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Trophic transfer efficiency (TTE) is usually calculated as the ratio of production rates between two consecutive trophic levels. Although seemingly simple, TTE estimates from lakes are rare. In our review, we explore the processes and structures that must be understood for a proper lake TTE estimate. We briefly discuss measurements of production rates and trophic positions and mention how ecological efficiencies, nutrients (N, P) and other compounds (fatty acids) affect energy transfer between trophic levels and hence TTE. Furthermore, we elucidate how TTE estimates are linked with size-based approaches according to the Metabolic Theory of Ecology, and how food-web models can be applied to study TTE in lakes. Subsequently, we explore temporal and spatial heterogeneity of production and TTE in lakes, with a particular focus on the links between benthic and pelagic habitats and between the lake and the terrestrial environment. We provide an overview of TTE estimates from lakes found in the published literature. Finally, we present two alternative approaches to estimating TTE. First, TTE can be seen as a mechanistic quantity informing about the energy and matter flow between producer and consumer groups. This approach is informative with respect to food-web structure, but requires enormous amounts of data. The greatest uncertainty comes from the proper consideration of basal production to estimate TTE of omnivorous organisms. An alternative approach is estimating food-chain and food-web efficiencies, by comparing the heterotrophic production of single consumer levels or the total sum of all heterotrophic production including that of heterotrophic bacteria to the total sum of primary production. We close the review by pointing to a few research questions that would benefit from more frequent and standardized estimates of TTE in lakes.

Estimates of secondary production depend on the efficiency of sampling methods in capturing abundances and body lengths of the entire macroinvertebrate community. The efficiency of common sampling methods in fulfilling these criteria is poorly understood. We compared the effects of a Surber sampler (250 µm mesh size) and a Freeze corer in capturing abundance, biomass, and secondary production of macroinvertebrates in a forested headwater stream. We then examined how the use of nets with different mesh sizes could affect estimates of secondary production. Macroinvertebrate abundance was three times lower, and biomass was three times higher with the Surber than with the Freeze corer. Neither method captured the entire length distribution, and incomplete sampling of body lengths and abundance resulted in underestimating total secondary production by 48% (Surber) and 49% (Freeze corer). We estimated that reducing the mesh size from 250 to 100 µm would reduce the underestimation of production from ~ 48 to ~ 12% due to the inclusion of smaller individuals. Our results improve the efficiency of common sampling methods, allowing a reliable quantification of the role of macroinvertebrates in stream ecosystem functioning.

Ponto-Caspian gobies have invaded many river systems in the northern hemisphere, but their colonization patterns often remain unrecognized at the early stages of invasion. Here, we analyzed the ongoing invasion of the round goby Neogobius melanostomus (Pallas, 1814) in the Elbe River (Germany) by combining literature research with electrofishing and environmental DNA (eDNA) sampling. Along the surveyed 517 km, abundances peaked in the upper and lower sections, while low or no goby abundances were detected in the middle section. Combining the results of the field survey and literature data, we inferred four dispersal modes, jointly appearing during the colonization. (1) Human mediated transport, which was responsible for the initial colonization of the tidal Elbe and for a jumpy expansion to the upstream sections. (2) Fast downstream colonization, which was particularly observed for juvenile individuals. (3) Slow upstream migration, which was also recorded for another goby species, the bighead goby Ponticola kessleri (Günther, 1861) and (4) Lateral invasion via a major shipping canal in the middle river section. The eDNA analyses generally supported these conclusions and showed a high sensitivity and thus increased detection strength at low goby densities. Our study showed that the location of the first occurrence, as well as the dispersal mode, strongly determines the invasion process of non-native gobies.

Human impacts, particularly nutrient pollution and land-use change, have caused significant declines in the quality and quantity of freshwater resources. Most global assessments have concentrated on species diversity and composition, but effects on the multifunctionality of streams and rivers remain unclear. Here, we analyse the most comprehensive compilation of stream ecosystem functions to date to provide an overview of the responses of nutrient uptake, leaf litter decomposition, ecosystem productivity, and food web complexity to six globally pervasive human stressors. We show that human stressors inhibited ecosystem functioning for most stressor-function pairs. Nitrate uptake efficiency was most affected and was inhibited by 347% due to agriculture. However, concomitant negative and positive effects were common even within a given stressor-function pair. Some part of this variability in effect direction could be explained by the structural heterogeneity of the landscape and latitudinal position of the streams. Ranking human stressors by their absolute effects on ecosystem multifunctionality revealed significant effects for all studied stressors, with wastewater effluents (194%), agriculture (148%), and urban land use (137%) having the strongest effects. Our results demonstrate that we are at risk of losing the functional backbone of streams and rivers if human stressors persist in contemporary intensity, and that freshwaters are losing critical ecosystem services that humans rely on. We advocate for more studies on the effects of multiple stressors on ecosystem multifunctionality to improve the functional understanding of human impacts. Finally, freshwater management must shift its focus toward an ecological function-based approach and needs to develop strategies for maintaining or restoring ecosystem functioning of streams and rivers. Generalitat de Catalunya, Grant/Award Number: 2017SGR0976; Portuguese Foundation for Science and Technology, Grant/Award Number: CEEIND/02484/2018 and UIDP/04292/2020

