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At least 127 records · Page 7Linked to original sources
USGS2024

Fishes move to transient local refuges, not persistent landscape refuges during river drying experiment

Anthropogenically driven flow intermittency is increasing in freshwater streams, with important implications for the management and conservation of aquatic ecosystems. Because most freshwater fishes are mobile, they are expected to emigrate from intermittent reaches, but this may not be true in streams transitioning from perennial to intermittent. Here, we attempt to determine if riverine fishes vacate drying reaches before intermittency or remain in local refuges. We implemented a controlled, in situ experimental flow reduction resulting in intermittency, reducing flows from ~1.0 to 0 m 3 /s over a 3-week period. We monitored fish and fish-habitat changes over a 5-week period before, during and after flow reductions. During flow reductions, total wetted habitat was ultimately reduced by 91%. Habitat loss over time was not equal among habitat types: pool habitat increased slightly as run habit was lost, and backwater and isolated pool habitats were not strongly related to discharge. Likewise, water depth in run habitats decreased faster than other habitats. Only the river carpsucker Carpiodes carpio appeared to move upstream during flow recession; seven other species remained within drying sites. Habitat loss negatively affected fish populations, but at a lower rate than that at which habitat was lost, until sites dried completely. Overall, two species, red shiner Cyprinella lutrensis and western mosquitofish Gambusia affinis , had increased population growth during the study, whereas the remaining species had population declines. Fishes were able to find transient local refuges during flow reductions but did not vacate sites before intermittent conditions. Accounting for lack of emigration from anthropogenic flow-intermittent reaches will be important for designing conservation actions for fishes threatened by increasing flow intermittency. Creation of refuge habitats may not be effective for widely dispersed species that do not actively seek refuge habitats, unless those habitats support enough individuals to maintain resilience following the intermittent conditions.

New Mexico↗
USGS2024

Network connectivity contributes to native small-bodied fish assemblages in the upper Mississippi River system

Effective management and conservation of fishes requires understanding habitat use across multiple life stages while ensuring necessary habitats are both available and accessible. Tributary habitats may play an important role in recruitment and dispersal of fishes in anthropogenically modified rivers such as the Mississippi and Illinois Rivers of the Midwest U.S.A. Identifying source locations that contribute to recruitment of fish populations can determine the importance of connectivity within river networks and pinpoint critical habitats that sustain fish populations. In the Upper Mississippi River System (UMRS), the environments that fish use in early life stages (i.e., natal origin) can be identified using otolith trace element analysis due to stability and distinctness in water chemistry (strontium: calcium [Sr:Ca]) among water bodies that is reflected in otoliths. Here, we used trace element analysis to determine natal origin of six small-bodied fishes including bullhead minnow ( Pimephales vigilax ), emerald shiner ( Notropis atherinoides ), gizzard shad ( Dorosoma cepedianum ), bluegill ( Lepomis macrochirus ), orangespotted sunfish ( Lepomis humilis ), and yellow perch ( Perca flavescens ) across six reaches of the UMRS (Pools 4, 8, 13, 26, the Open River of the Middle Mississippi River, and the La Grange Pool of the Illinois River). Otolith core Sr:Ca for fishes was quantified using laser ablation inductively coupled plasma mass spectrometry. Using the resulting Sr:Ca chemical signatures of otolith cores, natal origin (tributary, immigrant, or potential resident) was determined for individual fish based on family-specific relationships between otolith and water chemistries. We found that all species originated from tributaries and other reaches (i.e., were immigrants) to varying extents, which acted as evidence for network connectivity. Specifically, tributaries contributed up to 48% of individuals at a given reach. In certain reaches, Pool 26 and the Open River reach, up to 80% of individuals in a species immigrated from another mainstem river reach. Network connectivity was also important in both upstream and downstream directions. Contributions from network connectivity varied among species: bullhead minnow used less whereas orangespotted sunfish used more network connectivity than when all species were combined. Further, the use of network connectivity varied spatially where individuals captured in Pool 8 and the La Grange Pool less often and those from Pool 26 and the Open River more often originated from network connectivity compared to the whole assemblage across reaches. These results indicate that species' life history traits probably interacted with the physical environment, which differs spatially, to yield observed recruitment source patterns. Our results show that network connectivity contributes to established assemblages of native small-bodied fishes throughout the UMRS and underscore the role of interjurisdictional management in maintaining network connectivity to sustain fish populations.

