IntroductionThis study examines the dynamics and characteristics of seasonal fish migration into the floodplain of a large lowland river.MethodsFor continuous recording of the number of fish migrating upstream and downstream, and their size composition, the hydroacoustic complex “NetCor” was used. The system was permanently installed in spring 2022 on a floating platform in the Varpak River (a small tributary of the Irtysh River), mounted in a side-scan setup from the left bank.ResultsThe dominant role of water level and temperature for abundance dynamics of fish migrating into the floodplain is demonstrated. The earlier onset of high values for these factors positively correlate with the abundance of migratory fish. Their influence accounts for approximately 70%. On a daily basis, migration dynamics are determined by the daily variation in illumination. The abundance of upstream migrant fish correlated with diurnal variations in illumination. Migration intensity increases during periods preceding morning and, to a lesser extent, evening twilight. The lowest proportion of migratory fish was observed during the daytime, when illumination was highest.DiscussionThe influence of water temperature, water level and light on the dynamics of seasonal fish migration is revealed. At the same time, a number of specific features have been noted for seasonal migration to the floodplain: synchronous increases and decreases in the intensity of fish movements up and downstream are simultaneously recorded, while the general trend of migration to the floodplain is maintained; in the initial phase of the flood, the largest fish are the first to migrate into the floodplain, which is an adaptive behavioral response. The diurnal distribution pattern of migratory fish is a strategy which they use to counter visually oriented predatory fish species—zander, perch, and pike—to avoid optimal prey search conditions.
IntroductionHigh mountain streams are experiencing pronounced shifts in precipitation regimes and water chemistry; however, it remains unclear how organisms inhabiting these systems influence the transfer of chemical elements through food webs.MethodsWe measured concentrations of potassium and chlorine in algae, grazing mayflies, predatory stoneflies, and Alpine Bullhead (Cottus poecilopus) across seasons in an alpine stream.ResultsAlgae exhibited marked seasonal variation, with higher potassium accumulation during the main summer growth period despite relatively stable water concentrations. Grazing mayflies maintained stable potassium levels but showed increased chlorine during colder months when chlorine concentrations in water were elevated. Predatory stoneflies showed only minor seasonal variation, with a slight increase in potassium in winter. The Alpine Bullhead did not exhibit significant seasonal changes in potassium or chlorine accumulation in its skull. Following major flood events, potassium and chlorine concentrations tended to co-vary across trophic levels, whereas under baseflow conditions they often varied independently.DiscussionThese patterns suggest that different trophic groups regulate element accumulation according to their physiology, life history, and ecological roles. The decoupling of organismal and water chemistry indicates that environmental concentrations alone do not fully explain element distribution within alpine stream food webs. Our findings provide new insights into element dynamics and highlight the importance of biological mediation in high-mountain ecosystems under changing environmental conditions.
IntroductionTemporary freshwater habitats are characterized by cyclical flooding and drying that create highly dynamic conditions for fish community assembly. These environments can exhibit high spatial and temporal variability in community structure driven by local topography, distance to source habitats and interannual variation in rainfall, which affects connectivity dynamics. As such, they provide unique opportunities to test hypotheses about the processes shaping metacommunities. Here, we investigate these processes in temporary freshwater habitats of the Atlantic Forest.MethodsWe sampled 36 temporary pools and 36 roadside ditches throughout 2024, recording 1,545 individuals from 20 species. We tested if community composition differs between habitat type and rainfall periods and whether beta diversity components were associated with local environmental and spatial predictors.ResultsFish community composition did not differ significantly between habitats or hydrological periods, but temporal variation in spatial beta diversity was consistently high and influenced by turnover. During the dry period, nestedness was positively correlated with differences in volume for temporary pools and roadside ditches. Additionally, during the wet period, turnover in ditches was correlated with differences in pH. In temporary pools during the wet period, turnover increased with differences in distance to the nearest stream.DiscussionHigh turnover for both habitats and periods suggest spatial processes such as dispersal limitation and colonization history are key drivers of community structure in those temporary aquatic habitats. Nestedness correlations with differences in volume in the dry period indicate that greater habitat availability promotes species accumulation under low connectivity, where dispersal occurs possibly only by overland displacement. Meanwhile, turnover correlation with differences in pH for ditches in the wet period is consistent with mass effect dynamics under higher connectivity conditions, with pH acting as a proxy for stream water influx. Turnover correlation with differences in distance to the nearest stream for pools during the wet period suggest colonization by highly tolerant and capable of dispersal species in more isolated sites. Overall, our findings emphasize that beta diversity in temporary aquatic habitats is primarily structured by habitat availability and spatial processes interacting with hydrological dynamics, underscoring that effective conservation must target the entire floodable mosaic rather than isolated sites to maintain this dynamic metacommunity.
