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Body size is a master trait in aquatic ecosystems to complement traditional taxonomic diversity measures. Based on a dataset of fish communities from 40 Turkish lakes covering a wide environmental gradient and continental to dry cold steppe to Mediterranean climates, we elucidated the key variables controlling size diversity, geometric mean length and number of size classes in the fish community. We further examined how these three size measures were related to species diversity and species richness. A GLM analysis revealed that both size diversity and the number of sizes were strongly related to taxonomic diversity and richness. Furthermore, fish size diversity decreased with decreasing annual precipitation, while the number of size classes increased with increasing lake area but decreased with increasing salinity. The geometric mean length of fish decreased with total nitrogen and increased with altitude. The inter-relatedness between the number of size classes and lake area suggests an increase in fish niches with increasing ecosystem size, while fish are smaller and have fewer size classes in lakes with higher salinity. We conclude that size measures provide valuable integrating information on lake fish diversity; thus, they may complement, but not replace, more traditional taxonomic fish measures. [ABSTRACT FROM AUTHOR]

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 the strengths of predator–prey interactions. The aim of this study was to gain more insight into the relationship between fish body size distribution and trophic position in a wide range of European lakes. We used quantile regression to examine the relationship between fish species' trophic position and their log‐transformed maximum body mass for 48 fish species found in 235 European lakes. Subsequently, we examined whether the slopes of the continuous community size distributions, estimated by maximum likelihood, were predicted by trophic position, predator–prey mass ratio (PPMR), or abundance (number per unit effort) of fish communities in these lakes. We found a positive linear relationship between species' maximum body mass and average trophic position in fishes only for the 75% quantile, contrasting our expectation that species' trophic position systematically increases with maximum body mass for fish species in European lakes. Consequently, the size spectrum slope was not related to the average community trophic position, but there were negative effects of community PPMR and total fish abundance on the size spectrum slope. We conclude that predator–prey interactions likely do not contribute strongly to shaping community size distributions in these lakes. [ABSTRACT FROM AUTHOR]

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 latent variable approach using Bayesian Markov chain Monte Carlo algorithms (R package "BORAL"). We compared the contributions of six environmental predictors and the spatial organization of 772 European lakes in 209 river basins on the presence/absence of the 19 most frequent fish species and on the biomass and mean mass of the six dominant species. We inspected the residual correlation matrix for positive and negative correlations between species. Results: Environmental (50%) and spatial (10%) predictors contributed to the presence/absence assembly of lake fish communities, whereas lake size and productivity contributed strongly to the biomass and mean mass structures. We found highly significant negative correlations between predator and prey fish species pairs in the presence/absence, biomass and mean mass datasets. There were more significantly positive than negative correlations between species pairs in all three datasets. In addition, unmeasured abiotic predictors might explain some of the correlations between species. Main conclusions: Strong effects of species interactions on assembly of lake fish communities are very likely. We admit that our approach is of a correlational nature and does not generate mechanistic evidence that interactions strongly shape fish community structures; however, the results fit with present knowledge about the interactions between the most frequent fish species in European lakes and they support the assumption that, in particular, the mean masses of fish species in lakes are modified by species interactions. [ABSTRACT FROM AUTHOR]

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 theory of ecology to calculate size‐ and temperature‐corrected community energy demand (CEDom) and community production (CP) of omnivorous fishes in 227 European lakes from major habitat types (MHTs) of polar freshwaters, temperate floodplain rivers and wetlands, and temperate coastal rivers. We related CEDom with total phosphorus (TP)‐corrected chlorophyll a (Chl a) concentrations to evaluate a potential top‐down directed trophic cascade from fish to phytoplankton. Furthermore, we related Chl a with CP to demonstrate potential bottom‐up effects of phytoplankton on fish. For both analyses, we added the CED of piscivorous fishes (CEDpi) as a predictor to account for potential predation effects on the omnivorous fish community. CEDom was weakly positively related with TP‐corrected Chl a, but the strength of the relationship differed between MHTs. In contrast, CP was consistently positively related with Chl a in the entire dataset. CEDpi did not contribute to top‐down or bottom‐up relationships. The application of metabolic variables characterizing fish community feeding and production rates makes these results robust because the approach accounted for the usually neglected effects of fish size and temperature in across‐lake comparisons. Our results suggest that bottom‐up effects from phytoplankton on fish secondary production in lakes are substantially stronger than top‐down effects from fish on phytoplankton biomass. [ABSTRACT FROM AUTHOR]

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 ratios but low predator–prey abundance ratios; however, empirical evaluation of the effects of predator–prey interactions on nonlinear structures remains scarce. Here, we aim to empirically explore the pattern of the size‐specific residuals (i.e. deviations from the linear regression between the logarithmic fish abundance and the logarithmic mean fish size) by using size spectra of fish communities in 74 German lakes. We found that nonlinearity was strong in lakes with high predator–prey abundance ratios but at low predator–prey size ratios. More specifically, our results suggest that only large predators, even if occurring in low abundances, can control the density of prey fishes in a broad range of size classes in a community and thus promote linearity in the size spectrum. In turn, the lack of large predator fishes may cause high abundances of fish in intermediate size classes, resulting in nonlinear size spectra in these lakes. Moreover, these lakes were characterized by a more intense human use including high fishing pressure and high total phosphorus concentrations, which have negative impacts on the abundance of large, predatory fish. Our findings indicate that nonlinear size spectra may reflect dynamical processes potentially caused by predator–prey interactions. This opens a new perspective in the research on size spectrum, and can be relevant to further quantify the efficiency of energy transfer in aquatic food webs. [ABSTRACT FROM AUTHOR]

