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Algal biofilms in streams are simultaneously controlled by light and nutrient availability (bottom-up control) and by grazing activity (top-down control). In addition to promoting algal growth, light and nutrients also determine the nutritional quality of algae for grazers. While short-term experiments have shown that grazers increase consumption rates of nutrient-poor algae due to compensatory feeding, nutrient limitation in the long run can constrain grazer growth and hence limit the strength of grazing activity. In this study, we tested the effects of light and phosphorus availability on grazer growth and thus on the long-term control of algal biomass. At the end of the experiment, algal biomass was significantly affected by light, phosphorus and grazing, but the interactive effects of the three factors significantly changed over time. At both high light and phosphorus supply, grazing did not initially reduce algal biomass, but the effect of grazing became stronger in the final three weeks of the experiment. Snail growth was enhanced by light, rather than phosphorus, suggesting that algal quantity rather than quality was the main limiting factor for grazer growth. Our results highlight the role of feedback effects and the importance of long-term experiments in the study of foodweb interactions.

The rationale of most restoration strategies is that with reconstruction of natural habitats comes biodiversity, and ecosystem functioning and services will follow suit. Uncertainty and frequent failure in restoration outcomes, however, are recurrent and likely related to the complexity of ecosystem properties. Here, we propose ecological simplification as the general mechanism by which human impacts have modified cross-scale relationships among landscape complexity, integrity, and niche diversity in ecosystems. To manage and reverse the negative effects of ecological simplification, the interplay between research and management must quantify the large-scale complexity of reference to restore simplified systems and to link these measures to niche diversity quantified at finer scales. Because of their historical interaction with human societies, we use riverine floodplains as model ecosystems to review the causes and consequences of simplification and to discuss how contemporary restoration can minimize the effects of simplification on biodiversity, functioning, and services of riverine floodplains.

Spatial variations in light and nutrient availability usually lead to a heterogeneous distribution of periphyton biomass within streams, but the effects of nutrient enrichment on periphyton heterogeneity are still poorly understood. We tested phosphorus enrichment effects on the heterogeneity of periphyton quantity and nutritional quality in an environment with uneven light conditions. Natural periphyton was grown at either high or low P supply in circular running water flumes, each of which was half shaded and half in light. After 3 weeks, periphyton biomass was significantly more heterogeneous at high P than at low P supply, as algal production increased with light only in the P-enriched treatment. However, periphyton C:P ratio was equally homogeneous in both P treatments, despite the light differences within each experimental flume. Although this outcome seems to contrast with the light:nutrient hypothesis, it is likely that high-quantity periphyton patches acquired more P and depleted the available P for the low-quantity patches, leading to an even C:P ratio within each flume. Therefore, in an environment with heterogeneous light availability, nutrient enrichment has the potential to increase the heterogeneity of periphyton quantity without affecting the heterogeneity of periphyton quality, due to periphyton patches not being independent of one another.

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-la-belled 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.

This study addressed the influence of common shoreline engineering structures (off-bankline revetment, rip rap and wing dike) on richness, biomass and secondary production of native and non-native macroinvertebrates in the navigation channel and near-shore habitats in the Elbe River (Germany). Within the navigation channel, only marginal differences among engineering structures were observed, and non-native species were absent from all samples. At the shoreline, secondary production of non-native species was significantly greater at the rip rap and represented 59% of total secondary production in near-shore habitats. Conversely, secondary production of native species at the shoreline was 9-fold lower at the rip rap and more than twice the rates at the wing dike. Differences in secondary production among engineering structures were attributed to differential distribution of substrate types. Boulder substrates, the dominant substrate type in the rip rap, promoted contributions of non-native species while macrophytes and silt were associated with high contributions of native species at the off-bankline revetment. Our results reveal that the morphological configuration of engineering structures in large rivers not only controls the rate of secondary production for macroinvertebrates but also the contribution of non-native species to total community functioning.