[EN] Water diversion and pollution are two pervasive stressors in river ecosystems that often co-occur. Individual effects of both stressors on basal resources available to stream communities have been described, with diversion reducing detritus standing stocks and pollution increasing biomass of primary producers. However, interactive effects of both stressors on the structure and trophic basis of food webs remain unknown. We hypothesized that the interaction between both stressors increases the contribution of the green pathway in stream food webs. Given the key role of the high-quality, but less abundant, primary producers, we also hypothesized an increase in food web complexity with larger trophic diversity in the presence of water diversion and pollution. To test these hypotheses, we selected four rivers in a range of pollution subject to similar water diversion schemes, and we compared food webs upstream and downstream of the diversion. We characterized food webs by means of stable isotope analysis. Both stressors directly changed the availability of basal resources, with water diversion affecting the brown food web by decreasing detritus stocks, and pollution enhancing the green food web by promoting biofilm production. The propagation of the effects at the base of the food web to higher trophic levels differed between stressors. Water diversion had little effect on the structure of food webs, but pollution increased food chain length and trophic diversity, and reduced trophic redundancy. The effects at higher trophic levels were exacerbated when combining both stressors, as the relative contribution of biofilm to the stock of basal resources increased even further. Overall, we conclude that moderate pollution increases food web complexity and that the interaction with water abstraction seems to amplify this effect. Our study shows the importance of assessing the interaction between stressors to create predictive tools for a proper management of ecosystems. Ministerio de Economia, Industria y Competitividad, Gobierno de Espana, Grant/Award Number: GL2016-77487-R; European Social Fund; Diputacion Foral de Bizkaia; Serra Hunter Fellow; Labex, Grant/Award Number: ANR-10-LABX-41; H2020 European Research Council; Eusko Jaurlaritza; Consejo Nacional de Investigaciones Cientificas y Tecnicas; FRAGCLIM Consolidator, Grant/Award Number: 726176

Many large rivers used for navigation have lost their hydromorphological heterogeneity, which has led to the widespread loss of native biodiversity and the concurrent establishment of non‐native communities. While the effects on biodiversity are well‐described, we know little about how the loss of natural habitats and the restructuring of communities cumulate into effects on riverine food webs. We constructed binary and ingestion webs for benthic macroinvertebrates and their resources in the Elbe River (Germany) and compared if food chain length, food web complexity, robustness, ingestion rates, and consumer‐resource interaction strength differ among three shoreline engineering practices. Food webs at profoundly altered shorelines were significantly less complex and had significantly shorter food chains than the food web at the semi‐natural shoreline. However, food web robustness to a simulated loss of species was comparable at all shorelines. Total ingestion rates were up to eight times lower at highly altered shorelines due to significantly lower ingestion rates by native species. Predator–prey interaction strength was comparable among shorelines due to higher shares of non‐native predators, indicating that non‐native predators can be functionally equivalent to native predators. We attributed the observed food web differences to the absence of complex habitats at profoundly altered shorelines and the accompanied absence of specialized consumers. Our study provides empirical evidence that hydromorphological modifications reduce the efficiency of food webs to control organic matter dynamics and may ultimately affect the provisioning of riverine ecosystem services.

Intensification of agricultural land use leads to riparian clear cutting, which disrupts stream aquatic-terrestrial linkages through the loss of terrestrial particulate organic matter (POM)...

The role of hydromorphological degradation and temporal variation for food webs in human-modified rivers is still not fully evaluated. We tested the hypothesis that man-made engineering structures alter macroinvertebrate resource use in the Elbe River (Germany) in relation to seasonal variation. Stable isotopes (δ13C, δ15N) and mixing models revealed that dietary contributions of benthic organic matter (BOM) and phytoplankton were driven by engineering structure. Contributions of biofilm were driven by season, while contributions of terrestrial particulate organic matter (t-POM) were driven by both engineering structure and season. Contributions of t-POM were larger than those of phytoplankton in spring and summer, but not in autumn, which adds to the debate about the sources of organic matter fuelling riverine benthic food webs. Resource availability was not systematically related to resource use, indicating that factors other than resource limitation were responsible for the observed results. By demonstrating that human alterations determine consumer resource use independently from resource availability, our study links hydromorphological modifications to fluxes of matter in riverine food webs. Future studies should quantify organic matter fluxes from various autochthonous and allochthonous pathways in human-modified and natural rivers to allow for a robust synthesis of how hydromorphological modifications alter benthic food webs.

