Effects of density on the vital rates of a modular seaweed

The effects of density on the vital rates of the modular seaweed Ascophyllu m nodosum (L.) Le Jol. were investigated in 2 areas of the northern Atlanti c Ocean: the Isle of Man, in the Irish Sea, and Tjarno, on the Swedish west coast. The densities of plants (genets) were manipulated in the field, The impact of density on survival, growth and reproduction of plants > 2 cm wa s estimated over 1 yr. Percentage survival of A. nodosum was high and appar ently density-independent. This result is in accordance with other studies in land and marine systems where low mortality after establishment appears common and overpopulation is avoided by a balance in module density. The gr owth of A. nodosum was negatively density-dependent. The response of the mo dules (shoots) within plants was asymmetrical. Furthermore, the inception o f new shoots was enhanced at low densities on the Isle of Man. Apart from i ntraspecific competition, other density-dependent processes affected the pe rformance of the alga; the probability of breakage to smaller sizes increas ed with density in the Swedish area, As a result, the impact of density on the net plant growth was more patent at Tjarno than on the Isle of Man, In relation to reproduction, the results of this study suggest a positive effe ct of density on the annual reproductive effort of the alga. Nevertheless, the manipulation was carried out after the initiation of reproductive struc tures, Increasing density clearly reduced plant growth, and reproduction is size-dependent in A. nodosum. This indicates that, in successive reproduct ive periods, reproduction may be negatively affected by density. In contras t to the results of another previous work, our study shows a negative impac t of density on the net growth of A, nodosum and indicates the importance o f module length structure in the demographical processes of the species. No t only the effect of density but also the significance of density dependenc e in the dynamics of the species probably vary in these areas with very dif ferent physical domains.