Effects of density on the vital rates of a modular seaweed

The effects of density on the vital rates of the modular seaweed Ascophyllum nodosum (L.) Le Jol. were investigated in 2 areas of the northern Atlantic Ocean: the Isle of Man, in the Irish Sea, and Tjärnö, 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 was estimated over 1 yr. Percentage survival of A. nodosum was high and apparently 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 growth of A. nodosum was negatively density-dependent. The response of the modules (shoots) within plants was asymmetrical. Furthermore, the inception of new shoots was enhanced at low densities on the Isle of Man. Apart from intraspecific competition, other density-dependent processes affected the performance of the alga: the probability of breakage to smaller sizes increased with density in the Swedish area. As a result, the impact of density on the net plant growth was more patent at Tjärnö than on the Isle of Man. In relation to reproduction, the results of this study suggest a positive effect of density on the annual reproductive effort of the alga. Nevertheless, the manipulation was carried out after the initiation of reproductive structures. Increasing density clearly reduced plant growth, and reproduction is size-dependent in A. nodosum. This indicates that, in successive reproductive periods, reproduction may be negatively affected by density. In contrast to the results of another previous work, our study shows a negative impact of density on the net growth of A. nodosum and indicates the importance of module length structure in the demographical processes of the species. Not only the effect of density but also the significance of density dependence in the dynamics of the species probably vary in these areas with very different physical domains.