Benthic biofilms are often assumed to control terrestrially-derived dissolved organic carbon (tDOC) uptake in streams. We tested this by comparing 13C-enriched ryegrass leachate uptake in an agricultural and a forest stream, hypothesizing that a greater abundance of autotrophic biofilms in the agricultural stream would cause its whole-stream tDOC uptake to be comparatively low. We measured whole-stream and biofilm tDOC tracer uptake, metabolism, bacterial and algal diversity, and nutrient status of benthic epilithic biofilms, and assessed whole-stream hydromorphology. Whole-stream uptake of tDOC was six times lower in the agricultural (3.0 mg m−2 day−1) than in the forest (19.0 mg m−2 day−1) stream, and tDOC uptake velocity indicated lower tDOC demand in the agricultural (1.2 mm min−1) than in the forest (1.9 mm min−1) stream. The agricultural stream differed from the forest stream by slightly lower transient storage capacity and higher benthic biofilm bacterial abundance and production, lower biofilm biomass and lower biofilm molar C:N, C:P, and N:P ratios. Changes in epilithic biofilms contributed little to the differences in whole-stream tDOC tracer uptake between streams, as biofilm tDOC uptake only amounted to 4% and 13% of whole-stream uptake in the forest and agricultural stream, respectively. This comparison of a forest and an agricultural stream suggests that agricultural stressors have the potential to diminish both whole-stream tDOC uptake and uptake efficiency. Furthermore, the weak link between biofilm and whole-stream tDOC uptake implies that benthic biofilms characteristics are poor predictors for human impacts on tDOC uptake in agricultural streams and that hot spots of tDOC uptake are likely situated in the hyporheic zone or in the stream water column.

Littoral macroinvertebrates are an integral component of lake food webs, but their productivity may be affected by shoreline alteration. We hypothesized that human modification of lake shor...

The science of the total environment 769, 144324 (2021). doi:10.1016/j.scitotenv.2020.144324, Published by Elsevier Science, Amsterdam [u.a.]

The composition of littoral macroinvertebrate communities in lakes is governed by multiple natural and anthropogenic environmental influences interacting at different spatial scales. Since ecological assessment methods using littoral macroinvertebrates should respond specifically to a single stressor, knowledge on the unique effects of a given stressor is necessary. To effectively disentangle the effects of hydromorphology and trophic state requires analysing macroinvertebrate communities at lake sites with the full range of both stressors. We used a dataset of 98 lakes encompassing the entire gradient of geographical locations, lake types, hydromorphological degradation and trophic states in Central European lakes. We studied the unique and joint effects of hydromorphology and trophic state on macroinvertebrate richness, community composition and the Littoral Invertebrate Multimetric Index based on Composite Sampling (LIMCO). Variation partitioning analyses were conducted to test the importance of hydromorphology relative to trophic state across and within hydromorphological states (natural shorelines, hard and soft shore modifications) and trophic states (oligotrophic to hypertrophic states). At natural, hard and soft modification sites, hydromorphology explained 10, 16 and 19%, respectively, of the average unique variation of diversity, community composition and the LIMCO index, whereas trophic state explained on average 2, 5 and 5%, respectively. Similarly, in low, medium and high trophic state lakes, hydromorphology explained 10, 15 and 7%, respectively, of the average unique variation of diversity, community composition and the LIMCO index, whereas trophic state explained on average 0.3, 3 and 6%, respectively. Our results demonstrate that littoral hydromorphology was a more important driver of macroinvertebrate diversity, community composition and LIMCO than trophic state across hydromorphological states and trophic states. This indicates that multiple stressors in lakes act hierarchically on littoral macroinvertebrate communities and that the hydromorphological degradation of littoral zones is the primary driver for altered communities.

The assessment of the exposure of aquatic wildlife to complex environmental mixtures of chemicals originating from both point and diffuse sources and evaluating the potential impact thereof constitutes a significant step towards mitigating toxic pressure and the improvement of ecological status. In the current proof-of-concept study, we demonstrate the potential of a novel Aggregated Biomarker Response (ABR) approach involving a comprehensive set of biomarkers to identify complex exposure and impacts on wild brown trout (Salmo trutta fario). Our scenario used a small lowland river in Germany (Holtemme river in the Elbe river catchment) impacted by two wastewater treatment plants (WWTP) and diffuse agricultural runoff as a case study. The trout were collected along a pollution gradient (characterised in a parallel study) in the river. Compared to fish from the reference site upstream of the first WWTP, the trout collected downstream of the WWTPs showed a significant increase in micronucleus formation, phase I and II enzyme activities, and oxidative stress parameters in agreement with increasing exposure to various chemicals. By integrating single biomarker responses into an aggregated biomarker response, the two WWTPs' contribution to the observed toxicity could be clearly differentiated. The ABR results were supported by chemical analyses and whole transcriptome data, which revealed alterations of steroid biosynthesis and associated pathways, including an anti-androgenic effect, as some of the key drivers of the observed toxicity. Overall, this combined approach of in situ biomarker responses complemented with molecular pathway analysis allowed for a comprehensive ecotoxicological assessment of fish along the river. This study provides evidence for specific hazard potentials caused by mixtures of agricultural and WWTP derived chemicals at sublethal concentrations. Using aggregated biomarker responses combined with chemical analyses enabled an evidence-based ranking of sites with different degrees of pollution according to toxic stress and observed effects.