Illinois, Iowa, Minnesota, Missouri, Wisconsin↗
USGS2024

Effects of episodic stream dewatering on brook trout spatial population structure

Stream dewatering is expected to become more prevalent due to climate change, and we explored the potential consequences for brook trout ( Salvelinus fontinalis) within a temperate forest ecosystem in eastern North America. We estimated fish density within stream pools ( n = 386) from electrofishing surveys over 10 years (2012–2021) to compare a stream that exhibits episodic dewatering (Paine Run) against a stream of similar size that remains flow-connected (Staunton River) within Shenandoah National Park, Virginia (U.S.A.). Annual surveys encompassed fluvial distances ranging from 2.6 to 4.4 km in each stream. Mean annual fish density (fish/pool m 2 ) was not different between streams for juvenile or adult age classes, but spatial variation in density was greater in Paine Run for both age classes of fish. Paine Run also included a greater proportion of unoccupied pools than Staunton River and exhibited stronger spatial autocorrelation in fish density among nearby pools, suggesting dispersal limitation due to surface flow fragmentation. Fish density in pools increased during years with low summer precipitation, and this effect was observed in both streams but was stronger in Paine Run than Staunton River, further indicating the importance of fish movement into pools in response to low-flow thresholds. Our results indicate the importance of pools as ecological refuges during low-flow conditions and that episodic dewatering may affect extirpation risks for brook trout by sequestering more fish into fewer areas. Our findings also highlight the importance of hydrological variation within stream networks because downstream river gages could not predict the observed spatial heterogeneity in fish density or pool occupancy.

Freshwater Biology↗
USGS2024

Fish assemblage and functional trait responses to small-dam removal

Dams are one of the greatest threats to freshwater biodiversity and efforts to remove dams to restore riverine systems are increasing. However, dam-removal studies have primarily focused on taxonomic responses to large dam removals with little work on the functional trait responses of fish to small-dam removals; such a focus limits the application of results in regions with different taxonomic compositions. We explored taxonomic and functional trait responses of fish assemblages to two small-dam removals over 10 years using a Multiple Before After Control Impact design at a dammed and an undammed river. Eight life-history traits were used to calculate functional diversity (RaoQ) and determine the position of each fish species within a multivariate life-history space relative to three life-history strategy endpoints: opportunistic, periodic, and equilibrium. The distance of each species relative to these endpoints was used to calculate community weighted means (CWM), allowing us to examine the shift in life-history strategy of a given assemblage after dam removal. Based on life-history theory, we predicted a decrease in the CWM of equilibrium strategists and an increase in the CWM opportunistic strategists after dam removal. For the dammed river, we observed changes in assemblage structure at both the control and impact sites driven primarily by shifts from a lentic to lotic assemblage, with the most apparent change occurring in the formerly impounded sites. These changes tended to occur within 1 year, suggesting rapid colonization by lotic species after habitat change. By contrast, no change in assemblage structure was found for the undammed river, suggesting that dam removal was the primary driver of the shifts in assemblage structure observed in the dammed river. We found no change in the CWM of periodic strategists or RaoQ of all life-history traits at any site after dam removal. Based on CWM, life-history strategies shifted in response to dam removal at impounded sites where equilibrium strategists decreased and opportunistic strategists tended to increase after the impounded sites changed from a stable lentic environment to an unstable lotic environment, supporting predictions made by life-history theory. Our results suggest that small-dam removal may provide ecological benefits by restoring fish assemblages to a more natural riverine state and reversing the negative effects of dam construction on the ecosystem. We demonstrate that combining both trait-based and taxonomic approaches can improve our ecological understanding of the impacts of dam removal on fish assemblages and provide relevant data for local management.

