Natural Nanoparticles and Colloids in Forested Streams Across Europe: Seasonal Patterns and Impact of Soil Groups

Abstract

The element loss from forested ecosystems is largely reflected by the so-called dissolved phase (<450 nm) in stream waters. However, natural colloids (1 nm–1 μm), including natural nanoparticles (NNP, 1–100 nm), constitute a major but largely unexplored part of this dissolved phase. We hypothesized (a) that there is a systematic seasonal variation in colloidal concentrations across European catchments, and (b) that the colloidal concentrations are mainly controlled by site conditions, that is, prevalent reference soil groups. To test these hypotheses, the base flow of seven stream waters was sampled from a transect across Europe from Northern Finland to Portugal at monthly intervals for 1 year. Three colloidal size fractions (1–20 nm, >20–60 nm, and >60 nm) were identified using field flow fractionation coupled with inductively coupled plasma mass spectrometry and an organic carbon detector for the size-resolved detection of Al, Si, P, Ca, Mg, Mn, Fe, Zn, and organic C. Colloids accounted for up to 90%–100% of total element concentrations, emphasizing the importance of colloids for element cycling. Element concentrations showed seasonal patterns in stream waters. However, these patterns differed between the catchments with distinct soil groups: the catchments with dystrophic terrestrial soils exhibited peak flows in colloidal element concentrations during spring, whereas those with eutrophic terrestrial soils did during summer. By contrast, catchments with Gleysols showed season-unrelated single peaks of high element export. Peak concentrations were also influenced by the climatic regime, which in turn impacted colloid abundance.