European Freshwater Ecosystems Research Articles

Towards achieving a twenty-fold increase in the coverage of freshwater species distributions within protected areas in Europe

Freshwater ecosystems are threatened worldwide, with 1/3 of freshwater species being under extinction risk. Yet, systematic conservation planning has received little attention as a means to prioritize freshwater conservation areas. Here, we develop a comprehensive approach to spatially delineate key areas for freshwater conservation across Europe, considering directed-connectivity, climate change and human-related disturbances. We assess the adequacy of the current reserve network in protecting European freshwater ecosystems and provide guidelines for expansion. We conducted 13 prioritization scenarios for 18,300 sub-basins across Europe, accounting for the distribution of 1,668 freshwater species. Connectivity properties, metrics of climatic stability and human induced pressures were incorporated into the spatial prioritization framework. We highlight the critical importance of specific regions in Central and Southern Europe, the Mediterranean Basin, and parts of the Balkan Peninsula for future freshwater conservation efforts. However, our analysis also reveals the complete inadequacy of the current reserve network, as protected sub-basins only cover 4 % of the study area and encompass on average only 3 % of freshwater species distributions. In contrast, expanding current reserves to cover the top 17 % of high priority areas across Europe could substantially improve the overall coverage of species distributions, reaching an average of 61.5 % for all freshwater species and even exceeding 93.1 % for species of conservation interest (critically endangered, endangered and vulnerable species). The priority freshwater conservation network derived from the analysis provides specific spatial recommendations to help achieve conservation goals if the current reserve network is appropriately expanded.

Distribution of Non-Indigenous Crayfish Species in Estonia and Their Impacts on Noble Crayfish (Astacus astacus L.) Populations

Invasive non-indigenous crayfish species (NICS) are a major threat to the existence of native crayfish populations in European freshwater ecosystems. The discovery of signal crayfish Pacifastacus leniusculus, marbled crayfish Procambarus virginalis, and spiny-cheek crayfish Faxonius limosus in Estonia has increased the risk of extinction of Estonia’s only native crayfish species, the noble crayfish Astacus astacus. The aim of this study was to give an overview of the status, distribution, and impacts of P. leniusculus, F. limosus, and Procambarus virginalis on A. astacus populations and assess the effect of trapping on NICS abundance. Annual monitoring of crayfish has been carried out since 2008 as part of A. astacus conservation and the NICS eradication plan. In this study, we present data from nine sampling locations monitored from 2010 to 2022. The spread of NICS continues to increase beyond their distribution areas, and in two sampling locations P. leniusculus and A. astacus live in sympatry. Our results suggest that trapping has a limited effect on population abundance, as NICS have already caused the extinction of two A. astacus populations. However, intensive trapping should continue simultaneously with sensitive molecular techniques to monitor the spread of NICS.

Updating the Chronic Freshwater Ecotoxicity Database and Biotic Ligand Model for Nickel for Regulatory Applications in Europe.

Bioavailability has been taken into account in the regulation of nickel in freshwater ecosystems in Europe for over a decade; during that time a significant amount of new information has become available covering both the sensitivity of aquatic organisms to nickel toxicity and bioavailability normalization. The ecotoxicity database for chronic nickel toxicity to freshwater organisms has been updated and now includes 358 individual data points covering a total of 53 different species, all of which are suitable for bioavailability normalization to different water chemistry conditions. The bioavailability normalization procedure has also been updated to include updates to the bioavailability models that enable more sensitive water chemistry conditions to be covered by the model predictions. The updated database and bioavailability normalization procedure are applicable to more than 95% of regulated European surface water conditions and have been used to calculate site-specific criteria for a variety of different water chemistry scenarios, to provide an indication of how the sensitivity to nickel varies between different water types. The hazardous concentration for 5% of a species (HC5) values for this diverse selection of water types range from 1.6 to 36 µg L-1 , clearly demonstrating the importance of accounting for nickel bioavailability in freshwaters. This updated database and bioavailability normalization procedure provide a robust basis for the derivation of regulatory thresholds for chronic nickel toxicity in freshwaters such as predicted no-effect concentrations and Environmental Quality Standards and are protective of the results of several mesocosm studies. Environ Toxicol Chem 2023;42:566-580. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

The red alga Hildenbrandia rivularis is a weak indicator of the good ecological status of riverine habitats

