The Botany of Iceland - 01.12.1912, Síða 41

MARINE ALGAL VEGETATION
27
2. THE OCEAN.
As regards the ocean, the chief points are its movements, tem-
perature and salinity.
A. The Movements of the Ocean.
These are — tides, waves and currents. All these movements
of the ocean are of very great importance to the life of the algæ.
a. Tides. By the alternate rise and fall of the tide a part of
the shore is laid bare, and the vegetation growing there must then
be capable of maintaining life in the air for a longer or shorter
period. Those plants which grow liighest up in the zone thus left
dry, are exposed during the greater part of the period between the
one ílood-tide and the next, or for about 10—11 hours in every 12.
The plants occurring lowest down in the zone, on the other liand,
are not exposed during spring-tide for more than one hour in every
12, and they are submerged the whole time during neap-tide. Tlie
upper limit of the algal vegetation is, moreover, dependent on how
high the tide rises, i. e. the height of the flood-tide.
The Heiglit ofthe Flood-tide. The following data regarding
the height of the ílood-tide are taken froin “Den islandske Lods”
(1903) and from the alterations and additions to it which have been
published. The height of the flood-tide is greatest in SW. Iceland
and least in E. Iceland. The height of the flood-tide at spring-tide
is recorded as being about 14 feet from Reykjavík (SW. Iceland),
10—11 feet from NW. Iceland, 5—572 feet from the north coast
and 5 feet from E. Iceland.
In many places there is a great difference as regards the height
of the flood-tide during the spring and neap tides. To illustrate
this more fully I give the following figures from some localities on
the different parts of the coast: —
South Iceland, Vestmannaeyjar . . .
Eyrarbakki........
South-west Iceland, Reykjavík . . . ,
— Stykkishólmur
North-west Iceland, Dyrafjörður ..
North Iceland, Akureyri..........
East Iceland, Djúpivogur.........
Spring-tide Neap-tide
8—10 feet 4 feet
10 - 6 -
cir. 14 - 4 -
12 - 6 -
11 - 5-6 -
5l/2- F/2-
7V2- 2l/í-
From Elliðaey near Stykkishólmur in SW. Iceland the height
of the flood-tide during spring-tide is recorded as being 14 feet and