Thomas Mehner

Leibniz-Institute of Freshwater Ecology and Inland Fisheries | IGB

This publication brings together 16 leaflets (in English and in local Languages) developed for the DEMO-sites of the EU H2020 PONDERFUL project (2020-2024). These summary “leaflet” cover each of the 16 demonstration pondscapes located across various EU countries, as well as Turkey and Uruguay. The main objective of the DEMO-SITES leaflets is to (i...

Ponds occupy a large share of standing water worldwide and play an important role in providing various ecosystem services. There are concerted efforts of the European Union either to create new ponds, or to restore and preserve existing ponds as nature-based solutions to provide benefits to ecosystem and human well-being. As part of the EU PONDERFU...

Because marine protected areas (MPAs) are not equally effective across their areas, monitoring should progress from dichotomic (within vs. outside) to a finer spatial resolution. Here, we examine the effect of an Eastern Mediterranean no-take MPA on fishes across the MPA and into fished areas, using three methods: underwater visual censuses, acoust...

The trophic niche of aquatic generalist predators is influenced by ontogeny, habitat characteristics, availability and type of prey, and competitive interactions. Many interrelated lake characteristics influence the availability of prey and may thereby impact foraging niches and the trajectory of ontogenetic niche shifts. Our work uses Eurasian per...

Aim The maintenance of broad‐scale connectivity patterns is suggested as a sustainable strategy for biodiversity preservation. However, explicit approaches for quantifying the functional role of different areas in biogeographic connectivity have been elusive. Freshwaters are spatially structured ecosystems critically endangered because of human act...

We investigated the relative influence of local environmental and spatial factors in structuring the community composition of fish at 15 sampling sites along the longitudinal gradient of the Lower Niger River Basin (LNRB) in dry and rainy seasons using distance-based redundancy analysis and variation partitioning analysis. We collected a total of 3...

Intraspecific morphological variation may reflect phenotypic plasticity or adaptive divergence. While adaptive shape divergence may occur more likely among isolated populations with reduced gene flow, phenotypic plasticity may reflect morphological responses to heterogeneous environments, even in spatially connected populations. We evaluated both p...

The spatial isolation gradient of communities and the gradient in the species dispersal ability are recognized as determinants of biodiversity in metacommunities. In spite of this, mean field models, spatially explicit models, and experiments were mainly focused on idealized spatial arrangements of communities leaving aside the combining role of di...

Traditionally, population sizes, ecology, and threats to species and their habitats have been obtained by empirical scientific studies. However, the knowledge of local and indigenous communities worldwide has been acknowledged as an extremely rich and underused source of information on how the environment, biodiversity, and local conditions are cha...

Ponds and “pondscapes” (networks of ponds) are crucial habitats for biodiversity and for delivering multiple benefits to humans, so-called “Nature’s Contribution to People”, such as climate mitigation and adaptation to climate change, creation, and maintenance of habitat for biodiversity, water purification, flood mitigation and cultural benefits (...

An essential factor for aquatic conservation is genetic diversity or population divergence , which in natural populations reflects the interplay between geographical isolation with restricted gene flow and local evolution of populations. The long geological history of Africa may induce stronger among-population divergence and lower within-populatio...

We conducted a systematic evaluation of the correspondence in fish length data obtained from vertical hydroacoustics and gillnetting across seven European natural lakes differing in abiotic and biotic characteristics. Length data were analyzed as continuous size spectra characterized by their maximum-likelihood estimated exponents b. First, we exam...

An organism's body size plays an important role in ecological interactions such as predator-prey relationships. As predators are typically larger than their prey, this often leads to a strong positive relationship between body size and trophic position in aquatic ecosystems. The distribution of body sizes in a community can thus be an indicator of...

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 ra...

Diel vertical migration is widespread in planktonic organisms and fish in ponds and lakes. We discuss diel vertical migration of zoo- and phytoplankton as a habitat selection behavior, highlight its variation, and discuss the proximate factors and adaptive significance of diel vertical migration, how diel vertical migration patterns can be affected...

Africa's freshwater ecosystems are impacted by many threats due to climate change and several anthropogenic activities. In Côte d'Ivoire, the economy relies mostly on agriculture, which covers around 64% of the land surface. The study aimed to investigate Côte d'Ivoire fishermen's perception of climate change, its drivers and other anthropogenic ac...

Considerable theoretical work predicts that intraspecific trait variation can have profound ecological consequences by altering species interactions. Because of their high flexibility, behavioral traits may be especially relevant in mediating how species respond to one another, thus affecting food web dynamics and ecosystem functioning. However, em...

Background Sympatric speciation along ecological gradients has been studied repeatedly, in particular in freshwater fishes. Rapid post-glacial ecological divergence has resulted in numerous endemic species or ecologically distinct populations in lakes of the temperate zones. Here, we focus on the Baltic cisco ( Coregonus albula ) complex, to study...

Vendace is commonly assumed at low risk to recruitment overfishing. This assumption has been confirmed for boreal stocks but might not apply at lower latitudes. We evaluated the risk of recruitment overfishing at the southernmost extent of vendace populations by comparing fecundity, natural mortality, and fishing mortality of two stocks differing i...