Biofilm formation in bacteria is considered to be one strategy to avoid protozoan grazing. However, this assumption is largely based on experiments with suspension-feeding protozoans. Here we test the hypothesis that grazing resistance depends on both the grazers' feeding trait and the bacterial phenotype, rather than being a general characteristic of bacterial biofilms. We combined batch experiments with mathematical modelling, considering the bacterium Pseudomonas putida and either a suspension-feeding (i.e. the ciliate Paramecium tetraurelia) or a surface-feeding grazer (i.e. the amoeba Acanthamoeba castellanii). We find that both plankton and biofilm phenotypes were consumed, when exposed to their specialised grazer, whereas the other phenotype remained grazing-resistant. This was consistently shown in two experiments (starting with either only planktonic bacteria or with additional pre-grown biofilms) and matches model predictions. In the experiments, the plankton feeder strongly stimulated the biofilm biomass. This stimulation of the resistant prey phenotype was not predicted by the model and it was not observed for the biofilm feeders, suggesting the existence of additional mechanisms that stimulate biofilm formation besides selective feeding. Overall, our results confirm our hypothesis that grazing resistance is a matter of the grazers' trait (i.e. feeding type) rather than a biofilm-specific property.

Eukaryotic microbial life at abyssal depths remains \"uncharted territory\" in eukaryotic microbiology. No phylogenetic surveys have focused on the largest benthic environment on this planet, the abyssal plains. Moreover, knowledge of the spatial patterns of deep-sea community structure is scanty, and what little is known originates primarily from morphology-based studies of foraminiferans. Here we report on the great phylogenetic diversity of microbial eukaryotic communities of all 3 abyssal plains of the southeastern Atlantic Ocean---the Angola, Cape, and Guinea Abyssal Plains---from depths of 5,000 m. A high percentage of retrieved clones had no close representatives in genetic databases. Many clones were affiliated with parasitic species. Furthermore, differences between the communities of the Cape Abyssal Plain and the other 2 abyssal plains point to environmental gradients apparently shaping community structure at the landscape level. On a regional scale, local species diversity showed much less variation. Our study provides insight into the community composition of microbial eukaryotes on larger scales from the wide abyssal sea floor realm and marks a direction for more detailed future studies aimed at improving our understanding of deep-sea microbes at the community and ecosystem levels, as well as the ecological principles at play.

We measured the clearance rates of the filter-feeding bivalve Corbicula fluminea over a period of 2 years. Strong seasonal variations, such as a 50-fold increase from February to July, were observed. These variations were only poorly linked to temperature, as they could be found at both the ambient field temperature and a constant temperature of 15°C. Instead, peaks in the filtration activity corresponded to the spawning activity. Additionally, a strong interannual variability with much lower clearance rates in 2009 than in 2008 was identified. The low clearance rates were linked to a preceding period of low winter temperatures close to the lethal temperature of 2°C and the associated reduced reproduction rates. Our findings demonstrate that other factors besides temperature and body mass can strongly affect clearance rates. These results should be considered when predicting the effects of changing temperatures on the ecosystem impact of filter-feeding bivalves.

The concept of land-use intensity has been widely used to quantify human impacts in terrestrial systems, yet such approaches fail to capture the cumulative effects of land use on connected river networks. Here, we introduce the Land Use Intensity Index for Stream Ecosystems (LUIS), a spatially explicit, hydrologically informed index that integrates crop-specific inputs of nitrogen, phosphorus, and pesticides with river network structure. To evaluate the relationship between agricultural pressures and water quality, we used logistic regression and a random forest classification model to assess overall ecological status and biological quality elements across Germany. Pesticide pressure showed the strongest association with ecological status, exhibiting the highest effect sizes, R² values, and predictor importance across both low- and high-order streams. These associations were especially pronounced in low-order streams and the central highlands, likely due to their limited dilution capacity and direct exposure to runoff. Phosphorus showed secondary relevance, particularly in higher-order streams, while nitrogen’s effects were less distinct. LUIS is a valuable tool for assessing the compound-specific and combined pressures of agricultural land use on stream ecosystems. Its mechanistically informed yet conceptually flexible framework provides a solid basis for future development and integration into operational monitoring and management contexts. Pesticide pressure has the strongest association with ecological status in Germany particularly in low-order streams and the central highlands, as revealed by a new land use intensity index explicitly considering multiple hydrological information.

