspond to 20 division of the stage micrometer. These 60 divisions are equal to 200mm. In other words these 60 divisions occupy 200µm space of the stage micrometer while the total length of the scale is1000µm (1mm). Therefore one ocular division is equal to 200÷60 – 3.3µm.The number of divisions multiplied by the corresponding calibrated value gives the length of an organisms i.e. 30 x 3.3 = 99µm.