Intraspecific variation in the phlorotannin content of the brown alga Ascophyllum nodosum

Many brown macroalgae contain substantial concentrations of polyphenolic compounds, known as phlorotannins. Previous studies have shown that much of the variation in phlorotannin concentration is correlated with taxonomy and biogeography, but little is known about patterns of variation within species and populations. In this study, we examined intraspecific variation in the phlorotannin content of the intertidal seaweed Ascophyllum nodosum, by testing for differences within as well as between genetic individuals (genets) and populations, and between algal tissues of similar age from juveniles and adults. The results show that variation in the phlorotannin content of annual shoots within primary shoots and genets contributes substantially to the total intraspecific variation. This pattern probably indicates that the phlorotannin content is a highly plastic trait that can be significantly influenced by small-scale variation in extrinsic factors. There were also significant differences in mean phlorotannin content among study sites, and these differences varied significantly between the two study years. The extensive variation within and among genetic individuals and populations shows that it is crucial to use proper sampling designs in field studies comparing phlorotannin levels between different populations, taxa, or experimental treatments, in order to avoid false conclusions. Phlorotannin levels were significantly higher in adult annual tissue than in juveniles in five of six sites, with an overall mean phlorotannin content of 7.8% of dry mass for adult tissue and 5.9% of dry mass for juveniles. This result may be explained by a trade-off between growth and phlorotannin production and a lower mortality risk for larger individuals, leading to a strategy where it is more advantageous for a juvenile to invest in growth than to allocate resources to chemical defence.