The sum of benthic autotrophic and bacterial production often exceeds the sum of pelagic autotrophic and bacterial production, and hence may contribute substantially to whole-lake carbon fluxes, especially in shallow lakes. Furthermore, both benthic and pelagic autotrophic and bacterial production are highly edible and of sufficient nutritional quality for animal consumers. We thus hypothesised that pelagic and benthic transfer efficiencies (ratios of production at adjacent trophic levels) in shallow lakes should be similar. We performed whole ecosystem studies in two shallow lakes (3.5ha, mean depth 2m), one with and one without submerged macrophytes, and quantified pelagic and benthic biomass, production and transfer efficiencies for bacteria, phytoplankton, epipelon, epiphyton, macrophytes, zooplankton, macrozoobenthos and fish. We expected higher transfer efficiencies in the lake with macrophytes, because these provide shelter and food for macrozoobenthos and may thus enable a more efficient conversion of basal production to consumer production. In both lakes, the majority of the whole-lake autotrophic and bacterial production was provided by benthic organisms, but whole-lake primary consumer production mostly relied on pelagic autotrophic and bacterial production. Consequently, transfer efficiency of benthic autotrophic and bacterial production to macrozoobenthos production was an order of magnitude lower than the transfer efficiency of pelagic autotrophic and bacterial production to rotifer and crustacean production. Between-lake differences in transfer efficiencies were minor. We discuss several aspects potentially causing the unexpectedly low benthic transfer efficiencies, such as the food quality of producers, pelagic-benthic links, oxygen concentrations in the deeper lake areas and additional unaccounted consumer production by pelagic and benthic protozoa and meiobenthos at intermediate or top trophic levels. None of these processes convincingly explain the large differences between benthic and pelagic transfer efficiencies. Our data indicate that shallow eutrophic lakes, even with a major share of autotrophic and bacterial production in the benthic zone, can function as pelagic systems with respect to primary consumer production. We suggest that the benthic autotrophic production was mostly transferred to benthic bacterial production, which remained in the sediments, potentially cycling internally in a similar way to what has previously been described for the microbial loop in pelagic habitats. Understanding the energetics of whole-lake food webs, including the fate of the substantial benthic bacterial production, which is either mineralised at the sediment surface or permanently buried, has important implications for regional and global carbon cycling.

Ephemeral ponds are often dominated by species with both terrestrial and aquatic life phases. Such species have the potential to strongly alter the food web structure of ponds, particularly if they are predators. Here we experimentally tested the effects of salamander larvae (Salamandra salamandra) on invertebrate communities in ephemeral forest ponds. We repeatedly split two ponds into salamander enclosure- and exclosure-segments, and compared the diversity and biomass of potential prey organisms. We used stable isotopes of carbon (δ13C) and nitrogen (δ15N) of resources and consumers to characterise the food web structure. The presence of salamander larvae did not affect abundances of culicid larvae, their preferred prey. The population dynamics of most insect larvae was independent of the presence of salamander larvae, and was instead driven by the timing of hatching and emergence. However, a significant reduction resulting from salamander predation could be detected in the less abundant chironomid larvae. There was no substantial alteration of the food web structure as indicated by stable isotopes. However, the stable isotope results suggest a strong trophic subsidisation from the terrestrial system, which is probably the reason for the weak top-down effects of the salamander larvae on the invertebrate food web.

Background: Differences in the trophic niche often underlie ecological specialization of individuals and can promote ecological speciation of populations, but studies showing a link between differences in the trophic niche and genetic differentiation of populations are rare. On the island of San Cristóbal (Galapágos archipelago), a strong genetic differentiation between two relatively proximate populations (subspecies; Amblyrhynchus cristatus mertensi and A. c. godzilla) of marine iguanas along the coastline has been observed. Here, we explore whether this genetic differentiation is mirrored in the iguanas’ trophic niche. Results: Although, no significant difference in the number of consumed algal taxa between subspecies were detected, the Schoener index exhibited low diet overlap between A. c. mertensi and A. c. godzilla. The latter was also demonstrated by the PERMANOVA analysis with significantly different diet OTU composition from the fecal samples between subspecies. Stable isotope analysis revealed that subspecies identity was overall more important than site for the iguanas’ resource use. Conclusions: By applying a metabarcoding approach on feces samples in combination with stable isotope analysis of skin sheds, we found that A. c. mertensi and A. c. godzilla differ in their ecological niches. Moreover, stable isotope analysis indicated that marine iguana populations have low spatial foraging distances, which, together with the diet partitioning patterns, might explain, at least partially, the lack of gene flow between these geographically proximate marine iguana populations. Key words: diet analysis, trophic niche, metabarcoding, stable isotopes, marine iguanas, Amblyrhynchus cristatus