Our knowledge about the size of benthic red alga populations inhabiting European freshwater ecosystems is still insufficient. For many years, in Europe, Hildenbrandia rivularis (Rhodophyta, Hildenbrandiaceae) was believed to be a very rare species of crustaceous red algae, valuable for oligotrophic system biomonitoring. In this paper, we challenge this assumption and show the phenomenon of H. rivularis expansion (i.e., from mountains and highlands to lowlands) in Europe based on detailed data from Poland. We collected over 311 records of H. rivularis occurring in rivers, streams and lakes (not typical habitats for this alga) across the country, including historical data. Here, we show the detailed distribution of this endangered species in Poland, one of the largest European countries. To examine the mechanism of H. rivularis population expansion, physicochemical data from 122 aquatic ecosystems were investigated. A comparison between three subperiods (1860–1944, 1945–1999, and 2000–2019) revealed an increase in the average abundance of the H. rivularis population in the lowlands area. However, the studied species seems to be disappearing in mountain and upland ecosystems. We found altitude and water flow and quality (i.e., alkalinity, oxygenation and biogens) to be the main determinants of H. rivularis occurrence within the study area. The results indicate that this red alga can be described as an expansive species, and its occurrence does not always indicate good water quality and fast water flow. Surprisingly, in contrast to observations from the last century, the population of H. rivularis in Poland currently occurs mainly in habitats with eu- and hypertrophic waters. We point out here that in the zone with temperate climate influence, H. rivularis is no longer an obvious indicator for oligotrophic ecosystems. We suggest that climate warming and eutrophication–reoligotrophication may have contributed to the increased spread of H. rivularis in Europe over recent decades. In addition, we see no reason to classify H. rivularis as an endangered species in Poland. This species most often inhabits anthropogenic water systems and occurs at high abundance in waters with high concentrations of biogens. For these reasons, H. rivularis is more ubiquitous than a cosmopolitan species (i.e., with a wide range of occurrence but associated with only a specific type of habitat and specific environmental parameters). Thus, the value of H. rivularis as a bioindicator for oligotrophic water ecosystems is very low.

Cost‐saving population genomic investigation of Daphnia longispina complex resting eggs using whole‐genome amplification and pre‐sequencing screening

Resting stages of aquatic organisms that accumulate in the sediment over time are an exceptional resource that allows direct insights into past populations and addressing evolutionary questions. This is of particular interest in taxa that face relatively new environmental challenges, e.g., climate change and eutrophication, such as the Daphnia longispina species complex, a keystone zooplankton group in European freshwater ecosystems. However, genomic analysis might be challenging as DNA yield from many of these resting stages can be low and the material degraded. To reliably allow the resequencing of single Daphnia resting eggs from different sediment layers and characterize genomic changes through time, we performed whole-genome amplification to obtain DNA amounts suitable for genome resequencing and tested multiple protocols involving egg isolation, whole-genome amplification kits, and library preparation. A pre-sequencing contamination screening was developed, consisting of amplifying mitochondrial Daphnia and bacterial markers, to quickly assess and exclude possibly contaminated samples. In total, we successfully amplified and sequenced nine genomes from Daphnia resting eggs that could be identified as Daphnia longispina species. We analyzed the genome coverage and heterozygosity of these samples to optimize this method for future projects involving population genomic investigation of the resting egg bank.

The monitoring of diet and habitat preferences indicates competitive effect of exotic Ictalurus nebulosus on native fish under food-limited conditions

Due to the increasing globalization and ongoing introduction of exotic species, native species are confronted with competitors with unknown outcomes. Catfish Ictalurus nebulosus Le Sueur, 1819 is native to eastern regions of North America. The species was introduced to some European countries, including Poland, in the 1880s for hobby and farming purposes, and it is currently present in freshwater bodies of almost the whole Europe. In this study, stomach content analyses were used to study food and habitat partitioning between catfish and native fish coexisting in a nutrient-poor lake. The results suggest strong competition between catfish and native perch, resulting in the dominance structure of fish untypical for nutrient-poor lakes. Moreover, catfish, which are opportunistic food generalists, displayed a very narrow food niche and selectively foraged on the most nutritional food items. This may suggest that the foraging opportunity of catfish undoubtedly supports the spreading of this species among various habitats, but catfish may become specialists when colonizing some ecosystems. In the context of the current expansion of catfish towards the eastern periphery of Europe, monitoring of its population sizes, trophic niches and effects on lower trophic levels are essential for the management and mitigation of its potential impacts on invaded freshwater ecosystems in Europe.

