174 
BULLETIN OF THE BUREAU OF FISHERIES 
6. The arm of Cape Cod forms a permanent northern barrier for the southern 
neritic plankton but only a summer barrier for northern pelagic species. 
7. The proportion of benthonic animals occurring in the plankton of this 
region is much greater than that found in normal littoral plankton. After north- 
east storms Buzzards Bay types predominate; after southerly storms Vineyard 
Sound types are most plentiful. This is particularly noticeable in the case of 
amphipods. 
8. No correct impression of the relative abundance of the local benthonic 
fauna can be obtained from surface collections unless the distribution of each of 
these animals in the bay and sound is completely understood. 
9. A distinct periodicity in the occurrence of all the common animals of the 
region is clearly noticeable. The succession of species remains almost the same 
each year, the only variation being in the time of their appearance and disappear- 
ance. 
10. The planktonic animals of the region, with one exception, may be placed 
in two general groups — the summer community and the winter community. The 
coelenterates are the exception. For the most part these have a long spring maxi- 
mum and a short one in the fall. 
1 1 . The pelagic diatoms exert a very great influence on the zooplankton. When 
the greatest maxima appear most of the zooplanktonic forms disappear. There 
are possibly two reasons for this. First, the common species having these swarm- 
ing periods do not form the food of the zooplankton so far as I have been able to 
determine. During the maxima of the larger diatoms the smaller members of this 
group which are eaten by pelagic animals disappear, causing a scarcity in the 
food supply. This may account for the similarity in the time of disappearance of 
the larger forms and the small diatoms. Second, the great numbers of the diatoms 
filling the water apparently cause conditions unfavorable for animal life of any 
3ort. The macroplankton seems to be literally choked out. This, however, is 
hardly probable, and is offered merely as a possible explanation. 
12. Conditions favoring the increased production of one species of diatoms 
are also favorable for many others, provided that one does not become so abundant 
that almost all others disappear. The summer maximum often exemplifies the 
latter condition. The winter swarm usually consists of many species in which 
various forms predominate at different times. 
13. My observations on the distribution of pelagic diatoms lead me to disagree 
with the theory that all production takes place in deeper waters off the coast, the 
species occurring in the littoral waters being the result of winds and tides. Such 
factors no doubt account for the distribution of the various species, but the quan- 
titative distribution can not always be explained on that basis. 
All evidence points clearly to the fact that great production of floating diatoms 
takes place at the mouths of rivers where the largest amount of drainage from the 
land is emptied into the coastal waters. Peck's observations in Buzzards Bay also 
indicate that the greatest swarms are found where the greatest outwash from the 
land occurs. Buzzards Bay is a great reservoir in which pelagic diatoms accumu- 
late and multiply, and as a result the swarms carried into Great Harbor are often 
exceedingly large. 