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 diversity. Thus, the relationship between size diversity and species diversity may help to integrate variation at both intraspecific and interspecific levels. We studied the relationship between species diversity and size diversity as a measure of the degree of overlap in size among species and thereby the potential overlap in niches in a community. We hypothesized that the relationship between size diversity and species would be different across the European continent due to different levels of size overlap in fish communities. The data were derived from samplings of fish communities using standardised benthic gill nets in 363 lakes. At the continental scale, size diversity increased with species diversity; at the ecoregion scale, the slope of the relation changed across the continent, with the greatest mismatch occurring in northern Europe where communities comprised only one or a few species, but each of which exhibited a great range in size. There was an increase in slope towards the south with significant relations for four out of six ecoregions. The steeper size diversity‐species diversity slope at lower latitudes is attributable to a lower overlap in fish size and thus likely to finer niche separation. Our results also suggest that size diversity is not a strong surrogate for species diversity in European lake fish communities. Thus, particularly in fish communities composed of few species, measuring size diversity may help to detect potential functional variation which may be neglected by measuring species diversity alone. [ABSTRACT FROM AUTHOR]

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 expected strong geographical gradients to emerge in the occurrence of non-natives. We further hypothesised that the contribution by non-natives to the local fish community biomass was correlated with local richness and the trophic level of native and non-native species. Non-native fish species occurred in 304 of 1943 water bodies (16%). If the average number of occupied water bodies per country was weighted by number of water bodies per country, the grand mean occurrence of non-natives in Western Palearctic water bodies was 10%. Exotic (non-native to the Palearctic) and translocated (non-native only to parts of the Palearctic) species were found in 164 (8.4%) or 235 (12.1%) of the water bodies, respectively. The occurrence and local richness of non-native fish species increased with temperature, precipitation and lake area and were substantially higher in reservoirs than in natural lakes. High local biomass contributions of non-native species were strongly correlated with low richness of native species and high richness of non-native species, whereas the trophic level of the fish species had only a weak effect. Single non-native species rarely dominated community biomass, but high biomass contributions and thus strong community and ecosystem impacts can be expected if several non-native species accumulate in a water body. [ABSTRACT FROM AUTHOR]

The estimation of biodiversity is complex in remote areas like Andean-Amazon piedmont where the lack of knowledge is noticeable and most species have not yet been identified. An alternative to the taxonomical approach to assess biodiversity and community structure is to study the body size structure of the community. The present study aimed to identify systematic changes of the size structure of fish communities along the altitudinal gradient of pristine Andean streams. At the same time, we assessed if size structure of fish communities could be an alternative or complementary approach to traditional taxonomic metrics of ecological assessment. This study was conducted along two rivers of eastern Andean piedmont: Hacha River and Alto Madre de Dios basin. Our analysis showed lower values of total size range, maximum length and number of size classes in the highest places and increasing in lower reaches of the rivers. Concerning abundance size spectra, we found flatter slopes (indicating higher relative abundance of large fish) and lower coefficient of determination (i.e. less linear size spectra) and intercept at y-axis on upstream reaches. On the other hand, fish abundance, species richness and species diversity increasing from upstream to downstream. These systematic changes on size-related variables were similar using both basins together or independently and may be used in future studies as management reference conditions when assessing the effect of perturbations in freshwater ecosystems, while bypassing the error associated with the lack of taxonomic information. [ABSTRACT FROM AUTHOR]

Highlights • Small sized species of alien fish dominate the lower Ebro River fish community. • Normalized Abundance Spectra (NAS) of fish were evaluated monthly for one year. • NAS reflected hydro-peaking impacts on the fish community. • High hydro-peaking conditions may increase predatory efficiency of piscivorous fish. Abstract The European Water Framework Directive requires the integration of body size characters as an important part of fish-based bioassessment tools for freshwaters ecological status determination. The study of the entire fish community size-structure provides valuable information about food web capacity, food web stability and ecological efficiency of aquatic ecosystems. One of the most used representations of community size structure is the Normalized Abundance Spectra (NAS) that provides an approximation of the total fish abundance and food web capacity (through the parameter y-intercept) and an estimation of food web efficiency (through the slope of the linear regression). In this study we explored NAS of the lower Ebro River fish community by integrating data from monthly electrofishing samplings during a whole year (November 2014- October 2015). We found that the percentage of total alien and alien-prey individuals were directly related with y-intercept and inversely related with slope of NAS. This is because the bulk of the community consists of introduced species of small body length. Furthermore, we detected significant relationships between NAS-related parameters and the hydrological variables describing diel flow oscillations and daily flow variability. Based on this, we suggest that high flow variability conditions and, above all, high hydro-peaking conditions, caused a diminution of the total abundance of fish and a decrease of the proportion of small sized fish (i.e. lower y-intercept and flatter slopes of NAS, respectively). Finally, a significant interaction between hydro-peaking and the percentage of alien-predators suggests that high hydro-peaking conditions benefit predation by facilitating predator-prey encounters. This is reflected by strong linear relationship between NAS parameters and percentage of piscivorous at high hydropeaking conditions that disappears at low hydro-peaking fluctuations. We concluded that the high proportion of alien fishes and the presence of a hydropower generation plant that operates by hydro-peaking are important factors determining fish size structure in the lower Ebro River. [ABSTRACT FROM AUTHOR]