Abteilung Oberflächengewässer

Human activities significantly alter natural river flows, impacting ecosystem functioning and biodiversity worldwide. Hydropeaking, resulting from intermittent on-demand hydropower generation, introduces sub-daily flow fluctuations exceeding natural variability. While the effects of single hydropeaking events are well-studied, the cumulative impacts of frequent hydropeaking requires further exploration. This study aims to develop metrics that captures changes in habitat dynamics at the patch scale (i.e. individual micro-habitats within the habitat mosaic) due to reoccurring hydropeaking. Using hydrodynamic simulations, we introduce three patch-scale metrics to quantify habitat dynamics with high spatial (0.5 m) and temporal (10 min) resolution: (M1) Habitat probability within patches, assessing spatio-temporal diversity of habitats; (M2) Habitat shifts within patches, evaluating habitat persistence for sessile organisms (e.g. vegetation, invertebrates); and (M3) Spatial shifts of habitats, indicating habitat relocation affecting mobile species (e.g. adult fish). Using eight hydro-morphological scenarios representing different levels of anthropogenic modification of flow and morphology, we demonstrate that these metrics effectively quantify changes in habitat dynamics at patch-scale. The results highlight the ecological relevance of these metrics and their potentially utility for river management. By identifying areas susceptible to ecological impacts, these metrics may serve as tools for hydropeaking mitigation, enabling more targeted and spatially explicit habitat management and restoration.

Emissions of microbially produced methane (CH₄) from lake sediments are a major source of this potent greenhouse gas to the atmosphere. The rates of CH₄ production and emission are believed to be influenced by electron acceptor distributions and organic carbon contents, which in turn are affected by anthropogenic inputs of nutrients leading to eutrophication. Here, we investigate how eutrophication influences the abundance and community structure of CH₄ producing Archaea and methanogenesis pathways across time–resolved sedimentary records of five Swiss lakes with well–characterized trophic histories. Despite higher CH₄ concentrations which suggest higher methanogenic activity in sediments of eutrophic lakes, abundances of methanogens were highest in oligotrophic lake sediments. Moreover, while the methanogenic community composition differed significantly at the lowest taxonomic levels (OTU), depending on whether sediment layers had been deposited under oligotrophic or eutrophic conditions, it showed no clear trend in relation to in situ distributions of electron acceptors. Remarkably, even though methanogenesis from CO₂-reduction was the dominant pathway in all sediments based on carbon isotope fractionation values, taxonomic identities, and genomes of resident methanogens, CO₂-reduction with hydrogen (H₂) was thermodynamically unfavorable based on measured reactant and product concentrations. Instead, strong correlations between genomic abundances of CO₂-reducing methanogens and anaerobic bacteria with potential for extracellular electron transfer suggest that methanogenic CO₂-reduction in lake sediments is largely powered by direct electron transfer from syntrophic bacteria without involvement of H₂ as an electron shuttle.

The global nitrogen (N) cycle has been strongly altered by anthropogenic activities, including increased input of bioavailable N into aquatic ecosystems. Freshwater sediments are hotspots with regards to the turnover and elimination of fixed N, yet the environmental controls on the microbial pathways involved in benthic N removal are not fully understood. Here, we analyze the abundance and expression of microbial genes involved in N transformations using metagenomics and -transcriptomics across sediments of 12 Swiss lakes that differ in sedimentation rates and trophic regimes. Our results indicate that microbial N loss in these sediments is primarily driven by nitrification coupled to denitrification. N-transformation gene compositions indicated three groups of lakes: agriculture-influenced lakes characterized by rapid depletion of oxidants in the sediment porewater, pristine-alpine lakes with relatively deep sedimentary penetration of oxygen and nitrate, and large, deep lakes with intermediate porewater hydrochemical properties. Sedimentary organic matter (OM) characteristics showed the strongest correlations with the community structure of microbial N-cycling communities. Most transformation pathways were expressed, but expression deviated from gene abundance and did not correlate with benthic geochemistry. Cryptic N-cycling may maintain transcriptional activity even when substrate levels are below detection. Sediments of large, deep lakes generally showed lower in-situ N gene expression than agriculture-influenced lakes, and half of the pristine-alpine lakes. This implies that prolonged OM mineralization in the water column can lead to the suppression of benthic N gene expression.