Trade-offs between phlorotannin production and annual growth in natural populations of the brown seaweed Ascophyllum nodosum

1. The potential cost of polyphenolic (phlorotannin) production in the brown seaweed Ascophyllum nodosum was investigated by testing for phenotypic trade-offs between phlorotannin content and annual growth. Relationships between tissue nitrogen and phlorotannin content, as well as between tissue nitrogen content and annual growth, were also examined to evaluate the importance of the carbon-nutrient balance in explaining intraspecific variation in phlorotannins. 2. Annual shoots were collected from a total of 300 A. nodosum individuals in natural populations at three sites in each of two study areas, in 2 consecutive years. 3. Data were analysed using analyses of covariance (ANCOVAs), which showed that there was a significant negative relationship between phlorotannins and growth in both study areas in both years. Shoots with a relatively high phlorotannin content (> 9% dry weight) had a mean reduction in growth that varied from 25% to 54%, compared with shoots with relatively low concentrations (< 6% dry weight) of phlorotannins. 4. Predictions about variation in the cost of carbon-based chemical defences were supported by regression analyses with subsampled data sets that revealed that the trade-off between phlorotannins and growth was most pronounced in individuals with relatively high tissue nitrogen levels. 5. The relationship between tissue nitrogen and phlorotannin content, as well as between nitrogen content and growth, was weak and variable. 6. Production of phlorotannins in A. nodosum under natural conditions appears to involve a substantial cost in terms of individual growth. This cost varies spatially and temporally, at least partly as a consequence of variation in nutrient availability. 7. Tissue nitrogen content is not strongly related to phlorotannin concentration of A. nodosum individuals, in contrast with findings for a few other seaweed species.