Freshwater Biology

Benthic algae integrate catchment-and reach-scale pressures in streams. We assessed land-use effects on benthic algal assemblage structure, chlorophyll-a, ash-free dry mass (AFDM), and autotrophic index across 24 headwater streams in the Sondu-Miriu River basin, Kenya, and explored wet-dry patterns in two monitoring streams. We sampled across four land-use categories: natural forest (NF), tea and tree plantations (TTP), smallholder agriculture (SHA), and smallholder tea (SHT). Synoptic sampling occurred in August 2025, and one NF stream and one SHA stream were sampled five times between November 2024 and May 2025. Shannon and Simpson diversity were highest in NF streams and lowest in SHT streams. Chlorophyll-a and AFDM were highest in SHA streams; chlorophyll-a was lowest in SHT, while AFDM and autotrophic index were lowest in NF. Relative abundance patterns showed that SHA streams were dominated by Lyngbya and Stigeoclonium, whereas SHT streams were dominated mainly by Stigeoclonium; NF streams supported persistent diatom-rich assemblages. Non-metric multidimensional scaling showed land-use separation of benthic algal assemblages, while redundancy analysis indicated weak, non-significant evidence of environmental associations in the synoptic dataset. Composition and biomass metrics tracked land-use gradients and wet-dry patterns, supporting periphyton-based bioassessment for monitoring disturbance in Afrotropical headwater streams.

Passive sampling is the deployment of a collection material in the environment to continuously capture environmental DNA (eDNA) over time, offering the potential to integrate biodiversity signals while reducing the need for repeated active water collection. However, the mechanisms governing eDNA capture and retention on passive samplers remain poorly understood, limiting the interpretation of passive eDNA signals and their broader application. Here, we investigated the mechanistic performance of glass fibre passive samplers using controlled mesocosm experiments with three invasive freshwater bivalves: zebra mussels (Dreissena polymorpha), quagga mussels (Dreissena bugensis), and Asian clams (Corbicula fluminea). Specifically, we quantified eDNA accumulation dynamics, evaluated the contribution of different eDNA states, tested the persistence of captured eDNA, and compared passive sampler signals with conventional active sampling. Passive samplers rapidly accumulated target eDNA within hours of deployment, after which concentrations either plateaued or continued to increase depending on species. Sequential transfer of passive samplers between mesocosms containing different species showed that previously captured eDNA declined while new target eDNA accumulated to concentrations comparable to freshly deployed samplers, demonstrating continual turnover rather than permanent retention. Dissolved eDNA showed little evidence of accumulation beyond the concentration retained in the pore water within the membrane, suggesting that it is unlikely to be the dominant contributor to long-term passive sampler signals. Instead, the observed variability among replicate samplers, together with the physical properties of glass fibre membranes, suggests that membrane-bound and particulate eDNA are the primary contributors to passive eDNA capture. Collectively, these findings support a model in which glass fibre passive sampler signals reflect a dynamic equilibrium between ongoing eDNA capture and concurrent loss processes rather than cumulative accumulation over time. This mechanistic framework provides a foundation for interpreting passive eDNA data and informs the future development of passive sampling materials, deployment strategies, and biodiversity monitoring applications.

The rapid global expansion of dams has significantly reshaped freshwater ecosystems, yet long-term trophic dynamics and food-web stability within reservoirs remain underexplored. This study examines the reorganization of fish communities induced by the trophic surge following dam construction and river impoundment. We used the predator-to-consumer mass ratio (PCMR) as a proxy for trophic pyramid structure to track changes in fish biomass distribution across three large boreal reservoirs in Quebec, Canada, over 20 years, using data from before and after impoundment. We found transient top-heaviness events (i.e., PCMR values > 1; predator accumulation), characterized by a hump-shaped trajectory of the PCMR values in two reservoirs (Opinaca and Robert Bourassa). The third reservoir (Caniapiscau) showed a gradual increase in top-heaviness but did not exhibit a clear transient pattern. The magnitude and duration of top-heaviness varied across reservoirs, potentially due to differences in the system's capacity to receive and accumulate nutrients, which are driven by water residence time and reservoir size. This research provides a conceptual framework for understanding the long-term trophic dynamics of reservoirs, which can help inform the development of more effective management strategies to preserve ecological health in these ecosystems.