Climate-driven glacial melt is altering polar ecosystems. Shifts in hydrological regimes have cascading effects on limno-terrestrial ecosystems. In the McMurdo Dry Valleys (Southern Victoria Land, Antarctica), year-round ice cover isolates lentic habitats, yet seasonal melt along the lake perimeter forms open-water “moats” during the short austral summer provide transient hydrological connectivity among soils, benthos, and the stratified water columns of the dry valley lakes. To investigate how connectivity influences biological communities, we tracked biodiversity, phytoplankton photosynthesis, and physicochemistry along lateral transects in two McMurdo Dry Valley lakes, Fryxell and Bonney. These lakes, shaped by distinct basin features (bathymetry, streams) and ecological legacies (nutrient status, chemistry), exhibited contrasting degrees of limno-terrestrial connectivity. Our data reveal that lake-specific hydrological linkages restructure microbial and invertebrate communities. We conclude that climate-induced hydrological changes destabilize previously stratified systems, altering ecological interactions and fundamental ecosystem processes across Antarctic limno-terrestrial ecosystems. Our findings provide critical insight into how polar freshwater ecosystems may reorganize under future climate scenarios, informing predictions of microbial community resilience in extreme environments.
IntroductionLarge rivers exhibit spatiotemporal heterogeneity, where habitat structure affects spatial and temporal overlap between zooplankton and planktivores. In the central United States, the non-native, planktivorous silver carp (Hypophthalmichthys molitrix) dominates fish communities in complex riverscapes. Understanding how silver carp and native planktivores interact with zooplankton through space and time is essential for anticipating changes to trophic stability and energy pathways and can inform silver carp management. The objectives of this study were to assess spatiotemporal patterns in zooplankton and planktivorous fish densities, including investigating evidence of zooplankton diel vertical migration and whether planktivores display diel patterns in main channel and off channel habitat use.MethodsThe vertical distributions of three zooplankton taxa and the spatial distributions of silver carp and native planktivores were quantified over a 24-h period in main channel and off channel habitats of three locations in the Illinois River, USA during October 2018.ResultsPlanktivorous fishes > 30 cm total length were dominated by silver carp and were consistently denser in off channel habitats during the day and night. At some sites, densities of cladocerans, copepods, and rotifers differed between the main channel and off channel habitats. Cladoceran, copepod, and rotifer taxa did not exhibit daily changes in vertical distribution. Cladoceran taxa were denser in off channel habitats at night than during the day, while copepod densities were consistently higher near the bottom of the off channel habitat.DiscussionDeclining densities of off channel cladocerans during the day may have been due to movement from nearshore areas or the sediment, planktivory by diurnally feeding fishes, or drift. Unchanging diel rotifer densities support other research showing that these small-bodied taxa with short life cycles persist in the face of high planktivory. Increases in cladocerans during the night in off channel habitats may have been due to a combination of sediment use, decreased nighttime planktivore foraging rates, drift from upstream sources, water flow, and movement from lateral areas. Zooplankton subsidies from the off channel areas of the riverscape support high densities of invasive silver carp and native planktivores.
