212 BULLETIN OF THE BUREAU OF FISHERIES. 



nous substances in the sea bottom to a greater extent than is the case with the nitroge- 

 nous matter or (2) by the fixation of inorganic or free nitrogen by bacteria. 



It has been established beyond all doubt that nonnitrogenous substances of the 

 sea floor are to a very considerable extent destroyed by bacteria, at least one step in 

 the process being the fermentation of the pentoses. Another is the formation of methane 

 from the fermentation of cellulose. On the other hand, it is probable that the nitrog- 

 enous substances are acted upon to a lesser degree, due to the fact that they are com- 

 paratively easily transformed into humic compounds, which are less easily destroyed. 



It is also possible that the excremental action of the fauna contributes to render the 

 bottom richer in nitrogen. The nitrogenous portion of the bottom is indigestible, while 

 the nonnitrogenous matter contains considerable quantities of digestible pentosans. 

 Hence, when fed upon in the form of detritus by such organisms as mussels and oysters, 

 the nonnitrogenous matter would be removed and the nitrogenous portion returned 

 to the bottom. This was well illustrated by comparing the composition of oyster 

 excrement, 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 0.187 per cent, while that of the excrements was 0.71 per cent. 



That nitrogenous matter of the bottom can also be increased by the fixation of 

 inorganic nitrogen through the action of bacteria is likewise probable. The nitrogen 

 may be taken from the ammonia or nitrates dissolved in the water or from the free 

 nitrogen, which is also present in solution. Bacteria such as Azotobacter and Clostri- 

 dium, 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 vege- 

 tation is abundant. 



In addition to the above sources of nitrogen, it should be mentioned that the fauna 

 itself, by dying and forming 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 carefully filtered 

 and the total quantity of detritus and plankton measured. It was first weighed and 

 dried at ioo° C. and then weighed again. Samples were also subjected to microscopical 

 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 plankton. The weight of the dry matter in the residue varied between 9.6 and 

 72.3 mg. per 10 1. of sea water. No relation could be shown to exist between the weather 

 conditions and the amount of detritus in the water. The conclusion to be drawn from 

 these results is that sea water is rich in the quantity of detritus it contains. 



The next question which arises is, What value does this organic matter of the sea 

 bottom possess as a source of nourishment for the benthos or bottom fauna? 



Having assumed that the organic matter of the sea bottom forms a source of 

 nourishment for the majority of the fauna living in and near the bottom, Jensen con- 

 sidered it advisable to investigate the question as to how far suitable nourishment for 

 such fauna can be shown to exist among the substances of which the sea floor is 

 composed. Since eelgrass contributes most of the organic matter of the bottom, it was 

 natural to examine quite closely the chemical composition of this weed. It was found 

 to compare favorably with the composition of the common fodder grasses. Proteinw as 



