ABSTRACT Like other pathogens, parasitic fungal chytrids contribute to the collapse of phytoplankton blooms. When a bloom crashes, limiting nutrients become more freely available, facilitating subsequent blooms dominated by different species. Although this disease‐mediated control can enhance overall phytoplankton diversity, some studies suggest that parasites can increase host density at short timescales; some at long (equilibrium) timescales; still others report negative effects of parasites on host density. This article collates such theoretical and empirical studies, illustrates the mechanisms underlying the positive effects of chytrids on host density, and explores the conditions under which these effects occur. Lastly, I propose a hypothesis by which parasitic infections may increase host density and how these positive effects contribute to seasonal plankton bloom dynamics. At short timescales, chytrid exploitation of hosts reduces their fitness and can leave more nutrients available for uninfected phytoplankton (a cascading effect on nutrient availability). The parasites take 2–19 days to kill infected hosts—indicating their interaction durability—and release free‐living infectious zoospores that enable new infections. This lag delays death of the infected host and parasite attacks on susceptible hosts, and thus prolongs the period of growth for susceptible hosts on the available nutrients. A combination of the cascading effect and interaction durability can increase total host density relative to uninfected phytoplankton, leading to a phenomenon called the “hydra effect”. Here, I show that such an effect on host phytoplankton is possible when their maximum growth rate is sufficiently high and the parasite infection rate is sufficiently low. At longer timescales, with host death, chytrids release zoospores that are edible and nutritious for zooplankton. Predation of zoospores (the mycoloop) can boost zooplankton growth, then increase predation pressure on edible nonhost phytoplankton (apparent competition). A mycoloop therefore positively affects host density by reducing zoospore and host competitor densities. A mini‐review shows that parasitic chytrids can enhance the dominance of host phytoplankton under certain host–parasite trait combinations modulated by environmental conditions, even in the absence of the mycoloop. However, infections that turn inedible hosts into edible prey for zooplankton should reduce these positive effects. In conclusion, parasites of phytoplankton can play significant roles in host bloom initiation and termination, potentially controlling bloom timing and magnitude. This should be especially true in temperate regions, where the prevalence of parasitic fungi is predicted to increase over time with warming.
ABSTRACT The reconfiguration of flexible structures such as submerged aquatic plants is a key mechanism for reducing drag in environments with high variation in water velocities. In flowing water, submerged aquatic vegetation can form patches, which are discrete structures formed by many individuals at high density. While most studies have focused on isolated shoots, drag and reconfiguration remain largely unstudied in aquatic plant patches. We studied the drag and reconfiguration of patches and tested the effects of patch size, density, and plant flexural stiffness on reconfiguration. We investigated three aquatic plant species presenting contrasting properties in a flume experiment. Our results demonstrated that the reconfiguration of patches is similar to previous findings on isolated shoots, indicating a high reconfiguration capacity for patches. Moreover, the reconfiguration capacity differed between species, suggesting that a single reconfiguration coefficient cannot accurately estimate drag in models. Overall, patch reconfiguration was linked to patch fresh mass and density but not to plant flexural stiffness. By providing the first measurements of drag and reconfiguration of small patches in flumes, our results provide an initial basis for the large‐scale numerical modelling of patch drag in fluvial environments.
ABSTRACT Nutrient enrichment in shallow lakes and ponds can shift systems from clear water, macrophyte dominance to turbid, phytoplankton dominance. Because the identity and threshold of the limiting nutrients vary among sites, system‐specific evidence is needed to guide water quality management. We conducted 3‐day in situ nutrient‐enrichment bioassays in two wetland impoundments adjacent to Great Salt Lake (Utah, USA) to identify the limiting nutrient for phytoplankton growth, quantify growth responses to nitrogen (N) and phosphorus (P), estimate an operational nutrient threshold using Monod kinetics and evaluate bacterioplankton community and functional responses across the same nutrient gradient. Phytoplankton biomass (chlorophyll a ) was used to calculate growth rates, and bacterioplankton communities were profiled by 16S rRNA gene sequencing with inferred functional annotation. Across both wetlands, phytoplankton growth increased with N additions but not with P additions, indicating N limitation under high ambient P. Monod models yielded a half‐saturation constant of approximately 0.7 mg N L −1 (as NO 3 –N), which we interpret as an N threshold for phytoplankton growth in these impoundments. Bacterioplankton composition differed by site, with Cyanobiaceae dominating at both locations, and community evenness declining above the N threshold. Inferred metagenomes indicated a higher relative abundance of taxa carrying the microcystin synthetase gene mcyE above the N threshold, suggesting increased cyanotoxin potential. Together, these results support management actions that control N in addition to P and demonstrate the value of bioassay‐derived thresholds for informing numeric criteria in managed wetlands.
