STUDIES IN SUBLITTORAL ECOLOGY

III. LAMINARIA FOREST ON THE WEST COAST OF SCOTLAND; A STUDY OF ZONATION IN RELATION TO WAVE ACTION AND ILLUMINATION

¹ From the Department of Zoölogy, University of Bristol, England

1. On the shores of Carsaig Island, Scotland, the sublittoral region is densely forested with laminarian seaweeds down to a depth of at least 15 meters, and probably down to a much greater depth.

2. At the upper margin of the sublittoral region, exposed to air at low water of spring tides, there is a characteristic "sublittoral fringe."

3. The dominant brown algae of the sublittoral fringe have pliable stipes, so that they lie flat at low water and escape desiccation. The dominant brown algae of the true sublittoral region have tall erect stipes, which hold up the fronds to the light.

4. The Laminaria canopy at depths of 1-6 meters cuts off about 99 per cent of the available light. At 6-2 meters the forest is less dense, and relatively more light penetrates. There is therefore a sharp decline in illumination between the sublittoral fringe and the Laminaria forest just below it; but within the forest the illumination changes much less over a considerable range of depth.

5. Wave action is considered to be maximal in the sublittoral fringe, and to decrease gradually with depth.

6. The undergrowth-forming algae fall into two clearly defined groups—one group confined to situations of high illumination, such as the sublittoral fringe and the upper parts of Laminaria stipes; and the other to dark places, such as the rock surface in the depths of the forest and the lower parts of the Laminaria stipes.

7. The undergrowth of the Laminaria forest is practically uniform in composition within a vertical range of about 12 meters, and probably more.

8. Vertical distribution is determined chiefly by wave action in the case of one laminarian alga, two polyzoa, one hydroid, and probably other organisms.

9. It was found that on artificially denuded areas new Laminaria Cloustoni plants grew to form a forest 1 meter high in 12 months.

10. It is concluded that whereas the steeply graded zonation of the littoral region is to be ascribed in the main to desiccation, the more gentle zonation of the sublittoral region depends on illumination and on wave action.