Microorganisms, found in every environment, play an important role in recycling matter and providing energy to the ecosystem. Here, we used 18S and 16S metabarcoding to study bacteria, cyanobacteria and protist community diversity and functional ecology among different environments of typical lake, river, marine and soil from the Yangtze Delta of China. The results showed that the similarity of cyanobacteria and protozoa communities in soil and aquatic environments was both higher than that of bacteria and microalgae communities. While the diversity of cyanobacteria and bacteria in lake was higher than that in river, marine and soil, the diversity of microalgae and protozoa in river was higher than that in lake, marine and soil. The {beta} diversity of cyanobacteria and bacteria marked differences in lake from other environments. But The distribution of dominant families of cyanobacteria, bacteria, microalgae and protozoa is similar to diversity. While significant positive correlations were found among dominant species of cyanobacteria, protozoa, bacteria and microalgae in lake the dominant species among bacteria, microalgae and protozoa in river, marine and soil all showed more negative correlations. Our study provides the basis for understanding the functional ecology of microbes in the micro-food webs of different environments.

Hydropower development has emerged as a major driver of global environmental change, causing significant ecological damage to freshwater ecosystems and threatening aquatic biodiversity. However, the impact of cascade dams on fish communities in large river systems remains poorly understood, particularly in biodiversity hotspots such as China. Here, we used environmental DNA (eDNA) metabarcoding to assess the spatiotemporal patterns of fish diversity and community structure in the Ganjiang River, the largest tributary of the Yangtze River, in relation to cascade hydropower development. We collected water samples from 19 sites along the main stem and major tributaries in both wet and dry seasons, and analyzed the eDNA data using high-throughput sequencing and advanced bioinformatics pipelines. Our results revealed significant seasonal and spatial variations in fish diversity and community composition, with lower species richness and distinct assemblages in the impounded sections upstream of the dams compared to the downstream reaches. The fish communities were strongly influenced by environmental factors such as water temperature, pH, dissolved oxygen, and nutrients, which exhibited complex interactions with dam operation and land use practices. Spatial scale analyses further indicated that the impact of dams on fish diversity was most pronounced at the local scale, but also detectable at the regional scale. Our findings provide novel insights into the ecological consequences of cascade hydro-power development on fish communities and highlight the importance of considering spatial scales and environmental heterogeneity in assessing the impacts of human activities on freshwater biodiversity. Our study demonstrates the power of eDNA metabarcoding as a cost-effective and non-invasive tool for biodiversity assessment and conservation planning in large river systems facing multiple stressors and knowledge gaps.

Tidal flats cover less than 1% of the surface of our planet, but they provide important ecosystem functions and services, like carbon sequestration and nutrient (re-)mineralization, primary, sec-ondary, and food production, and habitats for a variety of species. Despite their importance, they are threatened by environmental stressors and anthropogenic impacts. Hence, a better under-standing of the interaction between macrozoobenthos, their metabolic activity, and their contri-bution to carbon and nitrogen cycling is important. Therefore, we conducted in-situ pulse-chase tracer experiments at a Pacific oyster (Magallana gigas) reef in the Eastern Scheldt (Southwest Dutch Delta, Netherlands) in summer and autumn 2020. During these experiments, we provided the submerged benthic community with 13C and 15N-enriched bacterioplankton and incubated it in seawater enriched in 1% deuterium oxide (2H2O) for one tidal cycle (~9 h), before the benthic community was kept at in situ conditions for another 14 d. Our aim was (1) to assess seasonal variations in 13C and 15N processing by macrozoobenthos, (2) to evaluate potential life-stage de-pendent differences in 2H uptake - a proxy for metabolic activity, and (3) to identify the macro-zoobenthos taxon which had the highest metabolic turnover rate (MTR). In summer, most bacte-rioplankton-derived 13C- and 15N was taken up by sponges (Halichondria panicea), limpets (Crepidula fornicata), and clams (Ruditapes philippinarum), whereas in autumn, H. panicea, C. fornicata and the sponge Hymeniacidon perlevis dominated 13C and 15N incorporation. Statisti-cally significant differences in 2H uptake between life stages (juvenile/ small vs. adult) was de-tected in Malacostraca (i.e., Rhithropanopeus harrisii), but not in Polychaeta or Bivalvia. There-fore, this difference between taxonomic classes might be linked to mold-intermolt periods. Sea-sonal differences in MTR of H. panicea and M. gigas (i.e., the species with the highest MTRs in summer and autumn) might be linked to seasonal environmental variability, such as temperature and food availability.