ABSTRACT Floodplain wetlands are characterised by dynamic hydrological regimes and seasonal patterns of wetting and drying. They support distinct communities of fish, many of which are adapted to dynamic and variable conditions. Seasonal hydrological variation and the degree of water permanence influence physicochemical conditions and local habitat, which together with the surrounding landscape strongly influence the distribution and persistence of fish within individual wetlands. This study aimed to evaluate how broad‐scale landscape and temporal environmental factors of wetland ecosystems and biological traits of wetland fishes can determine occurrence patterns in floodplain wetland fish assemblages. We analysed inundation and environmental data spanning 37 years (1986–2022) across floodplain wetlands in the southern Murray–Darling Basin (MDB), Australia. Fish occurrence records for 14 species (10 native and 4 introduced) were sourced from public databases and combined with remotely sensed estimates of inundation frequency and land‐cover data to assess species‐environment‐relationships. To better understand the potential influence of biological determinants of habitat use (i.e., biological traits and species interactions), we also incorporated species‐specific traits related to life history, trophic levels and potential population growth, as well as phylogenetic relatedness. We applied Joint Species Distribution Models (JSDMs) to evaluate how environmental and biological factors jointly shape wetland fish community structure. Fish assemblages were primarily structured by environmental conditions, with water temperature, wetland area, and inundation regime identified as influential factors. Species‐specific responses to these environmental filters were heterogeneous. Biological traits, including life history, trophic level and resilience, collectively explained 26% of the variance in species occurrences, highlighting their role in determining habitat use. Spatial effects, accounting for variation among wetland locations and catchments, also contributed significantly to community structure. Phylogenetic relatedness did not influence species co‐occurrence. After accounting for environmental, trait‐based, and spatial factors, residual species associations suggested the potential for biotic interactions such as competition or facilitation acting at different scales. Fish communities in floodplains wetlands of the southern Murray–Darling Basin (MDB) are predominantly shaped by environmental filtering, with hydrology (inundation) the dominant factor, moderated by species traits and spatial context. This underscores that effective conservation and restoration of floodplain fish communities will require management strategies that buffer thermal extremes and enhance wetland permanence, along with maintaining a mosaic of wetland areas and considering landscape connectivity to accommodate both environmental influences and species‐specific habitat requirements.
ABSTRACT Phenological shifts in animal migration are widely reported under climate warming, yet responses of warm‐temperate river fishes in regulated systems remain poorly resolved. Understanding how hydrological and thermal cues interact to structure migration timing is central to predicting ecological responses in large rivers and informing environmental water management. We analysed 31 years (1994–2025) of daily fishway monitoring data from the Murray River, south‐eastern Australia (5460 sampling days; 349,305 individuals), to quantify long‐term variation in migration phenology of five potamodromous fish species across juvenile and adult life‐stages, and to evaluate the relative roles of river discharge and water temperature in structuring migration timing. River water temperatures increased significantly over the study period, particularly during summer and autumn. Despite this warming, migration timing remained largely stable across species and life‐stages, with strong interannual variability driven primarily by episodic flow events rather than gradual temperature change. Only Murray cod ( Maccullochella peelii ) exhibited a modest advancement in migration timing, associated with low‐flow conditions. These results indicate that, in warm‐temperate regulated rivers, migration phenology is structured primarily by hydrological variability rather than temperature trends alone. Importantly, the apparent absence of directional phenological change represents an informative ecological signal, demonstrating that life‐history plasticity, facultative migration and flow regulation can buffer migratory timing against observed moderate climatic warming. Our findings highlight the context‐dependence of climate–phenology relationships in freshwater ecosystems and have direct implications for the timing of environmental flows and fish passage management in regulated rivers.