Plastic pollution and antimicrobial resistance (AMR) represent two critical environmental and health threats in aquatic environments. The combined presence in river water of microplastics (MPs), particles smaller 5 mm, and antibiotic-resistant bacteria (ARB), may enhance the spread of antibiotic resistances, as MPs provide the surface for ARB colonization and their delivery throughout the environment. In this study MPs were collected in a river of central Italy using a Manta-Net at two representative sites, and analyzed by Fourier Transform Infrared Spectroscopy (FTIR). Polyethylene (PE) was the most abundant polymer accounting for over 60% of the total detected plastic debris. River water sampled from the same sites was filtered to isolate third-generation cephalosporin (3GC)-resistant Enterobacteriaceae, which showed a high percentage (44%) of multidrug resistance profiles. Findings of this research demonstrate the co-existence of these threats within the Chienti river, suggesting a possible amplification of AMR diffusion in the environment.
Despite the success of the Sea Lamprey Control Program (SLCP) in reducing Sea Lamprey (Petromyzon marinus) abundance throughout the Laurentian Great Lakes, control remains a significant challenge in the Saint Clair-Detroit River System (SCDRS) due to the system's size, flow dynamics, and amount of potentially suitable spawning habitat. We used acoustic telemetry to identify repeated patterns in habitat use, presumed spawning areas, and migratory routes for 271 acoustic-tagged Sea Lamprey in the SCDRS during the 2016 and 2017 spawning seasons. Sixty-nine percent of tagged Sea Lamprey appeared to spawn in the Saint Clair River with many individuals displaying commonly shared movement histories in terms of timing and channel use. Probable spawning locations were associated with the main river channel upstream of major bifurcations (e.g., Stag and Fawn Islands) in the Saint Clair River and overlapped with areas of high bottom flow. Selection for low flow refugia during migration appears water temperature dependent. Traditional control methods are likely to be inefficient in the SCDRS, highlighting the need for novel supplemental strategies tailored to large river systems.
River sedimentation has emerged as a critical driver of water scarcity in semi-arid regions, yet most studies neglect sediment accumulation on riverbeds. This study investigates the magnitude, economic implications, and community adaptations to fluvial sedimentation in the Lower Lusitu River Channel, southern Zambia. A mixed-methods approach was used, combining geomorphological field sampling with qualitative focus group discussions across a 15 km downstream channel stretch. Sediment depth was measured at 68 points using a Graduated Sediment Depth (GraSeD) rod, spatially analyzed via the ArcMap 10.4 Inverse Weighted Distance interpolation plugin and further analyzed using polynomial regression. A novel participatory suitability rating of river sand and market data was used to estimate the economic value of sediment. Qualitative data were analyzed using thematic analysis. Results indicate that 2.86 million m3 (4.64 million tons) of sand, approximately equating to 68.34% of the original channel depth, occupied the riverbed, drastically limiting surface water flow. We argue that while the river appears dry from July to December, interstitial water persists throughout, supporting basic household and agricultural needs through 56 community-built wells, pools, and sand reservoirs on the riverbed. Socioeconomically privileged households utilize pumps and plastic container–reinforced wells, whereas poorer households rely on labor-intensive manual fetching. Notwithstanding the fact that interstitial water provides an alternative source of water during the water-stressful period from July to early December, health risks to about 20,000 people in 1,073 households with over 30,000 livestock, potential human–livestock conflicts, and inequities in water access are prevalent. Despite these challenges, the sediment's estimated market value of USD 11.2 million, based on a suitability score of 0.652, presents an underexplored economic opportunity. This study provides empirical evidence that sediment acts as both a barrier to water access and a potential economic asset. It challenges the notion of the Lusitu River as a seasonally dry river by demonstrating sustained subsurface hydrological activity through interstitial water. The findings highlight the potential for policy reform to legalize community-built water infrastructure and regulate sustainable sand extraction. Integrating community resilience strategies with sediment management could enhance water security and livelihoods in similar river systems facing sediment-induced water scarcity and stress.
