METHODS 



third method is the one now mostly used. This is just a patience-demanding 

 variation of the previous method. A sample of sea water, after adding a pre- 

 servative (see page 26 ), is placed in a vertical glass or plastic tube open at the top 

 hut attached at the bottom to a shallow trough of the same diameter which has 

 been fixed to a microscope slide. The join is made lightly with 'Vaseline' or a 

 similar waterproof seal, easily broken. The whole column is carefully set 

 aside and the sample gradually sediments out into the shallow trough. This 

 may take hours or days, according to the length of the tube. After settling 

 out has been completed, the surplus water is removed (usually by sucking it 

 out from above) leaving only the small amount of water in the trough with 

 the sample of plankton. The tube is then broken oft at the seal and the slide 

 ready for microscopical examination. 



Methods of observation of the living plankton in its natural environment 

 are now possible with special modern eqiupment, and extend from the simple 

 underwater camera and the frogman, to closed circuit television, and to 

 special underwater observation chambers like the French bathyscaphe 

 Trieste and the Russian research submarine, Severyauka. 



All these methods have their limitations, which are mostly serious, but are 

 of a different kind from other sampling methods. The obvious one is, of 

 course, that the small size of most planktonic organisms makes naked eye 

 or near-natural size observation difficult, if not impossible, so that it is confined 

 to the larger species only, except to record an undefined 'soup' which can vary 

 in its 'soupiness'. Visibility under water, even in the best conditions, is poor 

 compared with that in air. A further difficulty is the extreme transparency of 

 so many of the organisms while they are alive, though they rapidly become 

 opaque after death. Using photographic methods, a great deal depends on 

 chance, especially if the camera is operated from the deck of a ship and not by 

 a diver in person. The camera cannot be orientated and must be focused 

 beforehand. Only those organisms in actual focus can be seen whilst the 

 others tend to blur the picture. 



One advantage of the unaccompanied camera is that it can be operated 

 in conditions where a diver cannot work, because of depth, or darkness, or 

 weather. Underwater photography at night or in depths below adequate 

 light penetration must be accompanied by its own hghting unit. This can 

 be continuous flood lighting — which might radically change the natural 

 distribution by attracting or repelling the plankton — or by flash. Flash, 

 however, is a very concentrated form of light and this has its own peculiar 

 but severe hmitations. Most plankton organisms are fairly transparent, some 

 extremely so, and what one sees in a flash photograph is often merely the 

 high-lights reflected from their curved surfaces, a scries of minute replicas 

 of the flash and its reflector, varying in clarity with the accuracy of the focus 



25 



