PLANKTON OF THE WOODS HOLE REGION 107 
layer, which is differentiated from the deeper water masses by a lesser density and 
higher temperature. In seasons when there is great outwash from the land the 
neritic diatoms often form great swarms. In localities where upwelling or vertical 
circulation takes place under these conditions the surface layers, with their flora, 
are blown away from the shore and replaced by infertile water drawn up froni the 
bottom layers. The outwash of this fertile water is very favorable to the offshore 
plankton but causes a diminution of diatoms near the coast, the few that were not 
carried out having adverse conditions to combat. An inshore wind, on the other 
hand, heaps up the surface waters and is conducive to luxuriant plant growth. 
Repeated investigations (Gran, 1912; Nathansohn, 1909; Leder, 1917) of this prob- 
lem have confirmed the belief that often the rapid appearance and disappearance of 
diatom maxima is notso much a biological question as a hydrographical one. Gran 
and Nathansohn in 1909 observed, ‘‘We find an intensive plant life, and conse- 
quently also an intensive animal life, everywhere at the surface of the sea where an 
influx of water masses takes place, which has not, or at least has not immediately 
previous, served as a source of nourishment for phytoplankton.” 
Sometimes a diatom society is found in summer in the lower strata, with its 
higher density and lower temperature, which was present in the surface waters 
earlier in the spring. Such conditions are common among the zooplanktonic forms 
and are occasionally found among the diatoms. Miss Ogilvie found the same dia- 
toms in the lower strata off the south coast of Ireland in August as were present at 
the surface from January to April. This is an indication that certain neritic forms, 
which are apparently periodical in their occurrence, might remain as permanent 
members of the plankton if conditions of existence were more uniform. This is 
interesting in view of the fact that many investigators have considered that a resting 
period (spore formation) is a necessary part of the existence of truly littoral species. 
It is impossible in local waters accurately to determine the real relationship of 
the local conditions of existence and the development of the diatoms, because the 
currents often cause variations much greater than those actually due to conditions 
of existence. Gran (1912), realizing this, substituted a study of the rate of growth 
as a measure of production in place of quantitative chemical analysis of food mate- 
rials present in the water. In the vicinity of Woods Hole, where the currents are 
unusually strong, the production would have to take place at an extremely high rate 
in order to maintain itself were it not for the many “‘pockets”’ of quiet water which 
are supplied with abundant land outwash. In certain less protected sections of the 
coast this may be an important factor in the sudden disappearance of certain species. 
As soon as the rate of production declines the species is unable to maintain itself, and 
this inability to replace the numbers carried away by the currents may cause the 
maxima to disappear long before the food supply is exhausted. 
In dealing with the conditions of production it is very important to know just 
where the production of floating forms takes place before attempting to explain 
their appearance or disappearance. Two theories are now held. One contends 
that all production of pelagic neritic diatoms takes place off the coasts, the sudden 
swarming in inland’rivers and small bays being the result of tides and winds. The 
second theory is that production also takes place within certain limits in inland 
waters. To be sure, winds may blow quantities of diatoms into open harbors and 
