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Terrestrial diatoms as tracers in catchment hydrology: a review

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Diatoms are remarkable organisms. They are present in almost all habitats containing water (e.g., lakes, streams, soils, bark) and rank among the most common algal groups in both freshwaters and marine ecosystems. The ubiquitous character of aquatic diatoms has triggered countless applications as environmental tracers for studies in water quality, paleoclimate reconstruction and sediment tracing. However, diatoms also occur in the terrestrial environment. It is this plethora of diatom life‐forms that has recently triggered interest in their use as tracers of hydrological processes. The use of diatoms in catchment hydrology has been very limited. Part of the reason is that until recently, the taxonomy and ecology of terrestrial diatom assemblages were largely unknown. However, in the past decade, much work has been done to quantify terrestrial diatom reservoir size, dynamics, and potential depletion following precipitation events. Therefore, such terrestrial diatoms now hold promise for use in catchment hydrology—for tracing runoff flow sources and pathways across a wide range of spatial scales. Here we review the literature on terrestrial diatoms and describe the various sampling protocols that have been designed and tested for specific applications in hydrological processes research. We review and summarize the work on terrestrial diatom reservoir characterization, transport mechanisms and pathways to show how such diatom‐based tracer work might be possible at the catchment scale for rainfall‐runoff studies. Finally, we present a vision for future work that might take advantage of terrestrial diatoms in catchment hydrology and discuss the main challenges going forward.

(a) Automatic sampler (ISCO Automatic sampler (Teledyne Isco, Inc., US)) installed in the Weierbach catchment (Attert River basin, Luxembourg) used to collect drifting diatoms; (b, c) common landscape surrounding the streams in the Attert River basin (Luxembourg); (d–f) soil diatom sampling rings in (d) agricultural, (e) grassland/pasture, and (f) forested areas; (g, h) soil sampling ring filled with sparkling water (view from top) and (i) digestion of organic matter with hydrogen peroxide (H2O2) and hydrochloric acid (HCl).
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Samples with high content of aerial species are in general dominated by smaller species, which lead to a decrease in the relative biovolumetric content (µm3).
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3D scatter plot of detrended correspondence analysis (DCA) based on diatom data from drifting samples collected during rain events. The three main geologies are structuring the three axis (i.e., Weierbach (WEI): schists, low pH; Upper Huewelerbach (HUE): calcareous sandstone, high pH; Wollefsbach (WOL): marls, moderate conductivity and pH).
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Detrended correspondence analysis (DCA) ordination using diatom species data from six distinct geological units collected with ISCO samplers ISCO automatic samplers (Teledyne Isco, Inc., US) during rainfall events in the Attert River basin, Luxembourg. Each point represents a diatom species. Groups were defined using an indicator species analysis based on the subcatchments. Catchment abbreviations: HUE, Upper Huewelerbach; SCH, Schwebich; WOL, Wollefsbach; WEI, Weierbach; COL, Colpach; and ROU, Roudbach.
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(a) Relative abundance (in %) of F. virescens (FFVI) as related to the percentage of aerial species and (b) pH of water (data from all seven‐nested catchments in the Attert River basin).
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Examples of diatom cells observed with different instruments: (a) light microscope images of species belonging to distinct genera and classes, collected in the Weierbach catchment (Attert River basin, Luxembourg); (b, c) show raw live material collected with ISCO samplers ISCO automatic samplers (Teledyne Isco, Inc., US) following a rainfall event. Alive specimens (with chloroplasts) showing solitary and colonial habits; (d) water sample after digestion of organic matter showing ‘cleaned’ frustules in a permanent slide ready for observation and counting with a light microscope; (e, f) scanning electron microscope (SEM) images showing valves under high magnification for accurate identification of species.
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