Field. — Sources of Marine Food 185 



It is also possible that the excremental action of the 

 fauna contributes to render the bottom richer in nitro- 

 gen. The nitrogenous portion of the bottom is indigest- 

 ible while the non-nitrogenous matter contains consid- 

 erable quantities of digestible pentosans. Hence when 

 fed upon in the form of detritus by such organisms as 

 mussels and oysters the non-nitrogenous matter would 

 be removed and the nitrogenous portion returned to the 

 bottom. This was well illustrated by comparing the 

 composition of oyster excrements which consisted of 

 almost pure detritus, with bottom samples taken at the 

 same place where the oysters were found. The nitrogen 

 of the bottom samples amounted to .187% while that of 

 the excrements was .71%. 



That nitrogenous matter of the bottom can also be in- 

 creased by the fixation of inorganic nitrogen through 

 the action of bacteria is likewise probable. The nitro- 

 gen may be taken from the ammonia or nitrates dis- 

 solved in the water or from the free nitrogen which is 

 also present in solution. Bacteria such as Azotobacter 

 and Clostridium, which perform this function, are of 

 common occurrence on the bottom and a considerable 

 amount of nitrogen fixation has been shown to take place 

 where the vegetation is abundant. 



In addition to the above sources of nitrogen it should 

 be mentioned that the fauna itself, by dying and form- 

 ing detritus, also serves to increase the amount of 

 nitrogen in the sea floor. 



A determination of the total quantity of detritus and 

 plankton in sea water was also attempted. Ten liter 

 samples of sea water from various localities were care- 

 fully filtered and the total quantity of detritus and 

 plankton measured. It was first weighed and dried at 

 100° C. and then weighed again. Samples were also 

 subjected to microscopic examination to determine the 

 amounts of detritus and plankton organisms present. 



The results were that nearly all the samples showed 

 a greater proportion of detritus than of plankton. The 

 weight of the dry matter in the residue varied between 



