320 
BULLETIN OF THE BUREAU OF FISHERIES 
The fresh-water diatoms in the plankton were not abundant at the areas visited 
in Chesapeake Bay. This was expected, since usually the water at all these areas 
was brackish in character. Asterionella jormosa (Hass.), the tests of which occurred 
so commonly in oceanic bottom deposits off the mouth of the Delaware River accord- 
ing to Mann (1894), were found only here and there in small numbers but widely dis- 
tributed over Chesapeake Bay. The highest counts were obtained during the colder 
months and at areas near the mouth of the bay. Navicula borealis (Ehr.) Kutz, also 
mentioned by Mann, and Bacillaria paradoxa Gmel. (now called Nitzschia par adorn 
Gmel.) occurred about as abundantly as A. jormosa, but they were less widely dis- 
tributed. Campylodiscus echeneis Ehr. was taken in very small numbers in the lower 
half of the bay. Most, if not all, of these forms are littoral also (Gran, 1908). 
The so-called brackish water diatoms were represented in largest number by 
Raphoneis amphiceros Ehr. — a form found frequently in deposits off the mouth of the 
Delaware River by Mann. It was taken only in the southern half of the bay. An- 
other brackish-water form, Nitzschia sigma (Kutz) W. Sm., designated by Mann as 
“frequent” was found widely distributed over Chesapeake Bay but in every small 
numbers. Nitzschia plana W. Sm. and Pleurosigma balticum W. Sm. were very 
scarce, the former occurring only in the lower half of the bay and the latter only at 
area A near the mouth. 
The bottom and semibottom (tychopelagic) diatoms are abundantly represented 
in Chesapeake Bay. A form which is closely related to the tychopelagic group (classed 
above in the neritic northerly temperate group) is Skeletonema costatum (Greve) which 
according to Ostenfeld (1913) is found all the year round as a littoral-bottom form in 
European waters. It was very abundant and very widely distributed in Chesapeake 
Bay during the cruises of 1915 and 1916. It is known to be largely independent of the 
degree of salinity, and so its occurrence in considerable numbers at area X, almost as 
far north as Annapolis, is not surprising. As we shall see, its behavior is not that of 
a bottom form, for the highest counts, judging from the data for the cruise of October, 
1915, are not at the bottom. The condition just mentioned is in keeping with Osten- 
feld’s statement that this species multiplies to an important degree in the plankton. 
Skeletonema costatum was found in the plankton samples during all the cruises from 
October, 1915, to September, 1916. 
The highest counts were obtained during the October and January cruises, the 
numbers decreasing during the spring cruises until April, when the maximum was 
reached. This and similar conclusions for other forms as to the maximum and min- 
imum occurrence is based mainly on a surface and a deep-water count for each area, 
but such counts were made at nine widely distributed areas on each cruise. 
Ostenfeld and others have pointed out that Skeletonema costatum is extremely 
euryhaline — that is, adapted to a great range of salinity. In Chesapeake Bay it has 
been found in waters from 11.13 to 32.00 per mille. The range of temperatures, 1.0° 
to 26.1° C. is about as extreme as that of the salinities. 
During the cruise of October, 1915, when the largest numbers were found for 
Skeletonema costatum, there was evidence to show that the highest counts were near 
the mouth of the Potomac River. The surface counts were as follows: Area A, 900; 
J, 4,300; M, 1,800; an unlettered area near the middle of the Maryland and Virginia 
line directly in front of the mouth of the Potomac River, 35,800; L, 4,200; P, 28,500; 
and R, 25,800. At 9 meters the counts for the same areas were these: Area A, 200; 
J, 6,700; M, no count; the unlettered area, 26,600; L, 10,500; P, 33,300; and R, 
