In many ecosystems, detritus is the dominant source of energy and the driver of ecosystem functioning. In particular, in forested headwater streams, allochthonous detritus (e.g. leaf litter, dead wood) constitute the main energy source for detritivores and living primary producers contribute marginally to ecosystem metabolism and energy flows. We hypothesised that a low consumption of benthic diatoms, a high‐quality resource, could be of major importance for the growth of detritivores. In particular, these resources might represent an essential source of polyunsaturated fatty acids (PUFAs). In a microcosm experiment, three food resources were manipulated: alder (Alnus glutinosa: Betulaceae) leaf litter, fungal mycelium and a common benthic diatom. They were offered to juveniles of Gammarus fossarum (Crustacea: Amphipoda) as food resources, either alone or in combination, with each resource type being enclosed in agarose pellets. Juveniles were fed for 5 weeks in controlled conditions. Survival, feeding and growth rates were monitored. The fatty acids content of food resources and gammarids were also quantified. Our results showed that detritus alone permits survival, but not the significant growth of detritivores. The presence of diatoms in food resources was necessary to ensure a significantly positive mass growth of detritivores over the 5‐week experiment. More importantly, detritivores that did not receive algae in their food were generally unable to maintain their PUFA levels when compared to juveniles collected in the field. Gut‐content analysis of field‐collected G. fossarum showed that low amounts of benthic algae were always visible, indicating that most individuals fed at least for a small part on benthic algae. These minor, but high quality, food sources might therefore be essential for ensuring the growth and survival of detritivores. Our results clearly highlight the need to consider the functional importance of such minor food sources.

SUMMARY1. Organisms commonly face multiple stresses in ecosystems, among which are toxic substances that add to natural ecological constraints such as insufficient resource quality. The combined effects of these stressors on species and ecosystems remain poorly investigated. 2. We hypothesised that feeding on high-quality resources in terms of phosphorus (P) content will improve the physiological state of consumers and thus increase tolerance to episodic metal stress by silver (Ag). We used a detritivorous crustacean from streams, Gammarus fossarum, to test this hypothesis. 3. The experimental design involved two phases. In the first phase, gammarids were fed ad libitum for 15 days to evaluate the effect of food quality on their energetic status. Gammarids were placed in laboratory microcosms and allowed to feed on two leaf species, alder and sycamore, that were colonised by aquatic fungi and experimentally enriched with P. At the end of this feeding period, we determined (i) food consumption, (ii) energetic reserves and (iii) physiological and behavioural states. In the second phase, gammarids previously consuming food of different quality were exposed to dissolved Ag (0, 0.5 and 1 lg L À1) for 4 days to investigate the physiological and behavioural responses of the consumers. 4. Elevated P concentrations in leaf litter increased the energetic reserves and locomotor activity of G. fossarum. Gammarids exposed to Ag had a low glycogen content, regardless of the Ag concentration and leaf P concentration, suggesting significant energy allocation to several aspects of organism maintenance (e.g. detoxification or compensatory mechanisms). In most cases, this investment appeared to prevent gammarids from carrying out lipid peroxidation (indicating cell membrane damage), except for those gammarids feeding on leaves with the highest P concentration. 5. Higher energetic storage (measured as glycogen content), related to higher elemental food quality, did not lead to a systematic increase in consumer tolerance to Ag, gammarids being unable to compensate for negative effects of Ag at the concentrations tested.

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