EFFECTS OF NUTRIENT AVAILABILITY AND HERBIVORY ON POLYPHENOLICS IN THE SEAWEED FUCUS-VESICULOSUS

Research seeking to explain intraspecific variations in plant phenolic s has focused on two general paradigms, the resource availability (car bon/nutrient balance) and induced-defense models. We experimentally te sted both hypotheses to explain changes in phlorotannin concentrations in the brown alga Fucus vesiculosus. Fucus was collected monthly from two estuarine sites (Cape Cod, Massachusetts, USA) differing in nitro gen availability, and analyzed for polyphenolic levels and tissue-nitr ogen concentrations. In situ nutrient enrichment experiments were cond ucted to measure changes in polyphenolic levels related to N availabil ity. Simulated grazing experiments examined the possibility of induced increases in polyphenolic concentrations in Fucus vesiculosus. Paired choice experiments conducted in the laboratory examined feeding selec tivity by the snail Littorina littorea for either Fucus population. Fi eld measurements and experiments revealed temporal and site-related ch anges in tissue constituents. Polyphenolic concentrations were consist ently, sometimes two times, higher in Fucus vesiculosus from the low-N site compared with algae from the high-N site; tissue-N content was h igher in the population from the high-N site. For both populations, ti ssue N was inversely correlated with polyphenolic concentrations. Only Fucus from the low-N site showed a significant reduction in polypheno lic concentrations under experimental enrichment. Regression analysis revealed a significant inverse relationship (R2 = 0.71) between phenol ic concentrations and growth rates for Fucus at the high-N, but not th e low-N site. This suggests a growth cost associated with phlorotannin production for this phenolic-poor population due, perhaps, to C-limit ed growth. Phlorotannin concentrations increased significantly in clip ped Fucus at the low-N site in two of three simulated grazing experime nts, indicating an inducible response. We did not find a significant i nducible response for the high-N, phenolic-poor population, and our ch oice experiments revealed a clear preference for this population by Li ttorina littorea. Our results offer support for both the induced-defen se and carbon/nutrient balance hypotheses as explanation for variation s in phlorotannin concentrations in Fucus vesiculosus. We suggest that within-species variation in polyphenolics is due to a complex interac tion of environmental (nutrient availability, irradiance levels) and d efense-related (grazing activity) factors.