The interactive effects of adult canopy, germling density and grazing on germling survival of the rockweed Ascophyllum nodosum

Mortality soon after settlement is typically high and spatial distribution of germlings is usually very variable in seaweeds. This is the case for Asc ophyllum nodosum (L.) Le Jol, which is a very common brown alga on sheltere d rocky shores of the northern Atlantic. In this paper, the interactive eff ects of several factors on the survival of germlings of A. nodosum were inv estigated using an experiment carried out on the Swedish west coast. The ge neral hypothesis that the combination of littorinid grazing, adult canopy a nd germling density affect the survival of germlings was tested. Two densit ies of zygotes were seeded onto small outplant discs in the laboratory. Aft er 4 wk, the discs were transplanted onto the shore in plots, which were co mposed of all possible combinations of adult canopy and Littorinid grazing. Germling survival was calculated at 2 different points in time (after 5 an d 23 d on the shore). After 5 d, there was no effect of grazers in the pres ence of canopy. In contrast, in areas cleared of canopy, survival was extre mely low where grazers were present, but high where grazers were removed. A t this stage, higher survival was observed at low density under an adult ca nopy and the opposite trend was observed in cleared areas. Mortality rates were not constant over time. In the presence of gastropods, instantaneous m ortality rates were higher during the first 5 d than over the whole experim ental period. Thus, the first period constituted the mast vulnerable phase for A. nodosum germlings. After 23 d, there was a trend far germling surviv al to decrease in the presence of grazers, regardless of other factors. Thi s result suggests that grazers may affect the patterns of recruit distribut ion at later stages and the transition probabilities to macrorecruits.