Grazing and nutrients reduce recruitment success of Fucus vesiculosus L. (Fucales: Phaeophyceae)

Coastal eutrophication has resulted in the reduced abundance of the perennial macroalga Fucus vesiculosus L. due to negative effects of increased turbidity, spatial competition, and grazing. We conducted a factorial field experiment to explore the main and interactive effects of three factors on the recruitment success of F. vesiculosus. We manipulated (1) substrate occupancy by offering unoccupied substrates and substrates occupied by a natural assembly of macroalgal spores, periphyton and loose material, (2) nutrient availability by exposing substrates to ambient and enriched nutrient conditions, and (3) herbivory by exclosing the natural herbivore community and enclosing either small (Hydrobia spp.) or large (Theodoxus fluviatilis L.) snails. Microrecruit density of F. vesiculosus depended on the density of parent algae in ambient nutrient availability, whereas, under enriched conditions, such dependency disappeared. Microrecruit density was lower on previously occupied than on unoccupied substrates and at enriched rather than ambient nutrient levels. The lowest recruitment success resulted from a combination of these two treatment levels. The microrecruit density of F. vesiculosus decreased sharply with the density of filamentous algae indicating that competition with opportunistic species contributed to poor recruitment success. Microrecruit density did not differ between the grazer exclosure and the natural grazer community treatment level reflecting locally low grazer densities and consequent weak grazing pressure in the study site. However, the enclosure of snails at higher densities that still naturally occur in the nearby areas affected the recruitment success of F. vesiculosus. This effect was dependent on nutrient availability; at ambient levels, snail grazers had only a minor negative effect on microrecruit density whereas under enriched conditions, both snail species negatively affected recruitment success. Thus, instead of reducing competition and thereby benefiting the perennial slow-growing F. vesiculosus, negative grazing effects by both snail species increased under nutrient-enriched conditions.