ABSTRACT: The occurrence of microscopic algae that are endophytes and potential pathogens of kelps was quantified during 1994 in wild populations of Laminariasaccharina, L.hyperborea and L.digitata at Helgoland, North Sea. Sampling was designed to enable analysis of the influence of 4 fixed factors: species, date, wave exposure, and depth. Microscopic examination of, in total, 1224 thalli showed that the prevalence of infection by endophytic algae was 85%, much higher than was inferred by gross lesions alone. One tenth of the hosts, mostly L.saccharina, showed severe morphological changes, such as distorted stipes or a crippled lamina. One third showed weaker symptoms of endophyte disease, such as dark spots on the lamina or warts on the stipe. In most infected thalli, the infection was not evident macroscopically. Prevalence was high throughout the year with a minimum in spring. At a more exposed site, prevalence was higher and disease symptoms stronger than at a sheltered locality. Disease symptoms were more severe in shallower than in deeper water. Endophytes, mostly brown algae, were repeatedly isolated and identified in laboratory cultures. Endophytes were not strictly host-specific, but L.saccharina was predominantly infected by Laminarionemaelsbetiae, recently detected at Helgoland. This is the first epidemiological study comparing the prevalence and effects of kelp endophytes in different hosts at the same locality.

KEY WORDS: Disease · Kelp · Pathogen

Published in MEPS Vol. 146 (1997) on January 30
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1997