ABSTRACT Aquatic hyphomycetes are key decomposers in headwater streams, yet their temporal dynamics in tropical systems remain poorly understood due to the scarcity of long‐term studies. Here, we investigated the temporal variation of aquatic hyphomycete communities in a tropical Atlantic Forest stream over 5 years, evaluating the relative influence of intra‐ and interannual environmental factors on fungal richness, conidial density and community composition. We combined field sampling with multivariate and modelling approaches (ANOSIM, BRT, TITAN and GAMMs) to disentangle temporal patterns and identify key environmental predictors. Our results show that interannual variation exerted a stronger influence on community structure than intra‐annual dynamics. Water chemistry and leaf litter quality were the main factors associated with fungal richness and density, whereas seasonal signals had limited explanatory power. During seasonal environmental fluctuations, fungal communities showed remarkable temporal resilience and functional stability, with no collective community shift observed throughout the annual cycle. Indicator species such as Anguillospora filiformis , Tricelophorus monosporus and Amniculicola longissima displayed distinct responses to interannual environmental gradients, reinforcing their value as bioindicators of long‐term environmental change. These findings highlight the predominance of long‐term environmental variability over seasonal dynamics in structuring tropical fungal communities and emphasize the importance of long‐term datasets for understanding ecosystem functioning in tropical streams. This study provides key insights into the ecological roles of aquatic hyphomycetes in tropical streams by emphasizing the critical importance of long‐term approaches to unravel complex temporal patterns. Our results challenge the assumption of seasonality's dominance in tropical systems, offering a more nuanced understanding of fungal community responses to environmental change, which is essential for predicting ecosystem integrity under global climate change.
ABSTRACT Growth of ectotherms typically positively correlates with increasing temperature towards an optimal temperature range, followed by a decline in growth when temperatures exceed the optimal temperature range for that organism. When temperature is within or close to the optimal range, food availability and other environmental factors can play important roles in modifying growth rates. River regulation often alters both environmental conditions (e.g., flow, turbidity and temperature) and aquatic food webs, modifying the quality and quantity of resources available to consumers. Here we aim to understand how food availability, water temperature and other environmental factors are associated with fish growth (defined here as a change in length) in a large, regulated desert river. We measured growth of humpback chub ( Gila cypha ) at two river reaches located 240 km apart in the Colorado River within the Grand Canyon, Arizona, USA. We then fit Bayesian state‐space models of growth that account for variable time at large to test environmental predictors such as water temperature, flow, turbidity (which increases during tributary flooding and can be a proxy for allochthonous inputs) and gross primary productivity (a proxy for autochthonous food) using an inclusion parameter approach. Both river reaches are located below Glen Canyon Dam, which regulates flow, alters seasonal water temperatures and traps sediment, nutrients, and organic carbon; however, merging tributaries and a greater distance from the dam lead to differences in environmental conditions. In the upriver reach, where warm season water temperatures are below the optimal range for humpback chub most of the time, temperature was the primary driver of variation in growth according to inclusion parameter values and the strength of the standardised effect size. Turbidity was also included in the final model for the upper reach, but the inclusion parameter values and standardised effect size were smaller than for temperature. In the downriver, warmer reach, temperature still had the highest inclusion parameter value, but GPP had the highest standardised effect and nearly as high an inclusion parameter value. We conclude that temperature is the primary limiting factor on growth of humpback chub in the Grand Canyon; however, as water temperatures increase, other factors, especially GPP, are associated with temporal variation in growth rates. Over the past two decades, declines in reservoir elevations and the volume of summer releases have increased downstream water temperatures. If warm temperatures persist into the future, other factors may have increasing roles in regulating interannual variability in chub growth. Field studies of ectotherm growth often emphasise the role of water temperature; however, the role of resource availability is also increasingly recognised, especially in bioenergetics studies. Quantifying resource availability directly can be difficult, especially in large, remote rivers. Here, we show here that using proxies for resource availability (i.e., GPP as a proxy for autochthonous inputs) can yield useful insights regarding resource limitations with implications for management of federally listed fish species in a larger regulated river system.
ABSTRACT Climate change and species introductions are currently the main threats to freshwater systems. The combination of global warming and salmonid introductions could have an enormous impact on native fishes: particularly affected will be interspecific interactions, key to the structure and functioning of ecological communities. The aim of the present study was to assess the combined effects of species interaction and temperature on the feeding performance, using multiple predator functional response (FR) analysis of a native and a non‐native fish from Patagonia, and the overall effect on the native amphipod used as prey. We evaluated intra‐ and interspecific interactions between the native puyen grande Galaxias platei and the non‐native rainbow trout Oncorhynchus mykiss across four experimental temperatures using the functional response (FR) approach, which describes how prey consumption varies with prey density. FR curves were first estimated for individual fish and then for conspecific and heterospecific pairings. From the individual trials, we predicted ‘expected’ FR curves under the assumption of no interaction between predators. These were compared with the ‘observed’ FR curves in combined trials to quantify intra‐ and interspecific predator interactions. In addition, we calculated the functional response ratio (FRR) as a complementary index to assess prey impact under different experimental conditions. Both species showed hyperbolic‐shaped (type II, destabilizing for prey population) FR curves for all treatments. The single FR curves of O. mykiss were higher than those of G. platei at all temperatures except 21°C, at which prey consumption dropped off abruptly. The strongest effects on prey were observed at high temperatures for G. platei (19°C and 21°C) and at lower temperatures for O. mykiss (13°C and 16°C). The presence of a conspecific had no negative effect on G. platei . Temperature had no effect on the conspecific FR curves. Under heterospecific conditions, prey consumption of G. platei was not negatively affected by the presence of O. mykiss . In contrast, O. mykiss reduced its feeding performance in the presence of G. platei . The overall prey consumption of both species in the heterospecific treatment did not vary with temperature. There was no evidence of a negative intra‐ or interspecific effect on the feeding performance of G. platei . Increasing temperature had a strong negative effect on O. mykiss prey consumption, but not on G. platei , suggesting that under future warming conditions, this non‐native salmonid will be negatively affected first. From the perspective of the prey population, the predation effect is likely to be greatest in the habitats occupied by O. mykiss alone. Our study contributes to the understanding of complex interspecific interactions between native and non‐native species in relation to temperature, and highlights that non‐native species are not always competitively superior to native species. In addition, under global warming scenarios, the non‐native species may be more negatively affected than the native species. This could have implications not only for the southern hemisphere, where galaxiids occur, but also globally in regions where salmonids have established self‐sustaining wild populations and interact with other native fishes.
