Vertical Circulation off Ross Ice Shelf — Thomas 
241 
TABLE 1 (continued) 
Station Ed-8. Off Cape Crozier. 77°29'S, 169°34'E. 0 600 February 28, 1956. Depth, 300 m. Algae present 
throughout water column. 
Coscinodiscus borealis 
few 
none 
none 
Coscinodiscus janishu 
few 
few 
few 
Coscinodiscus sp. A 
few 
none 
none 
Coscinodiscus sp. B 
common 
few 
few 
Corethron criophilum 
many 
common 
common 
Eucampia valasistum 
few 
none 
none 
Trigonium arcticum 
none 
none 
many 
Thalassiosira hyalina 
few 
none 
none 
Thalassiosira sp. 
few 
none 
none 
Vragilariopsis antarctica 
few 
few 
none 
Disephanus speculum 
common 
few 
few 
Peridinium depressum 
many 
none 
none 
Tomopteris sp. 
few 
none 
none 
Copepods 
common 
few 
many 
Nauplii 
many 
many 
many 
Veligers 
many 
none 
none 
In addition to the three stations mentioned 
above one (Ed-8) was taken 1 km off Cape 
Crozier, Ross Island (Fig. 1). Opportunity did 
not permit taking additional stations offshore 
along the same meridian, normal to the current. 
The populations shown in Table 1 were liv- 
ing when taken and examined at stations Ed-1 
and Ed-8. They were preserved for later exami- 
nation at station Ed-2. The presence of diatoms 
at depths of 200-300 m and deeper engendered 
a desire to inquire into the reason for this ap- 
parent phenomenon. 
The author is grateful to Dr. W. L. Tressler 
and to Mr. J. Q. Tierney, both of the U. S. Navy 
Oceanographic Office, for data and help in 
identifying biota. 
DISCUSSION 
According to Sverdrup (1953^) the thresh- 
old requirement in energy for Coscinodiscus 
excentricus is 0.002 lys. min" 1 . Schreiber (1927) 
calculated 0.00172 lys. min" 1 for Biddulphia 
mohiliensis, the lowest energy requirement for 
a diatom of which there is a record. No micro- 
meteorological data were taken. However, List’s 
tables (1951) show an albedo of 14% under 
average conditions similar to those which ex- 
isted off the Ross Ice Shelf at the time of ob- 
servation on January 21, with 0.383 lys. min" 1 
refracted into the sea. Considerably less energy 
reached Cape Crozier on February 28 — 1 month 
later. To determine the compensation depth, 
the coefficient of absorption for visible radia- 
tion is calculated from the Poole- Atkins (1929) 
equation: 
-k ~ 1.17. D" 1 
where D = transparency by secchi disk (in 
meters ) . 
— k — 4.2 X 10" 3 cm with a transparency 
of 4. 
The compensation depth is found from the 
equation: 
z = (l/-k) In (I z / 0 ) 
where I z = threshold requirement in energy 
(1.72 X 10"° lys. min" 1 ), and I G = energy re- 
fracted into the sea (3.83 X 10" 1 lys. min" 1 ). 
Then: z = —12.65 m (compensation depth). 
Because transparency in Antarctic waters is 
primarily a function of population densities, 
— k =: 4.2 X 10" 3 is valid only for the upper 
4 m. However, considering the density of popu- 
lations at lower depths, it is axiomatic that 
Corethron criophilum, for example, cannot be 
fixing carbon by photosynthesis at depths in ex- 
cess of 300 m. Still, at station Ed-1 this diatom 
and Coscinodiscus sp. A occurred throughout 
all sampled layers of the water column. At sta- 
