INTBODUCTION 5 



as food by them weighs from twelve to eighteen times as much as they 

 do. In making an assessment of the annual crop of plankton, there- 

 fore, it is necessary to take the destructive processes into account as 

 well as the productive processes. 



In order to estimate the annual production of plankton it would be 

 necessary to determine the approximate rate of turnover in this ma- 

 terial, but the present data show only the actual quantity of organic 

 matter yielded by the plankton at the time the various observations 

 were made. The results show, however, that the greater part of the 

 material was derived from the protophyta and the protozoa ; these two 

 groups of organisms reproduce at a rather rapid rate so that the turn- 

 over in that part of the stock which they contribute would be equally 

 as rapid. 



Numerical data were also obtained for the entire series of plankton 

 catches, but they can be given only a general consideration at present. 

 The average weight per individual has been ascertained for only a few 

 of the characteristic plankton forms so that an assessment of the rela- 

 tive values of the different constituents cannot be made until further 

 determinations of this kind have been made. If the average weight 

 per individual were known for all of the different kinds of organisms 

 that make up the plankton, the numerical results would make it possible 

 to determine roughly, at least, the relative importance of the different 

 forms which make up the plankton complex. 



The chemical analyses show that, on an average, the crude protein 

 constitutes from a little more than 44 per cent to more than 57 per cent 

 of the dry organic matter in this fresh-water plankton ; in this respect 

 the material compares very favorably with some of the meats that are 

 used for human food. Crude protein constitutes about 47 per cent of 

 the dry organic matter in the edible portion of the hind quarter of 

 beef, for example, and about 37 per cent of that in the hind quarter 

 of mutton. This plankton material, therefore, must be given a high 

 rank as a source of protein food for other organisms. The edible por- 

 tion of fish, however, contains a higher percentage of protein than 

 either beef, mutton, or plankton; in the black bass, for example, the 

 crude protein makes up about 92 per cent of the dry organic matter of 

 the edible portion and in the brook trout about 90 per cent. 



The plankton yields a relatively small amount of fat or ether extract, 

 .averaging from about five per cent to somewhat more than seven per 

 cent. This is comparable to the percentages in black bass and brook 

 trout, for example, which are, respectively, eight per cent and ten per 

 cent of the dry organic matter in the edible portions. The percentage 

 of fat is much larger in beef and mutton ; it amounts to 53 per cent of 

 the dry organic matter in the edible portion of the hind quarter of 



