31
There is, however, another factor which appears to be of
considerable importance.
This ts the effect of distribution
of activity in the water with depth on the activity in the
plankton sample.
The plankton sample is taken from 4 water
layer at a depth of 200 meters to the surface.
The water sam-
ples are taken at discrete depths down to 150 meters.
The fol-
lowing uncertainties sre therefore inherent in the sampling
method.
First, there is not a uniform decrease of activity
with depth (Figs. 8 and 9).
There are, indeed, changes with
depth which range from a sharp decrease from the surface to
25 meters to the other extreme where there are sharp increases
at a derth of 100 metars.
are taken at discrete
Secondly,
since the water samples
intervals of 25 to 50 meters,
the pre-
sent data do not permit ruling out the possibility that extremes of activity in narrow regtons distort the total picture
at a given station elther by being missed completely or being
up-
li-
overemphasized.
And thirdly,
there {8 no guarantee that the
plankton is uniformly distributed throughout the depth which
is sampled by the plankton net.
There
is,
for example,
some
evidence that a change in the vertical distribution of the
plankton population occurs from hours of daylight to hours of
darkness.
The diurnal-nocturnal difference in ratio of activ-
ity in surface plankton (material on filter papers) to that in
the surface water is
significant at stations 1-30, which for
the most part show a sharp decrease in activity in the water
from the
surface to 25 meters,
and no subsequent increase.
No
such significant difference was found at the remaining stations,