104 
BULLETIN OF THE BUREAU OF FISHERIES 
this time RMzosolenia alata was the dominant diatom. In 1923-24 Nitzschia seriata 
occurred in such abundance that the zooplankton disappeared almost entirely from 
November 16 until February 1. During this period top and bottom collections ia 
the shallow water of the bay and sound yielded nothing but diatoms. The zoo- 
plankton was found to be fairly abundant in the deeper waters at the western end 
of the Sound. As soon as the diatoms declined in numbers the larger forms returned 
to the shallow water. Figures 8 and 9 show the relative abundance of the zooplank- 
ton and phytoplankton in 1922 and 1923. 
DIATOMS AND OTHER PLANTS 
The diatoms of this locality may be divided into two great groups — the pelagic 
and the bottom forms. In certain parts of Great Harbor the bottom diatoms are 
very abundant, and often large numbers occur in surface collections after storms or 
particularly strong winds. As no fresh-water streams of importance are found in 
the vicinity of Woods Hole to carry the various chemicals needed for diatom pro- 
Fig. 8. — Relative abundance of zooplankton and Fig. 9. — Relative abundance of zooplankton and phyto- 
phytoplankton in surface collections from May plankton in 1923. — — , zooplankton; — , phyto- 
to December, 1922. , zooplankton; plankton 
— , — phytoplankton 
duction, much essential material must be carried through the " Hole " from Buzzards 
Bay by the strong currents. For that reason bottom diatoms were found to be 
more abundant in eddies and pockets about the entrance of the bay than elsewhere 
in Great Harbor and not scattered about evenly on the bottom in shallow water, 
as might otherwise be expected. 
Together with the dinoflagellates, the pelagic diatoms make up the greater part 
of the phytoplankton of the region. On all but two occasions the former were far 
outnumbered by the latter. In every haul made during the year with a No. 20 net 
diatoms were found. They had regular seasonal variations which were very similar 
to those of previous years (figs. 10 and 11). There is a regularity in the quantita- 
tive variation as well as in the qualitative. The maximum of one year may be 
larger, smaller, earlier, or later than that of another, but the basic characteristics 
of the rhythm remain for the most part unchanged. An exception to this rule is 
