Stream Assemblages Research Articles

Long term trends of fish after liming of Swedish streams and lakes

Thousands of Swedish acidified lakes and streams have been regularly limed for about 30 years. Standard sampling of fish assemblages in lakes and streams was an important part of monitoring the trends after liming, i.e. sampling with multi-mesh gillnets in lakes (EN 14757) and electrofishing in streams (EN 14011). Monitoring data are nationally managed, in the National Register of Survey test-fishing and the Swedish Electrofishing Register. We evaluated long-term data from 1029 electrofishing sites in limed streams and gillnet sampling in 750 limed lakes, along with reference data from 195 stream sites and 101 lakes with no upstream liming in their catchments. The median year of first liming was 1986 for both streams and lakes. The proportion of limed stream sites with no fish clearly decreased with time, mean species richness and proportion of sites with brown trout (Salmo trutta) recruits increased. There were no consistent trends in fish occurrence or species richness at non-limed sites, but occurrence of brown trout recruits also increased in acid as well as neutral reference streams. Abundance of brown trout, perch (Perca fluviatilis) and roach (Rutilus rutilus) increased significantly more at limed sites than at non-limed reference sites sampled before and after 1986. The mean species richness did not change consistently in limed lakes, but decreased in low alkalinity reference lakes, and fish abundance decreased significantly in limed as well as in non-limed lakes.

Hydrologic connectivity driven natural stream fish assemblages in mountain streams in the Yangtze River basin: implications for stream fish conservation in monsoonal East Asia

The East Asian monsoon and related seasonal rain belts are assumed to be significantly variable at intraseasonal, interannual, and interdecadal time scales, and largely determine the hydrology and ecological processes of the streams and rivers in this region. This study investigated the spatiotemporal variation in fish assemblages and the relative importance of local habitat and regional landscape variables in two protected streams in the middle Yangtze River basin. We hypothesized that the stream fish assemblages in this area are strongly influenced by the monsoon climate, and display a spatiotemporal variation in response to changes of stream flow regimes and physical habitats. The results show that species richness and abundance varied with both season and stream size. The distance to the downstream confluence site was the top important environmental factor, followed by altitude and downstream link. The results suggest that the fish assemblages within this protected watershed are mainly affected by regional landscape variables such as longitudinal position and stream position, but less by local habitat variables including water temperature and habitat types. The study highlights the importance of tributary connectivity and continuity to stream fish diversity in this area, indicating that restoration activities should preferentially recover the connectivity between upstream and downstream.

Temporary pools provide stability to fish assemblages in Amazon headwater streams

AbstractTemporal variations in the environment may function as disturbances that affect population processes and change the structure and composition trajectories of communities. The presence of refuges may buffer such variations and also support the recovery of communities after disturbances. In this study, we tested the effect of temporary pools as seasonal refuges for the ichthyofauna in upland forest streams in the Brazilian Amazon. We sampled environmental characteristics and fish in five occasions over one seasonal cycle in eight stream stretches and their adjacent temporary pools in Reserva Ducke, Manaus. We recorded 27 fish species, 13 of which occurred in both habitats and represented 81% of the total fish abundance in the stream–pool system (3695 records of fish, 54% made in pools). During rainy periods, fish abundance in pools was higher than in the channels. Total number of records varied inversely among habitats over time with a reduction in the pools and an increase in the channels as the pools gradually desiccated. The number of species exhibited a similar pattern. When data from pools and channels were pooled, representing the entire flood zone, fluctuations in all the measured variables along the time were smoothed. The variability in fish assemblage composition was associated with variability in the volume of water in the entire system. These results indicate that temporary pools are an important habitat component of upland forest streams, providing resources and spatial refuges for the fish during rainy periods, and regulating the stability of these communities.

How do abiotic environmental variables shape benthic diatom assemblages in subtropical streams?

Diatoms (Division Bacillariophyta) commonly dominate benthic algal communities in upland streams and are the major primary producers that sit at the base of food webs. Given their ecological importance, the aim of the present study was to investigate the spatial patterns of freshwater biofilm biomass and diatom assemblage composition in response to differences in physical and chemical variables in the Bremer, Logan and Albert catchments in south-east Queensland. Patterns in benthic diatom assemblage structure were examined during base flow conditions and the focus was on relating the spatial hierarchical interrelationships among ultimate (i.e. catchments and geology) and proximate (i.e. flow, nutrients, light, conductivity, pH and turbidity) environmental drivers. The spatial difference observed in the benthic diatom assemblages was predominantly driven by proximate environmental drivers, which, in turn, were affected by ultimate drivers. Flow played a critical role in controlling both biofilm biomass and affecting the occurrence of diatom taxa. Nutrients and light also played roles as limiting resources structuring biofilm biomass and the relative abundance of diatoms. Diatom growth form, cell size and attachment mode can be used to create a more quantitative and predictive approach to establishing relationships between diatoms and environmental gradients.