ABSTRACT On remote oceanic islands, changes in land uses frequently involve the clearance of native forest, establishment of exotic commercial plantations and intensification of agricultural and livestock activities. These transformations often result in the alteration or removal of native riparian vegetation, with potentially strong effects on stream ecosystem structure and function. Aquatic decomposers and organic matter decomposition are particularly sensitive to land uses changes due to their dependence on terrestrial litter inputs. Here we assessed organic matter decomposition, and associated aquatic hyphomycetes and macroinvertebrates in Azorean streams under riparian forest modifications (cryptomeria plantations and pastures) in comparison with non‐impacted streams (native laurel vegetation). Ochroma pyramidale wood and Clethra arborea leaves were used as substrates enclosed in fine‐ and coarse‐mesh bags to assess organic matter decomposition driven by microbes alone and by the combined activity of microbes and macroinvertebrates, respectively. We found that organic matter decomposition was faster in streams surrounded by pastures due to higher dissolved nutrient concentrations and water temperature, while streams flowing through native riparian vegetation and cryptomeria plantations showed similar organic matter decomposition. Leaf decomposition was faster in coarse‐ than in fine‐mesh bags only in streams adjacent to cryptomeria plantations, a pattern associated with the higher shredder abundance compared with the other streams. Moreover, leaves decomposed faster than wood due to their lower toughness and greater susceptibility to biological degradation. Aquatic hyphomycete taxa richness was lower in pasture streams, but their reproductive activity did not significantly differ among stream types. Benthic macroinvertebrate taxa richness and abundance did not significantly differ among stream types. These findings highlight the complex interactions between land‐use transformation and organic matter decomposition processes emphasizing the importance of riparian management for maintaining ecosystem functioning. Moreover, our findings underscore the vulnerability of stream ecosystem functioning to riparian land‐use change on remote oceanic islands, where even moderate alterations in riparian vegetation can significantly affect key ecological processes.
Changes driven by climate change, biological invasion, land‐use change, pollution and overexploitation are decreasing the resilience of freshwater ecosystems worldwide. While the impact of these threats is widely recognised, a critical gap remains in systematically quantifying their relative effects across ecological levels, which is essential for linking impacts on multiple responses. This gap limits our ability to anticipate their potential additive effects and to establish baseline conditions for studies addressing interactive impacts on freshwater ecosystems. We conducted a meta‐analysis to assess the individual impacts of multiple threats on freshwater ecosystems at the population, community and ecosystem levels, integrating data across multiple response variables. Our findings revealed that individual threats show similar effects on specific ecological levels and key responses, leading to potential additive effects. Pollution and climate change consistently increased population‐level responses, whereas pollution and land‐use change increased productivity‐related processes at the ecosystem level. At the community level, biological invasion and land‐use change disrupted freshwater biodiversity, although all threats presented an overall decreasing effect on the related responses. Our results show that climate change, biological invasion, land‐use change, and pollution compromise freshwater ecosystems through similar magnitudes and directions of impact across ecological levels, revealing a consistent cross‐level convergence among threats. This cross‐level convergence provides an empirical basis for anticipating additive effects among threats, improving the design of target monitoring and management strategies for freshwater biodiversity.
