Hydromorphologic Sorting of In-Stream Nitrogen Uptake Across Spatial Scales
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Hydromorphologic Sorting of InStream Nitrogen Uptake Across Spatial Scales Ute Risse-Buhl,1* Christine Anlanger,1,2 Christian Noss,2,3 Andreas Lorke,2 Danielvon Schiller,4 and Markus Weitere1 1
Department River Ecology, Helmholtz Centre for Environmental Research GmbH – UFZ, Bru¨ckstraße 3a, 39114 Magdeburg, Germany; 2 Institute for Environmental Sciences, University of Koblenz-Landau, Fortstrasse 7, 76829 Landau, Germany; 3Federal Waterways Engineering and Research Institute, Kussmaulstrasse 17, 76187 Karlsruhe, Germany; 4Department of Evolutionary Biology, University of Barcelona, Av. Diagonal 643, 08028 Barcelona, Spain
ABSTRACT Nitrogen (N) uptake is a key process in stream ecosystems that is mediated mainly by benthic microorganisms (biofilms on different substrata) and has implications for the biogeochemical fluxes at catchment scale and beyond. Here, we focused on the drivers of assimilatory N uptake, especially the effects of hydromorphology and other environmental constraints, across three spatial scales: micro, meso and reach. In two seasons (summer and spring), we performed whole-reach 15N-labelled ammonium injection experiments in two montane, gravel-bed stream reaches with riffle– pool sequences. N uptake was highest in epilithic biofilms, thallophytes and roots (min–max range 0.2–545.2 mg N m-2 day-1) and lowest in leaves, wood and fine benthic organic matter (0.05– 209.2 mg N m-2 day-1). At the microscale, N uptake of all primary uptake compartments except wood was higher in riffles than in pools. At the
mesoscale, hydromorphology determined the distribution of primary uptake compartments, with fast-flowing riffles being dominated by biologically more active compartments and pools being dominated by biologically less active compartments. Despite a lower biomass of primary uptake compartments, mesoscale N uptake was 1.7–3.0 times higher in riffles than in pools. At reach scale, N uptake ranged from 79.6 to 334.1 mg N m-2 day-1. Highest reach-scale N uptake was caused by a bloom of thallopyhtes, mainly filamentous autotrophs, during stable low discharge and high light conditions. Our results reveal the important role of hydromorphologic sorting of primary uptake compartments at mesoscale as a controlling factor for reach-scale N uptake in streams. Key words: spatial hierarchy; ammonium uptake; epilithic biofilms; filamentous autotrophs; hydromorphology; environmental constraints; collective properties.
Received 20 December 2019; accepted 5 October 2020
Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s10021-020-00576-7) contains supplementary material, which is available to authorized users. Authors contributions: U.R.-B., C.A., C.N., A.L., D.v.S. and M.W. designed the study. U.R.-B., C.A., and D.v.S. performed research and analysed data, and D.v.S. contributed new methods. U.R.-B. and M.W. wrote the paper with comments from C.A., C.N., A.L., and D.v.S. *Corresponding author; e-mail: [email protected]
HIGHLIGHTS Hydromorphologic sorting was an i
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