Factors influencing depth distribution of soft bottom inhabiting Laminaria saccharina (L.) Lamour. in Kiel Bay, Western Baltic

Abstract

The kelp Laminaria saccharina dominates soft bottoms in 4–10 m depth in Kiel Bay. Experimental sporophytes transplanted to 2 and 5 m depth showed the typical annual growth pattern of Laminaria species. Surprisingly, 2 m plants died after the first resting phase, whereas 5 m plants survived and showed outgrowth of a new blade generation. Thalli at both depths were infected with the brown algal endophyte Strehlonema aecidioides, with host deformations being significantly stronger in 2 m plants. Growth rates of infected sporophytes were reduced. Exclusion of UV-light in 2 m depth resulted in less infected thalli. Discs excised from L. saccharina and cultivated in different photon fluence rates from 10–600 µmol m^-2 s^-1 did not differ in growth rate, photosynthesis or dark respiration. Hence, an exclusion of L. saccharina from shallow depths caused by high light cannot be concluded. We suggest the biological interaction with the endophyte S. aecidioides, amplified by UV light, to be most important for the exclusion of L. saccharina from shallow depths in the western Baltic.