Cryoconite holes are among the most biologically active and diverse limnic habitats on glacier surfaces. The apex cryoconite consumers, tardigrades and rotifers, are known to contribute significantly to the processing and composition of supraglacial (i.e., glacier surface) organic matter (OM). Yet, their specific food sources remain unknown. We experimentally added two contrasting isotopically labelled food sources, 13 C‐labelled algae and 15 N‐labelled bacteria, in four treatment setups to cryoconite holes in situ. After incubations, tardigrades and rotifers were collected and analysed for their carbon (δ 13 C) and nitrogen (δ 15 N) stable isotopic composition. δ 13 C and δ 15 N values of consumers from non‐labelled control samples were compared to those previously measured from the same glaciers, with seasonal variability in isotopic signatures found in some samples. Both consumers select their food and likely feed on both algae and bacteria, albeit the consumption efficiency of filter‐feeding rotifers was generally higher. Tardigrades showed a significant but low 13 C enrichment compared to control samples in treatments with 13 C‐labelled algae, while in treatments with 15 N‐labelled bacteria, both consumers showed comparably high enrichment in their δ 15 N. This study presents the first empirical study on food preferences for algae and bacteria in supraglacial tardigrades and rotifers and differences in their consumption rates, significantly deepening our knowledge of the consumers' role in supraglacial OM composition and nutrient flow through the cryoconite food web.
Ship‐induced waves significantly impact freshwater ecosystems, estuaries and tidal rivers by accelerating shoreline erosion and reducing aquatic biodiversity. Morphological protection measures are implemented to mitigate these adverse effects, but their effectiveness in preventing shoreline zone deterioration has not been systematically assessed. This meta‐analysis evaluates the performance of various measures in influencing hydraulic forces, abiotic processes and biological responses across primary producers, benthic invertebrates and fish. Our findings reveal that the effectiveness of these measures varies depending on the scale of investigation, ecosystem type and response variable. Offshore breakwaters were highly effective in reducing hydraulic forces in freshwater ecosystems (+756%) but had a limited impact in transitional ecosystems (+34%, not significant). In contrast, breakwaters positively influenced benthic invertebrates in transitional ecosystems (+6945%) but had negligible effects in freshwater ecosystems (+91%, n.s.). Increased shoreline complexity had overall positive effects in mesocosm‐based studies (+407%) but yielded limited benefits in freshwater field studies (+67%, n.s.). Moreover, the measures showed diverse responses across trophic levels within freshwater ecosystems. Shoreline complexity significantly benefited benthic invertebrates (+336%), while positive effects on fish were found for breakwaters (+74%) and side channels (+205%). These findings highlight the importance of implementing ecosystem‐specific management strategies tailored to the needs of target organisms. A combination of offshore breakwaters and increased shoreline complexity appears promising for mitigating the negative impacts of ship‐induced waves on all trophic levels in navigable rivers, lakes and estuaries.
ABSTRACT Zooplankton egg banks are crucial in rebuilding zooplankton populations in temporary water bodies when favourable conditions return after dry periods. However, their abundance, viability, and hatching success depend on many factors, with changes in land use and flooding regimes being potentially crucial drivers. Here, we studied a large‐river floodplain to compare the egg banks of soils across a land use gradient and at different elevations, the latter affecting inundation frequency and duration. We performed experimental manipulations at both the ecosystem scale (land use in the floodplain) and in the laboratory (incubation of soils of different origins). Six land use types were considered: (i) natural maple swamp, (ii) natural wet meadow, (iii) old forage cropping, (iv) recently sown forage cropping, (v) agri‐environmental corn/soybean cropping, and (vi) conventional corn/soybean cropping implemented in collaboration with agricultural producers. Egg banks were characterized through both (i) direct egg counts and (ii) by counting the individuals emerging following the incubation of soils in the laboratory, testing the effects of land use and flooding regime (duration, frequency) on hatching success. We found no significant difference in the abundance of resting eggs of different taxa in soils across the land use gradient in frequently inundated sites at low elevation. In contrast, at high elevation, where inundation periods are less frequent and shorter, rotifer and cladoceran egg density varied across land uses, with higher abundance in old forage cropping compared to agricultural cropping. Ostracods, rotifers and copepods were the most abundant taxa that hatched in the laboratory. Their abundance varied across land use types, with natural soils supporting higher abundances than agricultural soils, and forage cropping showing intermediate abundances. Frequently inundated sites (low elevation) had higher abundances of hatchlings than the less frequently inundated (high elevation) sites. Inundation duration in incubators strongly influenced ostracod hatching only, with an overall increase after 2 weeks of inundation and a decrease thereafter for most soil types. Different taxa dominated the egg and hatchling counts. For example, cladocerans were common in the egg bank but not in hatchling samples, while ostracods were absent from the former but dominated the latter. This apparent paradox suggests that egg viability varied along the land use gradient (e.g., due to agricultural practices during summer months), that the cues required for hatching differed across taxa, or that stress (land use) modulates the likelihood that ostracods enter dormancy as eggs versus juveniles/adults. Natural wetland habitats in floodplains favour the resilience of zooplankton communities facing a disturbance (i.e., the dry period) via a high potential recovery through the emergence of resting stages. In contrast, alterations to these habitats due to agricultural intensification in floodplains hampers zooplankton community resilience via decreased emergence success. Agricultural practices leading to changes in soil properties, which influence the viability of resting stages, are the plausible explanations for this phenomenon. It is paramount to conserve natural wetlands and implement sustainable land use practices in floodplains, to allow egg banks to rebuild zooplankton communities in floodplains and sustain their productivity and ecosystem services.
