The Botany of Iceland - 01.12.1912, Blaðsíða 110

96
H. JÓNSSON
—2 fathoms, down to a depth of 20 fathoms; and finally the
elittoral region stretches from the 20-fathom contour downwards.
Tliis division, unchanged in ils main features, is generally employed.
The boundary between the littoral and sublittoral regions is, I think,
commonly supposed to be rather sharplj7 defined in places with
tides, and less sharply defined where tides do not occur.
By this division into tliree regions the algal Benthos is divided
into three belts of different depth. Allhough the boundary lines thus
drawn may be described as floristic boundaries, as regards many
species, yet they cannot always be regarded as natural limits of
vegetation. If natural limits of vegetation are to be drawn, several
factors must be taken into consideration. From depth-records, pure
and simple, a somewhat clear idea may naturally be formed of the
conditions of light, but not of the salinity and warmth. If the
conditions of light, salinity and warmth in those layers of water
where the algal vegetation lives were known all the year round it
would be easy to characterize the limits of the vegetation. Tlie
great importance of the salinity to algal vegetation is well-known
and is emphasized by Rosenvinge (66), Svedelius (71), Börge-
sen (12) and Kylin (45), amongst others.
The ecological factors in the coastal waters round Iceland are
not so well known that I am able to draw the natural limits of
the vegetation by means of them. My starting point is, therefore,
the vegetation itself, and from the appearance of the vegetation it
is possible, to a certain extent, to form an opinion as regards the
ecological factors, in tlie same way as an opinion may be formed from
these concerning the appearance and composition of the vegetation.
The marine algal vegetation divides itself into several zones as
the Benthos does in fresh water. The divisions between the zones in
the sea are very distinct: some species seem to be spot-bound or
very sensitive to changes of level; other species may occur in two
or several zones; but it depends especially, I think, on the quality
of the water, the intensity of the light and, where there are tides,
on the period of exposure (in the littoral zone).
By studying the vertical distribution of the species and asso-
ciations, I have come to the conclusion that the marine algal vege-
tation may be divided into three zones almost parallel one with an-
other: the Littoral Zone, the Semi-Iittoral Zone and the Sublittoral Zone.
The Littoral Zone understood in a more restricted sense is
identical with the upper littoral zone and extends almost to the