This review explores biomass production and CaCO3 precipitation by charophytes (Charophyta) and their environmental implications. A hypothesis was developed that charophytes play a substantial role in sedimentary processes and that the long-term deposition of CaCO3 in the sediments contributes significantly to the sequestration of excess carbon in bottom sediments. The study shows that, compared to angiosperms, many charophytes produce significantly greater dry mass. Tall and branchy species produce higher dry weight (DW) per individual but form sparser communities while smaller and slender charophytes can produce lower DW but tend to create very compact carpets that balance species-specific differences in dry mass production. While charophyte dry mass frequently exceeds 1 kg DW m−2 or even 2 kg DW m−2 in temperate climate, the study from Mediterranean climate reports charophyte DW can exceed 4.1 kg m−2 and in extreme cases be up to 11.5 kg m−2. Compared to angiosperms some charophytes are highly efficient in utilizing bicarbonate as a carbon source for photosynthesis and precipitating calcium carbonate encrustation but it varies in space and time. This process contributes to water decalcification, impacting water chemistry and carbon cycling in aquatic ecosystems. Charophytes, particularly large species, play a significant role in carbon sequestration through the precipitation and deposition of CaCO3, and this review shows that charophytes can capture and deposit in the bottom sediments up to >300 g C m−2. However, different species exhibit varying degrees of carbonate deposition and recirculation. Therefore, although the hypothesis adopted was supported by the results of this review, further study is needed to fully understand the long-term impact of charophytes on carbon sequestration and the influence of climate change on their growth and permanent CaCO3 deposition.
The Sacramento-San Joaquin River system (SSJ) of California includes both riverine, delta, and estuarine habitats and is among the most modified aquatic ecosystems in the United States. Water development projects in the system are associated with declines of many native species, including White Sturgeon Acipenser transmontanus. We used White Sturgeon pectoral fin rays collected from 1983 to 2016 throughout the SSJ to assess long-term changes in growth and associations with thermal and hydrological conditions (i.e., temperature, discharge, salinity). Age and growth were estimated from 1,897 White Sturgeon varying in fork length from 25 to 210 cm and from age 0 to 33. Age structure varied through time with the oldest fish generally sampled during the mid-1980s. Growth of White Sturgeon in 1951–1970 was slower than growth of fish in 1971–1990 and 1991–2012. Growth of White Sturgeon during 1991–2012 was ~10% higher than during other time periods. Little variation in growth was explained by environmental covariates, suggesting that annual growth was likely influenced by factors not measured in our study. Alternatively, population structure and movement behavior of White Sturgeon in the SSJ may be such that the scale (i.e., spatial or temporal) of available habitat covariates was mismatched to the scale at which growth of White Sturgeon responds. Increased growth in recent times may be partly due to density-dependent processes in association with substantial declines in White Sturgeon population abundance over the last several decades. This research provides important information on long-term patterns in growth that contributes to the conservation and management of White Sturgeon in the SSJ and beyond.
Identifying commonalities in how fish navigate rivers near infrastructure will enhance water operations and design by improving our ability to predict engineering outcomes (e.g., barrier construction/removal, fish passage installation) in novel settings before the cost of real-world implementation. Evidence from intermediate-scale computer models (time scales of minutes to days and spatial scales <2 km) suggests that fish movement behavior in rivers is frequently governed by responses to one or more of the following hydrodynamic features: (1) flow direction (i.e., rheotaxis), (2) flow velocity magnitude, (3) turbulence, and (4) depth, plus (5) the integration of information over recent time periods (i.e., memory/experience). However, the lack of consistent modeling approaches, infrequent assessment of each response in isolation and combination, and a focus on a limited number of species means the generality of these responses is uncertain. We use a computer model, specifically a pattern-oriented modeling approach incorporating individual based models (IBMs), to apply responses to the four hydrodynamic features plus memory/experience in different combinations to study their value for reproducing the movement of an infrequently modeled species and lifestage, upriver migrating adult sea lamprey, Petromyzon marinus . The study site was the region downstream of the Sault Ste. Marie lock and dam complex located between Canada and the U.S.A on the St. Marys River joining Lake Superior and Lake Huron. Our analysis indicates that rheotaxis and a response to velocity magnitude as well as recent past experience improve sea lamprey spatio-temporal movement prediction compared to other, simpler forms of modeled behavior. Sea lamprey movement is also biased toward lower levels of turbulence (e.g., turbulent kinetic energy) or its precursor (i.e., the spatial gradient in water speed). A response to water depth was not found to be important, but the modeled domain was two-dimensional which limited our assessment. As similar responses to hydrodynamic features are found in very different fish, commonalities appear to underlie river navigation across a range of species and life stages that share the goal-oriented behavior of upriver and downriver movement. The systematic approach of our analysis highlights the accuracy trade-offs of each response, individually and in combination, that often accompany alternative behavioral formulations in a computer model of fish movement. The model structure provides a framework to which future findings from the analyses of additional species in different contexts can be added.