Boreal forested streams receive substantial inputs of terrestrial carbon (C) from riparian vegetation in the form of leaf litter, which is processed by litter-associated microorganisms. In addition to these leaf-litter C inputs, boreal streams also receive significant concentrations of dissolved organic carbon (DOC), which may represent an alternative major resource pool for litter-associated microorganisms. As leaf litter quality decreases over the process of decomposition, DOC may increasingly become a more suitable C source for the microbial communities. We investigated how DOC availability, litter type, and microbial succession influenced respiration and decomposition. Alder, birch, and spruce litter were incubated for 65 days in 12 outdoor experimental channels supplied with natural boreal stream water. Higher time-adjusted DOC concentrations were associated with higher respiration and lower relative mass remaining, whereas DOC flux received less support. Respiration and mass loss followed litter-specific trajectories, with birch showing a respiration peak that was not matched by proportional mass loss. Prokaryotic and eukaryotic communities differed among litter types and changed through time but explained process variation only for specific litter types and responses. Our results suggest that DOC can sustain litter-associated microbial activity and partly decouple respiration from litter decomposition, thereby affecting boreal stream C processing.

Quantitative stable isotope probing (qSIP) allows researchers to calculate taxon-specific carbon incorporation from sequencing of natural microbial communities, which can be used as a proxy for metabolic activity rates and subsequently as an input for biogeochemical modeling. While qSIP is widely utilized in soils to investigate the identity and metabolic activity of largely unculturable microbes, the application of qSIP in marine and aquatic ecosystems is more recent. Here, we investigated how bioreactor type (batch vs. chemostat) and carbon substrate complexity (single vs. multiple substrates) affect the incorporation of {superscript 1}3C-labeled glucose into rRNA after 24 hours using excess atomic fraction (EAF) as a proxy for metabolic activity rate. We found that the growth dynamics and community composition of the {superscript 1}3C-incorporating bacteria differed significantly for each treatment. EAF was positively correlated with both 16S gene copy number and a genomic index of copiotrophy in both batch treatments, but not in the chemostat, suggesting that chemostats dampen the competitive advantage of fast-growing copiotrophic taxa. Our results demonstrate that both substrate complexity and experimental regime influence qSIP-derived metabolic activity estimates and provide guidance for future applications of qSIP in aquatic environments.

Understanding the evolutionary history of species within a geographic context is key to historical biogeography, as it reveals how geological and climatic changes shaped biodiversity. This is especially important in ecologically significant regions like the Amazon and Orinoco basins. Together, they host the worlds greatest freshwater fish diversity ([~]3,500 species), sharing a common but not yet fully understood evolutionary history. The gilded catfish (Brachyplatystoma rousseauxii), an ancient species widely distributed as a metapopulation in Neotropics, is an important model for studying past connectivity, divergence, and historical processes shaping fish diversity between these basins. This study analyzed the genetic structure, connectivity routes, and demographic history of B. rousseauxii using nuclear (microsatellite and ddRADseq) and mitochondrial DNA. Population structure analyses and coalescent models indicate that B. rousseauxii populations from the Orinoco and Amazon basins are genetically distinct, with no evidence of current gene flow. However, our results support the occurrence of a possible secondary contact event after the divergence, with the Boa Vista population retaining the genetic signal of this process. The ancestral population split occurred at the Rupununi Portal around 2.54 Ma (ddRAD) or 1.31 Ma (mtDNA). Then, the species colonized the Branco and Orinoco Rivers [~]1.90 Ma (ddRAD) or 0.6 Ma (mtDNA), rapidly expanding in the Orinoco (>1.3 or >0.29 Ma), while colonization of the Amazon from the Branco River was more recent ([≤]1.0 or [≤]0.15 Ma). Population expansion signal was detected in the Orinoco ([~]0.20 Ma), whereas the Amazon remained stable. Our findings suggest that the rise of the Vaupes Arch in the Late Miocene does not explain the observed genetic divergence. Likewise, the Casiquiare Canal and Japura-Guaviare headwaters are not connectivity routes between basins. Instead, the Rupununi Portal, including the recent capture of the Branco River by the Negro River, was the last point of connection and played a key role in shaping B. rousseauxiis distribution. These findings provide insights into Neotropical fish biogeography and the historical configuration of the Orinoco and Amazon basins.