South Carolina↗
USGS2024

The effects of flow extremes on native and non-native stream fishes in Puerto Rico

Globally, freshwater fishes are among the taxa most vulnerable to climate change but are generally understudied in tropical island ecosystems where climate change is predicted to alter the intensity, frequency and duration of extreme flow events. These changes may impact stream ecosystems and native and non-native biota in complex ways. We compiled an extensive dataset of fish assemblages collected at 119 sites across the Caribbean island of Puerto Rico from 2005 to 2015. We coupled these data with stream flow indices and dam height to understand associations between flow and fish assemblage structure. Sixteen percent of sites contained exclusively non-native species, 34% contained exclusively native species, and 50% contained native and non-native species. We built generalised linear models and conducted all subsets model selection to identify extreme flow variables explaining variation in native and non-native species richness and biomass. We also built models to determine the combined effects of extreme flows and the presence of non-native species on native species richness and biomass. Extreme flows and dam height were important in explaining variations in native and non-native species richness and biomass. Model averages showed native biomass decreased by 0.42 kg/ha with a 1-m increase in dam height, by 0.05 kg/ha with 1 cm/s increase in maximum mean daily high flow and by 3.45 kg/ha with each additional day increase in maximum high flow duration, and increased by 2.06 kg/ha with each additional day increase in mean high flow duration. Model averages predicted that non-native biomass increased by 1.32 kg/ha with a 1-m increase in dam height and by 0.01 kg/ha with each additional day increase in mean high flow duration, and decreased by 0.36 kg/ha with each additional day increase in maximum high flow duration. Model averages also predicted an increase in native and non-native biomass of 0.71 gage and 0.06 kg/ha, respectively, with each additional day increase in maximum low flow duration. The combined effects of non-native species presence and extreme flows changed the relationship between maximum high and low flow durations and native biomass. Model averages showed that native biomass increased by 1.83 kg/ha with each additional day increase in maximum high flow duration and decreased by 2.52 kg/ha with each additional day increase in maximum low flow duration when non-native species were present. Native fishes may be able to better cope with longer maximum durations of low flows than expected when non-native fishes are absent. In mixed fish assemblages, extended maximum durations of high flows may act as a control of non-native species and dampen their negative effect on native species, but longer maximum durations of low flows may heighten the negative effects of non-native fishes. Our results are informative for tropical island ecosystems globally and can guide the management and conservation of native fishes, particularly when faced with the dual threats of climate change and non-native species. Managers may consider increasing efforts to conserve native fishes in Caribbean rivers by maintaining connectivity and habitat complexity while preventing non-native species introductions.

Freshwater Biology↗
USGS2024

Fish size structures in lakes of the Lower Mississippi River floodplain

The Lower Mississippi River has a floodplain that includes >1350 perennial lakes carved by shifts in river courses and other hydro-fluvial processes over eons. Notwithstanding their similar provenances, these waterbodies exhibit an immense variety of morphologies and successional stages that illustrate their natural trajectory from aquatic to forested wetlands. A result of this geographical, morphological and temporal diversity is dynamic and varied fish communities. We examined how size structures of fish communities in these floodplain lakes were associated with key in-lake and off-lake environmental drivers. Fish lengths were collected with standardised procedures in a sample of 30 of these lakes to construct a lake-by-length group matrix. Likewise, in-lake and off-lake environmental descriptors were collected to construct a lake-by-covariate matrix. Distance-based linear models were used to assess associations between fish size structure and environmental descriptors. Smaller fish were typically associated with increasing levels of turbidity, chlorophyll- a , phycocyanin and surrounding agriculture. Shallow, hypereutrophic floodplain lakes associated with agricultural landscapes and reduced connectivity experience harsh physicochemical environments. These conditions appeared to hinder the formation of sustained fish communities but may confer a survival advantage to juveniles or small short-lived species. Conversely, larger fish were associated with increasing lake depth, water clarity, connectivity, and extent of surrounding forests-wetlands. Enhanced stability and size structure were observed in communities residing in deeper and clearer lakes, suggesting that these conditions facilitated the development of longer-lived species spanning multiple age groups. The enhanced connectivity that facilitated this increased stability also permitted the presence of larger itinerant species. Size-structure assessments can serve as a valuable ecological and biodiversity indicator in floodplain lakes. Size-structure assessments could supplement and, depending on objectives, even supplant conventional taxonomic analyses, and enhance surveillance of this vast and important natural resource.