Effects of area and available energy on fish assemblages of tropical streams

A central issue in fish community ecology is to understand how the size of the drainage area and the available energy influence fish species diversity and their spatial distribution. In the present study, we tested whether the species–area relationship (represented by drainage area) and species–energy association (represented by algal biomass and organic matter) drive taxonomic and functional richness in a regional scale. The results indicated that fish assemblages of the two tropical neighbouring basins sampled responded differently to the size of drainage area. Whereas taxonomic richness was influenced by the size of the drainage area in Tocantins River basin streams, it was not affected in Araguaia River basin streams. Both taxonomic richness and functional richness of the fish assemblages were affected by available energy in the system. A possible explanation for these different responses is related to local conditions, such as the percentage of natural vegetation cover encountered in each basin.

Effects of land use on larval Odonata assemblages in cloud forest streams in central Veracruz, Mexico

This study analyzes the effect of stream water quality and its relationship with habitat characteristics on larval Odonata assemblages in streams found in tropical montane cloud forests, pastures, and coffee plantations in the upper La Antigua River watershed. The main factors that influenced larval Odonata assemblages in forest streams were low temperatures and high oxygen levels in water, coupled with a high diversity of the substrate and riparian vegetation; in pasture streams and coffee plantation streams, sedimentation strongly affected the composition of larval Odonata assemblages. In all the streams, species richness (0D) was higher during the dry season. The highest Shannon α diversity (1D) values were found in forest streams during the rainy season; however, in pasture and coffee plantation streams, 1D values were higher during the dry season. The low species turnover in forest streams suggests that these streams provide highly stable conditions for Odonata assemblages; however, in pasture and coffee plantation streams, the higher turnover was related to the more variable water and habitat conditions. Thus, the persistence of Odonata assemblages in the upper La Antigua River watershed is closely related to the conservation status of the tropical montane cloud forests.

Trends in studies of Brazilian stream fish assemblages

Studies about fish assemblages in Brazilian streams have grown in recent years, however, it remains unclear whether this increase is followed by increments in the diversity of addressed topics and theoretical frameworks adopted by researchers. We performed a systematic search for Brazilian studies on stream fish assemblages recording study region, publication year, objectives, and spatial and temporal scales adopted. The number of studies is unevenly distributed among regions. Most papers describe the general structure of local fish assemblages and their scientific objectives have not varied through time. Studies have been conducted mainly at small temporal and spatial scales, though the latter is increasing over time. We argue for the need of focusing on recently developed ecological theories and frameworks, and expanding the temporal and spatial scales of studies. These changes will improve regional and local conservation policies, and the visibility of aquatic Brazilian research in the global scientific community.

Spatial and seasonal dynamics of fish assemblage along river Soan, Pakistan and its relationship with environmental conditions

This study aims to discuss the seasonal and spatial dynamics of fish assemblage of River Soan in relation with environmental changes. Survey was conducted during pre monsoon and post monsoon seasons in 2008. Physico chemical and biological samplings were carried out at 26 sites which were selected along the whole stretch of the river. High concentration of total phosphorous and conductivity, low pH and dissolved oxygen were measured at urban part of midstream zone whereas high concentration of total nitrogen was measured at downstream zone. A total of 22 species were collected during the survey among which Barilius pakistanicus, Puntius ticto, Crossocheilus diplochilus and Labeo dero were distributed throughout the river stretch. Maximum fish diversity was recorded at suburban part of midstream while its urban part was appeared to be the most species poor part of the Soan River. Herbivore individuals constituted the dominant trophic group of fish assemblage. Significant seasonal and spatial changes in fish assemblage were determined at river downstream. Upstream- downstream differences in fish assemblage were more pronounced during pre monsoon and seem to be related to thermal gradient and hydrological changes. Seasonal variations in fish assemblage were significant only at downstream zone and were influenced by velocity, floodplains and migration patterns in fish assemblage. Understanding of spatial and seasonal trends in fish assemblage in relation with their environment is of great importance for the management and conservation of fish assemblage. Findings of this study will be helpful in future attempts to restore and conserve the aquatic assemblages of Himalayan streams.