Brine flies of the genus Ephydra (Diptera: Ephydridae) are ubiquitous extremophile animals whose larval forms are able to thrive in high-stress environments, including hypersaline and high carbonate waters. Despite their historical and ecological importance, little is known about their evolutionary history. Here, we focus on an understudied population of Ephydra living in former industrial salt evaporation ponds of the San Francisco Bay Area (SFBA), a region which has undergone significant environmental changes in the past century. We observed and collected Ephydra specimens and took salinity measurements of waters they inhabited during a 6-month period in 2024. We found that E. millbrae and E. gracilis demonstrate strong salinity preferences. We also analyzed DNA from Ephydra spp. specimens from distant sites and a timespan of ~40 years, generating the most comprehensive gene genealogy of the group to date based on a single mitochondrial locus. Our analysis of phylogenetic signals suggests that salinity envelopes are influenced by evolutionary relationships in Ephydra. This study provides new information about the natural history of extremophile flies in the SFBA and fills in current gaps in our knowledge about the evolutionary history of the genus more broadly.

The decline in fishery yields in Japan's enclosed coastal seas has been accompanied by a reduction in phytoplankton biomass, suggesting a widespread decrease in ecosystem productivity. Oligotrophication in Mikawa Bay has caused substantial declines in phytoplankton biomass and Manila clam (Ruditapes philippinarum) production. To assess whether external nutrient enhancement could stimulate phytoplankton growth, a controlled demonstration experiment using treated effluent from two sewage treatment plants (WWTPs) was conducted (spanning fiscal years 2022 to 2024) based on analysis using a three-dimensional non-hydrostatic model. Although effluent DIP and nitrogen concentrations were increased, model results showed that WWTP-derived nutrients represent only a minor portion of the total nutrient budget. Field observations revealed limited increases in surface water phosphorus, strong seasonal variability, and only temporary rises in chlorophyll-a and clam harvests. Monthly TP data demonstrated that seasonal DIP pulses, especially spring phosphorus bursts, dominate nutrient dynamics and overshadow external loading effects. Laboratory experiments further showed that phosphate is efficiently fixed by Fe(III) under oxidizing conditions and rapidly released during reductive dissolution of [FeOOH-HPO4]2- complexes, confirming iron as the principal regulator of internal P cycling. Because current numerical models omit iron-driven internal cycling, they cannot accurately predict bioavailable P or ecosystem responses. Overall, the study indicates that localized nutrient enhancement is insufficient to counter long-term oligotrophication, and effective management must integrate nitrogen limitation, internal Fe and P cycling, oceanic exchange, and seasonal variability in Ise and Mikawa Bays.

1. Atlantic salmon (Salmo salar) are an anadromous fish with a broad distribution from eastern North America to western Europe, with ecological, economic, and cultural significance. Despite their expansive range, wild populations across the North Atlantic have experienced decline. Survival to reproductive age is a management concern, with a focus on identifying factors affecting the juvenile freshwater life stage. Specifically, research is needed to identify potential bottlenecks limiting freshwater productivity. 2. We addressed this research gap with a case study on the Exploits River, Newfoundland - a watershed that receives one of the largest anadromous Atlantic salmon returns in North America. We aimed to identify biotic, chemical, and physical predictors of food resources for juvenile salmon (i.e., invertebrates) and food resources for invertebrates (i.e., periphyton). To test our hypotheses, we collected data on a suite of biotic (e.g., invertebrates and periphyton), chemical (e.g., total dissolved N), and physical (e.g., wetted width) variables at 42 sites within and around this watershed. 3. We observed evidence for relationships between invertebrate biomass and biotic and temporal variables. Specifically, invertebrate biomass was higher at sites with high periphyton biomass that were sampled later in the season. In addition, we observed that periphyton biomass was related to biotic, chemical, and physical variables. Here, we present evidence for bottom-up control, with increased periphyton biomass supporting increased invertebrate biomass. 4. We expect knock-on effects where increased invertebrate biomass results in increased juvenile Atlantic salmon biomass. Our large sample size revealed some predictors of invertebrate biomass and periphyton biomass. However, we found no evidence of correlation between invertebrate biomass and chemical or physical variables. The low productivity of our maritime boreal forest system could be an outlier in expected biotic, chemical, and physical relationships in streams. 5. Our findings provide an important step forward in furthering boreal stream ecology, producing novel invertebrate and periphyton knowledge for watershed managers.

