Trophic effects on the maintenance of biodiversity in the Fucus zone
Abstract: Ecologically important macroalgae with large thalli and complex structures are threatened worldwide due to anthropogenic activities such as pollution. In the northern Baltic Sea the salinity is very low; the only large algae producing complex structural habitats found there are the fucoid species Fucus radicans and F. vesiculosus. This thesis investigates the importance of these two species for the diversity of the faunal communities living in algal vegetation. The trophic effects of high nutrient supply and grazers on the survival, facilitation, and diversity of F. vesiculosus were also studied. Fucoid vegetation generally harboured a higher biomass and different composition of associated fauna m–2 than did vegetation types without fucoids. Through the occurrence of Pilayella littoralis, high nutrient supply had indirect negative effects on early stages of F. vesiculosus survival; these effects were exerted by both the presence of and exudates from P. littoralis. Another indirect effect of high nutrient supply was higher epiphytic load, but this did not have any impact on the growth of F. vesiculosus. Instead, grazers had a positive effect by reducing the biomass of fouling epiphytes, though the identity of the grazer species was crucial in this. Furthermore, when investigating the effect of grazers on the epiphyte biomass it became evident that only the presence of two species with complementary feeding habits effectively prevented overgrowth. The biodiversity of the epiphytes was affected by both nutrient level and grazing pressure, which indicates that the effect of grazers on algal diversity depends on resource supply. This thesis demonstrates that fucoids are important for hard bottom fauna, and that F. vesiculosus (particularly at early life stages, and to a lesser extent as adults) is sensitive to high nutrient loads. Furthermore, it was evident that specific species of grazers, i.e., Theodoxus fluviatilis, were able to facilitate the growth of this alga.
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