With the global nanotechnology market growing rapidly, nanomaterials are being increasingly released into aquatic environments, where they can undergo modifications and sedimentation, which will put benthic organisms at risk. Of particular interest is the study of nanomaterial effects on periphyton, a community of auto- and heterotrophic microorganisms embedded in an extracellular polymer matrix that covers submerged surfaces in aquatic ecosystems. Although periphyton assumes important functions in ecosystems, like primary production, little information is available on its sensitivity to nanomaterials and how these might be transferred to higher trophic levels in food webs. In the present thesis, effects of short- and long-term exposures to citrate-coated silver nanoparticles (AgNP, 35 nm) and silver ions (Ag+, dosed as AgNO3) were assessed on periphyton in microcosm studies for periods between 2 hours and 21 days. After exposure, primary production, secondary production, photosynthetic yield, microbial respiration and potential extracellular enzyme activity were measured. Moreover, addition of an aquatic snail, Physa acuta, feeding on periphyton allowed the assessment of effects of AgNP on an aquatic herbivore and trophic transfer efficiency. Contaminated (AgNP and AgNO3) and uncontaminated periphyton was offered to snails in a 5-day experiment to determine silver uptake and the mortality, feeding rate and reproduction of the snails. Our findings show that all endpoints measured in periphyton were affected by both AgNP and AgNO3 during short-term exposure, but AgNO3 was more toxic on a total silver mass basis than AgNP. Concentrations causing a 50% reduction in the different endpoints (EC50) ranged from 17 to 83 µM AgNP and from 2 to 4 µM AgNO3. Exposure of periphyton in the presence of silver ion ligands to complex Ag+ (ca. 1% of total silver in AgNP suspensions) prevented AgNP toxicity to most endpoints, confirming that the observed toxic effects were caused by the bioavailable Ag+. An exception was the extracellular enzyme leucine aminopeptidase, which was directly affected by AgNP. Concentrations of AgNP and AgNO3 that caused low toxicity during short-term exposure also caused effects on periphyton functions upon long-term exposure, with algae and bacteria being differently affected. Periphyton exposed to 10 µM AgNP for 21 days showed a 63% decrease in primary production and a 246% increase in bacterial secondary production per unit of total periphyton biomass. The community composition exposed to AgNP or AgNO3 differed from that of the control and also from each other. Furthermore, AgNP-exposed communities were more tolerant to subsequent AgNP exposure. Trophic transfer of silver from contaminated periphyton to snails occurred efficiently, independently of the silver form. Feeding rate of snails on AgNO3-exposed communities was 50% lower than on controls, and disturbance of digestion was evident upon feeding on both AgNP- and AgNO3-exposed communities. No changes in the mortality or egg production of the snails were observed. However, the hatching success of eggs from snails that fed on contaminated periphyton was reduced by more than 79%, and 28-day old embryos showed malformations. Overall, this study clearly identified damaging effects of AgNP on periphyton, with consequent indirect impacts on a consumer, trophic relationships and stream ecosystem processes.