Interactive Impacts of Silver and Phosphorus on Autotrophic Biofilm Elemental and Biochemical Quality for a Macroinvertebrate Consumer.

Autotrophic biofilms are complex and fundamental biological compartments of many aquatic ecosystems. In particular, these biofilms represent a major resource for many invertebrate consumers and the first ecological barrier against toxic metals. To date, very few studies have investigated the indirect effects of stressors on upper trophic levels through alterations of the quality of biofilms for their consumers. In a laboratory study, we investigated the single and combined effects of phosphorus (P) availability and silver, a re-emerging contaminant, on the elemental [carbon (C):nitrogen (N):P ratios] and biochemical (fatty acid profiles) compositions of a diatom-dominated biofilm initially collected in a shallow lake. We hypothesized that (1) P and silver, through the replacement of diatoms by more tolerant primary producer species, reduce the biochemical quality of biofilms for their consumers while (2) P enhances biofilm elemental quality and (3) silver contamination of biofilm has negative effects on consumers life history traits. The quality of biofilms for consumers was assessed for a common crustacean species, Gammarus fossarum, by measuring organisms' survival and growth rates during a 42-days feeding experiment. Results mainly showed that species replacement induced by both stressors affected biofilm fatty acid compositions, and that P immobilization permitted to achieve low C:P biofilms, whatever the level of silver contamination. Gammarids growth and survival rates were not significantly impacted by the ingestion of silver-contaminated resource. On the contrary, we found a significant positive relationship between the biofilm P-content and gammarids growth. This study underlines the large indirect consequences stressors could play on the quality of microbial biomass for consumers, and, in turn, on the whole food web.

High stoichiometric food quality increases moulting organism vulnerability to pollutant impacts: An experimental test with Gammarus fossarum (Crustacea: Amphipoda).

Headwater organisms are most often simultaneously faced with multiple stressors such as low resource quality and pollutants. Higher food quality has been hypothesized to enhance the tolerance of organisms to pollutants, but the interactive effects of food quality and pollutants on species and ecosystems remain poorly studied. To better understand these interactive effects, we experimentally manipulated the phosphorus (P) content of two leaf litters with contrasted carbon quality (alder and maple). During four weeks, individuals of the detritivorous crustacean Gammarus fossarum were exposed to low levels of cadmium ([Cd] = 0, 0.35 or 0.70 µg L-1) while being fed with one of the leaf P treatments. When organisms were not exposed to Cd, their high survival rate was more driven by the carbon quality of the resource (litter species) than by its stoichiometric quality. In contrast, their number of moults and growth rates were primarily increased by the P content of resources. When exposed to Cd, G. fossarum survival rate was reduced, but this effect was largely magnified by a higher P level in resources. Our results showed that despite positive effects of resource stoichiometric quality on organism life history traits (growth, survival), a resource of high stoichiometric quality might be detrimental for organisms exposed to low and environmentally realistic levels of pollutants. Two non-exclusive hypotheses are proposed to explain these results. First, organisms fed on the highest quality resource exhibited the highest moulting frequencies (moults being the most critical life cycle step of arthropods), which could have rendered them more sensitive to pollutants. Secondly, the metabolism of organisms fed on higher quality resources was potentially enhanced, increasing the uptake of dissolved Cd by gammarids. This study suggests that species sensitivity to pollutants might be underestimated in ecosystems facing both nutrient constraint and pollutants.