Storm-event-transport of urban-use pesticides to streams likely impairs invertebrate assemblages.

Insecticide use in urban areas results in the detection of these compounds in streams following stormwater runoff at concentrations likely to cause toxicity for stream invertebrates. In this 2013 study, stormwater runoff and streambed sediments were analyzed for 91 pesticides dissolved in water and 118 pesticides on sediment. Detections included 33 pesticides, including insecticides, fungicides, herbicides, degradates, and a synergist. Patterns in pesticide occurrence reveal transport of dissolved and sediment-bound pesticides, including pyrethroids, from upland areas through stormwater outfalls to receiving streams. Nearly all streams contained at least one insecticide at levels exceeding an aquatic-life benchmark, most often for bifenthrin and (or) fipronil. Multiple U.S. EPA benchmark or criterion exceedances occurred in 40 % of urban streams sampled. Bed sediment concentrations of bifenthrin were highly correlated (p < 0.001) with benthic invertebrate assemblages. Non-insects and tolerant invertebrates such as amphipods, flatworms, nematodes, and oligochaetes dominated streams with relatively high concentrations of bifenthrin in bed sediments, whereas insects, sensitive invertebrates, and mayflies were much more abundant at sites with no or low bifenthrin concentrations. The abundance of sensitive invertebrates, % EPT, and select mayfly taxa were strongly negatively correlated with organic-carbon normalized bifenthrin concentrations in streambed sediments. Our findings from western Clackamas County, Oregon (USA), expand upon previous research demonstrating the transport of pesticides from urban landscapes and linking impaired benthic invertebrate assemblages in urban streams with exposure to pyrethroid insecticides.Electronic supplementary materialThe online version of this article (doi:10.1007/s10661-016-5215-5) contains supplementary material, which is available to authorized users.

Mussel Assemblages in Streams of Different Sizes in the Neches River Basin of Texas

Abstract Freshwater mussel composition and abundance in stream ecosystems are influenced by changes in environmental and hydrologic forces related to stream size. Mussel assemblages in headwater areas tend to be depauperate but distinctive, with species richness and abundance increasing rapidly with increasing stream size. We compared the mussel assemblages in 3 subwatersheds of the Neches River Basin of east Texas: the small Attoyac Bayou (4th order), the intermediate-sized Angelina River (5th order), and the large mainstem of the upper Neches River (6th order). We conducted timed searches at 75 sites and found 12,860 mussels comprising 26 species. As in other river systems, the smallest stream segment, the Attoyac Bayou, had several species more adapted for low flow rates and stagnant conditions. The largest stream segment, the upper Neches River, had the highest diversity and abundance of unionids. We found that mussel abundances increased as the streamsegment size increased. In addition, we found a ch...

Macroinvertebrate assemblages in agricultural, mining, and urban tropical streams: implications for conservation and management.

The study evaluated the response of macroinvertebrate assemblages to changes in water quality in different land-use settings in Manyame catchment, Zimbabwe. Four land-use categories were identified: forested commercial farming, communal farming, Great Dyke mining (GDM) and urban areas. Macroinvertebrate community structure and physicochemical variables data were collected in two seasons from 41 sites following standard methods. Although not environmentally threatening, urban and GDM areas were characterised by higher conductivity, total dissolved solids, salinity, magnesium and hardness. Chlorides, total phosphates, total nitrogen, calcium, potassium and sodium were significantly highest in urban sites whilst dissolved oxygen (DO) was significantly higher in the forested commercial faming and GDM sites. Macroinvertebrate communities followed the observed changes in water quality. Macroinvertebrates in urban sites indicated severe pollution (e.g. Chironomidae) whilst those in forested commercial farming sites and GDM sites indicated relatively clean water (e.g. Notonemouridae). Forested watersheds together with good farm management practices are important in mitigating impacts of urbanisation and agriculture. Strategies that reduce oxygen-depleting substances must be devised to protect the health of Zimbabwean streams. The study affirms the wider applicability of the South African Scoring System in different land uses.

Interactions between hydrology and water chemistry shape bacterioplankton biogeography across boreal freshwater networks.

