Effects of Nutrient Availability and Herbivory on Polyphenolics in the Seaweed Fucus Versiculosus

Abstract

Research seeking to explain intraspecific variations in plant phenolics has focused on two general paradigms, the resource availability (carbon/nutrients balance) and induced—defense models. We experimentally tested both hypothesis to explain changes in phlorotannin concentrations in the brown alga Fucus vesiculosus was collected monthly from two estuarine sites (Cape Cod, Massachusetts, USA) differing in nitrogen availability, and analyzed for polyphenolic levels and tissue—nitrogen concentrations. In situ nutrient enrichment experiments were conducted to measure changes in polyphenolic levels related to N availability. Stimulated grazing experiments examined the possibility of induced increases in polyphenolic concentrations in Fucus vesiculosus. Paired choice experiments conducted in the laboratory examined feeding selectivity by the snail Littorina littorea for either Fucus population. Field measurements and experiments revealed temporal and site—related changes in tissue constituents. Polyphenolic concentrations were consistently, sometimes two times, higher in Fucus vesiculosus from the low—N site compared with algae from the high—N site; tissue—N content was higher in the population from the high—N site. For both populations, tissue N was inversely correlated with polyphenolic concentrations. Only Fucus from the low—N site showed a significant reduction in polyphenolic concentrations under experimental enrichment. Regression analysis revealed a significant relationship (R2 = 0.71) between phenolic concentrations and growth rates for Fucus at the high—N, but not the low—N site. This suggests a growth cost associated with phlorotannin production for this phenolic—poor population due, perhaps, to C—limited growth. Phlorotannin concentrations increased significantly in clipped Fucus at the low—N site in two of three simulated grazing experiments, indicating at inducible response. We did not find a significant inducible response for the high—N, phenolic—poor population, and our choice experiments revealed a clear preference for this population by Littorina littorea. Our results offer support for both the induced—defense and carbon/nutrient balance hypotheses as explanation for variations in phlorotannin concentrations in Fucus vesiculosus. We suggest that within—species variation in polyphenolics in due to a complex interaction of environmental (nutrient availability, irradiance levels) and defense—related (grazing activity) factors.