Do littorinids affect the survival of Ascophyllum nodosum germlings?

Abstract

The survival of germlings of the brown alga Ascophyllum nodosum on the west coast of Sweden were investigated in two field experiments. In the first experiment the role of Littorina fabalis, L. littorea, L. obtusata and L. saxatilis for the survival of 5 days old germlings were tested at two spatial scales, by comparing plots cleared of Littorina spp with plots holding the natural composition of the snails. The second experiment tested the effect of Littorina littorea and the exclusion of mesoherbivores other than littorinids on the survival of 13 days old germlings placed in cages. The treatments were cages with and without L. littorea and two mesh sizes (1.4 and 6.3 mm respectively), where the finer mesh was thought to exclude the mesoherbivores. There were no detectable effects of Littorina spp in the first experiment despite a low survival of the germlings. Significant differences in the survival of germlings were found between plots within a shore on a spatial scale of 2 to 30 m, but no significant differences were found between shores about 1 km apart. In the second experiment L. littorea had a negative effect on the survival of the germlings, and there was a lower level of survival of the juveniles in the cages with a larger mesh size compared to those with a finer mesh size. It is suggested that the difference in effect between the cages with different mesh sizes is due to the difference of accessibility for mesoherbivores of different sizes and that it is not caused by physical alterations by the cages. Our experiments showed that Littorina spp had no effect on the survival of germlings of A. nodosum unless in cages at abundances higher than those naturally occurring in the area at this time. Further, the experiments suggest that other mesoherbivores such as isopods and amphipods are important grazers on A. nodosum germlings.