Fluvial networks are globally relevant for the processing of dissolved organic matter (DOM). To investigate the change in molecular DOM diversity along the river course, high-field FTICR mass spectrometry and NMR spectroscopy of riverine DOM as well as bacterial abundance and activity were measured in a third order stream along a land-use gradient from pristine, agricultural to urban landscapes. DOM composition showed a clear evolution along the river course with an initial decrease of average oxidation and unsaturation followed by an increased relative abundance of CHNO and CHOS compounds introduced by agriculture and waste water, respectively. DOM composition was dominated by rather unsaturated CHO compounds (H/C ≤ 1) in headwaters and by more aliphatic molecules at downstream sites. Oxygenated functional groups shifted from aromatic ethers and hydroxyl groups to aliphatic carboxylic acids and aliphatic hydroxyl groups. This massive dislocation of oxygen significantly increased the diversity of atomic environments in branched aliphatic groups from headwater to downstream DOM. Mass spectra of DOM enabled the detection of compositional relationships to bacterial abundance and activity which was positively related to more aliphatic components (H/C > 1) and negatively related to unsaturated components. FTICR mass and NMR spectra corroborated the initial decline in DOM molecular diversity predicted by the River Continuum Concept (RCC) but demonstrated an anthropogenic increase in the molecular diversity of DOM further downstream. While the high DOM molecular diversity in first order headwater streams was the result of small scale ecosystem plurality, agriculture and waste water treatment introduced many components in the lower reaches. These anthropogenic influences together with massive bacterial oxidation of DOM contributed to a growth of molecular diversity of downstream DOM whose composition and structure differed entirely from those found in pristine headwaters.

Human lake shore alterations often result in a substantial decrease of littoral and riparian habitat diversity and physical complexity, but the intensity at which shore alterations affect biodiversity may differ among European geographical regions. We tested if the response of littoral macroinvertebrate communities to human shoreline alterations is consistent among geographical regions. We compared community composition and diversity of human altered with those of unmodified littoral zones from 51 lakes across seven European countries in four geographical regions based on pooled composite as well as habitat-specific macroinvertebrate samples. Taxon richness and community composition differed among shore types and different habitats in all geographic regions, with morphological alteration having an overall negative effect on macroinvertebrate taxon richness. In addition, habitat heterogeneity also had a strong effect on littoral communities, with highest taxon richness found in the structurally complex macrophyte habitats in all regions. Average proportional densities of Diptera and Oligochaeta taxa generally increased in morphologically altered shores in all geographical regions, while Bivalvia, Crustacea, Ephemeroptera, Gastropoda and Trichoptera showed comparatively lower numbers in many anthropogenically altered sites. Furthermore, taxon richness was positively correlated with habitat diversity. We were able to relate changes in littoral communities to anthropogenic shoreline alterations, and linked the effect to the loss of habitats and habitat complexity. The results of our study demonstrate that littoral macroinvertebrates respond consistently negative to the influence of morphological alterations across European geographical regions in terms of biodiversity. While macroinvertebrates have previously been identified to be useful descriptors of morphological change in single countries/regions, we can now validate that they can be used to assess the ecological status of lakes in terms of morphological alterations across European regions. Our results can be used to further improve ealready existing WFD-compliant multimetric indices, for example by including taxa groups, which show a strong reaction to shoreline alterations. This could be supported by the inclusion of a suit of indicator taxa reflecting the loss of complex habitats such as macrophytes in the lake littoral.

Accidental spills or illegal discharges of pesticides in aquatic ecosystems can lead to exposure levels that strongly exceed authorized pesticide concentrations, causing major impacts on aquatic ecosystems. Such short-term events often remain undetected in regular monitoring programs with infrequent sampling. In early spring 2015, we identified a catastrophic pesticide spill with the insecticide cypermethrin in the Holtemme River, Germany. Based on existing pre-event macroinvertebrate community data, we monitored the effects and recovery of the macroinvertebrate community for more than two years after the spill. Strong short-term effects were apparent for all taxa with the exception of Chironomidae and Tubificidae. Effects could also be observed on the community level as total abundance, taxa number and biomass strongly decreased. Total abundance and taxa number showed a fast recovery. Regarding long-term effects, the total biomass remained substantially below the pre-contamination level (76%) until the end of the study. Also the abundances of three taxa (Gammarus, Leuctra, Limnius Ad.) did not return to levels prior to the spill even after 26 months. This lack of the taxon-specific recovery was likely due to their long generation time and a low migration ability due to a restricted connectivity between the contaminated site and uncontaminated stream sections. These factors proved to be stronger predictors for the recovery than the pesticide tolerance. We revealed that the biological indicators SPEARpesticides and share of Ephemeroptera, Plecoptera and Trichoptera (EPT) are not suitable for the identification of such extreme events, when nearly all taxa are eradicated. Both indicators are functioning only when repeated stressors initiate long-term competitive replacement of sensitive by insensitive taxa. We conclude that pesticide spills can have significant long-term effects on stream macroinvertebrate communities. Regular ecological monitoring is imperative to identify such ecosystem impairments, combined with analytical chemistry methods to identify the potential sources of spills.