Myxobolus cerebralis, the parasite responsible for salmonid whirling disease, was unintentionally introduced to and became established in Colorado in the 1990s. Mortality of young-of-year fish due to infection by M. cerebralis resulted in recruitment failure and subsequent significant declines in Rainbow Trout (Oncorhynchus mykiss) populations. The complex multistage lifecycle of M. cerebralis makes it difficult to eradicate and manage, and hatchery control strategies do not work in the wild. A viable method that has been utilized for wild populations is enhancing host resistance. Myxobolus cerebralis resistant Rainbow Trout were discovered at a hatchery in Germany and subsequently incorporated into Colorado's brood stock program. Since 2004, M. cerebralis resistant strains have been stocked into all major Colorado coldwater drainages to re-establish Rainbow Trout populations after whirling disease-related declines, with documented survival and reproduction of stocked disease resistant fish. Genetic population assignment tests (via putatively neutral microsatellite markers) were used to monitor the stocked populations and indicated that, after only a few years, many of the individuals in these populations unexpectedly assigned to genetic strains that were historically susceptible to M. cerebralis. To further investigate the genetic composition of these fish, a single nucleotide polymorphism (SNP) panel was used to determine the percent genetic composition of resistant strain in these individuals. Microsatellites and SNPs provided similar results, indicating a low percentage of ancestry from the resistant strain in these fish, but they continued to survive exposure to M. cerebralis, suggesting that these individuals possessed genetic loci necessary for resistance. Finally, a quantitative trait locus (QTL) region (termed WDRES-9) was used to identify individuals with alleles associated with disease resistance. Implementation of the WDRES-9 QTL test allowed for more accurate determination of M. cerebralis resistant individuals within wild populations and better described their variability in resistance. Overall, reintroductions and genetic monitoring required a suite of tools to understand the effects of M. cerebralis exposure on the genetic resistance of wild fish populations over time.
Introduction Identifying patterns in the primary limiting nutrients of basal trophic levels such as benthic algae can inform the prediction of potential ecological responses to anthropogenic nutrient loading. In coastal wetlands of the Laurentian Great Lakes, reduced concentrations of reactive nitrogen species such as ammonium and nitrate may limit algal growth, especially when nutrient loading is minimal. However, the response of benthic algae to macronutrient inputs remains understudied, especially in Lake Superior coastal wetlands. Methods We conducted nutrient amendment assays using nutrient diffusing substrate devices in 25 coastal wetlands along the southwestern shore of Lake Superior in the spring, summer, and fall. These assays allowed us to investigate seasonal and regional variation in nutrient limitation status and the relationship between nutrient limitation, in situ water quality (dissolved and total nitrogen and phosphorus, water temperature, dissolved oxygen, specific conductivity, and total suspended solids), and watershed land use. Results We found that nitrogen limitation was common, particularly during summer, with 60% of wetlands exhibiting this condition, while phosphorus limitation was not observed in any wetland during any season. The strongest N limitation was found in wetlands of the Apostle Islands National Lakeshore where watershed land cover was almost entirely natural. Wetlands with more developed watersheds, including those of the St. Louis River Estuary, had a lower degree of N limitation ( p = 0.003). Nitrogen limitation was observed in spring, summer, and fall, but was most pronounced in the summer. Discussion These findings suggest that N limitation predominates in these Lake Superior coastal wetlands, contrasting with the well-documented phosphorus limitation of the lake's pelagic zone. Our study also highlights the potential for anthropogenic N loading to stimulate excessive benthic algal growth in Lake Superior coastal wetlands, particularly in more developed regions. These findings are consistent with those for coastal wetlands in other regions of the Great Lakes and support the need for continued monitoring and targeted mitigation of both nitrogen and phosphorus loading to shoreline habitats of large lakes.