DNA barcodes evidence the contact zone of eastern and western caddisfly lineages in the Western Carpathians

The region of the Western Carpathians is, among other aspects, very important for survival and diversity of European freshwater fauna due to the presence of a large number of (sub)mountain springs and streams. However, these ecologically and faunistically diversified habitats are still understudied in the context of genetic diversity and population structure of their inhabitants. This study focuses on genetic diversity and distribution patterns of the caddisfly Rhyacophila tristis, common and widespread representative of mountain freshwater fauna. Analysis of the COI mitochondrial marker revealed presence of the western and eastern lineages, with samples from both lineages being grouped in BOLD (Barcode of Life Data System) into separate BINs (Barcode Index Numbers). Our data indicates that eastern lineage (BIN_E) is more closely related to the Balkan populations than to co-occurring western lineage (BIN_W), and that the contact zone of the lineages passes through the W Carpathians. The study revealed phylogeographic and demographic differences between lineages, supporting hypothesis of their evolutionary independence and specific ecological preferences. The obtained genetic data of the R. tristis population from W Carpathians improved our knowledge about population genetics of this aquatic species and can contribute to understanding the state and evolution of biodiversity of freshwater ecosystems in Europe.

Weak evidence of spatial segregation between the vulnerable southern water vole (Arvicola sapidus) and the two main invasive mammals of European freshwater ecosystems

ABSTRACT Niche differentiation by spatial segregation facilitates the coexistence of species sharing ecological preferences, which can buffer the impact of biological invasions on native species. The introduction of two semi-aquatic rodents, the coypu Myocastor coypus and the muskrat Ondatra zibethicus, to most freshwater ecosystems across Western Europe, has been pinpointed as a cause for the decline of the southern water vole (SWV) Arvicola sapidus. We investigated the co-occurrence of these three species in a river catchment of northwest France and whether spatial habitat segregation took place at two spatial scales, using hierarchical modelling accounting for imperfect detection. At a large spatial scale (river catchment), the occupancy rate of the SWV was 0.52 ± 0.06, i.e. noticeably smaller compared to coypu (0.58) and muskrat (0.80). We found no evidence of a negative effect of the presence of the two invasive rodents on SWV occurrence. At a smaller spatial scale (SWV home range), we found weak evidence of spatial segregation in habitat use with a negative, although not significant, effect of muskrat. Overall, our results suggest that riparian habitats in the study area allow the southern water vole to coexist with two larger invasive rodents, provided that hygrophytic vegetation is preserved alongside rivers.

Identifying genome level variation in generalist and specialist bacteria and their relationship to abiotic conditions in European freshwater ecosystems

Microbial biodiversity is one of the increasingly studied fields, yet we are unable to accurately pinpoint the intricacies of how the biotic landscape changes with its abiotic counterpart. Every organism reacts differently to physical changes in its surrounding(Zeglin (2015)). While there are some generalist microbes that can grow in limited capacity in many different types of environment, there are also some specialists that can flourish in very specific environments(Monard et al. (2016)). We hypothesize that both generalists and specialists have developed advanced strategies that allow them thrive under diverse conditions or in very specific niches, and that these changes should be traceable on the genomic level. We expect to see variations in factors like genome sizes, number of genes, GC content, metabolic pathways that yield growth and replication advantages in nutrient deficient niches and defense mechanisms. We also expect to identify genomic streamlining in organisms that survive in nutrient deficient environments. We are currently studying the genetic basis of why these generalists are so universally present though in smaller numbers, and what allows the specialists to efficiently exploit the abiotic factors of their specific environment in order to flourish. To pursue this, we have sampled and sequenced metagenomes from 47 lakes spread across Europe with strongly diverging environmental conditions such as pH, temperature, organic and nutrient content, elevation, and conductivity. We’ve identified reads from more than 9000 taxonomically unique organisms, of which 700 were identified as generalists and 1200 as specialists, based on niche width permutations. In total we have assembled 313 high quality Metagenome Assembled Genomes (MAGs). Which now allow us to get a more detailed insight into the genetic basis and specific adaptations of these organisms.