Disentangling the mechanisms shaping bacterioplankton communities across freshwater ecosystems requires considering a hydrologic dimension that can influence both dispersal and local sorting, but how the environment and hydrology interact to shape the biogeography of freshwater bacterioplankton over large spatial scales remains unexplored. Using Illumina sequencing of the 16S ribosomal RNA gene, we investigate the large-scale spatial patterns of bacterioplankton across 386 freshwater systems from seven distinct regions in boreal Québec. We show that both hydrology and local water chemistry (mostly pH) interact to shape a sequential structuring of communities from highly diverse assemblages in headwater streams toward larger rivers and lakes dominated by fewer taxa. Increases in water residence time along the hydrologic continuum were accompanied by major losses of bacterial richness and by an increased differentiation of communities driven by local conditions (pH and other related variables). This suggests that hydrology and network position modulate the relative role of environmental sorting and mass effects on community assembly by determining both the time frame for bacterial growth and the composition of the immigrant pool. The apparent low dispersal limitation (that is, the lack of influence of geographic distance on the spatial patterns observed at the taxonomic resolution used) suggests that these boreal bacterioplankton communities derive from a shared bacterial pool that enters the networks through the smallest streams, largely dominated by mass effects, and that is increasingly subjected to local sorting of species during transit along the hydrologic continuum.

Development of Standardized Stream‐Dwelling Crayfish Sampling Methods at Site and Drainage Scales

AbstractMethods for assessing stream fish, freshwater mussel, and insect assemblage structure, presence, and status have existed for several decades. Such methods are lacking for assessing crayfish. The current study addresses that deficiency by testing robust sampling methods to assess species richness, distribution, and density at both site and drainage scales. We tested the ability of timed search methods to capture all known species of crayfish at a site and of a kick seine to assess the mean density of crayfish (per m2) in riffles. Riparian crayfish burrows were excavated to capture additional species not captured with the other methods. Our data and methods allow for assessments of the inherent trade‐off between sampling time and resources and statistical confidence. An example sampling regime of 3.5 h of effort per site required seven 15‐min timed search samples and captured at least 80% of the crayfish species richness, with 85% confidence across all sites tested. Twelve kick‐seine samples yielded crayfish density estimates within 50% of an estimated true density at 70% confidence for all species with densities &gt;0.5/m2. In the drainage‐scale component, we employed a sampling effort prescription derived from our site‐scale data across seven drainages in Missouri. To achieve 100% of the species richness sampled at 97.5% confidence in wadeable second‐ to fourth‐order streams, researchers will need to sample one site for every 18 river kilometers of second‐ to fourth‐order streams occurring in a drainage. We believe our methods can provide a repeatable and statistically supported tool for researchers assessing crayfish assemblages in wadeable streams across the Midwest.Received March 9, 2015; accepted October 14, 2015

Benthic Macroinvertebrate Assemblages in Kryal and Rhithral Lake Outlet Streams in the North Cascade Mountains

Abstract Alpine aquatic environments are one of the harshest on earth and are sensitive indicators of climate change. In Washington, the most glaciated state in the contiguous United States, relatively little is known about the effect of loss of glacial water sources on macroinvertebrate communities. Our study compares the water quality conditions and composition of benthic macroinvertebrates from five rhithral (snowmelt-fed) and five kryal (glacially-fed) lake outlet streams in the North Cascade Mountains, WA. Kryal streams were characterized by lower water temperatures, higher discharge rates, higher turbidities, and less stable in-stream channels compared to rhithral sites. A total of 24 985 specimens representing 96 macroinvertebrate taxa were collected. Rhithral lake outlet streams had significantly higher densities and supported more taxa than kryal sites (9049 individuals m-2 and 82 taxa versus 821 individuals m-2 and 38 taxa). Chironomidae dominated macroinvertebrate populations at all sites, alth...

How oil palm cultivation is affecting mayfly assemblages in Amazon streams

The effects of the implantation of oil palm plantations on a series of environmental variables and the diversity and distribution of mayfly nymphs was investigated through a survey of 23 streams located in areas of plantation and natural forest in the eastern Amazon basin. Composition and richness of mayfly genera were compared among streams in order to verify which genera respond to the habitat disturbance gradient and determine thresholds for each variable. The genera number was higher in forest streams, although genus composition was similar between treatments. Only 9 of the 114 physical variables measured differed significantly between plantation and forest, and only three limnological variables were significantly different. Threshold Indicator Taxa ANalysis indicated that four genera were significantly associated with these variables. Of these, Miroculis and Ulmeritoides were associated with the largest number of variables. Given the different options available for use of soil in the Amazon basin, oil palm plantations appear to be one of the least deleterious for native fauna. In the Brazilian Amazon basin, government-sponsored zoning for the establishment of oil palm plantations as a strategy for the recuperation of degraded land thus appears to be a viable option for the reintegration of areas left damaged by agricultural activities, as long as the process is implemented adequately and monitored systematically. Also, legislation must be respected, in particular the permanent preservation of riparian zones, which may function as effective buffer zones to the runoff of pesticides and sediments.