Ecological forecasts often rely on established relationships between the ecological process of interest and predictor variables that are more easily measured. Proliferations of benthic (i.e., bottom-dwelling) cyanobacteria have been increasingly observed in rivers globally and are an emerging ecological forecasting issue as they pose a public health threat due to the production of potent neurotoxins known as anatoxins. Controls on these benthic cyanobacteria are poorly understood, thus predicting or forecasting their extent and anatoxin production is a significant challenge. Here, we measured benthic cyanobacterial cover and anatoxin concentrations for two common taxa associated with anatoxins (Microcoleus and Anabaena) at biweekly to weekly intervals during June to September in 2022 and 2023 in three northern Californian rivers. We then built predictive models to test how incorporation of a biotic predictor (river reach-scale gross primary productivity [GPP]) affected predictive accuracy in addition to widely measured abiotic predictors (i.e., nutrients, discharge, and temperature). Temporal patterns in taxon-specific benthic cyanobacterial cover and anatoxin concentrations were highly variable among rivers and between taxa. While Microcoleus cover peaked in rivers during periods of relatively low GPP, there were no clear relationships between GPP and Anabaena cover nor anatoxin concentrations of either taxon among rivers. Among multiple reaches sampled weekly in the South Fork Eel River, magnitudes of taxon-specific benthic cyanobacterial cover and anatoxin concentrations differed, but the timing of peak taxon-specific cover and anatoxin concentrations were generally consistent. Furthermore, Anabaena displayed a hysteresis relationship where increases in cover were followed by increases in anatoxin concentrations while Microcoleus lacked such relationship. While incorporating GPP as a covariate improved our predictions of Anabaena cover, we had more success predicting Microcoleus cover than Anabaena cover due to its strong negative relationship with discharge. In contrast, our models predicting Anabaena anatoxin concentrations outperformed those predicting Microcoleus anatoxin concentrations due to the hysteresis relationship between Anabaena cover and anatoxins. Overall, our predictive modeling results highlight the application of ecological forecasting for benthic cyanobacterial cover and anatoxin concentrations in rivers and demonstrate the importance of incorporating taxon-specific predictors into future forecasts of benthic cyanobacteria.

Accurate ecological forecasting is increasingly essential for understanding and managing ecosystem responses to climate variability and anthropogenic pressures; however, prediction remains difficult in lakes because key biological variables, such as phytoplankton biomass, exhibit nonlinear dynamics, observational noise, data sparsity, and non-stationarity. This study aimed to test whether large pre-trained time-series foundation models can improve forecasts of lake phytoplankton dynamics, focusing on chlorophyll-a (Chl.a) and water-quality variables in two ecologically contrasting Japanese lakes, the deep-stratified Lake Biwa and the shallow nutrient-rich Lake Kasumigaura. Using extensive monthly monitoring records spanning up to 30 years, we benchmarked two time-series foundation models - Transformer-based Chronos-T5 and probabilistic Lag-Llama - against 15 statistical (AR, ARIMA, SARIMA, Prophet), machine learning (Random Forest, XGBoost, KNN, SVR), and deep learning (LSTM, CNN, TCN, and SSA-hybrid) approaches. Chronos-T5 achieved superior reproduction accuracy and stability across diverse environmental variables and outperformed all other tested models. This performance stemmed from its ability to capture long-term dependencies and complex temporal patterns through large-scale pre-training. We further identified an optimal training window of approximately 14 years, balancing data sufficiency with regime stability, beyond which model accuracy diminished owing to ecological regime shifts. Our findings highlight the transformative potential of time-series foundation models for ecological forecasting, providing a scalable, data-driven framework that can underpin robust early warning systems and adaptive management strategies for aquatic ecosystems worldwide.

