CHAPTER 2 



Methods 



The individual constituents of plankton, even when abundant, only occupy 

 a small part of the total volume of water, so that their collection is essentially 

 a method of separating them from the water in the most convenient way. 

 The method chosen will depend on what is required, and from where; it 

 is a matter of selection and extraction. As we have seen in the introduction, 

 plankton consists of a very wide range of forms from minute plants of 

 some i/io,ooo inch to the larger jellyfish, and quite obviously what is suitable 

 for selecting from one end of this range would be hopeless for the other. 

 If we want to concentrate our organisms by passing the water through some 

 sort of net or filter we must choose a suitable mesh, and this can range from 

 the finest microfilters to the coarse mesh of a fishing net. The finer the filter 

 the longer it takes the water to drain through it, so that whereas filter paper 

 can be used to filter about a cupful of sea water — which might well contain 

 50,000 of the minutest plants and fifty of those about i/ioo inch — if we want 

 to sample the large jellyfish as much water as possible must be filtered. To 

 do this as big a net as one can handle with a large mesh must be used. The 

 larger the mesh the less resistance to the water and the faster we can pull it 

 using the same energy, thus filtering more water. 



By far the most diverse in form, and thus the most interesting, of the 

 plankton organisms arc those towards the middle of the range, i.e. from 

 about 1/200 inch to i inch and for these we want neither a filter paper, which 

 will not deal with the quantity of water we want fast enough, nor a large 

 coarse net. Except for very casual sampling, we need to choose the size of 

 mesh required for our special purpose, and to be satisfactory our net must 

 have a reasonably constant mesh. If one looks at a linen handkerchief 

 through a microscope (Plate III) the strands are fairly regularly placed but 

 they occupy more space than the holes which are small and extremely 

 irregular. A coarser net of stramin (jute) or a 'cheese cloth' (Plate III) has a 

 much larger ratio of holes to material but again the holes arc unsatisfactory, 

 this time due to the fluffy nature of the strands so that small fibres extend 

 into the spaces and choke them. Fortunately for the planktologist just the 

 right material is commercially available; the flour millers use it to sift their 

 flour into various degrees of fineness. It is called 'bolting cloth' and is made 

 of silk or nylon; the meshes are remarkably constant and the strands are 

 kept the right distance apart by the special twists put in during manufacture 