Selection and application of trophic magnification factors for priority substances to normalize freshwater fish monitoring data under the European Water Framework Directive: a case study

BackgroundThe European Water Framework Directive (WFD) requires the monitoring of biota—preferably fish—to check the compliance of tissue concentrations of priority substances (PS) against substance-specific environmental quality standards (EQSs). In monitoring programs, different fish species are covered, which often are secondary consumers with a trophic level (TL) of about 3. For harmonization, a normalization of monitoring data to a common trophic level is proposed, i.e., TL 4 (predatory fish) in freshwaters, so that data would be sufficiently protective. For normalization, the biomagnification properties of the chemicals can be considered by applying substance-specific trophic magnification factors (TMFs). Alternatively, TL-corrected biomagnification factors (BMFTLs) may be applied. Since it is impractical to derive site-specific TMFs or BMFTLs, often data from literature will be used for normalization. However, available literature values for TMFs and BMFTLs are quite varying. In the present study, the use of literature-derived TMFs and BMFTLs in data normalization is studied more closely.ResultsAn extensive literature evaluation was conducted to identify appropriate TMFs for the WFD PS polybrominated diphenyl ethers (PBDE), hexachlorobenzene, perfluorooctane sulfonate (PFOS), dioxins and dioxin-like compounds (PCDD/F + dl-PCB), hexabromocyclododecane, and mercury. The TMFs eventually derived were applied to PS monitoring data sets of fish from different trophic levels (chub, bream, roach, and perch) from two German rivers. For comparison, PFOS and PBDE data were also normalized using literature-retrieved BMFTLs.ConclusionsThe evaluation illustrates that published TMFs and BMFTLs for WFD PS are quite variable and the selection of appropriate values for TL 4 normalization can be challenging. The normalized concentrations partly included large uncertainties when considering the range of selected TMFs, but indicated whether an EQS exceedance at TL 4 can be expected. Normalization of the fish monitoring data revealed that levels of substances accumulating in the food web (TMF or BMF > 1) can be underestimated when relying on fish with TL < 4 for EQS compliance assessment. The evaluation also revealed that TMF specifically derived for freshwater ecosystems in Europe would be advantageous. Field-derived BMFTLs seemed to be no appropriate alternative to TMFs, because they can vary even stronger than TMFs.

Two lineages of the invasive New Zealand mudsnail Potamopyrgus antipodarum spreading in the Baltic and Black sea basins: low genetic diversity and different salinity preferences

The New Zealand mudsnail Potamopyrgus antipodarum is one of most widespread invasive species worldwide. Two phenotypically diverse but genetically mostly uniform clones have been reported in Europe, typically occuring allopatrically. It has been suggested that salinity may determine distributions of the two lineages in Europe, but the hypothesis remains speculative. The snail has recently expanded its distribution area into Central European lakes, but the clonal affinity and ecological tolerances of the species have not yet been assessed. In this study, we use mitochondrial and nuclear genetic markers to explore the diversity of P. antipodarum in 19 locations in Lithuania, Poland, Ukraine and Finland. We first confirm congruence between the independently reported mitochondrial and nuclear lineages, now called multi-locus lineages T and Z. Both lineages were found in the studied area, and, in agreement with earlier studies, both showed extremely low genetic diversity. Only lineage T occured in freshwater ecosystems in Central Europe, whereas only lineage Z was found in brackish waters of the Black Sea basin. Both lineages occur in the Baltic Sea, although mostly allopatrically. It could be expected that the lineage T will expand its distribution area in the Central European freshwater ecosystems, but further studies are needed to understand the origins and occurrence of the lineage Z in the Baltic Sea and southern Ukraine.

Cyanobacteria from freshwater lakes in the Azores archipelago, Portugal: data from long term phytoplankton monitoring.

