HIGH PREVALENCE OF INFECTION BY ENDOPHYTIC BROWN-ALGAE IN POPULATIONSOF LAMINARIA SPP (PHAEOPHYCEAE)

The occurrence of microscopic algae that are endophytes and potential pathogens of kelps was quantified during 1994 in wild populations of L aminaria saccharina, L. hyperborea and L. digitata at Helgoland, North Sea. Sampling was designed to enable analysis of the influence of 4 f ixed factors: species, date, wave exposure, and depth. Microscopic exa mination of, in total, 1224 thalli showed that the prevalence of infec tion by endophytic algae was 85 %, much higher than was inferred by gr oss lesions alone. One tenth of the hosts, mostly L. saccharina, showe d severe morphological changes, such as distorted stipes or a crippled lamina. One third showed weaker symptoms of endophyte disease, such a s dark spots on the lamina or warts on the stipe. In most infected tha lli, the infection was not evident macroscopically. Prevalence was hig h throughout. the year with a minimum in spring. At a more exposed sit e, prevalence was higher and disease symptoms stronger than at a shelt ered 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 h ost-specific, but L. saccharina was predominantly infected by Laminari onema elsbetiae, recently detected at Helgoland. This is the first epi demiological study comparing the prevalence and effects of kelp endoph ytes in different hosts at the same locality.