Triggered by the adoption of the Water Framework Directive, a variety of fish-based systems were developed throughout Europe to assess the ecological status of lakes. This paper provides a comprehensive overview of all existing systems and summarizes sampling methods, fish community traits (metrics) and the relevant anthropogenic pressures assessed...

Trophic interactions in the pelagic area of lakes and the opposing effects of fish feeding (top-down) and phytoplankton biomass (bottom-up) on zooplankton communities are central topics in limnology. We hypothesized that zooplankton size distributions should be a more sensitive approach to disentangle top-down and bottom-up effects than the commonl...

Net sampling by trawling and hydroacoustics was used to methodologically compare size spectra (SS) of the pelagic fish community in a deep lake across 12 years of sampling. Hydroacoustic SS were generated based on either single-echo detections (SEDs) or tracked-echo groups (TEGs) from 20 cross-lake transects. Trawl SS were obtained by a midwater tr...

Aim: We tested whether there is a strong effect of species interactions on assembly of local lake fish communities, in addition to environmental filters and dispersal. Location: Seven hundred and seventy-two European lakes and reservoirs. Time period: 1993-2012. Major taxa studied: Nineteen species of freshwater fishes. Methods: We applied a laten...

Die „Tesla Gigafactory Berlin-Brandenburg“ wird in der 1. Ausbaustufe 1,4 Mio. m3/a Wasser benötigen. Das geklärte Abwasser mitsamt der problematischen Inhaltsstoffe soll in nur schwach durchflossene, intensiv genutzte Berliner Gewässer eingeleitet werden. Wie sind solche Industrieansiedlungen aus Sicht der Gewässerforschung zu bewerten?

Monitoring the qualitative status of freshwaters is an important goal of the international community, as stated in the Sustainable Development Goal (SDGs) indicator 6.3.2 on good ambient water quality. Monitoring data are, however, lacking in many countries, allegedly because of capacity challenges of less-developed countries. So far, however, the...

Fish community feeding and production rates may differ between lakes despite similar fish biomass levels because of differences in size structure and local temperature. Therefore, across-lake comparisons of the strength and direction of top-down and bottom-up fish–phytoplankton relationships should consider these factors. We used the metabolic theo...

We summarize the scientific knowledge about lake fish communities with respect to taxonomic and functional diversity, to patterns of abundance and biomass, and to size structure. The majority of studies have been conducted in temperate areas of the Palearctic and Nearctic, but we added more scarcely available data about fish communities from Arctic...

“Research for the Future of our Freshwaters“ is IGB’s guiding principle. This involves giving objective and evidence-based information and advice to policymakers, authorities, associations, industry, educational institutions and the public. Within the institute’s publication series called IGB Outlines, which includes the IGB Policy Brief, IGB disse...

Understanding drivers of freshwater fish assemblages is critically important for biodiversity conservation strategies, especially in rapidly developing countries, which often have environmental protections lagging behind economic development. The influences of natural and human factors in structuring fish assemblages and their relative contribution...

It is well documented that aquatic ecosystems may be subsidized by naturally derived terrestrial carbon sources. In contrast, the intentional or unintentional subsidy of animal populations by human-derived feed resources and their ecosystem effects are poorly studied. We added ground baits of the type, amount and temporal duration commonly applied...

Planktivorous fish predation directly affects zooplankton biomass, community and size structure, and may indirectly induce a trophic cascade to phytoplankton. However, it is not clear how quickly the zooplankton community structure and the cascading effects on phytoplankton recover to the unaffected state (i.e. resilience) once short-term predation...

Background The diversity and phylogeny of whitefish of the genus Coregonus is complex, and includes many endemic species of high conservation concern. However, because of commercial importance of whitefish fisheries, stockings and translocations have occurred repeatedly, which challenges the identification of local populations as conservation units...

The pace-of-life syndrome (POLS) hypothesis predicts variation in behaviour and physiology among individuals to be associated with variation in life history. Thus, individuals on the “fast” end of POLS continuum grow faster, exhibit higher metabolism, are more risk prone, but die earlier than ones on the “slow” end. Empirical support is nevertheles...

Size structure of organisms at logarithmic scale (i.e. size spectrum) can often be described by a linear function with a negative slope; however, substantial deviations from linearity have often been found in natural systems. Theoretical studies suggest that greater nonlinearity in community size spectrum is associated with high predator–prey size...

The density of organisms declines with size, because larger organisms need more energy than smaller ones and energetic losses occur when larger organisms feed on smaller ones. A potential expression of density‐size distributions are Normalized Biomass Size Spectra (NBSS), which plot the logarithm of biomass independent of taxonomy within bins of lo...

• Trophic interactions are important pathways of energy and matter fluxes in food webs and are commonly quantified using stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N). An important prerequisite for this approach is knowledge on the isotopic difference between consumer and resource (trophic discrimination, Δ¹³C and Δ¹⁵N). The range and mechan...