The overarching issue we address here is how to extract clear and actionable ecological and management insights from real-world field data that often do not satisfy traditional statistical assumptions. Toward this goal, we developed a general 12+6 step adaptive management framework tool. We applied this framework tool to existing biodiversity monitoring data to create a proof-of-concept result that addresses the overarching question of “ why might a specific native stream fish taxon be present or absent at specific locations? ” Our multi-step framework tool links established steps and steps that are unique to our framework through weight-of evidence (WOE) integration, an approach that combines quantitative results from multiple visualization and statistical procedures. The systematic use of all steps in our framework can provide improved conservation outcomes compared to a single analysis. Advantages accrue from our approach because our framework tool refines the overarching goal into related sub-questions, applies a specific quantitative procedure to each sub-step, combines results from all sub-questions using a WOE integration, identifies testable questions that elucidate ambiguities and gaps revealed through WOE integration, and proposes practical field methods for obtaining this clarifying information through future research and data collections. The process of considering multiple visualizations and analyses as individual pieces of a shared puzzle offers a new way to approach the use of existing data. Our team-based approach transforms the collection and analysis of existing data into a series of field tests that can guide future actions (e.g., data collection-analysis events, restoration initiatives, research). Habitat and impact regressors will vary with taxa and system, but our structured process tool has broad generality for a range of conservation issues in which freshwater systems are threatened by human impacts.
Nitrate removal via microbial denitrification in floodplains is an important ecosystem service that can potentially help mitigate anthropogenic nitrogen inputs from rivers. However, floodplain denitrification estimates can vary by four orders of magnitude, making it difficult to quantify the social value of floodplain nitrogen mitigation potential. Constraining floodplain denitrification rates requires innovative experiments that mimic overland flooding of oxic water vs. infiltration of sometimes anoxic hyporheic waters to bound the rates. We incubated soils of varying textures and corresponding hydraulic connectivity from four field sites across the Dogtooth Bend of the middle Mississippi River, contrasting their varied rates of denitrification using novel deep injection compared to traditional surface delivery of oxic or anoxic river water. Averaged across all soil types, denitrification rates as nitrogen (N) gas production followed an anoxic-injection hierarchy of anoxic deep > anoxic surface > oxic deep > oxic surface treatments. Rates in sand ranged from 101 to 592 mg N/m 2 /day compared to diffusion-limited clay, which ranged from 166 to 448 mg N/m 2 /day. The chemical stoichiometry of nitrate (NO 3 -N) loss to N gain indicated apparent nitrification that replaced approximately 62% of N removed by denitrification even in anoxic treatments, modifying net rates for oxic-surface-injection vs. anoxic-deep-injection treatments to 31 to 176 (sand) and 81 to 162 mg N/m 2 /day (clay), respectively. Combining net denitrification bounds with the daily inundation exceedance probabilities for the 140 km 2 of connected floodplain at Dogtooth Bend indicates, on average, between 70 and 385 tons of N may be removed annually from floodwater during the growing year. While the potential nitrogen removal equates to a small percentage ( ≤ 0.06%) of the river's nitrogen load, economically, the estimated monetary value of N mitigation is worth US$156–$4,106/ha/growing season compared to net profits for soybeans and corn of US$79 and US$88/ha/yr, respectively. Thus, N mitigation across the Dogtooth Bend could rival the agricultural use of floodplain lands.