BackgroundThe Azores are oceanic islands located in the Northern Atlantic Ocean and are particularly rich in aquatic systems, ranging from freshwater, brackish, marine and thermal habitats. Due to the increase in local anthropogenic pressures and global warming, several azorean lakes began to reveal signs of eutrophication that led to the implementation of monitoring programmes and management strategies on the most impacted lakes. Later, the Water Framework Directive (2000/60/EC) demanded the establishment of biomonitoring programmes for European freshwater ecosystems and the limited Azorean monitoring programmes were extended to a larger set of lakes. Since the establishment of the aquatic systems monitoring programme in the Azores archipelago, lakes have been regularly sampled, producing innumerous unpublished records of cyanobacteria that are difficult to access.New informationHere we present the occurrences of cyanobacteria in Azorean lakes that result from 22 years of phytoplankton monitoring in a total of 1948 cyanobacteria occurrences from 968 phytoplankton sampling events on Azorean lakes done between 1996 and 2018 as part of regional inland aquatic ecosystems monitoring programmes. Forty two cyanobacteria taxa were identified in those events, 28 species and 14 at genus level. This information is crucial for conservation, biodiversity studies and lake management, as some of the cyanobacteria species present are bloom-forming and have the ability to produce toxins. This will also allow for the identification of invasive species and possible targeted control and mitigation programmes, according to the species present in the Azorean lakes.

Portraying fisheries and ecological status of a Mediterranean lake

Background. Gaps of knowledge in lentic systems and discrepancies in official fishery statistics biased fisheries state and inhibits the development of conservation strategies. For that reason the integration of conventional (official data from fisheries landings) and unconventional (fishermen knowledge) sources of data, accompanied with the use of historical archives will outline the framework of the monitoring of freshwater resources.The presently reported study integrated local ecological knowledge of fishers, historical data, field surveys and scientific data to present better management options for a Mediterranean lentic system (Volvi Lake, northern Greece). Materials and methods. On-the-spot interviews were conducted with the professional fishers of Volvi Lake (northern Greece) in parallel with field surveys of environmental parameters during 2014&ndash;2015 and complemented with archival freshwater-related information. Results. The results of the presently reported study point to commercial fisheries as a declining activity, with little scope for a future, due to internal and external threats. This diagnostic can probably be generalized beyond the case study to other freshwater ecosystems in Europe, where decreasing fisheries productivity due to overfishing, habitat loss, and pollution, coupled with low economic productivity has led to an increased marginalisation of freshwater fisheries. Conclusion. The reported difficulties of freshwater fisheries could be bypassed through creation of fishers&rsquo; typology regarding their dependence on fisheries. Upgrading the methodological approach followed by the official reporting system in freshwaters might also facilitate the fulfilment of the requirements of the EU Water Framework Directive.

Chemical surveillance in freshwaters: small sample sizes underestimate true pollutant loads and fail to detect environmental quality standard exceedances

BackgroundChemical surveillance in surface waters is crucial to identify potential threats to the health of freshwater ecosystems. Usually, the concentrations of pollutants are highly variable over the course of the year and often result in non-normally distributed data sets. Therefore, the European Water Framework Directive recommends measuring, e.g. priority substances at least 12 times a year to achieve an acceptable accuracy level for the estimation of the true mean annual loads. However, in Europe priority substances are often measured much less frequently. In this context, the aim of the present study was to analyze how sample size, temporal variability and skewness of the data sets influence the accuracy of the mean annual load estimation and the assessment of annual average environmental quality standards. For this purpose, sample size simulations using weekly composite samples of benzo(a)pyrene, 4-tert-octylphenol, fluoranthene and di(2-ethylhexyl) phthalate, selected as representatives for priority substances, were carried out.ResultsThe sample size simulations showed two general patterns: the accuracy of the mean annual load estimation increased with increasing sample size and skewness and temporal variability were more apparent in smaller sample sizes. In right-skewed data sets, small sample sizes led, on average, to a systematic underestimation of the true mean annual load whilst in a few cases these led to an overestimation. Although the study was carried out on priority substances, results can be transferable to other pollutants. Furthermore, in small sample sizes a considerable proportion of the simulated means failed to detect annual average environmental quality standard exceedances.ConclusionsThe results of the present study indicate that the usage of small sample sizes is likely to result in an underestimation of the true mean annual pollutant loads in chemical surveillance and scientific research, thus potentially jeopardizing the validity of results. Therefore, it is recommended to avoid the usage of small sample sizes for the determination of mean annual pollutant loads. Furthermore, priority substances should be sampled according to the European Water Framework Directive guidelines at least 12 times/year to improve the assessment of the threat posed by pollutants to freshwater ecosystems in Europe.

