SEA WATER
233
uniform salinity cooling in its surface layers, when the den—
sity of this Surface water increases and it sinks downwards.
Upwelling of water occurs when a deep current meets a
submerged bank or the shelving bottom of coastal regions :
it can also be brought about by persistent off—shore winds,
which blow the surface water outwards and this is replaced
by water from below (Fig. 48).
The upwelling of water
is of the greatest importance, for “the deep waters of the
sea
abound in those nutrient salts, the phosphates and
,
nitrates, and when water from deeper
to the
surface it brings with it '
.
_
-
-
00 shore uwnd
this
store of nutr1ment
»
,
/
to
replenish
the
—-»
—->
-—>
—>
—*
impoverished
surface ’
-
'
/
/
waters (see page 246).
_
<"
<—
".*"
"‘“
/%
This
is
why, in the
’l/I/Il/I/Il/Ill/IlljIlll/I/
A
'
region
of
submerged
"'/”"'
shelves and in Coastal
areas,
life is so abun—
'
j
_
dant.
_,
_)
—à
’
__
Apart. _from
these
:
great _c1rculatory cur—
.
.
rents in the» ocean there are other periodic movements
brought about by the attraction of the sun and moon on
-
the waters.
These are the tides. .
'
.
.
.
-
The mass of the moon eXerts an attracting force on the ,
particles on the earth. "This force is greater on those
particles which lie closest to the moon, but it is extremely
’
small, being only one ten—millionth of the earth’s pull on
those same particles.
Let us imagine the earth to be
Stationary and covered uniformly all over with water."
__
The pull of the moon on the water immediately beneath