Summary The biochemical composition of primary food resources may affect secondary production, growth, reproduction and other physiological responses in consumers and may be an important driver of food‐web dynamics. Changes in land use, riparian clearing and non‐point nutrient inputs to streams have the potential to alter the biochemical composition of periphyton, and characterising this relationship may be critical to understand the processes by which environmental change can affect food webs and ecosystem function. We conducted a manipulative, in situ experiment to examine the effect of light and nutrient availability on stream periphyton biomass, nutrient content, stoichiometry and fatty acid composition. Greater light increased periphyton biomass [chl‐a, ash‐free dry mass (AFDM)], periphyton carbon concentrations and monounsaturated fatty acids (MUFAs), but decreased saturated fatty acids (SAFAs). Greater light availability also increased levels of <20C polyunsaturated fatty acids (PUFAs), but decreased quantities of several long‐chain (20–22 C), highly unsaturated fatty acids (HUFAs). Nutrient (+N, +P) addition had no significant effect on periphyton biomass in the study streams, but did increase periphyton carbon content. For fatty acids, despite non‐significant effects on periphyton biomass, nutrient additions resulted in an increased ratio of SAFA to PUFA, greater concentrations of stearidonic acid (18:4ω3) and near‐significant increases in α‐linolenic acid (18:3ω3). Nutrient additions also resulted in decreased quantities of the physiologically important docosahexaenoic acid (DHA; 22:6ω3) and increased the ratio of omega‐3 to omega‐6 fatty acids. An interaction between nutrient addition and greater light also resulted in decreased percentages of another physiologically important fatty acid, eicosapentaenoic acid (EPA; 20:5ω3). Results from this study suggest that increased light and nutrients, a condition common in urban and agricultural catchments, would increase 18C PUFA intermediates that would be available in periphyton food sources, but decrease the availability of essential 20‐22C HUFAs. If stream consumers are unable to elongate PUFAs into essential HUFAs, there may be important effects on food webs, including shifts by invertebrate consumers from herbivory to omnivory or carnivory, or changes in the species composition of aquatic consumers. Our results demonstrate that fatty acid measures were more sensitive to changes in light and nutrient conditions than more traditional measures of biomass, nutrient content and stoichiometry. We recommend routine measurement of fatty acid content in monitoring the effect of changing environmental conditions in rivers and streams, particularly studies on food quality and ecosystem health.

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In this study, a bioassessment was conducted to determine the effectiveness of best management practices (BMPs) implemented in farms in the Upper Delaware River watershed, NY (USA). Diatom and macroinvertebrate communities were analyzed across 17 low-order streams, designated as reference, BMP, or non-BMP. Streams lacking improvements (non-BMP) had significantly greater specific conductance, pH, TDP, NH 4 ? -N, and NO 3 --N than did reference streams. Diatom model affinity (DMA) values were significantly greater in reference and BMP streams than in non-BMP streams; non-BMP streams bordered on a ''severely impacted'' rating. The Trophic Diatom Index (TDI) varied two-fold among stream classes, with non-BMP [ BMP [ reference. TDI and DMA values were highly correlated, and both varied significantly with conductance, TDP, NH 4 ? -N, and NO 3 --N. Macroinvertebrate taxa, EPT richness, and Simpson's diversity did not differ significantly among stream classes. Macroinvertebrate metrics (HBI, Bioassessment Profile, Percent Model Affinity) varied by stream class, but none indicated greater water quality in BMP sites. Nonetheless, each correlated significantly with conductance and TDP in the directions predicted by each model. Our data suggest that diatoms are more sensitive to moderate increases in nutrients, conductivity, and pH in highgradient agricultural streams, and may be more useful in assessing stream management practices.