The Botany of Iceland - 01.12.1912, Side 113

MARINE ALGAL VEGETATION
99
10. The Community of Rhodymenia.
11. The Polysiplionia urceolata-association.
12. The Community of Corallina.
13. The Crustaceous Alga-association.
c. The Sublittoral Zone.
14. The Community of Laminariaceæ.
15. The Desmarestia-association.
16. The Deep-water Community of Florideæ.
17. The Lithothamnion-association.
18. The Community of Crustaceous Algæ.
B. The Sea-grass Vegetation.
1. The Zostera-association.
A. The Marine Algal Vegetation.
a. The Littoral Zone.
The Littoral Communities. The littoral vegetation is com-
posed of several communities. These are left dry during low-tide,
with the exception of the pool-vegetation; but the period during
wliich the different communities are left dry varies greatly. I am
not prepared to state accurately the period of exposure, but I pre-
sume that the lowest littoral communities will be laid bare for
about 1—2 hours under normal conditions, while the uppermost
connnunities will be submerged i'or about one liour, and will he
laid bare for about 11 hours. On exposed coasts the period of ex-
posure is shortened by the beat of the waves. The ecological factors
differ not a little in the uppermost and lowermost part of the littoral
zone, and because of this difference the vegetation is divided into
longitudinal belts along the coasts. The littoral vegetation falls na-
turally into three divisions: the light-loving or strictly littoral com-
munities, the shade-Ioving communities, and the pool-vegetation.
In the following description of the vegetation, the communities in
eacli of the three zones are arranged as far as possible according to
depth, and in such a way, that the uppermost come first and the
lowest come last. This rule, however, cannot always be adhered to.
aa. The Photophilous or strictly Littoral Communities.
These communities are found in places in the littoral zone
which are directly exposed to light during low-tide.
They are composed of blue-green, green, brown and red algæ; the
7*