When colonizing new habitats, the evolution of populations can be affected by their demographic history and structure, including their effective population sizes and the time and nature of their divergence from an ancestral population. The deglaciation of Southern Greenland following the last glacial maximum offers the opportunity to comparatively study colonization events of fish populations among lakes of varying size and altitude, the latter of which partially reflects their geological age and isolation. This study investigates the demographic history of threespine stickleback (Gasterosteus aculeatus) from 17 freshwater lake populations in comparison to a marine population sampled across four fjord sites in Southern Greenland using whole-genome data. Our findings suggest that divergence of freshwater from ocean populations was concurrent with ice sheet retreat and lake formation, which occurred ~4-13 thousand years ago. In all 17 freshwater populations, demographic modeling revealed a population decline upon lake colonization and prolonged periods of gene flow between freshwater and marine populations. We also found that bigger lakes had larger effective population sizes. These findings emphasize the interplay between local landscape and demographic history in shaping the population genetic structure of Southern Greenland stickleback populations. Our results underscore the relevance of such small scale variation in demographic history across the landscape for understanding postglacial colonization into freshwater ecosystems.

Context Pumping stations pose a threat to fish globally through land use change, habitat fragmentation and entrainment risk, with the catadromous and critically endangered European eel particularly impacted. Objectives/methods Establish, model, assess and understand the present-day distribution of European eel and resident fishes in 152 pumping station catchments in a once extensive wetland (The Fens) using eDNA metabarcoding (855 samples over two and half years), with specific focus on anthropogenic influences on hydrological connectivity and habitat quality. A removal survey design maximised confidence in negative results while minimising time and consumable costs. Results Eel occurrence upstream of pumping stations was low (occupancy = 28.3%) and positively associated with catchment area, fish species richness and natural hydrological connectivity (gravity drainage or flooding) and negatively associated with distance from the tidal limit. Fish species richness replaced catchment area and improved model performance, potentially acting as a biotic indicator of habitat quality and connectivity. Pumped catchments with manually operated upstream water transfers had reduced eel presence, potentially linked to the direction of water flow or the timing of operation. By contrast, fish species richness increased in these catchments during summer, suggesting displacement into unsuitable long-term habitats. Physical habitat maintenance had no detectable effect on eel occurrence or fish species richness. Conclusions This study provides the first landscape-scale assessment of European eel distribution and drivers of occurrence in pumped river catchments. The highly novel and comprehensive insights have implications for European eel conservation as well as infrastructure and catchment management, including compliance with legislation (EC Regulation No. 1100/2007).

Physiological adaptation across environmental gradients can contribute to ecological speciation by limiting performance outside locally optimal habitats. Intertidal systems provide strong natural thermal gradients, yet the extent to which thermal physiology contributes to divergence across shore height remains poorly resolved. We investigated cardiac thermal performance in two ecotypes of the marine snail Littorina saxatilis occupying different shore heights along the Galician coast (NW Spain): a wave-adapted ecotype on the lower and mid- shore and a crab-resistant ecotype on the mid- and upper shore. Using infrared photoplethysmography, we quantified heart rate responses in both a reciprocal field transplant experiment and laboratory thermal ramping trials. In the field, the Wave ecotype exhibited significantly higher heart rates than Crab ecotype snails under native mid-shore conditions and after 1 day of exposure to upper-shore conditions. However, after 4 days of exposure to the upper shore, Wave ecotype snails showed a marked reduction in cardiac activity, whereas Crab ecotype populations maintained stable heart rates across transplant locations and durations. In laboratory ramping experiments, Crab ecotypes displayed lower baseline cardiac activity and greater thermal insensitivity across the rising phase of the thermal response curve, while the Wave ecotype exhibited higher cardiac performance and an earlier decline in heartrate at high temperatures. Together, these results demonstrate pronounced ecotype divergence in cardiac thermal physiology and suggest that chronic exposure to upper-shore conditions compromises cardiac performance in the Wave ecotype. Such physiological differences likely contribute to vertical zonation and the evolution of barriers to gene flow between these ecotypes.

