Observation of live plankton

Observing living zooplankton enables you to see how they use their swimming and feeding appendages and how they capture and consume food items. The colours and translucence of freshly caught zooplankton are amazing. You can capture live plankton around a bright light at night, or sample the contents of a gently towed plankton net. Live zooplankton cannot tolerate any trace of formalin or preservative or the heat of a lamp.

Figure 4.8 Compound microscope for phytoplankton. a) Viewing a plankton sample should be relaxing, without squinting or using only one eye. By adjusting your chair height you should have a straight back and neck. Adjust the eye-pieces to suit your own inter-ocular distance (see Figure 4.9b; by closing each eye separately you should have an unobstructed view); after adjusting the coarse and fine focus knobs for one eye, you may also need to twist one of the eye-piece's individual focus adjustments. b) Method for preparing a wet mount for a compound microscope.

Figure 4.8 Compound microscope for phytoplankton. a) Viewing a plankton sample should be relaxing, without squinting or using only one eye. By adjusting your chair height you should have a straight back and neck. Adjust the eye-pieces to suit your own inter-ocular distance (see Figure 4.9b; by closing each eye separately you should have an unobstructed view); after adjusting the coarse and fine focus knobs for one eye, you may also need to twist one of the eye-piece's individual focus adjustments. b) Method for preparing a wet mount for a compound microscope.

Figure 4.9 a) Dissecting microscope for Zooplankton, b) ensuring the eye pieces are adjusted to suit your inter-ocular distance.

For large living zooplankton, use a wide-mouth pipette to place a small volume of the sample into a clean Petri dish. It is best to observe large copepods and cladocerans under the dissecting microscope at low magnifications (less than x40), as then they remain focused in the larger depth of field and they are less able to swim out of the microscope's field of view (Figures 4.8, 4.9). An anaesthetic (for example, a few drops of MgCl2 solution, soda water, clove oil or some ice) will slow the activity of larger zooplankton.

For small living zooplankton, use a pipette to place a small volume of the sample into a clean observation chamber, such as a counting chamber. A counting chamber can be made with two glass cover slips placed 3-10 mm apart, and a few drops of the sample placed between. Then gently place an intact cover slip over the sample, resting on the two beneath. The water will be held in the small chamber to prevent zooplankton specimens from being squashed between the slide and the cover slip. If an inverted microscope is available, you may observe living zooplankton held in a small volume of water on the glass slide without placing a cover slip.

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