Andreas Bruder

Leaf-litter breakdown in tropical streams: is variability the norm?

Many forested headwater streams are heterotrophic ecosystems in which allochthonous inputs of plant litter are a major source of energy. Leaves of riparian vegetation entering the stream are broken down by a combination of biotic and abiotic processes and, in most temperate and boreal streams, provide food and habitat for dense populations of detritivorous invertebrates. However, tropical streams in different parts of the world show substantial variability in the number and diversity of leaf-shredding detritivores (hereafter detritivores). We used data obtained with standardized methods from multiple streams in Africa, the Americas, Asia, and Australia to test the hypothesis that this variability would lead to differences in the relative role of detritivores and microorganisms in the breakdown process. We also tested the hypotheses that variability in litter breakdown rates changes with litter type (native litter mixtures vs nonnative alder [Alnus glutinosa]) and is higher across regions within than outside the tropics. We found that litter breakdown rates were highly variable across sites, with no consistent pattern within geographic areas, although litter consumption by detritivores was negligible at several sites, all in America. Geographic patterns of litter breakdown also varied between litter types, with higher breakdown rates for alder than for native litter in most but not all regions. When litter breakdown rates at the tropical sites were compared to previously reported values from temperate and boreal regions, we found that differences in variability between tropical and temperate sites were inconsistent, with great differences among studies. Further global-scale studies will be needed to assess the extent to which latitudinal changes in the diversity and composition of microbial and detritivore assemblages contribute to variability in litter breakdown rates.

Evaluation of mitigation measures to reduce hydropeaking impacts on river ecosystems – a case study from the Swiss Alps

New Swiss legislation obligates hydropower plant owners to reduce detrimental impacts on rivers ecosystems caused by hydropeaking. We used a case study in the Swiss Alps (hydropower company Kraftwerke Oberhasli AG) to develop an efficient and successful procedure for the ecological evaluation of such impacts, and to predict the effects of possible mitigation measures. We evaluated the following scenarios using 12 biotic and abiotic indicators: the pre-mitigation scenario (i.e. current state), the future scenario with increased turbine capacity but without mitigation measures, and future scenarios with increased turbine capacity and four alternative mitigation measures. The evaluation was based on representative hydrographs and quantitative or qualitative prediction of the indicators. Despite uncertainties in the ecological responses and the future operation mode of the hydropower plant, the procedure allowed the most appropriate mitigation measure to be identified. This measure combines a basin and a cavern at a total retention volume of 80,000m3, allowing for substantial dampening in the flow falling and ramping rates and in turn considerable reduction in stranding risk for juvenile trout and in macroinvertebrate drift. In general, this retention volume had the greatest predicted ecological benefit and can also, to some extent, compensate for possible modifications in the hydropower operation regime in the future, e.g. due to climate change, changes in the energy market, and changes in river morphology. Furthermore, it also allows for more specific seasonal regulations of retention volume during ecologically sensitive periods (e.g. fish spawning seasons). Overall experience gained from our case study is expected to support other hydropeaking mitigation projects.

High-throughput amplicon sequencing and stream benthic bacteria: identifying the best taxonomic level for multiple-stressor research

Disentangling the individual and interactive effects of multiple stressors on microbial communities is a key challenge to our understanding and management of ecosystems. Advances in molecular techniques allow studying microbial communities in situ and with high taxonomic resolution. However, the taxonomic level which provides the best trade-off between our ability to detect multiple-stressor effects versus the goal of studying entire communities remains unknown. We used outdoor mesocosms simulating small streams to investigate the effects of four agricultural stressors (nutrient enrichment, the nitrification inhibitor dicyandiamide (DCD), fine sediment and flow velocity reduction) on stream bacteria (phyla, orders, genera, and species represented by Operational Taxonomic Units with 97% sequence similarity). Community composition was assessed using amplicon sequencing (16S rRNA gene, V3-V4 region). DCD was the most pervasive stressor, affecting evenness and most abundant taxa, followed by sediment and flow velocity. Stressor pervasiveness was similar across taxonomic levels and lower levels did not perform better in detecting stressor effects. Community coverage decreased from 96% of all sequences for abundant phyla to 28% for species. Order-level responses were generally representative of responses of corresponding genera and species, suggesting that this level may represent the best compromise between stressor sensitivity and coverage of bacterial communities.