Smart biodiversity indicators are needed not only for assessing and managing biodiversity (change) but also for informing target setting for environmental policies. This need has created a plethora of indicator frameworks, which are widely debated in terms of their design and usefulness. We propose that these discussions would benefit from more clearly separating different types of indicators and more operationally linking them to management targets and biodiversity goals. Decision makers often consider the multitude of biodiversity indicators as being complicated, whereas scientists emphasize that they barely reflect how complex biodiversity is. It is therefore important to differentiate clearly between indicators for diagnosing (the drivers of) biodiversity change and indicators for steering policies. While the former must be scalable in time and space and reflect the full complexity of biodiversity, the latter need long‐term visions and simplicity. Management targets such as proportions of protected areas or mitigation of biodiversity drivers have the advantage of being operation‐oriented, well quantifiable and immediately responsive to change. Achieving a management target, however, does not necessarily equal achieving a biodiversity goal. Likewise, management indicators that track how well management targets are achieved cannot replace diagnostic or steering indicators. Solution: Biodiversity goals such as ‘bending the curve’ can only be achieved by closely linking diagnosis, steering and management, while accepting that their distinct design principles need to be addressed. Being aware and accepting such tailored types of different indicators and management targets could strongly improve the interplay between science and policy to reverse negative biodiversity trends in the future. Zusammenfassung Intelligente Biodiversitätsindikatoren werden benötigt, um Zustand und Veränderung der Biodiversität zu bewerten und zu steuern, aber auch als Grundlage für die Zielsetzung in der Umweltpolitik. Dieser Bedarf hat zu einer Vielzahl von Indikatorsystemen geführt, deren Gestaltung und Nützlichkeit kontrovers diskutiert werden. Wir schlagen vor, diese Diskussionen durch eine klarere Trennung der verschiedenen Arten von Indikatoren und eine stärkere operative Verknüpfung mit Managementzielen und Biodiversitätszielen zu bereichern. Entscheidungsträger halten die Vielzahl der Biodiversitätsindikatoren oft für kompliziert, während Wissenschaftler betonen, dass sie kaum die reale Komplexität der Biodiversität widerspiegeln. Daher ist es wichtig, klar zwischen Indikatoren zur Diagnose (der Trends und Treiber) von Biodiversitätsveränderungen und Indikatoren zur Steuerung von Politikmaßnahmen zu unterscheiden. Während erstere zeitlich und räumlich skalierbar sein und die gesamte Komplexität der Biodiversität widerspiegeln müssen, erfordern letztere langfristige Visionen und Einfachheit. Managementziele wie der Anteile geschützter Gebiete oder die Minderung der Einflussfaktoren auf die Biodiversität haben den Vorteil, dass sie handlungsorientiert und gut quantifizierbar sind sowie unmittelbar auf Veränderungen reagieren. Das Erreichen eines Managementziels ist jedoch nicht unbedingt gleichbedeutend mit dem Erreichen eines Biodiversitätsziels. Ebenso können Managementindikatoren, die verfolgen, wie gut Managementziele erreicht werden, Diagnose‐ oder Steuerungsindikatoren nicht ersetzen. Biodiversitätsziele wie ‘bending the curve’ können nur durch eine enge Verknüpfung von Diagnose, Steuerung und Management erreicht werden, wobei ihre unterschiedlichen Gestaltungsprinzipien berücksichtigt werden müssen. Das Bewusstsein für und die Akzeptanz solcher unterschiedlicher Indikatoren und Ziele könnte das Zusammenspiel zwischen Wissenschaft und Politik erheblich verbessern, um negative Biodiversitätstrends in Zukunft umzukehren. To understand, measure and mitigate biodiversity change, science and policy need to differentiate between diagnostic indicators that capture as many different biodiversity facets as needed, outcome‐oriented steering indicators to guide policy making and management indicators that measure the success of conservation and restoration efforts. None of these indicator classes can act as a replacement for the others, but we would improve communication about biodiversity change by acknowledging these differences, and it would improve both biodiversity research and management if the interrelationships between these indicator types were well understood.