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.

Lake shores are characterised by a high natural variability, which is increasingly threatened by a multitude of anthropogenic disturbances including morphological alterations to the littoral zone. ...

Agricultural and urban land use has dramatically increased over the last century and one consequence is the release of anthropogenic chemicals into aquatic ecosystems. One of the rarely studied consequences is the effect of land use change on internal concentrations of organic micropollutants (OMPs) in aquatic invertebrates and its effects on their genotype diversity. Here, we applied population genetic and internal concentrations of OMPs analyses to determine evolutionary implications of chemical pollution on Gammarus pulex populations from a natural and two agricultural streams. Along 14 consecutive months sampled, 26 different OMPs were quantified in G. pulex extracts with the highest number, concentration, and toxic pressure in the anthropogenically stressed stream ecosystems. Our results indicate distinct internal OMP profiles and changes in both genetic variation and genetic structure in streams affected by anthropogenic activity. Genetic variation was attributed to chemical pollution whereas changes in the genetic structure were attributed to environmental disturbances, such as changes in discharge in the impacted stream ecosystems, which worked both independently and in tandem. Finally, we conclude that human-impacted streams are subjected to severe alterations in their population genetic patterns compared to nonimpacted stream ecosystems.

Length–mass relationships are widely used to estimate body mass from body dimensions for freshwater macroinvertebrates. The relationships are influenced by environmental conditions and should be applied within ecosystems and geographic regions similar to those for which they were estimated. However, very few relationships exist for littoral macroinvertebrates, and thus we provide length–mass relationships for macroinvertebrates from lakes of the Central European lowlands. We compared log-linear and nonlinear methods for fitting length–mass relationships and tested the smearing factor for removing bias in mass predictions from log-linear models. We also estimated conversion factors to correct for mass changes during ethanol preservation and assessed the transferability of our results to different geographical regions. We showed that the log-linear approach gave better results in fitting length–mass relationships, while residuals showed that nonlinear models over-predict the mass of small individuals. The smearing correction factor successfully removed bias introduced by log transformation, and relationships transferred well between lakes in the same and different geographical regions. In total, 52 bias-corrected length–mass relationships are provided for littoral macroinvertebrates that are applicable also to lakes in geographic regions with similar environmental conditions, such as the Central European lowlands or the temperate lowland zone of America.

Hydromorphological alterations of lake shores constitute an important pressure on Central European lakes. The local ecological effects of such alterations can be assessed by multimetric assessment methods based on eulittoral macroinvertebrate communities. However, such tools so far only enable a local assessment even if the European Union Water Framework (EU WFD) requires an assessment at whole-lake level. Since due to time and financial constraints it is not feasible to perform small-scale macroinvertebrate assessments over the whole length of the lake shore, the site-specific ecological assessment result needs to be extrapolated. In this study, we analyse the use of a physical habitat survey method including both ground surveys and aerial photo analysis (HML = HydroMorphology of Lakes survey protocol) to enable an extrapolation from local to whole lake ecological assessment. For that purpose, we correlated individual macroinvertebrate metrics as well as results of a multimetric macroinvertebrate-based index (I) with the areal cover of surrounding natural shore elements, (II) with the hydromorphological impact within an encircling rectangle around the macroinvertebrate sampling site, and (III) with the hydromorphological impact in the adjacent epi-, euand sublittoral subsegments. Building up on this, an ecological whole lake assessment was achieved by averaging multimetric index scores calculated for each subsegement based on a regression equation between the multimetric index and the hydromorphological impacts in the three subsegments. This whole lake assessment approach can most likely be applied to geographical regions outside Central Europe and lake types not analysed here (fluvial lowland lakes, prealpine/alpine lakes), but more studies are necessary to verify this. Eventually, the extrapolation presented here may complement existing lake assessments under the EU WFD solely focusing on water quality so far.

