The Botany of Iceland - 01.12.1912, Síða 42

28
H. JÓNSSON
during neap-tide 7 feet, and it is also recorded from the latter place
that the low-water at neap-tide lies about 3 feet above that at
spring-tide.
When extraordinary conditions prevail the height of the flood-
tide may be still greater; thus, 18—19 feet has been recorded from
Reykjavík and 18 feet from Stykkishólmur.
From this it is seen that the height of the ílood-tide varies
greatly, which must necessarily affect the aigal vegetation in several
respects, especially as concerns the upper limit of its growth. Thus
there is a great difference in the highest flood-mark (at spring-tide)
and in the lowest flood-mark (at neap-tide). Tlie tidal wave or the
tide, moreover, shows irregularities, because neither the highest
flood-mark (spring) nor the lowest flood-mark (neap-tide) is constant.
The limit of the flood-tide fluctuates perpetually between a high-
mark and a low-mark. The liigh-mark or the highest limit of the
flood-tide is seen distinctly on rocky coasts from the action of the
sea on the rock. On flat coasts, the high-mark can be distinguished
by detached algæ and various otlier bodies which accompany the
tidal wave and remain at the highest level reached by the water.
This high-mark lies considerably higher than the upper limit of the
algal vegetation.
The low-mark is not as easy to distinguish as the high-mark,
but it will almost coincide with the upper limit of the Pelvetia-
Fucus spiralis association. Above this the Bangia association (JJlo-
thrix, Bangia, and others) is found, the extreme limit of which will
almost coincide with an average water-level which, however, does
not lie midway between low-mark and high-mark, but rather nearer
the former. The upper limit of the algal vegetation thus lies some-
what above the limit of the flood-tide at neap-tide. The same rule
holds good, of course, on a very exposed coast. That the place is
exposed means that there is a heavy swell, which causes the sea
to rise higher up on the coasts, both at neap-tide and at spring-tide.
By the upper limit of the marine algal vegetation is meant that
boundary line above which marine algæ do not occur in the form
of associations; on the other hand, no account is taken of single
individuals or groups of individuals being found higher up, in
crevices or pools, as such an occurrence must be considered accidental,
because they are carried up to this height with the high water or
by far-reaching breakers. They do live, of course, but do not thrive,
and have evidently gone beyond their real area of distribution. In