The Botany of Iceland - 01.12.1912, Qupperneq 142

128
H.JÓNSSON
noted that, when the semi-littoral species occur in the upper littoral
zone, they are usuallv confined to the pools, a fact which proves
that they do not essentially belong to the upper liltoral zone.
b. The Semi-littoral Zone.
The upper boundary of this vegetation is just at the lower
boundary of the Fucus-be\l. The vegetation covers the lower littoral
zone, and extends, usually in the form of stragglers, below the limit
of low-tide, down between the uppermost extensions of the vegeta-
tion of the Laminaria-be\t, which in many places reach right up
to the limit of low-tide. The semi-littoral communities consequently
grow side by side with the Lamúia/'/a-associations below the limit
of low-tide, to a depth of about 10 metres; thev also occur as epi-
phytic vegetation on the Laminariœ at the deptli mentioned.
Thus, the semi-littoral zone is that between the lower edge of
the Fucns-belt and tlie Laminaria-belt. Tlie breadth of the zone
depends on the slope of the coast. If the coast is steep the zone
is narrow, but if the coast slopes gently, it is broad. In tliis zone
no single dominant community of extremely social and large species
occurs, like, for example, the Fncaceœ-community in the littoral
zone, and the Lani/nar/aceœ-community in the sublittoral zone. The
zone appears to lie too low for Fucaceœ and too high for La-
minariaceœ.
Green and brown algæ occur in abundance in the semi-littoral
zone, but the Rhodophyceœ are most richly represented. The semi-
littoral associations occur both laid bare during low-tide and also
continuously submerged to the depth mentioned.
It is easy to follow the upper boundary of the zone, even in
places where the Fucaceœ are absent. The lower boundary can also
be ascertained with comparative ease, if the large Laminariœ only
are followed, and if tlie observer is not led astray by the small La-
minariœ, which may occur in the depressions and pools of the
lower littoral zone.
The semi-littoral zone comprises several associations which play
a rather considerable part in the vegetalion.
By authors who describe the algal vegetation on coasts where
there is a change of lide these associations are generally considered
to belong partly to the littoral and partly to the sublittoral vegeta-
tion. On coasts where there is no tide, the littoral vegetation has,