PLANKTON OF THE GULF OF MAINE 
473 
into fine fragments. The largest amounts of eelgrass (Zostera) leaves are also thrown 
off around the shores of the gulf during the autumn and early winter; but these are so 
tough and decay so slowly that great accumulations of their fragments are still to be 
found the following spring, especially in the deeper channels that cut the mud flats 
where fields of this plant flourish, and it may be several years before they are reduced 
to the state of fine silt. Thus, the amount of nitrogen in solution in the sea water 
tends to increase during the winter, while conversely the denitrifying bacteria (which 
are known to exist in the sea) are less active at low than at high temperatures. 
Rain and snow falling on the surface of the gulf likewise add nitrogenous com- 
pounds to its waters, for they wash out ammonia from the air and nitric acid formed 
during electrical discharges. But the amount of nitrogen dissolved in rain is much less 
in temperate than in tropical climates, Muntz and Marcano’s (1889 and 1891) 
analyses showing larger amounts (an average of 2.23 milligrams nitric acid and 1.55 
milligrams ammonia per liter) in the rain water at Carracas, Venezuela, than have 
been found in Continental Europe or in England. No nitrogen analyses have been 
made of the rain water that falls on the Gulf of Maine, or, so far as I can learn, for 
any neighboring part of North America, but probably it does not differ much from 
European analyses — that is, is in the neighborhood of 0.2 milligram nitric acid and 
0.5 to 0.9 of ammonia per liter. 
SILICA 
The obvious dependence of diatoms on silica (which is present in only very 
minute quantities in sea water) for the construction of their shells has naturally 
tended to focus attention on the fluctuations in concentration of that substance as 
probably governing the abundance of marine diatoms, and several recent authors, 
among them Michael (1921), have definitely accepted it as the chief determinant. 
Diatoms require much more silica than nitrogen, the disparity between these two 
substances being much greater in the dry matter of these plants than in the sea water 
in which they live. Evidently it would be impossible for diatoms to form their 
silicious frustules without a sufficient supply of silica; in fact such a failure, with 
resultant abnormal forms, has actually been recorded by Allen and Nelson (1910) for 
experimental cultures, while these were undergoing rapid multiplication. 
Sources for dissolved silica . — We might naturally expect to find the land drainage 
from an area as largely composed of felspathic rocks and of glacial debris as is the 
watershed of the Gulf of Maine, much richer in dissolved silica than the sea water, 
an expectation confirmed by several analyses of the waters of several New England 
rivers and springs made by the United States Geological Survey, as well as for river 
waters in other parts of the world. Thus, according to Fuller (1905, p. 53), 12 repre- 
sentative springs in various parts of the State of Maine carry from 5.1 to 15.1 parts of 
silica (as Si0 2 ) per million, the average for all 12 being about 10 parts per million, which 
is about five times as much as the sea water off Gloucester at the season of its highest 
concentration (p. 476). Spring waters, of course, undergo various and rapid modifica- 
tions on their way first to the rivers and then to the sea, a river being "the average of 
all its tributaries plus rain and ground water, and many rivers show also the effects of 
contamination from towns and factories” (Clark, 1916, p. 64). Nevertheless, Clark’s 
(1916, p. 71) analyses of the water of the Androscoggin a few miles above tide water 83 
!J Average of 38 analyses of weekly samples taken between Apr. 25, 1905, and Jan. 16, 1906. 
