PLANKTON OF THE GULF OF MAINE 
469 
planktonic diatoms may be grown with the aid of appropriate nutriants (including 
ammonium sulphate) than in normal sea water, and that exhausted cultures of dia- 
toms may be temporarily revived by adding nitrogen in appropriate combination, 
which would hardly be the case were the diatoms able to avail themselves directly 
of the nitrogen gas dissolved in the water . 80 Therefore it may be assumed that 
diatoms, probably also peridinians, Phaeocystis, Halosphasra, etc., require a supply 
of ready combined nitrogen for their existence. 
The elemental nitrogen absorbed by the water from the air may serve as the 
source of this combined nitrogen through the medium of the bacteria just mentioned. 
These nitrogen-fixing bacteria have been found in the Baltic and in the North 
Sea, in bottom muds from many localities; also on the surfaces of a great variety 
of fixed algae, including Fucus and Laminaria, and on the surfaces of planktonic 
organisms; likewise in the Indian Ocean (Keutner, 1905; Keding, 1906). Hence, 
they are probably cosmopolitan in such situations and may be expected to prove as 
widespread in the Gulf of Maine as they are in the North Sea region, though they 
have not been actually detected there as yet. The two genera, Clostridium and 
Azotobacter, have been found to exist under the most diverse physical conditions, 
and they may well prove of prime importance in the economy of life in the narrow 
coastwise belt where fixed algas flourish, though this is still a matter of conjecture, 
as is the extent to which their activities depend on symbiosis with other bacteria or 
with algae. But since they have never been detected free in the sea water it is not 
likely that their activities contribute much directly to the supply of nitrogen avail- 
able for the use of planktonic plants on the high seas. 
However this may be, normal sea water is extremely poor in nitrogen in com- 
binations utilizable by plants — that is, as ammonia, nitrates, or nitrites — the chief 
sources for the latter in coastwise seas such as the Gulf of Maine being the drainage 
from the land and the decomposition of organic matter in the sea. 
It has long been appreciated by biologists that northern rivers, especially those 
that flow from countries with heavy rainfall and much cultivated land and those that 
are polluted with organic wastes, do bring down to the sea vast amounts of this dis- 
solved nitrogenous nourishment (Gran, 1915). It has been calculated from the nitro- 
gen content of the Khine (averaging 2 to 3 milligrams of nitrogen, in the form of 
dissolved compounds, per liter) that the North Sea receives annually not less than 
390,000,000 kilograms (383,000 tons) of combined nitrogen in this way (Brandt, 1899, 
p. 230; Johnstone, 1908, p. 282). 
The greater part of the watershed of the Gulf of Maine being timbered, not 
cultivated, and less densely settled than the countries bordering the North Sea, 
its river waters might be expected to prove less rich in nitrogen than the Rhine water; 
and the many analyses made by the United States Geological Survey prove such to 
be the case, with the rivers of Massachusetts richer in nitrogen than those of Maine. 
Thus the Charles River, a short distance above Boston, has been found to average 
about 0.879 part of nitrogen — as ammonia, nitrates, and nitrites — per million of 
water , 81 the Merrimac 0.524 part per million in its lower course above Haverhill, and 
80 Allen and Nelson (1810) give an extended discussion of this subject. 
81 Massachusetts Board of Health, 1890, examination of water supplies 
