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 arc 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 tlit' 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 

 arc 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 (18S9 and 1S91) 

 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 fomi their 

 silicious frustules without a sufficient supply of silica; in fact such a failure, with 

 resultant abnormal forms, lias 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 ram 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 



u Average of 38 analyses of weekly samples taken between Apr. 25, 1905, and Jan. 16, 1906. 