Particulate organic matter (POM) characteristics and variability have been widely studied along the land-ocean aquatic continuum, yet, gaps remain in quantifying its source composition, fluxes, and dynamics at the river-estuary interface. POM in rivers consists of a complex mixture of sources, derived both from locally produced (i.e. phytoplankton) and from adjacent ecosystems (e.g. terrestrial POM). Each source differ in its trophic and biogeochemical characteristics, hence impacting its integration into local food webs, its transfer to estuaries and sea, and its contribution to biogeochemical processes. In this study, we use a robust approach based on in situ POM to characterize river POM end-members, to quantify POM composition and dynamics, and to identify the related key drivers. This study was performed at the River-Estuary interface of one of the main rivers in Western Europe (the Loire River, France). For 3 years, we conducted bimonthly measurements of carbon and nitrogen isotopic (δ 13 C, δ 15 N) and elemental (C/N) ratios to quantify the contribution of two sources (phytoplankton and terrestrial POM) to the POM mixture and calculated annual fluxes of particulate organic carbon (POC) and nitrogen (PN) sources. Throughout the year, POM consisted of ~65% phytoplankton and 35% terrestrial POM. The mean annual export fluxes were 40.6 tPOC/year and 2.45 tPN/year over the studied period, with half of it originating from phytoplankton (53 and 55% for POC and PN, respectively). We observed a clear seasonal pattern in POM composition: phytoplankton predominated from March to October, in relation to high primary production, while terrestrial contributions were the highest from November to February, driven by greater autumn-winter hydrodynamics. Our study illustrate the interest of such an approach to quantify POM composition in aquatic system and estimate source fluxes, and provide fundamental results for estimating seasonal baselines in food webs, establishing biogeochemical budgets, and quantifying POM exports to estuarine and marine environments. Applying this methodology across a broad spectrum of aquatic systems should enhance our understanding of biogeochemical processes and organic matter transformation along the land-ocean continuum and illustrates the contribution of these ecosystems to global biogeochemical cycles.
Fish from Cambodia's Tonle Sap Lake are economically, culturally, and nutritionally significant for people in the Lower Mekong Basin, providing income, livelihoods, and protein. Fish in this system generally migrate toward upstream Mekong River in dry season and return in early wet season. However, drivers of fish migration from Tonle Sap Lake to the Mekong River are not well-understood. In this paper, we utilized Mixed Effects Random Forest to predict the catch weight of six fish species migrating from the Tonle Sap Lake to the Mekong River using precipitation, lunar cycle, and hydrologic conditions like river stage, streamflow, flow magnitude, and timing as predictors. As a surrogate for fish migration, we used daily fish catch weight from 2002 through 2008 at the bagnet, or Dai , fisheries along Tonle Sap River, a migration corridor connecting Tonle Sap Lake to the Mekong River. We found that migration of large fish was mainly cued by streamflow and flow magnitude, while smaller fish migrate depending on the combination of streamflow and flow timing. Streamflow less than average cumulative flow was the most important driver for migration of Pangasianodon hypophthalmus , and Cirrhinus microlepis . Migration of Cyclocheilichthys enoplos and Osteochilus melanopleurus was highly dependent on the number of low- and minimum-flow days. Cumulative flows, period of high flow and water level were the main predictors of the small mud-carp Henicorhynchus entmema's migration, while individuals of Labiobarbus leptocheilus migrated out of the Tonle Sap Lake depending on the number of days after 7-, 30-, and 90-day minimum flows. These results suggest that flow characteristics can be used to aid conservation and adaptive management of Cambodia's Dai fisheries.