Diel variation in the structure of fish assemblages in south western Amazon streams

Abstract: Aim We investigate the influence of luminosity, habitat conservation and pluviometric periods in fish assemblages of in pasture and forest small streams in western amazon. Methods Sampling was conducted every two months from July 2013 to April 2014 in nine first- and second-order streams using seine nets and dip nets during the day and night. Fish composition, richness and total abundance were determined for each sampling period. The PERMANOVA was used to evaluate the effects of land use, season, and photoperiod, on fish assemblages. Fish assemblage structure for each stream in the presence and absence of photoperiod was ordered by NMDS analysis. Results In the light period, 3,484 specimens from 69 species were collected, while 4,574 specimens from 71 species where collected in the dark period. No significant differences in abundance and species richness were recorded between the presence and absence of luminosity periods, rainy and dry seasons and streams in forest and deforested areas. We found evidence of the dark phase composition and richness of exclusive species (22% of species collected were found at night), which were greater than in the light period (20% of species). Conclusion Despite our failure to identify any nycterohemeral segregation, the results complement existing knowledge of regional ichthyofauna and help provide a better understanding of the distributional, behavioral and functional ecological patterns of fish assemblages.

The impact of wildfire on stream fishes in an Atlantic-Mediterranean climate: evidence from an 18-year chronosequence

The predicted increase in wildfires associated with climate change poses a risk to freshwater biodiversity that may be exacerbated by river regulation. We studied the effects of wildfire and river management on the fish assemblages of Atlantic-Mediterranean streams in northern Portugal. Employing a chronosquence survey covering an 18-year gradient of impact-recovery from major fire events (ca. 100% catchment burnt), we assessed the ecological response with respect to time since wildfire, interpreting fish assemblages in the context of species traits and characteristics of the river habitat. Non-burnt sites (N = 18; surveyed 4 years previously) were compared to burnt sites (N = 14), two of which were part of the non-burnt set, thus providing a Before-After Impact comparison (BAI; N = 2). Across burnt sites richness and abundance were not related to time since wildfire. BAI revealed a contrast in the response of different species that corresponded to descriptive evidence from the chronosequence of burnt sites. As resource specialists, Salmo trutta were negatively impacted by wildfire; Iberian endemic cyprinids, characterized by generalist traits, demonstrated resistance. Habitat structure was a key determinant of wildfire-impact, increasing with channel slope and the degree of channelization. The low abundance of migratory taxa (S. trutta and Anguilla anguilla ) at burnt sites suggested the importance of fish mobility to post-fire recovery. These data demonstrate that trait profiles and habitat descriptions provide pragmatic information for the management of rivers in fire-susceptible regions and suggest that the rehabilitation of these upland stream habitats might enhance ecological resistance and resilience to catchment wildfire.

Effects of forest management on physical habitats and fish assemblages in Iberian eucalypt streams