Invasion of non-native species is considered a major threat to global biodiversity. Here we present a comprehensive overview of the occurrence, richness and biomass contribution of non-native fish species in 1943 standing water bodies from 14 countries of the Western Palearctic, based on standardised fish catches by multi-mesh gillnetting. We expec...

Trochine et al. 2018 (Ecosystems) ESM

In ectotherms, anthropogenic warming often increases energy requirements for metabolism, which can either impair growth (when resources are limiting) or lead to higher predator feeding rates and possibly stronger top–down trophic interactions. However, the relative importance of these effects in nature remains unclear because: (1) thermal adaptatio...

Body size of consumer species is a fundamental trait that influences the trophic ecology of individuals and their contribution to the functioning of freshwater ecosystems. However, the relationship between body size and trophic ecology can be highly variable both within and between closely-related and similarly-sized species. In this study we compa...

Lake ecosystems are deeply integrated into local and regional economies through recreation, tourism, and as sources of food and drinking water. Shifts in lake phytoplankton biomass, which are mediated by climate warming will alter these benefits with potential cascading effects on human well-being. The metabolic theory of ecology suggests that warm...

Body size, coupled with abundance and taxonomy, may help to understand the mechanisms shaping community structure. Since the body size of fish is closely related to their trophic niche, size diversity (based on individual body size) of fish communities may capture intraspecific variations in fish trophic niches that are not detected by species dive...

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 qua...

Here we provide the metadata for an overview on fish species presence/absence in 1943 Palearctic (Europe + Turkey) lakes and reservoirs. The data have been obtained by standardized multi-mesh gillnet fishing, primarily to fulfill the requirements of the European Water Framework Directive (WFD). The species list encompasses about 100 species, a few...

In fisheries science, stock density is one of the fundamental factors that affect growth and condition. However, reproductive traits like fecundity and also the size and quality of offspring (via maternal effects) can be modified by stock density. In the short-living freshwater fish species vendace (Coregonus albula), the impact of stock density vi...

Animals typically display among-individual differences in behavior that are consistent over time (i.e., personality). These differences are often triggered by variable individual responses to environmental stress factors experienced during life, such as competition for resources and risk of predation. While the causes underlying animal personality...

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 (...

Stable HCN isotope data used in this study. (XLSX)

Mixing model source contributions to invertebrate consumers in Gollinsee and Schulzensee. (PDF)

Animals adjust their behaviour over time and contexts to cope with ecological and environmental variation. However, the presence of consistent between-individual differences in behaviour (i.e. personality) suggests that individual behaviour may be less flexible than previously thought. Here, we tested whether the size of the experimental tank and t...

We studied fish size structure by using mean size, size diversity, and the slope of linear size spectra of six common European fish species along large-scale environmental gradients. We further analyzed the response of these three size metrics to environmental variables and to density-dependent effects, i.e., relative estimates of abundance (catch...

This study tested for links among behaviour, state and life-history variables as predicted by the pace-of-life hypothesis in adult pike Esox lucius. First, a standardized open-field behavioural assay was developed to assess individual behaviour of wild-captured adult E. lucius. Behaviour within the standardized assay predicted swimming behaviour in...

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 di...

Enhancement stocking is a widespread but rarely evaluated practice in German vendace, Coregonus albula L. fisheries, which is precautionarily carried out to buffer yield fluctuations. However, it is assumed that stocking of larval vendace can elevate year-class strength only in the presence of substantial reproductive deficits, which so far cannot...

Planktivorous and benthivorous fish have been documented to influence the density and size structure of their prey communities in lakes. We hypothesized that piscivorous fish modify their prey fish communities in the same way and sought to find evidence for such predation effects from a comparison across 356 lakes located in nine European ecoregion...

Turbidity, caused by suspended particles in the water column, induces light scattering and shifts in the wavelengths of light. These changes may impair the ability of fish to use physical cues and hence may modify social interactions. We experimentally investigated the social interactions of guppies, Poecilia reticulata, in clear and turbid water....

Large scale studies on individual size distribution of fish have primarily been done at community level, but little attention is paid at species level. We described fish size structure by using mean size, size diversity and the slope of linear size spectra of six common European fish species to compare whether these metrics responded to the same la...

Food-web effects of winterkill are difficult to predict as the enhanced mortality of planktivorous fish may be counterbalanced by an even higher mortality of piscivores. We hypothesised that a winterkill in a clear and a turbid shallow lake would equalise their fish community composition, but seasonal plankton successions would differ between lakes...

Body size has been widely recognized as a key factor determining community structure in ecosystems. We analysed size diversity patterns of phytoplankton, zooplankton and fish assemblages in 13 data sets from freshwater and marine sites and found only weak relationships, indicating that predation and competition are not the only determinants of size...

Recent studies have indicated that in fish populations performing diel vertical migrations (DVM), some individuals do not migrate but reflect a resident phenotype, a pattern named as partial DVM. I present data on fish densities and the proportion of residents in Lake Stechlin (Germany) as obtained by annual midwater trawling over four discrete dep...