A Preliminary Study of the Water. Loss of Laminaria digitata During Intertidal Exposure

THREE digitata species are found of Laminaria growing at Z . Port saccharina Eynon y L on . Clous the Gower tonii and coast, Z. digitata are ou d rowing at Port on on the ower coa t, where the present work was carried out. Both Z . saccharina and Z . Cloustoni live in deeper water than Z . digitata , although an occasional specimen of Z. Cloustoni may be found in deep pools within the Z. digitata zone. At Port Eynon, large parts of the L. digitata zone are exposed by the diurnal tidal oscillations, which is not the case with the other two species. Judging from the tidal drift, Z. saccharina is not as abundant as Z. Cloustoni . Thus, among the species of Laminaria growing at Port Eynon, the conditions arising from an intertidal exposure affect Z. digitata almost exclusively. The Z. digitata zone at Port Eynon is situated below the lowwater mark of neap tides, and in the lower part of the zone Laminaria is the dominant and only large alga. It grows only in rocky situations, and is abundant in the deep pools about Skysea Point. The duration of intertidal exposure varies with the extent of the tidal oscillation, being greatest during the new moon spring tides. Also, individual plants situated in different parts of the Laminaria zone will be exposed for varying lengths of time. Some plants are barely exposed at all, even during spring tides, while few are ever exposed for longer than two and a half hours at a time. Although Z. digitata may dominate over considerable areas of rocky substratum near low water, yet there is rarely a massing together of individuals to form dense communities, and thus 4 mutuality ' does not play a significant part as it undoubtedly does with Ascophyllum nodosum .