Introduction Global change is a multi-faceted issue putting many species at risk. The broad range of potentially interacting environmental stressors is problematic for effective and efficient conservation and management. In freshwater systems, habitat degradation and introduced species have been repeatedly recognized for their extensive impacts on native ecosystems. However, the simultaneous impacts of these environmental stressors on naturally depauperate and inherently vulnerable communities are poorly understood. Methods In southern New Zealand, the fish communities in 14 tributaries of three lowland lakes were surveyed to quantify the within- and between-community changes along gradients of habitat complexity and abundance of introduced species, specifically brown trout ( Salmo trutta Linnaeus) and redfin perch ( Perca fluviatilis Linnaeus). Results Stable isotope analyses identified that trophic diversity increased with habitat complexity and an abundance of native eels ( Anguilla spp.) but was unaffected by introduced species. Within each community, only perch exhibited distinct dietary shifts along all environmental gradients, whereas trout and the native fish had consistent, generalist diets. When supported with length–weight regressions, these impacts became increasingly size-dependent. For example, among the native fish, only the larger eels were unaffected by habitat and achieved greater body conditions with increased numbers of eels and perch; however, more trout were detrimental to eel body condition. In contrast, the smaller bodied natives, including elvers, all had improved body conditions from increased habitat complexity and reduced numbers of trout and perch. For the introduced species, perch weights were consistent regardless of the local environment due to their variable diet, but larger trout generally increased in weight with reduced habitat complexity and greater numbers of introduced fish, although high eel densities were detrimental. Discussion Overall, our results highlight how the responses to environmental stressors, even in depauperate communities, are complicated and generally species-specific. Nonetheless, habitat degradation had the most wide-ranging negative impacts on native fish, with perch numbers only affecting the smaller bodied natives and trout only affecting one native species. We conclude that focusing on habitat restoration in conservation strategies will provide the most efficient and effective use of resources, although the realized benefits for native species will be limited if introduced species are overly abundant.
Oxbow lakes, formed from abandoned river meanders, are unique wetlands that play a crucial role in supporting aquatic biodiversity and sustaining local communities through their ecosystem services. This study focuses on the Beledanga oxbow lake, highlighting its ecosystem services and the critical role of hydrological connectivity with the adjacent river for maintaining ecological functionality and sustainability. Using the Millennium Ecosystem Assessment framework, the study assessed provisioning, regulating, cultural, and supporting services through field surveys, stakeholder consultations, and secondary data analysis. Results show that the lake provides essential provisioning services, such as fisheries and irrigation, which support local food security and income generation. It also plays a role in regulating services like water management and carbon sequestration, while its biodiversity underpins vital ecological functions, including nutrient cycling and habitat provision. However, challenges such as seasonal water scarcity and macrophyte infestation threaten the lake's productivity and connectivity with the river. The study emphasizes the need for collaborative efforts involving local stakeholders to devise water management strategies and remove macrophytes, utilizing government initiatives like the Mahatma Gandhi National Rural Employment Guarantee Act (MGNREGA) for funding. Additionally, the establishment of robust monitoring systems for sustainable fisheries management and incentive programs for conservation efforts are recommended. The research highlights the importance of hydrological connectivity in sustaining the lake's ecosystem services and enhancing biodiversity. By integrating local knowledge and stakeholder engagement, this study provides valuable insights into the sustainable management of oxbow lakes and highlights the broader significance of river-oxbow lake connectivity in wetland conservation efforts. The findings also have implications for broader wetland conservation policies in similar ecosystems.
Biodiversity surveys of aquatic systems often include DNA metabarcoding analyses of environmental samples that are collected through filtration of large volumes of water. The standard practice of sterile collection and filtration in or near the field sites is challenging to implement in remote locations, and filtration of large volumes is a limiting step, especially for water from highly productive systems or with high suspended sediment loads. Recent trials have shown that passive samplers can be effective for aquatic metabarcoding to document metazoan diversity, but that this approach needs to be trialed under a wider variety of conditions and across more diverse taxa. Here we assess the utility of passive sampling for documenting the diversity of bacteria in six tropical aquatic environments (one lake, one reservoir, two mountain streams and two blackwater rivers). We find that passive collectors generally recover significantly higher diversity of Bacteria compared to filtered samples, despite capturing significantly less overall DNA than active water filtering. However, the communities captured by the two methods show significant differences within sites, with only 26% of the Bacteria ASVs recovered by both methods. These differences were largely driven by relative abundances of taxa within Actinobacteriota, Campilobacterota, Desulfobacterota, and Proteobacteria. Our results demonstrate that passive collectors can be a cost-effective solution for monitoring aquatic microbial diversity but that the two methods are not interchangeable. Additional work is necessary to understand the selectivity of both passive collectors and active water filtering for eDNA studies.