Arkansas, Louisiana, Mississippi, Missouri, Tennes↗
USGS2024

Global assessment of aquatic Isoëtes species ecology

Isoëtes are iconic but understudied wetland plants, despite having suffered severe losses globally mainly because of alterations in their habitats. We therefore provide the first global ecological assessment of aquatic Isoëtes to identify their environmental requirements and to evaluate if taxonomically related species differ in their ecology. The assessment resulted in an extensive new database on aquatic Isoëtes , ecological niche analyses, and descriptive species accounts. We compiled a global database that includes all known environmental data collected from 1935 to 2023 regarding aquatic Isoëtes. We then evaluated the environmental drivers of 16 species using 2,179 global records. Additionally, we used hypervolume analysis to quantify the ecological niches of the two species with the greatest number of records, finding significant differences and evidence that Isoëtes echinospora occupies a wider ecological niche than Isoëtes lacustris . Fifty-nine species (30% of the c. 200 Isoëtes species known today) were categorised as aquatic and were mainly reported in the Americas and northern Europe. About 38% of the aquatic species are threatened with extinction or are endemic to a small region, according to the International Union for Conservation of Nature's Red List in 2023. Many species were determined to be sensitive to certain water physical and chemical factors, generally preferring oligotrophic conditions such as low total phosphorus, moderate total nitrogen, moderate to low pH, and low conductivity. This analysis includes ecological data in the assessment of rare/threatened aquatic plants globally. This new database and the ecological analyses completed defined the ecological requirements of several species and identified knowledge gaps, which can aid management actions and future research. This paper highlights ecological significance and environmental sensitivities of aquatic Isoëtes . The current level of knowledge is inadequate for a large proportion of known taxa. We affirm the extreme need to support global, collaborative initiatives on which to build future conservation strategies.

Freshwater Biology↗
USGS2024

Anthropogenic and environmental risk factors of salmonid predation in a tidal freshwater delta

Water diversions that support agricultural and municipal use result in fish mortality through entrainment and impingement. Additionally, this infrastructure may attract both predators and prey fishes, thereby increasing predation rates and prey mortality near these anthropogenic contact points. The Sacramento–San Joaquin Delta (the Delta) in California's Central Valley is a tidal freshwater ecosystem that exports large volumes of water for municipal and agricultural use while at the same time providing valuable migratory and rearing habitat for imperilled fishes. Emigrating juvenile salmonids experience high mortality in the Delta, with predation by non-native fishes contributing substantially. Therefore, this study had three main objectives. First, we determined if small water diversions aggregated piscivorous fishes like other similar structures in freshwater ecosystems. Second, we determined how small diversions may influence juvenile salmon predation risk in conjunction with other known predation risk factors (e.g. predator abundance, temperature and depth). Third, we assessed the predator assemblage, abundance and distribution to determine the likely predator composition in objectives one and two. Throughout the spring of 2021, we used ARIS (adaptive resolution imaging sonar; Sound Metrics) sonars to compare piscivore abundance at 30 water diversions in the north Delta to shorelines adjacent to diversions that did not contain these structures. We used predation event recorders (PERs) to assess the predation risk juvenile salmonids were exposed to, with linear distance (m) from diversions, and other predation risk factors in the north Delta. Finally, we used a boat electrofishing survey to determine the piscivore assemblage and compare spatial trends in black bass ( Micropterus spp.) CPUE and relative abundance throughout these waterways. Piscivore abundance was greater near small water diversions than at adjacent shorelines and the predation risk of juvenile salmonids increased with diversion proximity. Additionally, predation risk increased with increasing piscivore abundance and decreasing water depth. The north Delta predator assemblage was dominated by black basses ( Micropterus spp.), which likely drove the negative relationship of predation risk with water depth, given habitat requirements of these species. Furthermore, increasing smallmouth ( Micropterus dolomieu ) and spotted bass ( Micropterus punctulatus ) abundance in our northern study sites may have weakened temperature effects on predation, given metabolic requirements of these species. Our work demonstrated that small water diversions are likely to increase mortality of endangered salmonids, and that the north Delta predator assemblage was different than recorded by previous work in this system, changing predation risk factors. Although more work is needed to determine the population level impacts of diversions, the ubiquitous distribution of these structures warrants management solutions to reduce mortality from this source. These results indicate that in addition to entrainment and impingement, water diversions may increase mortality of small-bodied fishes by attracting predators and elevating predation risk. Given the continual human demand for freshwater, predator–prey interactions should be considered along with entrainment and impingement when assessing intake infrastructure mitigation, especially when diversions co-occur along migratory routes and essential habitat of imperilled fishes.

