PLANKTON OF THE WOODS HOLE REGION 175 
14, Temperature is the dominant factor in governing the seasonal distribution 
of all local pelagic animals. It also determines whether oceanic species entering 
the region shall perish at once or live long enough to become an important factor 
in the local fauna. Three general conditions cause the appearance of the pelagic 
animals—winds, tides, and the food supply. Salinity forms barriers in some locali- 
ties, but not at Woods Hole. Once introduced into the region, the organisms remain 
until the temperature becomes unfavorable or the food supply is exhausted, and 
then they must leave or perish. Food is also an important factor in causing the 
disappearance of a species during a period of favorable physical conditions. This 
is probably the limiting factor of the summer diatom season. Temperature governs 
the breeding seasons of all planktonic and benthonic animals of this region. The 
temperature prevailing at the time of the extrusion of the eggs is not often the im- 
portant factor, for the eggs are usually thrown off as soon as ripe, provided the 
conditions are not too unfavorable. After the eggs have been deposited in the 
waters the existing temperature plays a part in determining whether the incubation 
period will be long or short. The determination of an early or a late breeding 
season, then, depends upon the temperature at some previous date when a warming 
or cooling of the water started the development of the sex products. This fact 
must be considered when interpreting the appearance of certain larve in the 
plankton. 
15. Reactions to changes of temperavire are tur one cuvse part More evident 
among planktonic animals than among benthonic forms. Bottom dwellers, par- 
ticularly sessile forms, in order to maintain themselves must be able to withstand a 
great range of temperature. Unusually low temperatures often kill large numbers, 
but as a rule both the larve and adults are extremely hardy. This is not true in the 
case of planktonic forms. Certain species, such as Calanus finmarchicus, although 
preferring cold water, are able to stand sudden rising or falling temperatures and 
appear to survive as well in water of 22° C.as at 0° C. Most pelagic animals, how- 
ever, particularly the phytoplankton, disappear as soon as the temperature condi- 
tions become unfavorable. 2 
16. The annual distribution of the diatom maxima of the American coast is 
very similar to that of the eastern Atlantic waters in that the seasons of the greatest 
swarms retreat farther and farther from the warmest months as one approaches the 
Tropics. A similarity in the seasonal variation in Kuropean and American waters 
of the same latitude is particularly noticeable, conditions at Woods Hole correspond- 
ing to those in the Adriatic Sea. The great effect of the arm of Cape Cod on the 
local plankton is again evident here, for within 20 miles of Massachusetts Bay, 
with conditions similar to the Norwegian Sea, conditions comparable to those of the 
Mediterranean and Adriatic Seas are found in Buzzards Bay. 
17. The distribution of the plankton of the western Atlantic coast is little 
understood, and the number of animals new to the region taken during the past 
year indicates that most of the eastern Atlantic coast pelagic species probably will 
be found here also. 
