Nutrient Enhancement Increases Performance of a Marine Herbivore via Quality of Its Food Alga

Herbivore performance and reproductive output are expected to covary with qualitative changes in the host plant. The availability of nutrients may be especially important for the quality of algae as food through its effect on both primary and secondary metabolism and within-plant variation. We evaluated the effect of the nutrient enrichment of the brown alga Fucus vesiculosus on the fecundity, mass gain, intermolt duration, and food consumption of females of the herbivorous isopod Idotea baltica. We reared the isopods on algae grown either under enhanced nutrient conditions or as controls for the entire duration of the intermolt preceding mating and oviposition. The herbivores fed on nutrienttreated algae achieved higher growth rates, consumed more food, and laid more and larger eggs than the controls. The nutrient treatment increased the amount of insoluble sugars and reduced both the total carbon content in the alga and the physical toughness of the thallus. On the other hand, the treatment had no effect on the amount of total nitrogen or phlorotannins in the algal tissue. This implies that the clear increase in herbivore performance and fecundity may be due to quantitative changes in the carbohydrate metabolism rather than to the lower content of feeding deterrents in the tissue of F. vesiculosus. We suggest that eutrophication may potentially improve the quality of F. vesiculosus for herbivores. The reproductive success, growth, and food consumption of I. baltica increase on algae maintained in a nutrient-rich environment; this is likely to be reflected in herbivore density and grazing pressure. This in turn may have potential ecological consequences for the maintenance of viable F. vesiculosus belts, thereby contributing to the structure of littoral algal assemblages in marine ecosystems.