Angling as a source of non-native freshwater fish: a European review

In a context of the growing concern about the impact of biological invasions, our objective is to review the role of those non-native species that have primarily been introduced for angling purposes in at least one European country. We are particularly interested in: (1) the relative role of sport fish species in the context of non-native species introductions; (2) assessing the relative importance of different fish taxa; (3) identifying similarity patterns in the composition of the angling fish species introduced throughout the continent, and (4) assessing the underlying factors that drive their diversity in Europe. According to our results, 23.6% of the freshwater fish introduced into Europe during the last century were released primarily for angling purposes. The species composition differed among countries, with a higher diversity of introduced angling species in larger countries and in those with a greater GDP per capita, along with a lower latitude. This review stresses that angling was a significant pathway for the introduction of invasive fish species into Europe in the last century. Furthermore, some of the introduced angling species had severe environmental impacts on many European regions. However, introductions of non-native angling species are still occurring. Therefore, existing EU regulations need better enforcement as well as to increase public awareness regarding invasive fish. This will help to preserve biodiversity and improve the sustainability of current angling schemes in increasingly managed European freshwater ecosystems. However, non-native fish could make angling sustainable, although not for biodiversity generally.

When alien catfish meet—Resource overlap between the North American Ictalurus punctatus and immature European Silurus glanis in the Arno River (Italy)

AbstractDue to the increasing globalisation and ongoing introduction of alien species specifically regarding European freshwater ecosystems, native and already present alien species will be confronted with competitors with unknown outcomes. One such case is the situation of the European catfish Silurus glanis introduced in the Arno River (Central Italy), a species sought after by anglers, which is facing competition from the later introduced alien North American channel catfish Ictalurus punctatus. Large catfish species are highly valued among anglers, but their interspecific interactions and potential ecosystem‐level impacts are still poorly known. We used stomach contents and stable isotope analyses to study niche partitioning between these two alien catfish species, coexisting in the Arno River. The results suggest partial niche segregation, with immature S. glanis showing a narrower dietary and isotopic niche and a slightly higher trophic position than I. punctatus. Monitoring the catfish population sizes, trophic niches and effects on lower trophic levels are essential for future management and mitigation of their potential impacts on invaded freshwater ecosystems.

Status, trends, and future dynamics of freshwater ecosystems in Europe and Central Asia

ABSTRACTThis review is part of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) report on Europe and Central Asia (ECA) and provides a critical assessment of issues facing decision-makers, including freshwater biodiversity and ecosystem trends as well as drivers of change. Freshwater systems are well established as the most threatened ecosystem type in the ECA region, with the quantity and quality of habitats and abundance of many species rapidly declining. Only about half (53%) of the EU's rivers and lakes achieved good ecological status in 2015 (as defined by the Water Framework Directive in terms of the quality of the biological community), and many lakes, ponds, and streams are disappearing as a consequence of agricultural intensification and inefficient irrigation and urbanisation, combined with climate change. The situation regarding freshwater biodiversity remains highly critical in ECA as many species remain threatened with extinction, including >50% of known species for some groups (e.g., molluscs, amphibians). Drivers of ECA freshwater taxa include the destruction or modification of their habitat, including water abstraction, which affects ∼89% of all amphibian threatened species and ∼26% of threatened freshwater invertebrate species. Of particular concern is the lack of data for freshwater invertebrates. Current status is available for only a minority of species, and the impact of alien invasive species is often unknown, especially in Central Asia. Based on current freshwater biodiversity trends, it is highly unlikely that ECA will achieve either the respective Aichi biodiversity targets by 2020 (i.e., targets 2 to 4, 6 to 12, and 14) or Target 1 of the Biodiversity Strategy.

