Environmental and spatial factors drive diatom species distribution in Alpine streams: Implications for biomonitoring

Abstract

Given their documented capacity to track changes in environmental conditions and human alterations, benthic diatom communities are at present widely used in biomonitoring programs to evaluate stream water quality. However, it is becoming more and more evident that species ecological preferences are not the only drivers of diatom community composition, since dispersal-related processes also play a role. This is particularly compelling in Alpine streams, where orographic conformation and humanrelated impacts limit dispersal of organisms. In addition, several environmental variables may influence diatom community in pristine or impacted sites. We here investigate the differential role of environmental and spatial factors in driving the community assemblages of diatoms in streams of the Eastern Italian Alps, focusing on both taxonomic and functional composition. We analysed data from 110 samples collected on two different geological substrates, i.e. calcareous and siliceous, during the last eight years of biomonitoring programs, among which 64 collected in reference sites and 46 in impacted sites. We first evaluated whether diatom communities in reference and impacted sites are differentially shaped by environmental and spatial factors, highlighting the major role of spatial constraints in both of them. In particular, anthropogenic disruption of longitudinal connectivity in streams likely shaped impacted communities, as demonstrated by the increasing abundance of motile 2 taxa, which are associated with physical disturbance. Conversely, reference communities were mostly affected by spatially structured environmental variables, especially those related to streambed lithology. We then compared the taxonomic and functional composition of diatom communities between the two geological substrates in both reference and impacted sites to better highlight the differential role of this factor. Our results demonstrate that lithology strongly drives diatom community composition in reference but not in impacted sites, confirming our previous observations. The analysis of functional traits, however, highlighted how differences were due not only to the geological substrates, but also to other environmental variables, like flow velocity. Overall, the effect of the spatial component on the structure of diatom assemblages can represent a background noise in the framework of the river quality assessment, and this should be taken into account especially in those countries, like Italy, covering a broad range of mountain areas.

Diatom communities and ecological status classification in the upper Po River basin

“…Among the latter, TP and SRP were included in the ordination diagram, unlike the diagram obtained for the Alpine macrotype. The higher variance explained by the two shared components in more pristine sites agrees with our previous findings in the Eastern Alps (Falasco et al, 2019), while the different role of phosphorus between the two macrotypes can be explained by the high occurrence of low values that is particularly marked in R-A2.…”

“…In recent years, many studies demonstrated that not only environmental local factors but also spatial ones may play an important role in diatom species distribution and abundance. Indeed, the spatial component has been recognised as one of the most important drivers for diatom communities especially in mountain areas, where geographical and topographical processes may influence the species dispersal (Dong et al, 2016;Piano et al, 2017;Falasco et al, 2019). Although the influence of spatial factors increases with geographical distance (Heino et al, 2010), it can be high also at the regional scale (Bottin et al, 2014).…”

Planktic diatom responses to spatiotemporal environmental variation in high‐mountain tropical lakes

“…Lake environmental variables explained ~30% of the total variance of diatom community composition. The proportion of explained variance is low when compared to other types of fresh waters, such as rivers in north‐east Spain (Leira & Sabater, 2005), but similar to other diatom datasets from mountain regions, such as the Alps and Pyrenees (Falasco et al., 2019; Rivera‐Rondón & Catalan, 2020). Environmental variability affecting diatom communities also is a function of the independent effects of space (i.e., distance among sites), as indicated by the variance partitioning analyses, particularly connectivity to other water bodies in the heterogeneous glacial landscape (Zaharescu et al., 2016).…”

A Digital Light Microscopic Method for Diatom Surveys Using Embedded Acid-Cleaned Samples

“…Diatoms are unicellular microalgae characterized by ornate silica shells (frustules) composed of two thecae, silicate half-shells fitting into each other, and their end plates (valves) often displaying species-specific morphologies. Diatom communities quickly respond to environmental changes and are thus widely used in freshwater ecosystem research and ecological quality assessment [3][4][5].…”