Identifying the characteristics of climate refugia for multiple species is becoming increasingly urgent. We used repeated, fish community surveys in streams of the northern Great Plains, USA to characterize occupancy change between historical and contemporary surveys. We then identified variables correlated with community change. Across all sites, average community turnover was 56%, native species persistence was 60%, and contemporary surveys consisted of 37% colonizing species (native or introduced). Community turnover was driven about equally by species loss and colonization. Individual species showed highly variable trends in occupancy with native plains minnow (Hybognathus placitus), northern redbelly dace (Chrosomus eos), longnose sucker (Catostomus catostomus), and flathead chub (Platygobio gracilis), declining substantially, and introduced smallmouth bass (Micropterus dolomieu) and native channel catfish (Ictalurus punctatus) increased considerably. While factors associated with individual species persistence and colonization varied, warm temperatures and agriculture were most associated with community change. Importantly, cool water temperatures appear to buffer fish communities from turnover at sites moderately impacted by agriculture. Protection of cool-water refugia will thus be critical for persistence of fishes in agricultural regions.

AimTo test whether dispersal ability determines the relative importance of environmental filtering and distinct dispersal pathways in structuring stream macroinvertebrate metacommunities. LocationNzoia River catchment, Lake Victoria basin, western Kenya. TaxonBenthic macroinvertebrates (family level), grouped by dispersal ability into strong fliers, weak fliers, passive aerial dispersers, and obligate aquatics. MethodsMacroinvertebrates were sampled seasonally (2018 to 2021) at 55 sites. Using distance-based redundancy analysis and variation partitioning, we compared the influence of physicochemical water quality on community structure, representing environmental filtering, with the influence of three sets of spatial factors representing distinct dispersal pathways: Euclidean distance (overland dispersal pathway), land-use resistance (land-use mediated dispersal pathway), and Asymmetric Eigenvector Maps built on river-network connectivity (along-watercourse dispersal pathway). In addition, distance-decay relationships and turnover-nestedness partitioning were examined for the full assemblage and each dispersal group. ResultsCommunity assembly shifted predictably along the macroinvertebrate dispersal-ability gradient. Environmental filtering dominated in strong fliers and retained an independent influence in weak fliers and obligate aquatics, but contributed no independent influence in passive aerial dispersers. The overland dispersal pathway structured the three aerial groups. The land use mediated dispersal pathway constrained weak fliers and passive aerial dispersers, and the along-watercourse dispersal pathway structured passive aerial dispersers and obligate aquatics. Passive aerial dispersers showed the strongest dispersal signal, with the overland and along-watercourse pathways contributing equally, and in this group beta diversity reflected richness gradients (nestedness) rather than species replacement. Main conclusionsDispersal ability determined whether environmental filtering or a specific dispersal pathway governed community composition, and no single pathway captured assembly across the full range of dispersal abilities. Dispersal traits and connectivity must therefore be considered jointly when studying and managing aquatic biodiversity in human-modified Afrotropical catchments.

Animal movements can transport materials across ecosystem boundaries, while simultaneously modifying habitats through physical engineering. We integrated GPS telemetry, camera trapping, sediment measurements and physicochemical sampling of ponds on Santa Cruz Island, Galapagos, to evaluate how giant tortoises connect and shape terrestrial and freshwater ecosystems. Telemetry revealed 2,254 visits by 21 tortoises to 12 ponds, with visits lasting up to 21.38 h and showing strong seasonal and diel variation, while camera traps showed peak pond use in January and during early morning and late afternoon. Greater tortoise abundance was associated with higher ammonium, dissolved organic carbon and total phosphorus concentrations, while ponds with persistently greater tortoise abundance had higher total suspended solids. Departing tortoises carried a mean 505.9 g of sediment, yielding a conservative potential export of 8.20 kg per tortoise per year and tens of tonnes annually. These findings identify giant tortoises as mobile zoogeochemical engineers linking terrestrial and freshwater ecosystems through physical and biogeochemical pathways.