California↗
USGS2024

Juvenile coho salmon growth differences track biennial pink salmon spawning patterns

1. Spawning Pacific salmon ( Oncorhynchus spp.) provide marine-derived resources (MDR) to freshwater food webs in the form of eggs, flesh and maggots that consume salmon carcasses, all of which positively impact stream-dwelling fish growth. Pink salmon ( O. gorbuscha ) are widely distributed throughout coastal catchments along the North Pacific Ocean and display increased spawning abundances in odd years, owing to a fixed 2-year life history. While many studies have found that foraging and growth of stream-dwelling salmonids are improved by increased adult salmon spawning abundance, few studies have investigated the importance of alternating pink salmon spawning abundance between years. 2. Here, we examined how patterns of pink salmon spawning abundance impact the foraging and growth of juvenile coho salmon ( O. kisutch ). First, we used bioenergetic simulations to generate a hypothesis that coho salmon growth would increase during odd relative to even years. We then collected empirical juvenile coho salmon diet and growth data from a Southeast Alaska catchment in 2021 (pink salmon spawning) and 2022 (no pink salmon spawning). Field data were compared against simulation predictions to understand impacts of biennial pink salmon spawning patterns on juvenile coho salmon growth. 3. Empirical growth data revealed similar patterns to bioenergetic simulations. Age-1 coho salmon grew 16.6 mm longer and 5.5 g heavier on average in 2021 compared to 2022. Age-0 coho salmon displayed minor growth differences between years. 4. These results support bioenergetic model predictions and suggest that patterns of pink salmon spawning abundance can impart interannual growth disparities to juvenile coho salmon. Moreover, we show that distinct spawning characteristics of Pacific salmon species are important when understanding patterns of MDR transfer and growth responses in stream fishes.

Alaska↗
USGS2025

Using long-term ecological datasets to unravel the impacts of short-term meteorological disturbances on phytoplankton communities

Extreme meteorological events such as storms are increasing in frequency and intensity, but our knowledge of their impacts on aquatic ecosystems and emergent system properties is limited. Understanding the ecological impacts of storms on the dynamics of primary producers remains a challenge that needs to be addressed to assess the vulnerability of freshwater ecosystems to extreme weather conditions and climate change. One promising approach to gain insights into storm impacts on phytoplankton community dynamics is to analyse long-term monitoring datasets. However, such an approach requires disentangling the impacts of short-term meteorological disturbances from the effects of the seasonal trajectories of meteorological conditions. To this end, we applied boosted regression tree models to phytoplankton time series from eight relatively large lakes on four continents, coupled with a procedure adapted to detect and quantify rare events. Overall, the patterns and potential drivers we identified provide important insights into the responses of lakes to short-term meteorological events and highlight differences in the response of phytoplankton communities according to lake morphological characteristics. Our results indicated that deepened thermoclines and lake-specific combinations of drivers describing altered thermal structures caused deviations from the typical trajectories of seasonal phytoplankton succession. For shallow polymictic lakes, shifts in phytoplankton succession also depended on changes in light availability. Overall, our study highlights the value of long-term monitoring to improve our understanding of phytoplankton sensitivity to short-term meteorological disturbances.

Freshwater Biology↗
USGS2025

Waterfowl endozoochory: Traits drive plant-bird dispersal interactions in North America

Plant biogeography and ongoing changes in plant distributions are influenced by long-distance dispersal (LDD) of seeds, and migratory waterfowl (Anatidae: Ducks, geese and swans) are particularly important for the dispersal of plants in and around wetlands through gut passage (endozoochory). However, this dispersal is not contemplated by classical dispersal syndromes; hence, currently we have limited understanding of what angiosperm traits predict seed dispersal by waterfowl and any differences due to avian trait variation among waterfowl species. We conducted a literature search for data on the presence of seeds in the alimentary canal or faeces of North American waterfowl as a proxy for dispersal interactions, since an important fraction of these seeds survives gut passage. We present a list of North American flowering plant species ( N = 536) from 95 families potentially dispersed by 38 waterfowl species, together with traits of these plants and their vectors. Owing largely to limited sampling effort, 47% of plant species and 35% of plant genera recorded were only found in a single waterfowl species. A suite of analytical approaches revealed major differences in the traits of plants whose seeds are ingested and likely dispersed between foraging groups (guilds) and in relation to other waterfowl traits. Diving ducks and sea ducks are associated with sublittoral plants that have submerged or floating leaves, and hence with the classical hydrochory syndrome. Dabbling ducks and herbivorous waterfowl (notably geese) are associated with terrestrial herbaceous plants, and also with the unspecialised dispersal syndrome common in terrestrial plants. Plants with larger seeds or of greater height are more likely to be dispersed by dabbling ducks, whereas shorter plants are more likely to be dispersed by geese than by other waterfowl. Network analyses of interactions between waterfowl species and plant traits were generally consistent with these results, but also provided evidence that not all geese, dabbling ducks, or sea duck species are alike in their interactions with plants along the terrestrial–aquatic gradient, but instead occupy distinct dispersal niches. The traditional classification of waterfowl into guilds therefore does not accurately reflect their differing roles in the ingestion and dispersal of plant seeds. Aquatic plant species had more vector species than terrestrial ones. Detailed studies of waterfowl endozoochory and more trait determinations for North American plants are needed to facilitate predictions of LDD as waterfowl population sizes and migration patterns vary in the future with climate change.