Stable isotope techniques are widely applied in aquatic food web research to infer trophic position or to estimate the relative contributions of different dietary resources. The accurate consideration of carbon (C) and nitrogen (N) discrimination in organisms is a critical prerequisite for such studies. Isotopic discrimination of invertebrates may differ systematically between temperate and tropical freshwater environments, but there are little data available on discrimination factors of tropical invertebrates. Here, we analyzed the C (Δ13C) and N discrimination (Δ15N) of eight taxa of benthic freshwater invertebrates from a Southeastern Brazilian tropical catchment, six predator and two shredder species. Predators showed a high variability in Δ13C (−1.5 to 1.3; min–max), but shredders exhibited a lower variability and had negative Δ13C values (−2.1 to −1.8). Values of Δ15N were also highly variable among both predators and shredders, but shredders had negative and lower values (−0.9 to −0.1) than predators (0.0 to 9.1). Tissue turnover rates were equal to or higher than 0.02 d−1 for all invertebrates, suggesting that experiment durations of 50 days may be sufficient for future isotopic discrimination experiments with tropical freshwater invertebrates. Our results suggest that Δ13C enrichment, and to a minor degree also Δ15N enrichment, may not always occur in tropical freshwater invertebrates.

Elevated nitrate concentrations are a thread for water supply and ecological integrity in surface water. Nitrate fluxes obtained by standard monitoring protocols at the catchment outlet strongly integrate spatially and temporally variable processes such as mobilization and turnover. Consequently, inference of dominant nitrate sources is often problematic and challenging in terms of effective river management and prioritization of measures. Here, we combine a spatially highly resolved assessment of nitrate concentration and fluxes along a mesoscale catchment with four years of monitoring data at two representative sites. The catchment is characterized by a strong land use gradient from pristine headwaters to lowland sub-catchments with intense agricultural land use and wastewater sources. We use nitrate concentrations in combination with hydrograph separation and isotopic fingerprinting methods to characterize and quantify nitrate source contribution. The hydrological analysis revealed a clear dominance of base flow during both campaigns. However, the absolute amounts of discharge differed considerably from one another (outlet: 1.42 m3 s− 1 in 2014, 0.43 m3 s− 1 in 2015). Nitrate concentrations are generally low in the pristine headwaters (

This article provides an overview about the Bode River catchment that was selected as the hydrological observatory and main region for hydro-ecological research within the TERrestrial ENvironmental Observatories Harz/Central German Lowland Observatory. It first provides information about the general characteristics of the catchment including climate, geology, soils, land use, water quality and aquatic ecology, followed by the description of the interdisciplinary research framework and the monitoring concept with the main components of the multi-scale and multi-temporal monitoring infrastructure. It also shows examples of interdisciplinary research projects aiming to advance the understanding of complex hydrological processes under natural and anthropogenic forcings and their interactions in a catchment context. The overview is complemented with research work conducted at a number of intensive research sites, each focusing on a particular functional zone or specific components and processes of the hydro-ecological system.

Hydrogen stable isotopes (δ2H) have recently been used to complement δ13C and δ15N in food web studies due to their potentially greater power to separate sources of organic matter in aquatic food webs. However, uncertainties remain regarding the use of δ2H, since little is known about the potential variation in the amount of exchangeable hydrogen (Hex) among common sample materials or the patterns of δ2H when entire food webs are considered. We assessed differences in Hex among the typical sample materials in freshwater studies and used δ2H, δ13C and δ15N to compare their effectiveness in tracing allochthonous matter in food webs of two small temperate lakes. Our results showed higher average amounts of Hex in animal tissues (27% in fish and macroinvertebrates, 19% in zooplankton) compared to most plant material (15% in terrestrial plants and 8% in seston/periphyton), with the exception of aquatic vascular plants (23%, referred to as macrophytes). The amount of Hex correlated strongly with sample lipid content (inferred from C:N ratios) in fish and zooplankton samples. Overall, the three isotopes provided good separation of sources (seston, periphyton, macrophytes and allochthonous organic matter), particularly the δ2H followed by δ13C. Aquatic macrophytes revealed unexpectedly high δ2H values, having more elevated δ2H values than terrestrial organic matter with direct implications for estimating consumer allochthony. Organic matter from macrophytes significantly contributed to the food webs in both lakes highlighting the need to include macrophytes as a potential source when using stable isotopes to estimate trophic structures and contributions from allochthonous sources.

