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

Abstract

Mortality soon after settlement is typically high and spatial distribution of germlings is usually very variable in seaweeds. This is the case for Ascophyllum nodosum (L.) Le Jol, which is a very common brown alga on sheltered rocky shores of the northern Atlantic. In this paper, the interactive effects of several factors on the survival of germlings of A. nodosum were investigated using an experiment carried out on the Swedish west coast. The general hypothesis that the combination of littorinid grazing, adult canopy and germling density affect the survival of germlings was tested. Two densities of zygotes were seeded onto small outplant discs in the laboratory. After 4 wk, the discs were transplanted onto the shore in plots, which were composed of all possible combinations of adult canopy and Littorinid grazing. Germling survival was calculated at 2 different points in time (after 5 and 23 d on the shore). After 5 d, there was no effect of grazers in the presence of canopy. In contrast, in areas cleared of canopy, survival was extremely low where grazers were present, but high where grazers were removed. At this stage, higher survival was observed at low density under an adult canopy and the opposite trend was observed in cleared areas. Mortality rates were not constant over time. In the presence of gastropods, instantaneous mortality rates were higher during the first 5 d than over the whole experimental period. Thus, the first period constituted the mast vulnerable phase for A. nodosum germlings. After 23 d, there was a trend far germling survival to decrease in the presence of grazers, regardless of other factors. This result suggests that grazers may affect the patterns of recruit distribution at later stages and the transition probabilities to macrorecruits.