NATURE ADRIFT 



end of a jetty. If the ring is light and the speed of towing about 2 knots, the 

 net will rise to the surface due to the resistance of the water. If a sub-surface 

 or deep water sample is wanted you must attach a weight, using a short 

 length of cable (Fig. 3 ; la). If the net is over-weighted or the speed too slow 

 and it goes to the bottom, the weight touches first, it then no longer pulls 

 the net down and this prevents the net from tearing on the rocks or sharp 

 shells. 



If, however, you want as a first trial to do something much simpler, try 

 this. Take to a rock pool a small muslin bag a few inches across (a cheap 

 'tiddler' net from the local seaside shop will do if it has a fme mesh) and a 

 jar. Use the jar to scoop the water from amongst the small weeds in the pool 

 and filter it through the bag. After about ten scoops fill the jar with clean 

 pool water and empty into it the contents of the bag. Strictly speaking this 

 will not be true plankton because most of the organisms you have will 

 normally live amongst the weeds and not drift about, but they will be the 

 first cousins of plankton and you will be able to recognize their relationships 

 from a study of Chapters 4 to 6. 



Now let us consider the equipment used by the marine biologists. 

 Basically the most used apparatus is the simple net illustrated in Fig. 3 ; i, with 

 various additions and modifications. If the biologist does not need accurate 

 figures, then he uses such a net unmodified because of its simplicity and 

 cheapness. While such a net is being towed through the water, the plankton 

 organisms caught are partly washed down to the tip and partly lie against the 

 meshes. The more plankton there is the sooner the net becomes clogged so 

 that it cannot pass all the water in its path and its catching power is gradually 

 reduced. This means that the exact amount of water filtered cannot be known 

 and careful counts of the organisms caught mean very little. If the filtration 

 of the net is improved, the counts mean more. Increasing the length of the 

 net improves the filtration but makes it all the more difficult to handle and 

 more difficult to ensure that the catch is taken out of the net and not left to 

 spoil the next sample. A better way is to reduce the size of the opening with 

 a cone of canvas or metal as in the Hensen net (Plate Wb). A much better 

 way, but a more expensive one, is to use a flow-meter in the net, an instrument 

 which works in the same way as the meter in a petrol pump. It can be cali- 

 brated by towing it and the empty ring, with no net, through a known 

 length of water and assuming a complete flow through the ring. There are 

 of course snags to this too; if the vanes of the meter were big enough to fill 

 the whole of the opening it could be very accurate, but it would increase the 

 warning effect of the net, so spoiling the sample ; it M'ould also be extremely 

 liable to injury when being hauled aboard in bad weather. As the net 

 gradually chokes, some of the water that enters is forced out again as an 



14 



