PLANKTON OF THE WOODS HOLE REGION 103 
the bay becomes colder than that ofthe Sound. That has an important effect on 
the plankton of Woods Hole, for as long as the bay remains warmer than outside 
waters all animals carried into it will survive and be carried through the passage 
during the flood tides. As soon as the temperature drops in the bay the tropical 
forms entering it will perish and not return to the Sound. Such a condition is very 
noticeable in the fall, when all Gulf Stream forms suddenly disappear from the 
plankton of Great Harbor, although they may be taken in abundance in the Sound 
or at Katama Bay throughout October and November. The arrival of southern 
forms is usually noted as soon at Woods Hole as in the Sound. 
In some parts of the eastern Atlantic a great many animals having a double 
breeding season are found. These forms appear in the spring and again in the fall. 
At Woods Hole examples of this group are limited to one phylum—the Ceelenterata. 
Figure 26 (p. 124) shows clearly that almost all Hydromedusz common to this region 
have been taken both in spring and fall, the spring swarm being the largest and 
lasting for the longest time. The Scyphomeduse rarely have this double period- 
ical appearance. During the past year, however, early ephyre of Aurelia flavidula 
were taken in November and again in April. The Ctenophora have been taken in 
small numbers throughout the winter, but usually swarm in the fall and early 
spring. 
I know of no permanent planktonic animals at Woods Hole having this biannual 
distribution. Certain copepods are most abundant in the fall or spring, but never 
at both seasons. Such forms are also plentiful in the winter and have been included 
in the winter plankton. Two species of importance belong to this group— Tortanus 
discaudata from December to June and Pseudodiaptomus coronatus from August to 
January. 
Under normal conditions the zooplankton, although varying considerably in 
its constituent parts, is always abundant at Woods Hole. The dominant winter 
or summer form of one year may be totally absent the next, but some other species 
usually takes its place. During the winter of 1899 and 1900 Temora longicornis 
formed the greater part of the plankton, while in the fall of 1922 and the spring of 
1923 hardly an adult specimen was found, the dominant species that year being 
Pseudocalanus elongatus. In the winter of 1923 the temperature of the water re- 
mained so high that neither of these winter species had appeared by December 29. 
Centropages hematus and Acartia bifilosa constituted the bulk of the collections. 
Temperature and weather conditions, no doubt, determine to what extent the north- 
ern forms pass south of Cape Cod and enter local waters. Almost no cold-water 
species were found in the early winter of 1923. Such diatoms as Rhizosolenia alata, 
Skeletonema costatum, and Dityliwm brightwelli appeared rarely or not at all, and 
even the cod apparently sought deeper water, for no young were taken in surface 
collections. 
Normal diatom maxima have no noticeable effect on the larger planktonic 
forms. When the unusually large swarms of phytoplankton appear, however, the 
zooplankton decreases rapidly and may even totally disappear for a time. Such 
conditions are often found during the summer maxima of Rhizosolenia semispina. 
Usually the winter maxima do not affect the largerforms. In the winter of 1922-23 
the phytoplankton and zooplankton were both abundant at the same time. At 
