BIOLOGICAL STUDY OF CHESAPEAKE BAY WATERS 
325 
records show, did not have access to it. There was no marked maximum for these 
forms during April such as was characteristic for many neritic forms, and in most 
cases their occurrence was confined to the late summer, fall, winter, and early spring, 
when the s alini ty was highest near the mouth of the bay. Whether such a condition 
can be related without question to the shifting of the highly saline North Equatorial 
Stream (Johnstone, 1923) toward the north during late summer and early fall must 
remain a subject for further investigation. 
Two species belonging to the oceanic, boreal, Arctic group have been found: 
Chaetoceras decipiens Cleve and Rhizosolenia semispina Hensen. The former, which 
is such a co mm on form in the North Atlantic Ocean, but which was only fairly num- 
erous at Woods Hole according to Fish was taken in small numbers near the mouth 
of the bay at areas G and A. It occurred there during the months of June, July, 
and September, which is about the same time it was found commonly in the southern 
part of the North Sea, as pointed out by Ostenfeld. The specimens occurred in the 
surface layers where the salinities and temperatures ranged from 22.11 to 23.59 per 
mille, and 19.9° to 24.3° C., respectively. It is necessary to state, however, that the 
counts were not only small but included both dead and living specimens. R. semi- 
spina, a common summer form according to Bigelow and Fish during the summers of 
1914 and 1922 in the region of Woods Hole, occurred sporadically during 1916 in the 
Chesapeake. A few specimens, only in one sample, taken at area J during January 
were found. 
By far the most abundant oceanic temperate diatom found in Chesapeake Bay 
during the year 1916 was Rhizosolenia alata Brightw., possibly/, gracillima, although 
Wolfe has not mentioned this form of R. alata in his list. Ostenfeld has pointed out 
that this form is better able to stand neritic conditions and that it has been found in 
the lower layers of the Baltic Sea. During 1916 in Chesapeake Bay there was no 
indication of a maximum in April such as was characteristic for many of the neritic 
diatoms, nor were there any high counts in the region of the mouth of the Potomac 
River. It occurred in largest numbers in and near the mouth of the bay. At area G, 
during January, the counts at the surface and 18 meters were 200 (salinity 23.40 
per mille, temperature 4.1° C.) and 12,700 (32.5 per mille, 5.9° C.); during March at 
the same depths the counts were 600 (28.15 per mille, 3.7° C.) and 2,700 (30.5 per 
mille, 3.4° C.); and during April the counts at those depths were almost negligible. 
Rhizosolenia stylijormis Brightw., which is such an important oceanic diatom, was 
found in small numbers in the lower part of the bay during December, 1915, January, 
March, and September, 1916. The highest count was at area F, in the mouth of the 
bay, during the September cruise. Its occurrence in largest numbers during that 
cruise indicates that its seasonal distribution is similar to that in European waters. 
Thalassiothrix jrauenjeldii Grun. was taken in very small numbers. It was found in 
the lower part of the bay, in the fall only. 
The oceanic tropical diatoms were represented in 1916 by Planktoniella sol 
(Brightw.). Like other oceanic diatoms, it was found in very small numbers and 
almost invariably only in the fall and winter — that is, during the time of high salinities. 
The factors governing the distribution and abundance of diatoms have been for 
many years and still are subjects of investigation. Currents, light intensity, tem- 
perature, salinity, and the chemical composition of the sea water have been recognized 
as important factors which have to be taken into consideration. All of these have 
received attention in a general way during the last 50 years, but within the last 5 years 
