Suitable specifications for an amateur marine plankton net require that the shape and mesh size are carefully considered.
Towing a net behind a boat, lowering it over the side, or simply throwing it into the sea from the shore and then dragging it in will capture marine plankton. There are standard designs for plankton nets, and mesh size determines what will be caught. Once captured, plankton can be kept alive for a short while in a vacuum flask which will stop the sea-water warming up too quickly.
Standard Design of Plankton Nets
All plankton nets are conical in shape, with mouth to length ratios of from 1:3 to 1:5. Small nets (five to eight inches in diameter and 15-20 inches long) are suitable for the amateur.
The mouth is held open by a rigid circular ring, and pulled through the water on a single line which ends in a three-piece ‘bridle’ where it attaches to the mouth-ring.
The netting is always made from ‘Nitex’ (a registered trademark). This nylon netting is treated so that the square mesh openings of known dimensions remain constant whatever you are doing with the net.
The pointy end of the net has a PVC collecting bottle (or ‘bucket’) which is easily detachable – this is where the captured animals end up.
Mesh Sizes of Plankton Nets
Smaller mesh sizes catch most things. A ‘small’ mesh of 80 microns (a micron, or micrometre is one thousandth of a millimetre) will catch larger phytoplankton and everything bigger than that. These nets tend to fill up fast!
‘Coarse’ mesh of 363 microns lets all the phytoplankton and many smaller zooplankton through – retaining only the larger animals.
For the amateur a ‘medium’ mesh size of 153 microns is best since it lets microscopic organisms through and keeps everything visible to the naked eye.
Keeping Plankton Alive to Observe Later
Planktonic plants and animals (such as copepods ) are very sensitive to temperature change and will die rapidly if the water begins to warm up. This is not a problem if they are to be observed immediately, but if the idea is to make a few hauls and the take the whole sample to a place where the living animals are to be observed and maybe photographed later, then they need to be kept at a constant temperature. Any sort of vacuum flask will do this, but for use in the field a stainless steel vacuum flask is highly recommended.
Once the captured animals have arrived at the place where they are going to be looked at they can be poured or pipetted into a small flat glass dish (Petri dish recommended) on top of a black card. Oblique lighting will help make them visible against this dark background (but will quickly cook them). A hand lens (X10) is very useful at this stage. Photographing planktonic animals is a challenge, and videoing them even more so!