Freshwater Biology↗
USGS2025

Exposure to ultraviolet radiation induces escape hatching of Cisco (Coregonus artedi) embryos

Cisco (Otoonapii in Ojibwe; Coregonus artedi Lesueur, 1818), is a widely distributed stenothermic freshwater fish whose embryos typically incubate under ice and in the dark. We used Cisco as a model organism for testing the potential of UV-induced escape hatching behaviour. Owing to reduced ice cover and increased water transparency in north temperate lakes, these experiments provide insights into the resilience of coregonine embryos if exposed to ultraviolet radiation (UV-B; 280–320 nm). Eyed Cisco embryos were exposed to artificially sourced UV-B through a series of experiments that measured the hatching rate and fitness (heart rate and pigmentation pattern) 2 days after hatching and under cold [6.6°C] and warm [8.6°C] conditions. These experiments supported an extension of the escape hatching behaviour hypothesis, whereby UV-B exposure induced earlier (ca 30 days) and more punctuated hatching of Cisco embryos, independent of an increase in water temperature. UV-B exposure produced more larvae with irregular pigmentation patterns and reduced heart rates (by about 20%) – both of which could be indicative of reduced fitness. UV-induced escape hatching adusts the fundamental framework in which we characterise fish embryo resilience to increased UV-B exposure and the potential consequences of reduced ice cover. Earlier hatching from UV-B exposure could increase the recruitment bottleneck of these fish by reducing survivorship of the post-hatched larvae.

Minnesota↗
USGS2025

Effects of riparian forest thinning on resident salmonid fishes in coastal northern California catchments

Resource managers are interested in whether thinning second-growth forests may be a viable restoration strategy for stream and riparian habitats, but may be concerned about the potential impacts that increases in stream temperature associated with riparian thinning treatments may have on cold-water salmonid fishes. We evaluated the effects of riparian forest thinning on resident populations of coastal cutthroat trout ( Oncorhynchus clarkii clarkii ) in coastal northern California catchments using a manipulative field experiment with a replicated before-after-control-impact design (pre-treatment data collected in 2016, thinning treatments occurred in 2017, and post-treatment data collected in 2018). Experimental thinning treatments reduced riparian shade by 20%–30% along five 150–200 m stream reaches. To provide a process-based evaluation of the implications of riparian thinning for coastal cutthroat trout, we combined seasonal observations of trout density, biomass, and growth with bioenergetics modelling. Cutthroat trout density increased by 8%–31% and biomass increased by 27%–111% in thinned reaches 1 year post-treatment, but responses varied widely across sites and seasons so did not always differ statistically. Growth rates of cutthroat trout varied more among seasons than among reach types (upstream reference, thinned, and downstream), peaking in spring and overwinter relative to summer. Bioenergetics modelling indicated that cutthroat trout responded to thinning-induced increases in stream temperature and shifts in prey energy density via higher consumption rates (i.e., fish fed more frequently) in thinned reaches. Additionally, reach-scale consumption estimates indicated that the energy intake of cutthroat trout increased at the population level in thinned reaches. However, thinned reaches exhibited relatively small changes in consumption, suggesting that riparian thinning was unlikely to enhance growth opportunities for cutthroat trout, supporting our empirical growth observations. Collectively, our field experiment suggests that salmonid fishes may be resilient to thinning second-growth riparian forests when treatments do not substantially increase water temperatures. Moreover, our results highlight that pairing empirical data with bioenergetics modelling can provide valuable insights into the mechanisms driving fish responses to riparian forest restoration.