Saprolegniosis, also known as water mould, is a lethal disease caused by Saprolegnia oomycetes that threatens freshwater animals including fish, amphibians, and crustaceans, as well as aquaculture. It causes biodiversity loss and population declines, including those of endangered fishes. Controlling saprolegniosis in aquaculture is based on chemicals, which are often expensive, ineffective and hazardous to the environment. Filter-feeding bivalves (Mollusca) remove pathogens, along with other particles, from the water. Therefore, testing of the capacity of freshwater mussels (Unionida) in biofiltering Saprolegnia spp. is important to find out if the mussels could provide an environmentally friendly and sustainable solution to mitigate the problems caused by Saprolegnia. We tested (1) the capacity of swan mussels (Anodonta cygnea) to filter particles (spores and hyphal fragments) of Saprolegnia ferax and S. diclina, and (2) the effect of mussel biofiltration on the concentration of infective spores and the germination rate of S. diclina spores, and (3) the effectiveness of mussel biofiltration to prevent saprolegniosis in fish eggs. Mussels effectively filtered Saprolegnia particles from the water: particle concentrations decreased in mussel aquaria compared to controls within 1 h of start and the difference remained until the end of the 24 h experiment. The number of colonies of S. diclina on oomycete-optimized culture plates was lower in samples originating from the water and sediment of mussel aquaria than in samples from control aquaria. Furthermore, mussel biofiltration reduced the germination rate of S. diclina spores in both water and sediment compared to controls. Additionally, we found that the proportion of fish eggs infected with saprolegniosis was much lower when the eggs were exposed to mussel aquaria water compared to control aquaria water. Our results show that freshwater mussels can reduce the concentration of infective Saprolegnia particles and the infectivity of remaining particles, and protect fish eggs from contracting saprolegniosis, thus highlighting the effect of valuable ecosystem service 'pathogen removal' provided by the mussels. Our findings encourage deployment of freshwater mussel biofiltration as a nature-based solution to mitigate the problems caused by Saprolegnia in freshwater ecosystems, aquaculture and fish conservation.
Ephemeral wetland pans in savanna landscapes support highly dynamic food webs whose structure is closely governed by hydroperiod. In these systems, variation in inundation length influences predator establishment, basal resource availability and the strength of aquatic-to-terrestrial linkages. We used stable isotopes of consumers (including fish and macroinvertebrates) and basal resources to examine how trophic position, reliance on aquatic carbon and isotopic niche width varied across two contrasting hydroperiod states: high hydroperiod in the wet season and low hydroperiod in the late dry season in six ephemeral pans of the Kruger National Park, South Africa. We predicted that during the high hydroperiod, ephemeral pans would support more stratified trophic structures and stronger assimilation of aquatic basal resources. Our results showed that trophic positions were consistently higher and more stratified under high hydroperiod conditions, especially among predators and fish. In contrast, predators and fish showed lower, more overlapping trophic positions across consumer groups in low hydroperiods. Isotopic niche widths (as measured by standard ellipse area) showed inconsistent responses among pans, with some pans exhibiting wider niches during low hydroperiod conditions. This variation likely reflects the ecological heterogeneity and resource unpredictability characteristic of rapidly drying habitats. These results accentuate the sensitivity of ephemeral wetland food webs to hydrological fluctuations and provide a foundational case study for understanding trophic responses to changing hydroperiods in African savanna ecosystems.
