FACTORS INFLUENCING DEPTH DISTRIBUTION OF SOFT-BOTTOM INHABITING LAMINARIA-SACCHARINA (L) LAMOUR IN KIEL BAY, WESTERN BALTIC

The kelp Laminaria saccharina dominates soft bottoms in 4-10 m depth i n Kiel Bay. Experimental sporophytes transplanted to 2 and 5 m depth s howed the typical annual growth pattern of Laminaria species. Surprisi ngly, 2 m plants died after the first resting phase, whereas 5 m plant s survived and showed outgrowth of a new blade generation. Thalli at b oth depths were infected with the brown algal endophyte Streblonema ae cidioides, with host deformations being significantly stronger in 2 m plants. Growth rates of infected sporophytes were reduced. Exclusion o f W-light in 2 m depth resulted in less infected thalli. Discs excised from L. saccharina and cultivated in different photon fluence rates f rom 10-600 mu mol m(-2) s(-1) did not differ in growth rate, photosynt hesis or dark respiration. Hence, an exclusion of L. saccharina from s hallow depths caused by high light cannot be concluded. We suggest the biological interaction with the endophyte S. aecidioides, amplified b y UV light, to be most important for the exclusion of L. saccharina fr om shallow depths in the western Baltic.