California↗
USGS2025

Challenges and solutions for measuring taxonomic richness of aquatic invertebrates in wetlands

Measurements of biodiversity are crucial to assessing the ecological integrity of ecosystems. However, adequately describing the range of organisms existing in habitats can be challenging, especially for the taxonomically rich invertebrates. We analysed six large datasets designed to describe the taxonomic richness of aquatic invertebrate assemblages in depressional freshwater wetlands from various regions or locations (in North America, southern Africa and South America). Three of the datasets targeted large numbers of wetlands (57–163 sites), sampled once and the other three datasets repeatedly sampled over longer time periods (5–23 years), but targeted a smaller number of wetlands (10–18). We estimated the total number of invertebrate taxa that likely existed for each target area (Chao estimator), as opposed to how much of that richness was actually collected by each effort (using taxon-accumulation curves). The most ambitious effort (17 wetlands, 23 years, 5–6 samplings per year) captured 95% of the aquatic invertebrate taxa projected to occur in the study area; none of the other five efforts captured appreciably more than 80% of the projected total richness per study area. Findings suggest that capturing 90% or more of regional taxa is truly laborious and should not be a primary goal for efforts to sample the invertebrate fauna in wetlands. Objectives for sampling wetland invertebrates should be tailored to address what is realistic, knowing that as much as 30% of taxa may be missed by even ambitious efforts. As a potential solution, we suggest setting feasible objectives for wetlands macroinvertebrate assessments. We recommend researchers try to reach a 70% study area richness target by sampling 60–80 wetlands once or smaller sets of wetlands for 2–3 years.

Freshwater Biology↗
USGS2025

Ice Age biogeography corresponds with current climate vulnerability of freshwater fishes

1. Both local environmental factors and historical biogeography shape ecological communities, but determining which historical biogeographical patterns correspond with contemporary climate vulnerability is an underused conservation method. The historical colonization patterns of freshwater fishes following the Pleistocene (“Ice Age”) glaciations offers an ideal model for comparing historical biogeography and climate-change vulnerability. 2. We used current thermal niches and future stream-temperature projections to estimate the climate vulnerability of 29 Great Plains and Rocky Mountain fishes that we classified as either early or late colonists of the region in the wake of glacial retreat (~19,000 years ago). 3. Ninety-three percent of the most vulnerable species were amongst the earliest colonists of the region and we consider them “postglacial-pioneer species”. Median predicted site loss (number of historically occupied sites predicted to become too warm by end-of-century) was 0% for late colonizing species and 33% for early colonizing species. 4. We provide empirical evidence that postglacial-pioneer fishes are uniquely vulnerable to climate change, and we suggest this may apply to many taxa from formerly glaciated regions. More broadly, we demonstrate that evaluating the relationship between current species-environment patterns and historical biogeography may be a fruitful avenue for future climate change and conservation research.

North America↗
USGS2025

Groundwater structures fish growth and production across a riverscape

Landscapes are composed of habitat patches and conditions that vary across space and time. While habitat variability and complexity can support important ecological processes and ecosystem services, the dynamic nature of habitats can also constrain organismal growth and production as optimal conditions are fleeting. In riverine ecosystems, groundwater discharge to streams stabilises water temperature and flow regimes, thus mediating how habitat complexity is expressed. Yet, how stable habitats structure growth and production within the broader landscape matrix is not well understood. In this study, we explored the effects of groundwater on spatiotemporal variation in growth and production for juvenile Yellowstone cutthroat trout ( Oncorhynchus virginalis bouvieri ) across the upper Snake River catchment, Wyoming, USA. We combined machine learning techniques and remotely sensed landscape data to estimate groundwater availability across the river network, which we linked to stream temperature regimes and conspecific density. We then used Bayesian hierarchical models to quantify the effects of temperature, density and groundwater on spatiotemporal variation in fish growth and production in 52 focal reaches. Finally, we predicted body size trajectories and trends in total production continuously over both space and time to understand the effect of groundwater at the riverscape scale. Groundwater discharged to streams where topography changes abruptly in valley-bottom areas underlain by coarse glacial deposits. Groundwater stabilised temperature regimes and was associated with high trout densities. Temperature and density, in turn, interacted to influence growth rates: growth increased strongly with temperature, but this effect was reduced when density was high. Accordingly, variation in groundwater availability among stream reaches diversified growth and production regimes. In reaches with low groundwater availability, growth and production declined over time from summer maxima. In contrast, in reaches with high groundwater availability, temporal trends in growth and production were hump-shaped—peaking in autumn—and mean production was greater. At the riverscape scale, temporal asynchrony in growth rates generated convergent spatial variation in growth capacity, but—when combined with density—led to the formation of distinct hotspots of production. Our results demonstrate how groundwater, an important driver of aquatic ecosystem heterogeneity, structures trout growth and production across space and time. Importantly, rare, but stable habitats may disproportionately affect ecological processes and serve as key sources of population diversity at larger spatial scales.

