246 . ’
.
THE SEAS
'
which; reach maturity during the course of the summer
_
months do so' at the expense of the great owering of
dñfting plants that occurred in the spring.
These for
some reason die down in the summer, _to be followed by a
smaller and shorter outburst in thé autumn.
Toexplain
these Seasonal changes we must
back still further, and
nd the causes for these curious maxima of plant abundance.
It is common knowledge that to obtaingood -crops'on
land we must manure the ground; of rstimportance in
manure for plant growth arè these chemical bodies, the
phosphates and nitrates. 'Sèa water contains these in
7
minute quantities in solution.
..
At the beginningof
and indeed throughout the previous winter months, these
'
salts are present in greatest quantities, but it is not until
;
the spring that the
diatoms are.able to utilize this manure
to any great extent; for plants are dependent for» their
activities upon light, and during the winter the sun’s
rays are very feeble, and owing to the sun’s low 'altitude
little of its light penetrates into the water and much is
reected from the surface.
In the spring months howèver
the sun monnts in the sky. and its light increases in strength.
The sun—’s energy now . “ becomes availableîor the plants’
activities and they start to grow; 'With the'large-available
,
fund of manurial salts the growth is rapid ; but the food
supply is not inexhaustible and within 'a month it is almost
…
all used up
(Fig.
_
52) and the great crop perforce dies
down.
Through0ut the summer months there may be
very. small sporadic outbursts of plant life where the Water
has perhaps been -te'mp_ÿorarily enriched in nutrient material
b_yñ the presence of large shoals Of animals ”whose excreta
manure the surrounding water, but the renewed vigour of.
,
the diatoms in the autumnstill remains to be explained.
The cause of this has been found -to lie in the physiCal
properties of the sea. water. .During the hot
.
summer