Growth Strategies of Three Laminaria Species (Phaeophyceae) Inhabiting Different Depth Zones in the Sublittoral Region of Helgoland (North Sea)

On European coasts, Laminaria digitata does not inhabit the deeper kelp zone; this is dominated by L. hyperborea and (locally) L, saccharina. Sporophytes of three Laminaria spp. were cultivated from unialgal gametophyte cultures in the laboratory and transplanted into the field in February, at depths of 2,4.5 or 7 m below M.L.W.S. Near Helgoland, the deepest kelp (L, hyperborea) occurs at 8 m, and older plants of L. digitata are not found below 2 m depth. Photosynthesis was light-saturated in all three species, irrespective of cultivat.ion depth, at photon flux densities above about 150 pE m-2s-1 (corresponding to an irradiance of 30 W m-2). The experimental plants of L. digitata and L. saccharina produced much bigger blade areas in spring and summer at all depths than L. hyperborea. In July, the latter species completely ceased blade growth and L. saccharina reduced its growth rate very much. L. digitata, however, first reduced its growth rate in September (by about 50 */D), so that at the beginning of the dark season, from October onwards, its blades consisted of much younger tissue, resulting in a lower content of reserve materials per plant than in the case of the other two species. The long-persisting, high growth rate in L. digitata may be interpreted as adaptation to life in the sublittoral fringe; however, it obviously prevents the species from perennial colonization of the deepest parts of the kelp zone. The finding that the three Laminaria spp. reduced or ceased growth at different times during the year, under otherwise identical conditions of temperature, nutrients, light intensity or light quality, suggests the possibility that none of these factors actually trigger the seasonal growth behaviour of these species.