Smaller individuals tend to be more abundant than larger individuals. Variations in this pattern can be assessed through size spectra analyses, which describe the relationship between abundance and body size. The parameters estimated from these analyses-midpoint height (expected abundance at the midpoint of the size class axis) and slope-can indicate the effects of natural environmental variability and anthropogenic impacts on biological communities. In this study, we analysed a long-term dataset on fish communities in the Upper Paran & aacute; River floodplain to assess temporal variations in size spectra slopes and midpoint heights, as well as their responses to varying levels of impact from damming, variation in water level, and different proxies of productivity (total phosphorus, total nitrogen, and chlorophyll-a concentrations). We also tested interactions between levels of impact, productivity, and time. We found that size spectra slopes were more negative at less impacted sites, where the smallest size classes also occurred predominantly. Water level was positively correlated with the abundance of the fish community (represented by the midpoint height) and the occurrence of the smallest size class. Total phosphorus concentration was negatively correlated with the slope. Over time, we observed a reduction in the magnitude of the slope, related to a decrease in the abundance of the smallest size classes. Our results highlight the importance of floods for fish recruitment and suggest that size spectra estimates are not always consistent indicators of natural change or anthropogenic impact, as they may respond in complex ways to both sources of variation
Morphological features play a critical role in the ability of an organism to navigate its environment. Organisms living in rivers and streams must handle a spectrum of habitats that span from dynamic flows in headwater creeks to the regularly high flows of large rivers. Freshwater mussels (order Unionoida) are an interesting group of organisms that possess a variety of hypothesized shell morphology traits adapted to life across the stream size spectrum. Fusconaia flava (family Unionidae) is a species that exhibits a pattern of clinal variation, known as Ortmann's Law of Stream Position, where shell width increases with stream size. Although speculations have been made about the causes and consequences of Ortmann's Law of Stream Position, few experiments have been conducted to help explain why some freshwater mussel species exhibit this phenotype-environment association. We conducted a comparative morphometric study and a fluvial experiment to identify how intraspecific mussel morphology and behaviour vary along a stream size gradient. We used micro-computed tomography scanning to digitize shells from hydrologically diverse habitats, then measured the width, size and median thickness of each shell. Multiple regression was used to test the association between shell width and thickness while accounting for shell size, and linear regression models were used to describe the relationships between shell traits and hydrology. In a corresponding flume experiment, we measured valve-gaping activity and observed presumptive burrowing behaviour of wild-collected F. flava that exhibited 'compressed' (small-river morph) and 'inflated' (large-river morph) shell morphologies found in the upper and lower reaches of the Gasconade River in central Missouri, USA. We scaled flume flows to mimic observed low and high streamflows on the Gasconade River and used Hall sensors to record valve movements. We found shell thickness and shell width were strongly associated with one another and increased with aspects of stream size (i.e., Strahler stream order, flow rate and catchment area). The flume experiment demonstrated the feasibility and mechanistic potential of using wild-collected mussels in behaviour experiments. We successfully integrated flow scaling via Froude numbers and demonstrated the sensitivity of Hall sensors for continuous behavioural monitoring. Valve-gaping activity of 'compressed' mussels did not appear to have an affinity to a specific flow regime, but visually they made significant spatial movements, whereas 'inflated' mussels exhibited pronounced changes in valve-gaping activity under higher flow conditions but remained more or less stationary. However, our data were not capable of definitive behavioural comparisons and instead proved the value of this setup for future, larger-scale experiments. Given the ongoing and projected increase of droughts and catastrophic flood events due to climate change, our results provide insight into how morphological trait variation and valve-gaping activity together allow individuals to respond to streamflow fluctuations. Additionally, our integration of comparative morphometrics and flume experiments provides a promising proof of concept for testing phenotype-environment associations.
Phytoplankton community responses to nutrient-management interventions in agricultural catchments remain poorly understood, particularly with respect to functional group transitions. Petit-lac-Saint-Fran & ccedil;ois, a hypereutrophic northern temperate lake, underwent agricultural nutrient-mitigation measures beginning in 2011, providing an opportunity to examine long-term ecological change. This study assessed phytoplankton community dynamics over a 10-year period (2011-2020) to determine how nutrient reductions influenced taxonomic composition, cyanobacterial functional groups and ecological regime shifts. Seasonal and annual phytoplankton biovolume, class-level composition and cyanobacterial functional traits were analysed, and Sequential t-test Analysis of Regime Shifts was applied to identify transitions. Nutrient fractions (TKN, TN, NH4+, PP, DOP, DP, TP) and stoichiometric ratios (TN:TP) were evaluated relative to ecological transition dates (August 2014 and August 2017). Cyanophyceae dominated the community (88.6% of annual biovolume), with strong seasonal contrasts between summer (91.6%) and winter (8.6%). Although overall class composition remained stable, substantial restructuring occurred within cyanobacteria. Nitrogen-fixing-capable taxa increased sharply, with regime shifts in their proportional abundance from 0.28 to 0.71 (August 2017) and then to 0.89 (June 2018). Species richness increased by 50.8% after August 2014, while evenness declined by 26.0%. Colonial Microcystis (42.8% pre-2015) was replaced by filamentous Dolichospermum (45.2% post-2015). Dissolved nutrient fractions (DP, DOP) declined during the initial community reorganisation (2014), whereas particulate fractions (PP, TP) and TN:TP ratios shifted during the later morphological transition (2017). Phytoplankton communities responded rapidly to nutrient-management interventions, particularly through functional reorganisation within cyanobacteria. However, the persistence of cyanobacterial dominance and the rise of nitrogen-fixing filamentous taxa indicate that full lake restoration will require measures beyond nutrient reduction alone. This study provides a rare long-term assessment of how nutrient-management actions influence cyanobacterial functional structure in a hypereutrophic lake. By linking specific nutrient fractions to distinct ecological transition points, it clarifies how dissolved and particulate nutrients are associated with different phases of community change. These results refine the understanding of cyanobacterial responses to shifting nutrient regimes and offer broadly applicable insights for managing eutrophic lakes under agricultural pressure.
