Seasonal variation in the relationship between growth rate and phlorotannin production in the kelp Ecklonia radiata.

Most theories for the evolution of plant chemical defences assume that defences are costly. In order to investigate the potential cost of phlorotannin production in the brown alga (kelp) Ecklonia radiata, I measured the correlation between changes in phlorotannin levels and growth rates for E. radiata in two seasons, spring and autumn. In spring, when both phlorotannin levels and growth rates in E. radiata were highest, there was a significant negative relationship between changes in phlorotannin levels and growth, consistent with a cost of phlorotannin production. No relationship was evident in autumn, indicating that moderate levels of phlorotannins and growth could be maintained simultaneously. These and related results suggest that the cost of defence will vary as a function of (1) endogenous patterns of growth or metabolite production and (2) variation in available resources. Evidence for a cost of phlorotannins in E. radiata adds to the paradox of the maintenance of high levels of phlorotannins in Australasian brown algae.