Ecosystems are generally linked via fluxes of nutrients and energy across their boundaries. For example, freshwater ecosystems in temperate regions may receive significant inputs of terrestrially derived carbon via autumnal leaf litter. This terrestrial particulate organic carbon (POC) is hypothesized to subsidize animal production in lakes, but direct evidence is still lacking. We divided two small eutrophic lakes each into two sections and added isotopically distinct maize litter to the treatment sections to simulate increased terrestrial POC inputs via leaf litter in autumn. We quantified the reliance of aquatic consumers on terrestrial resources (allochthony) in the year subsequent to POC additions by applying mixing models of stable isotopes. We also estimated lake-wide carbon (C) balances to calculate the C flow to the production of the major aquatic consumer groups: benthic macroinvertebrates, crustacean zooplankton, and fish. The sum of secondary production of crustaceans and benthic macroinvertebrates supported by terrestrial POC was higher in the treatment sections of both lakes. In contrast, total secondary and tertiary production (supported by both autochthonous and allochthonous C) was higher in the reference than in the treatment sections of both lakes. Average aquatic consumer allochthony per lake section was 27–40%, although terrestrial POC contributed less than about 10% to total organic C supply to the lakes. The production of aquatic consumers incorporated less than 5% of the total organic C supply in both lakes, indicating a low ecological efficiency. We suggest that the consumption of terrestrial POC by aquatic consumers facilitates a strong coupling with the terrestrial environment. However, the high autochthonous production and the large pool of autochthonous detritus in these nutrient-rich lakes make terrestrial POC quantitatively unimportant for the C flows within food webs.

Summary 1. Shoreline development and the associated loss of littoral habitats represent a pervasive alteration of the ecological integrity of lakes and have been identified as major drivers for the loss of littoral biodiversity world-wide. Little is known about the effects of shoreline development on the structure of, and energy transfer in, littoral food webs, even though this information is urgently needed for management and mitigation measures. 2. We measured macroinvertebrate biomass and analysed potential food resources using stable isotopes (δ13C, δ15N) and mixing models to compare the complexity and the trophic base of littoral food webs between undeveloped and developed shorelines in three North German lowland lakes. 3. The lower diversity of littoral habitats found at developed shorelines was associated with lower diversity of food resources and consumers. Consequently, the number of trophic links in food webs at developed shorelines was up to one order of magnitude lower as compared with undeveloped shorelines. 4. Mixing model analysis showed that consumer biomass at undeveloped shorelines was mainly derived from fine particulate organic matter (FPOM) and coarse particulate organic matter of terrestrial origin (CPOM). The contribution of CPOM to consumer biomass was twofold lower at developed shorelines, and consumer biomass was mainly derived from FPOM and suspended particulate organic matter. 5.Synthesis and application. Shoreline development impacts the flow of organic matter within littoral food webs primarily through the reduction in littoral habitat diversity. These effects are exacerbated by clearcutting of the riparian vegetation, which disrupts cross-boundary couplings between the riparian and the littoral zone. Lakeshore conservation should focus on preserving the structural integrity of the littoral zone, while restoration of coarse woody debris, reed and root habitats can be a cost-efficient measure to improve degraded lakeshores. The local effects of shoreline development demonstrated in this study might lead to whole-lake effects, but future studies are needed to derive thresholds at which shoreline development has consequences for the structure and functioning of the entire ecosystem.

Littoral macroinvertebrates are increasingly used for assessing the ecological status of lakes according to the EU Water Framework Directive. This requires harmonised sampling methods, but information on the appropriate spatial scale of the sampling as well as on the adequate sample sizes are mostly lacking. In this study, we compared the spatial variability of littoral (

Summary 1. Ship-induced waves disturb benthic invertebrate assemblages colonizing littoral zones of lakes and rivers. However, the impact of ship-induced waves on invertebrates has rarely been quantified, and the influencing factors have not been addressed. 2. In an experimental wave tank, five benthic invertebrate species, Bithynia tentaculata, Calopteryx splendens, Dikerogammarus villosus, Gammarus roeseli and Laccophilus hyalinus, were exposed to waves of increasing shear stress (0.43–2.19 N m−2). Mean number of detached individuals was recorded for five littoral habitats [coarse woody debris (CWD), reeds, sand, stones and tree roots], representing different levels of structural complexity as quantified by their fractal dimensions (FD). 3. Results showed that detachment of invertebrates was significantly related to shear stress in all habitats except tree roots. Detachments averaged for the five species were significantly lower in habitats with a high degree of structural complexity, decreasing in the habitat sequence: sand, CWD, stones, reeds and tree roots. 4. Consistent with their different morphologies and methods of attachment to substrates, the five species displayed differences in their response to hydraulic stress that were dependent on habitat. 5. The increasing sheltering effect of structural habitat complexity was mirrored by increasing dissipation of the kinetic energy of waves; i.e. the FD of the habitat was positively correlated with shear stress reduction due to the flow resistance of the habitat. 6. Network habitats such as tree roots provided the best sheltering conditions against hydraulic disturbance, because they combined good refuge availability for all studied invertebrate species and maximal dissipation of kinetic wave energy. Consequently, persistent anthropogenic impacts, such as lakeshore modification or long-term exposure to ship-induced waves, which cause disappearance of complex littoral habitats such as tree roots or dense reed belts, will drastically increase the adverse effects of boating and ship traffic on littoral invertebrate assemblages.