Light, nutrients and the fatty acid composition of stream periphyton

While the balance of light and nutrients is known to influence the food quality of herbivores by altering algal phosphorus and nitrogen content, the combined effects of light and nutrients on fatty acid synthesis in freshwater periphyton are relatively unknown. In this study, we manipulated light and phosphorus concentration in large, flow-through experimental streams to examine their effects on both elemental stoichiometry and fatty acid content in periphyton. Two levels of phosphorus (4 and 80μgL^-1) and three of light (17, 40, 110μmolphotonsm^-2s^-1) were applied in a factorial design in two separate experiments. Diatoms dominated periphyton communities in both experiments, comprising >95% of algal biovolume. Periphyton growth in the streams was simultaneously affected by both resources, even at low rates of supply. Periphyton C/P and C/N ratios increased with light augmentation and decreased with phosphorus enrichment, and consistent with the light:nutrient hypothesis (LNH). Light effects were strongest in streams with low phosphorus concentrations. Periphyton fatty acids reflected the dominance of diatoms:palmitic (16:0), palmitoleic (16:1ω7) and eicosapentanoic (20:5ω3) were the principal saturated (SAFA), monounsaturated (MUFA) and polyunsaturated fatty acids (PUFA), respectively. Linoleic (18:2ω6) and linolenic (18:3ω3) acids, characteristic of chlorophytes and cyanophytes, were rare, comprising <2% of total fatty acids. Periphyton fatty acid profiles were highly sensitive to light and phosphorus. The proportion of fatty acids comprised by SAFA and MUFA increased with light augmentation and decreased with phosphorus enrichment, whereas PUFA decreased with light and increased with phosphorus. Light effects on fatty acid composition were strongest in phosphorus-poor streams. PUFA declined with increasing light/phosphorus ratios in the streams, whereas 'energy' fatty acids (16:0 and 16:1) increased. The ratio of SAFA/PUFA was strongly and positively correlated with C/P and C/N ratios. SAFA and MUFA, normalised to dry mass, increased two- to threefold with increasing light, while PUFA normalised to dry mass was not significantly affected by light. Similarities in the responses of fatty acids and elemental stoichiometry to light and phosphorus treatments suggested that they were influenced by a common mechanism. Both components of food quality appeared to be sensitive to light-regulated rates of carbon fixation which, when coupled with insufficient supplies of phosphorus, caused diatom cells to store surplus carbon in SAFA, MUFA and other carbon-rich compounds that diluted both essential fatty acids and mineral nutrients.