Wyoming↗
USGS2025

Potential thiamine deficiency of phytoplankton across a productivity gradient and seasons in Ohio lakes

Although nitrogen and phosphorus deficiency of algal blooms have been the focus of substantial attention, organic nutrients can limit algal growth in aquatic systems. Growing evidence indicates thiamine (vitamin B 1 ) can influence the community of primary producers in marine systems, but comparatively little is known about the effect of thiamine on freshwater algal productivity. We conducted 106 nutrient deficiency experiments with water from 39 Ohio lakes of varying trophic status during the growing seasons (April–October) of 2008–2009. Specifically, we tested the response of phytoplankton biomass (as chlorophyll a , chl- a ) relative to controls to added nitrogen (N), phosphorus (P), thiamine (Th), or combinations of N + P and N + P + Th. Next, we compared the chl- a growth response of treatment/control to published thresholds based on frequentist approaches and compared the conclusions with Bayesian model results that focused on probability of a response. Although N + P addition was consistently associated with the largest chl- a response, we found evidence of a thiamine influence on phytoplankton growth in some experiments. The Bayesian approach suggested thiamine may become more limiting as the growing season progresses. By late in the growing season, there was an 85% probability of a positive algal growth response to thiamine addition. Understanding the role of thiamine or other overlooked nutrients is not likely to alter the prevailing understanding of nutrient deficiency in freshwater ecosystems. However, we present evidence that freshwater phytoplankton may experience thiamine deficiency and suggest limnologists consider thiamine when exploring resource deficiencies.

Ohio↗
USGS2026

Seasonal environmental conditions and river morphology shape summer phytoplankton communities

1. Phytoplankton form the base of large river food webs but there are limited studies on the nature and drivers of communities over longer time scales. Further, climate change is projected to favor taxa associated with harmful algal blooms, but our knowledge of the timing, locations, and drivers of cyanobacteria in rivers lags that of lakes and marine environments. 2. We used a summer phytoplankton community dataset collected from 2010-2020 across main channel, side channel and backwater areas in the La Grange reach of the Illinois River to assess: 1) How much do summer phytoplankton communities across diverse aquatic areas within a large river vary from year to year?, 2) What environmental conditions are associated with that variation?, and 3) Do cyanobacteria respond differently than the full phytoplankton community? 3. We found greater differences in phytoplankton communities across years than among aquatic areas. Further, we showed that annual variation in phytoplankton communities was synchronous among taxa in the same aquatic area, rather than specific to each taxonomic group or taxa-area combination. After accounting for this spatial variation in annual dynamics, phytoplankton reflected river conditions across seasons, responding positively to summer total phosphorus and invasive carp abundance and negatively to winter discharge and spring silica to nitrogen ratio. Annual variation in cyanobacteria was similarly distinct among main channel, side channel and backwater areas but was best explained by the ratio of summer air temperature to discharge, representing conditions that favored growth over transport. 4. These results show that annual variation in summer phytoplankton and cyanobacterial communities both reflected the spatial diversity of the river landscape and responded to conditions that occurred at different spatial and temporal scales. Further, in this highly modified river basin, we showed that a mix of anthropogenic pressures including eutrophication, invasive species, and potential changes to winter and spring conditions affected the phytoplankton communities present in the summer. Thus, when evaluating long-term change and the potential for harmful algal blooms in rivers it is important to consider spatial diversity of phytoplankton communities, how their sensitivity to environmental change may vary across the river landscape, and the suite of human modifications acting on those communities.

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