The best way to concentrate protists from the water column is by using a plankton net. These are conical nets made of a mesh which end in a small receiving jar. Biggers nets with wider openings are used where the plankton is sparse. The pore diameter of the mesh can vary – down to 10 microns or so, and a pore size that suits the target organisms must be sought (that is, marine diatoms may be collected with a 100 mesh net whereas synurophytes in a freshwater pond may require a 20 µm mesh net). Nets that are made from fine mesh are more easily to get clogged, and need more care. Protistologists tend to prefer nets that have 20 or 50 µm (microns) diameter mesh. We tend to use other techniques (such as filtering through a coffee filter) for smaller organisms.

Make your own plankton net.

Typical plankton net. Image by D J Patterson.

Typical sampling gear includes:

• buckets
• 50 - 500 ml wide-mouthed flasks / jars
• plankton net
• wash bottle to flush fabric of net
• tape - for containers
• indelible pens - to write on tape

Some folk like to take notes, GPS, temperature, pH etc. etc.

Procedure

Can be done from shore, but this is like to lead to shoreline debris getting caught up in the net. Throwing the net from a jetty or boat is better

• The net must be flung into the water, typically allowed to sink so that is just below the surface (although this can be adjusted) and then must be towed for a distance / period of time. The more water that flows through the net the more will be collected. However, if this is excessive, the net will get blocks. The towing must be gentle, just enough to keep the net floating rather than sinking. if the net is pulled too forcefully the water will tend to sweep around it rather than through it. The operation may be repeated several times. Sometimes the only way of towing the net through the water is to allow it to sink vertically and then pull it up, but in this case be careful not to get bottom debris in the net.

Throwing the net. The rope is collected in loose loops, and the net thrown with the support ring vertical. If the net captures a lot of air on landing in the water, pull it back and try again. Image by D J Patterson.

• This net can be used to sample the surface of the water, simply by slowly pulling it along the top of the water, or it can be dropped to a certain depth in the water (from a pier or boat for example) and pulled vertically upward to sample the water column.
• Remove net while spinning / twisting it so that the remaining water washes any collection from the side of the net and this will then flow into the receptacle at the bottom of the net
• You can use a squirt bottle – squirting from the outside, to release any captured material from the mesh
• This approach can be quite brutal on the specimens, and many species in the material once concentrated will not remain viable for very long. SO, if the material is not being fixed, it should be processed quickly
• To transport organisms back to the lab, empty the concentrated sample from the test tube into a slightly larger jar, you may wish to add water from the environment if the material is too concentrated, and cover. The temperature of the sample should not be allowed to change very much. Covered jars can be kept in a large bucket and covered with water from the environment to keep the temperature from changing too much.
• Samples can be observed under a dissecting scope, or pipetted onto a microscope slide and observed under high power.

A net that has been towed for a couple of minutes has turned orange inside because of the accumulated plankton. Image by D J Patterson.

Cleaning

• After sampling, you can flush the net with water or squirt water through the fabric. When returned to shore, the net should be cleaned by forcefully squirting freshwater from the outside inwards through the fabric of the net.

Cleaning the net with a squirt bottle, or flushing an open-ended net wth water. Images by D. J. Patterson

Notes

• Remember to hang on to your end of the piece of string
• When finished force clean water backwards through the net to clean out any pores
• Hang the net up to dry, don't allow it to dry folded as creases may crack
• You can use more simpler things to concentrate samples, for example coffee filters

Image by D. J. Patterson