Multiple‐stressor effects on leaf litter decomposition and fungal decomposers in agricultural streams contrast between litter species

Agricultural land use commonly exerts multiple stressors on the functioning of stream ecosystems, including leaf litter decomposition and the utilization of this resource in stream food webs. If stressors interact, their cumulative effects on biotic responses cannot be predicted from knowledge of individual stressor effects, posing challenges for management and restoration of ecosystems. We examined the individual and interactive effects of four common agricultural stressors and the role of litter quality on leaf litter decomposition and fungal decomposers. In 128 outdoor, flow-through mesocosms, we manipulated levels of nutrients, a nitrification inhibitor (dicyandiamide), deposited fine sediment and flow velocity. Interactions among these stressors can ensue because, for instance, they jointly affect physicochemical conditions around leaf litter colonized by fungi such as concentrations of dissolved oxygen and nutrients. The two litter species used, deciduous birch and evergreen mahoe, showed contrasting decomposition dynamics, and these differences influenced their response to stressors. Fungi were important for birch litter decomposition but played a minor role for mahoe. Overall, flow velocity reduction and deposited fine sediment had the strongest, mainly negative effects on fungi and litter decomposition, probably as a consequence of reductions in dissolved oxygen available to fungi. However, fine sediment substantially increased mahoe litter mass loss, pointing at fungi-independent processes being relevant for its decomposition. Although interactions among stressors were uncommon, they showed effects of the same magnitude as stressor main effects. Potential mechanisms underlying interactions included reductions in dissolved oxygen and changes in microbial community composition. Knowledge of the effects of multiple agricultural stressors and of litter quality on litter decomposition and litter-associated fungi is crucial for management of forested riparian corridors, which have been shown to efficiently mitigate impacts of agricultural stressors on streams. The contrasting responses of the litter species used in our study warrant consideration of species composition of the riparian vegetation. (Less)

Improving estimates of primary production in lakes: a test and a case study from a peri-alpine lake (Lake Lugano)

Abstract Although primary production (i.e., the carbon fixed by autotrophs through photosynthesis) is assessed extensively by limnologists, its measurement presents methodological challenges. The objectives of this study were to (1) address some of these challenges by proposing an improvement to current mathematical models used to calculate cumulative (e.g., seasonal or annual) production rates from instantaneous (e.g., hourly) production values, and (2) explore the response of primary production to 3 decades (1983–2014) of nutrient management in Lake Lugano (Switzerland and Italy) as a case study. Objective 1 was prompted by difficulties in estimating the photosynthetic parameters Pbmax, the maximum photosynthetic potential, and Iopt, the optimum light intensity, in days without photoinhibition. We developed a model extension to simulate these parameters from irradiance and total phosphorus (TP). To this end, we adapted equations previously used in marine studies and calibrated and tested them against values of primary production measured using the carbon-14 (14C) light-and-dark bottle method during 1983–2014. A Nash-Sutcliffe efficiency of 0.53 and a ratio between the root mean square error and the standard deviation of the observed values (RSR) of 0.68 indicated a satisfactory performance of our model extension. Therefore, we suggest that our modelling approach presents a step toward a more precise quantitative estimate of primary production. Concerning objective 2, in Lake Lugano, annual rates of primary production (measured as C) declined by 21.26 mg m−2 d−1 from 1983 to 2014. The environmental drivers included TP in spring, summer, and autumn, and water temperature in winter. These results suggest that primary production in Lake Lugano was limited mainly by TP availability and therefore seemed to be responding to nutrient management.

Schwall und Sunk — ein kurzer Überblick

Im vorliegenden Artikel wird versucht, den aktuellen Wissenstand zu den Auswirkungen von Schwall und Sunk auf die aquatische Gemeinschaft zusammenzufassen. Künstliche Pegelschwankungen verursachen kurzund langfristige Auswirkungen auf die Fließgewässerökologie. Auf Basis von Indikatoren lassen sich diese Auswirkungen bis zu einem gewissen Grad abschätzen und daraus möglichst e ziente Sanierungsmaßnahmen ableiten. Abschließend wird der Stand der Schwallsanierung in der Schweiz kurz beschrieben.