Patterns of reproductive effort in the brown alga Ascophyllum nodosum

The pattern of allocation of reproductive biomass in the brown alga Ascophyllum nodosum (L.) Le Jol. was investigated in 2 populations on the Swedish west coast. An analysis of the demography of A. nodosum in these stochastic environments suggested 2 specific hypotheses about the reproductive allocation. First, the annual reproductive effort [annRE = reproductive biomass/(reproductive biomass + net annual growth)] should increase with increasing individual size, and second, the annRE should be higher in one of the populations, with a higher degree of environmental stochasticity. The study showed that the annRE was not significantly different between the populations and the overall mean was 53%. The study showed a significant increase of annRE from a mean of 33% for small individuals to 74% for large ones. It was also shown that this increase with size was a nonlinear relation. Combining these findings with those on the demographic behaviour of these populations allowed a discussion of the cost of reproduction. A small individual that grows has a better chance of survival than a small individual that reproduces and thus has a higher cost of reproduction than do larger individuals, for which the mortality risk is not reduced by allocating more resources to vegetative growth. For large individuals which allocate more than 70% of their net growth to reproduction the level seems to be high, but it is suggested that the high level is due to the stochastic recruitment pattern and the low cost of reproduction for large individuals. The variation in annRE decreased with increasing size of individuals which may point to a selection of 'optimal' individuals, but other factors such as the more diverse background of small individuals may also be important.