Fatty Acids as Dietary Tracers at the Base of Benthic Food Webs

Abstract

Fatty acid (FA) analysis is a powerful ecological tool for examining trophic relationships among marine organisms. Its application in benthic food webs may be limited because many benthic organisms consume a highly mixed diet, and FA metabolism of benthic invertebrates may obscure dietary markers. This thesis examines the use of FA as dietary tracers for studying the diets of benthic invertebrates and the fate of primary production in rocky subtidal food webs. In Chapter 2, I review the use of FA for studying benthic marine food webs, and suggest that field studies using FA analysis should also include data from controlled feeding experiments, gut contents, or stable isotope analysis to provide more reliable results. In Chapter 3, I compare FA composition among sea urchins fed four natural algal diets in a controlled feeding study. These sea urchins substantially modified their dietary FA but differed in their overall FA composition according to diet. In Chapter 4, I use FA to trace the invasive alga Codium fragile ssp. fragile and the native kelp Saccharina longicruris through two trophic transfers in an experimental food web. Substantial signal attenuation occurred with each trophic transfer, suggesting that FA analysis may be of limited use for tracing benthic primary producers in field studies. In Chapter 5, I use FA in conjunction with stable isotope analysis and gut contents analysis to investigate the contribution of detrital kelp to the diet of sea urchins in habitats adjacent to kelp beds. FA analysis was unable to distinguish among sea urchins at different distances from the kelp bed, but the results of all analyses indicated that the availability of kelp detritus declines with distance from the kelp bed, and that sea urchins in low-productivity habitats rely on both kelp detritus and benthic diatoms. In Chapter 6, I summarize the overall findings and suggest experimental and statistical methods to address some of the problems associated with using FA analysis to study trophic relationships in benthic food webs.