In Portugal, more than 150,000ha of eucalypt plantations are managed under best management practices (BMP), in compliance with international certification systems. Some of these forestry areas extend to streams, and the riparian areas are thus protected from productive land uses. However, many other eucalypt plantations present riparian zones that are not managed under BMP and are often occupied by agricultural uses. To test the effects of forest management on physical habitats and fish assemblages in Iberian eucalypt streams, we selected three different stream reach types: (1) reaches in catchments dominated by native vegetation associations, and presenting semi-natural riparian zones (reference-REF); (2) reaches surrounded by certified eucalypt plantations (i.e., managed under BMP), and consequently, presenting protected riparian buffer zones (PRZ); and (3) reaches surrounded by eucalypt plantations (dominant) and some oak forests, and presenting disturbed riparian zones with agricultural land use (DRZ). All sites represented small, low-gradient, warmwater streams in Central Portugal (Tagus basin), and their upstream land cover was dominated by natural/semi-natural uses. PRZ sites were abiotically similar to REF sites, although PRZ sites generally presented lower potential cover for fish and tended to support a riparian vegetation that was a little more fragmented. PRZ sites supported fish assemblages that were similar to those in reference reaches, although at the functional level we found a higher abundance of native invertivores in REF sites. DRZ reaches, on the other hand, differed substantially from the other two groups. DRZ sites presented disturbed riparian vegetation (removal or substitution of native trees by exotic and/or non-woody species), eroded and modified channels, and a degradation of stream habitats (lack of riffle–poll complexes, higher silt contents, and lower potential cover for fish). The fish assemblages were also quite different, with a higher abundance of alien specimens (mainly Gobio lozanoi) and generalist spawners, and a lower proportion of native invertivores and lithophils. Riffles and morphological alteration explained the greatest proportion of variance in species composition. Our study showed that protecting stream riparian zones by implementing BMP may mitigate the effects of eucalypt forestry on Iberian fish assemblages. However, given that there are a few differences in physical condition and biological integrity between streams associated with certified plantations and reference streams we suggest additional riparian management measures in the certified areas, such as restoration of the native vegetation, removal of alien plants, and improvement of the stream habitat.

Disentangling the responses of boreal stream assemblages to low stressor levels of diffuse pollution and altered channel morphology

Non-point diffuse pollution from land use and alteration of hydromorphology are among the most detrimental stressors to stream ecosystems. We explored the independent and interactive effects of morphological channel alteration (channelization for water transport of timber) and diffuse pollution on species richness and community structure of four organism groups in boreal streams: diatoms, macrophytes, macroinvertebrates, and fish. Furthermore, the effect of these stressors on stream condition was evaluated by Ecological Quality Ratios (EQR) from the national Water Framework Directive (WFD) assessment system. We grouped 91 study sites into four groups that were impacted by either diffuse pollution or hydromorphological alteration, by both stressors, or by neither one. Macroinvertebrate richness was reduced by diffuse pollution, whereas other biological groups were unaltered. Hydromorphological modification had no effect on taxon richness of any of the assemblages. Community structure of all groups was significantly affected by diffuse pollution but not by hydromorphology. Similarly, EQRs indicated negative response by diatoms, macroinvertebrates and fish to diffuse pollution, but not to hydromorphological alteration. Agricultural diffuse pollution thus affected species identities and abundances rather than taxonomic richness. Our results suggest that channelization of boreal streams for timber transport has not altered hydromorphological conditions sufficiently to have a strong impact on stream biota, whereas even moderate nutrient enrichment may be ecologically harmful. Controlling diffuse pollution and associated land use stressors should be prioritized over restoration of in-stream habitat structure to improve the ecological condition of boreal streams.

Disturbance history influences stressor impacts: effects of a fungicide and nutrients on microbial diversity and litter decomposition

Summary Streams draining agricultural catchments are severely degraded by multiple stressors, including nutrient enrichment and pesticides. The understanding of how such stressors interact to alter ecosystem structure and function, and how responses of biota reflect their longer‐term disturbance history, remains limited. We conducted a multifactorial stream microcosm experiment to investigate how elevated nutrients and a fungicide (azoxystrobin) interact to affect multiple variables associated with leaf decomposition: the biomass, sporulation rate and diversity of aquatic hyphomycete decomposers, litter decomposition rates and detritivore growth. We further manipulated decomposer species composition by using three distinct fungal assemblages drawn from streams with contrasting histories of agricultural disturbance: a forest stream, a mixed land‐use stream subject to nutrient enrichment but little pesticide use, and an agricultural stream subjected to both intensive nutrient and pesticide use. We also varied the presence of the detritivorous isopod Asellus aquaticus. The fungicide azoxystrobin reduced both biomass and diversity of aquatic hyphomycetes and growth of A. aquaticus, and had negative knock‐on effects on leaf decomposition and fungal sporulation. These impacts further varied with nutrient concentration. Impacts of the fungicide differed markedly among the three fungal assemblages. The agricultural assemblages were dominated by tolerant species and showed some capacity for maintaining processes under pesticide exposure, whereas diversity and functioning were strongly suppressed in the forest stream assemblage, which was dominated by stress‐intolerant species. Pesticides, in interaction with other agricultural stressors, can impact microbial diversity and key ecosystem processes underlying the delivery of ecosystem services from streams. The extent of such impacts vary according to the longer‐term disturbance history of the biota, and might be most acute when agricultural activity expands into previously uncultivated catchments, as is currently occurring in many regions of the world.