Hydrological connectivity and sediment-driven disturbance structure microbial communities in river ecosystems, but most studies focus on single habitats. Consequently, the assembly mechanisms of cyanobacterial and eukaryotic microalgae across interconnected habitats in sediment-laden rivers remain poorly understood. We investigated longitudinal and vertical assembly patterns of planktonic and benthic microalgae across 44 sections of the Yellow River (22 natural river and 22 reservoir-affected sites) using 16S/18S rRNA gene amplicon sequencing combined with neutral modelling, network analysis and path modelling. We hypothesized that cyanobacteria and eukaryotic microalgae adopt distinct adaptive strategies in response to hydrodynamic disturbance and habitat heterogeneity in sediment-laden river. We found that microalgal communities differed markedly between planktonic and benthic habitats, with sediment transport enhancing benthic cyanobacterial abundance. Deterministic processes dominated community assembly, particularly in benthic habitats. Along longitudinal and vertical gradients, cyanobacteria showed stronger dispersal limitation and habitat coupling than eukaryotic microalgae and reservoir sections exhibited weaker planktonic-benthic connectivity. These findings indicate that hydrodynamic disturbance and sediment dynamics differentially regulate cyanobacterial and eukaryotic assembly across habitats in sediment-laden rivers, highlighting the importance of cross-habitat perspectives for understanding microbial organization in large river systems.
For more than 50 years, Lake Ontario has experienced continuous ecological change. The causes and consequences of these changes are only partially understood. Existing monitoring and management programs collect lake-wide data and provide results and recommendations on perceived lake-wide processes, potentially overlooking regionally important dynamics. We propose a framework that focuses on evaluating regional dynamics in food web structures by partitioning a large lake into six smaller ecological regions (i.e., ecoregions) based on multiple abiotic properties (bathymetry, thermal structure, tributary influence, trophic state, and anthropogenic influence). To evaluate this ecoregion approach, we investigated differences in trophic position (delta 15N), carbon sources (delta 13C), resource partitioning (isotopic niche size), and community metrics for round goby (Neogobius melanostomus), alewife (Alosa pseudoharengus), and lake trout (Salvelinus namaycush) in Lake Ontario sampled between 2009-2014 and 2018. We observed that despite having different trophic roles and using different carbon pathways, the three species responded similarly to differences in bathymetry, nutrient levels, and thermal dynamics, resulting in greater variation among than within an ecoregion. Estimates of trophic position were low (3.2 +/- 0.1) for round goby in ecoregions that had higher nutrient loading compared to those with lower (4.1 +/- 0.2). While trophic position for alewife (3.1-3.7) and lake trout (4.4-5.2) followed similar patterns among ecoregions, indicating these species respond similar to abiotic variation within the lake. Macrohabitat use derived using delta 13C was either benthic (alpha < 0.70) or pelagic (alpha > 0.30) for all three species in ecoregions that had higher nutrient loading and less heterogeneous bathymetry. Fish from ecoregions with lower nutrient loading and more heterogeneous bathymetry used a mix of benthic and pelagic carbon sources, had higher trophic positions, and smaller isotopic niches that resulted in less trophic redundancy. This study demonstrates that food webs in Lake Ontario operate in more discrete heterogenous spatial units (i.e., ecoregions) than as a single, large homogenous mixture. An ecoregion spatial framework, as our study developed and deploys, provides the specificity to support regional resource decision-making (i.e., watershed and fisheries) and reduces the biases created by a whole lake approach.
