The separate and interactive effects of light intensity and nutrient availability on the growth, protein content, concentration of C-based secondary metabolites, and susceptibility to herbivory of two brown seaweeds, Dictyota ciliolata (Dictyotales) and Sargassum filipendula (Fucales), were assessed in the context of the carbon/nutrient balance (CNB) hypothesis. Responses of Dictyota and its terpenoid secondary metabolites were often at variance with predictions of the CNB hypothesis while several responses of Sargassum and its phenolic secondary metabolites were often predicted by the CNB hypothesis. Findings for these. and other, seaweeds parallel recent terrestrial studies suggesting that the CNB hypothesis rarely predicts responses of terpene producing plants, but more commonly predicts responses of phenolic producing plants. In a microcosm experiment, nutrient addition significantly increased Dictyota growth but had no effect on concentrations of sterols, proteins, terpenes, or susceptibility to herbivores. In a field experiment, nutrient addition did not affect growth or protein levels, generally increased terpenoid metabolites, but did not affect the susceptibility of Dictyota to herbivores. Increased light in outdoor microcosms resulted in more growth, reduced terpene concentrations, and increased protein content in Dictyota. The high-light plants with more protein and less terpenes were preferentially consumed by the sea urchin Arbacia punctulata, but were eaten significantly less readily by the amphipod Ampithoe longimana. In a field experiment, higher light intensity increased growth but had no effect on secondary metabolites, protein content, or the alga's susceptibility to herbivores. For Sargassum filipendula, plants that settled from spores and grew naturally in an experimentally shaded environment had lower concentrations of phlorotannins than less-shaded plants, but this difference did not affect the plants' susceptibility to amphipod grazing. Branches of S. filipendula transplanted to a heavily shaded environment grew less than branches transplanted to a less shaded environment (i.e., light limited growth); however, the concentration of protein and phlorotannins, and the susceptibility of plants to amphipod grazing did not differ. Nutrient addition had no effect on protein or phlorotannin concentrations, but significantly increased the palatability of plants in the higher light treatment. Although the CNB hypothesis has been influential and provides a theoretical template for predicting how various stresses will affect plant defenses and susceptibility to herbivory, ecologists need to consider additional factors in predicting the responses of plants to environmental variation.
