PLANKTON OF THE WOODS HOLE KEGION 
105 
found in the appearance of certain oceanic forms, which will be discussed later. 
Extremely unusual physical factors may even eliminate a part of the cycle, but as 
soon as normal conditions are restored the progression continues as before. Such 
was the case in December, 1923, during an unusually warm period (fig. 11). Al- 
though quantitatively the winter diatom maxima remained approximately the same 
as in the previous year, qualitatively it was very different (fig. 10).^ 
To understand the seasonal distribution we must know something of the nature 
of the various forms that enter Great Harbor. The individual species of neritic 
phytoplankton are much more widely distributed than the zooplankton, and factors 
governing their appearance and disappearance are for the most part quite different. 
As in the case of land plants, the diatoms are able to form organic substances from 
Fig. 10. — Seasonal distribution of most abundant 
diatoms occurring in surface collections from 
June to December, 1922. Rhizosolenia ex- 
cluded. , Chaetoceros; . , Corethron 
raldiviae; — — , Ditylium brightwelU; 
Leptocylindrus danicus; , NHzschia 
seriata; — ... — , Skeletonema costatum 
Fig. 11. — Seasonal distribution of most abundant diatoms 
occurring in surface collections of 1923. Rhizosolenia ex- 
cluded. , Chaetoceros; , Corethron valdivice; 
— ... — , Ditylium brightwelU: , Leptocylindrus 
danicus: — • — , Nitzschia seriata; , Skeletonema 
costatum 
the various inorganic chemicals. Together with the littoral marine flora they form 
the basic source of food supply in the sea. Since all plankton animals are consumers 
and depend solely on the organic materials produced by the plants, the importance 
of the diatoms and the necessity for information regarding the sources of their 
production can not be overestimated. 
Obviously the two fundamental necessities for diatom growth are sunlight and 
food material. Secondary factors, such as temperature, salinity, necessary resting 
periods, etc., limit the geographical and seasonal distribution of certain species but 
do not usually affect the group as a whole. Physical conditions fatal to one species 
may be particularly favorable to another. Sunlight limits the vertical range of the 
species to the narrow zone penetrable by the light (photic zone) . This usually does 
not form a limiting factor of production in local waters or littoral plankton in general. 
' See paragraph 1, page 119. 
8242°— 25t — 