Plasticity in habitat use of two native Ponto-Caspian gobies, Proterorhinus semilunaris and Neogobius fluviatilis: implications for invasive populations

The Ponto-Caspian region is an important source area for some invasive gobiid fishes. These fishes have colonised several freshwater ecosystems in Europe, as well as in North America. As knowledge on their habitat utilisation in their native range remains limited, the seasonal habitat uses of Western tubenose goby, Proterorhinus semilunaris and monkey goby Neogobius fluviatilis were studied in four natural lakes in the Marmara Region (NW Turkey). Habitat use of both species was highly variable between the lakes and seasons, with P. semilunaris showing higher plasticity. In general, the main habitats used by P. semilunaris were shallow waters in littoral areas that had vegetation cover, whereas N. fluviatilis consistently used sandy substratum in deeper waters that lacked vegetation. These results suggested there was high plasticity in the habitat utilisation of these two gobiids, especially in P. semilunaris, which potentially facilitates their ability to establish and invade novel environments. The habitats that are especially vulnerable to P. semilunaris invasion are medium size substrates which provide individuals with sufficient interstitial space for refuge. Waters providing differing habitats, such as sandy substrata, might inhibit their colonisation, but are potentially more vulnerable to N. fluviatilis invasion.

Global warming may lower thermal barriers against invasive species in freshwater ecosystems – A study from Lake Constance

European freshwater ecosystems are increasingly invaded by exotic animal and plant species. Apart from increased connectivity between previously separated watersheds, the increasing temperature of the hydrosystems favors the spread of exotic species. The freshwater fauna of Central Europe is still shaped by the cold-adapted animal assemblages resulting from the last glaciation. It is less diverse, and the species are putatively less performant competitors, compared to the warm-adapted, species-rich fauna of the Ponto-Caspian realm, from which many current aquatic invaders are coming. Our study analyses potential mechanisms explaining the coexistence between one of the most impacting aquatic invaders of the past decades, the ‘killer shrimp’ Dikerogammarus villosus and the previously dominating amphipod Gammarus roeselii in Lake Constance, using laboratory predation experiments and field surveys. Our results indicate two key drivers for coexistence: low winter temperatures and the substrate structure of the alga Chara sp. At temperatures below 6 °C, the predation pressure on G. roeselii was strongly reduced; G. roeselii can therefore disperse throughout the littoral in winter, avoiding predation by D. villosus. Artificial heating of a section of the lake shore, however, resulted in local extinction of G. roeselii by D. villosus. The macroalga Chara sp. completely inhibited predation by D. villosus on G. roeselii. Climate change scenarios indicate that global warming might destroy this thermal refuge during winter until 2085. For the survival of G. roeselii it will then be crucial, which part of the Chara population will maintain epigeic plant parts during winter. The complex interplay between thermal and physical refuges for native species in the context of climate change and changing trophic status of freshwater systems, as disentangled by our study, shows that ecosystem management and restoration strategies need to better consider multiple stressors (and their rather complex mitigation strategies).

Species turnover and invasion of dominant freshwater invertebrates alter biodiversity–ecosystem‐function relationship

AbstractFreshwater ecosystems rely on allochthonous resources. Integration of these subsidies depends on diversity of both terrestrial resources and aquatic shredder and decomposer communities, but the diversity effects on leaf litter breakdown and decomposition are less clear in aquatic than terrestrial ecosystems. We need a better understanding of this relationship because aquatic communities are rapidly changing with species invasions and anthropogenic impacts. Here, we experimentally disentangled the effects of leaf and shredder richness on leaf litter breakdown by macroinvertebrates in mesocosm experiments using three species of amphipods, a dominant guild of crustaceans in European freshwater ecosystems. Increased leaf richness led to lower‐than‐predicted leaf consumption by native shredders, with mixed evidence of resource‐switching or prioritization of preferred food items within a leaf mix. Higher shredder species richness never promoted leaf consumption rates compared to predictions from relevant single‐species experiments, and instead sometimes substantially decreased leaf consumption. We then conducted a meta‐analysis of leaf litter consumption rates by seven widely distributed amphipod species (the three used in the experiments and four additional species). As expected based on our own experiments, nonnative amphipod species generally had lower biomass‐adjusted leaf litter consumption rates, although their larger body size led to higher per‐individual leaf consumption rates. Contamination of the water by metals, pesticides, and other chemicals additionally significantly decreased leaf litter consumption by multiple native and nonnative species compared to unpolluted systems. While the meta‐analysis suggested that litter consumption, and thus breakdown, would decline if native shredders are replaced by nonnative heterospecifics, complete species replacement is not the only outcome following immigration in a meta‐community context. Our experiments suggest that breakdown rates could remain reasonably high where native species coexist with nonnative arrivals. Experiments that neglect the ecological realism of species coexistence will necessarily mischaracterize effects on ecosystem functioning.