Lipids and fatty acids were analyzed in the herbivorous minnow (Campostoma anomalum) and its predominant food (periphyton) in three streams in eastern Tennessee (East Fork Poplar Creek, Hinds Creek and White Oak Creek). Fatty acid markers were used to explore the trophic relationship between the fish and its algal food. C. anomalum are lipid-rich fish, with a high potential for uptake of hydrophobic pollutants. The mean lipid content of the minnows was 6.5 % of the wet weight in the carcasses, and 26.5 % of the digestive tract. The concentrations of polychlorinated biphenyls (PCBs) in the fish carcasses and guts were directly proportional to the lipid contents of these body portions. Microscopic examination of periphyton in the guts, and the analysis of fatty acid markers showed that green algae dominated periphyton ingested by the minnows in East Fork Poplar Creek (∼90 % of the ingested algal biomass was from chlorophytes). The fatty acid profiles of the minnow guts (including their gut contents), and their lipid stores in East Fork Poplar Creek also reflected extensive ingestion of green algae indicated by an abundance of a fatty acid marker α-linolenic acid (18:3ω3), at 13 % of the total fatty acids. A relatively low biomass of diatoms, and a correspondingly low concentration of a fatty acid characteristic of diatoms (eicosapentaenoic acid, 20: 5ω3) occurred in periphyton from both Hinds Creek and White Oak Creek. Nevertheless, taxonomic and lipid analyses of the minnow gut contents showed that in White Oak Creek diatoms constituted a disproportionately large share of the minnows' diet. The discrepancy in fatty acid profiles between the portion of periphyton in this stream and the gut contents of C. anomalum suggests that the minnows do not consume all portions of the periphyton equally. These results emphasize the applicability of fatty acid analysis to reveal trophic linkages in aquatic ecosystems.
