PROGRESS IN BIOLOGICAL IliTQUIRIES, 1923. 19 



Great Harbor, the salinity averaging about 31.5 throughout the year. 

 Water temperature is the dominant factor in governing the seasonal 

 distribution and the breeding periods 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. 



As in the case of the bottom-living animals, the plankton of this 

 region is made up of a complex of faunas. The region about Woods 

 Hole forms the northern limit of many southern species, the southern 

 limit of many northern species, and a pocket where oceanic animals 

 blown in by strong southerly winds are deposited. The arm of Cape 

 Cod forms a permanent northern barrier for the southern coastal 

 plankton, but only a summer barrier for northern pelagic animals. 

 A distinct periodicity in the occurrence of all connnon animals of 

 the region is clearl}' noticeable. The succession of species remains 

 the same each year, the only variation being in the time of their ap- 

 pearance and disappearance. The planktonic animals of the region, 

 with one exception, may be placed in two general groups — the sum- 

 mer comnninity and the winter community. The coelenterates form 

 the exception : for the most part these have a long spring period of 

 maximum frequency and another short one in the fall. 



Three general conditions affect the appearance of the pelagic ani- 

 mals — winds, tides, and the food supply. Salinity forms barriers in 

 some localities, but not at Woods Hole. Once introduced into the 

 region, the organisms remain until the temperature becomes un- 

 favorable or the food supply is exhausted; then they must leave or 

 perish. Food is also an important factor in causing the disappear- 

 ance of a species during a period of favorable physical conditions. 



Temperature governs the breeding seasons of the plankton and 

 benthonic animals of this region. The temperature prevailing at 

 the time of the extrusion of the eggs is not often the important factor, 

 for the eggs are usually thrown off as soon as ripe provided the con- 

 ditions are not too unfavorable. After being deposited in the waters 

 the existing temperature plays a controlling part in determining 

 whether the incubation period will be long or short. The determina- 

 tion of an early or late breeding season depends on the temperature 

 at some previous date when a warming or cooling of the water started 

 the development of the sex products. These facts must be considered 

 when interpreting the appearance of certain larvae in the plankton. 



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 will probably be found here also. 



A full report of the work of the year, compiled with complete 

 records of plankton collections by the late Yinal IST. Edwards cover- 

 ing the years 1893-1907, inclusive, is nearly completed. 



ECOLOGY or FRESH-WATER LAKES. 



Cooperation with the Wisconsin Geological and Natural History 

 Survey in important investigations of the fundamental conditions of 

 fish life in lakes has been continued. These studies are directed by 

 Dr. E. A. Birge, president of the University of Wisconsin, and the 

 work was executed by Chancey Juday and Dr. H. W. Rickett, assisted 



