Stream Mesocosms Research Articles

Fish metacommunity responses to experimental drought are determined by habitat heterogeneity and connectivity

Summary Drought disturbances can have strong but variable effects on aquatic communities and little is understood about the impacts of drought, fragmentation and habitat reconnectivity on the dynamics of intermittent stream fish metacommunities. We performed two experiments using outdoor stream mesocosms to test the effects of drought‐mediated connectivity and habitat heterogeneity on realistic stream fish assemblages at local (pool/patch) and regional (stream unit) scales under non‐drought versus drought conditions (Exp1) and under drought conditions with ‘pulse’ flow connectivity versus ‘non‐pulse’ (Exp2). Survivorship, ‐diversity and γ‐diversity differed little at the unit level between treatments and experiments, but significant interactions between treatments and pool position were observed for species richness and abundances at the pool‐level. Specifically, drought (Exp1) and non‐pulse (Exp2) treatments had consistently higher species richness and abundances in deeper downstream pools due to downstream‐biased immigration during the onset of drought and higher residency among fishes in downstream pools. Species‐specific responses in these treatments resulted in downstream pools that were characterised by species pre‐adapted to lentic conditions (e.g. sunfishes), whereas upstream pools were characterised by smaller‐bodied top‐water and pelagic species. Non‐drought (Exp1) and pulse (Exp2) treatments showed no difference in richness or abundances among pool positions and assemblages were generally well‐mixed, indicating that connectivity (even a brief pulse) of pool refugia was important for determining local and regional assemblage structure and mediating the impacts of drought. These experiments demonstrate that stream fish assemblages responded strongly to reduced flows and fragmentation, and that metacommunity dynamics were structured by differential emigration and immigration rates and directionality among heterogeneous and variably connected pool habitats.

Multiple‐stressor effects on stream invertebrates: a mesocosm experiment manipulating nutrients, fine sediment and flow velocity

Summary Land‐use changes have degraded ecosystems worldwide. A particular concern for freshwater biodiversity and ecosystem function are stressors introduced by intensified agriculture. Typically several stressors affect freshwater ecosystems simultaneously. However, the combined effects of these multiple stressors on streams and rivers are still poorly understood, yet of critical importance to improve freshwater management. We investigated responses of benthic macroinvertebrates to three globally important agricultural stressors affecting streams (nutrient enrichment, fine sediment deposition and reduced current velocity), using 64 stream mesocosms (full‐factorial 2 × 2 × 2 design, eight replicates of each treatment combination) established on the banks of the Breitenbach Stream (Hesse, Germany). The experiment ran for 1 month (16 days of colonisation, 14 days of manipulations), and all invertebrates in the mesocosms were collected at the end of this period. Fourteen of the 17 studied invertebrate response variables were affected by one or more stressors each. Negative effects on richness or abundance of pollution‐sensitive Ephemeroptera, Plecoptera and Trichoptera (EPT) taxa were particularly common. Overall, both sediment addition and stream flow velocity reduction had pervasive and strong effects. Responses to sediment addition were mostly negative, whereas decreased current velocity reduced several EPT metrics but increased the abundances of some of the other common taxa. Nutrient enrichment had few effects, but these were consistently negative. Combined stressor effects were mainly additive, with only two interactions found in total, both between reduced velocity and nutrients (on the crustacean Gammarus spp. and ceratopogonid midges). This finding implies that multiple‐stressor responses may be predicted from knowledge of single‐stressor effects in this stream community (unlike the often synergistic or antagonistic responses observed elsewhere). However, further taxon‐specific responses and interactions among stressors may have been obscured by limited taxonomic resolution, especially for the numerically dominant Chironomidae. Genetic approaches are required to address this limitation in the future.

Effects of pulsed atrazine exposures on autotrophic community structure, biomass, and production in field-based stream mesocosms.

The authors performed a multiple-pulsed atrazine experiment to measure responses of autotrophic endpoints in outdoor stream mesocosms. The experiment was designed to synthetically simulate worst-case atrazine chemographs from streams in agricultural catchments to achieve 60-d mean concentrations of 0 μg/L (control), 10 μg/L, 20 μg/L, and 30 μg/L. The authors dosed triplicate streams with pulses of 0 μg/L, 50 μg/L, 100 μg/L, and 150 μg/L atrazine for 4 d, followed by 7 d without dosing. This 11-d cycle occurred 3 times, followed by a recovery (untreated) period from day 34 to day 60. Mean ± standard error 60-d atrazine concentrations were 0.07 ± 0.03 μg/L, 10.7 ± 0.05 μg/L, 20.9 ± 0.24 μg/L, and 31.0 ± 0.17 μg/L for the control, 10-μg/L, 20-μg/L, and 30-μg/L treatments, respectively. Multivariate analyses revealed that periphyton and phytoplankton community structure did not differ among treatments on any day of the experiment, including during the atrazine pulses. Control periphyton biomass in riffles was higher immediately following the peak of the first atrazine pulse and remained slightly higher than some of the atrazine treatments on most days through the peak of the last pulse. However, periphyton biomass was not different among treatments at the end of the present study. Phytoplankton biomass was not affected by atrazine. Metaphyton biomass in pools was higher in the controls near the midpoint of the present study and remained higher on most days for the remainder of the study. Ceratophyllum demersum, a submersed macrophyte, biomass was higher in controls than in 20-μg/L and 30-μg/L treatments before pulse 3 but was not different subsequent to pulse 3 through the end of the present study. Maximum daily dissolved oxygen (DO, percentage of saturation) declined during each pulse in approximate proportion to magnitude of dose but rapidly converged among treatments after the third pulse. However, DO increased in controls relative to all atrazine treatments during the last 17 d of the experiment, likely a result of metaphyton cover in the pools. Finally, atrazine significantly limited uptake of PO4(3-) and uptake and/or denitrification of NO3(-) but only during pulses; percentage of dose removed from the water column was >85% for P and >95% for N after pulse 3 through the end of the present study. Collectively, only DO and metaphyton biomass differed at the end of the present study and only slightly. Some other endpoints were affected but only during pulses, if at all. The high levels of primary production and accumulation of algal biomass in all streams suggest that effects of pulses of atrazine at the concentrations used in the present study appear transient and likely do not represent ecologically significant adverse outcomes to periphyton, phytoplankton, and aquatic macrophytes, particularly in agricultural streams subjected to high nutrient loads.

Biofilm scrubbing for restoration—algae community composition and succession in artificial streams

Photoautotrophic biofilms play a pivotal role in self-purification of rivers. We took advantage of the biofilm’s cleaning capacity by applying artificial stream mesocosms, called algae turf scrubberTM (ATS), to reduce the nutrient load of a highly eutrophicated backwater in Vienna (Austria). Since purification strongly depends on benthic algae on the ATS, we focused on the algae community composition and succession. Estimation of coverage, photographic documentation for micromapping, species identification and pigment analyses were carried out. Already one week after exposition, 20–30 different taxa were recorded, suggesting a rapid colonization of the substrate. In total around 200 taxa were identified, mainly belonging to Chlorophyta, Bacillariophyceae and Cyanoprokaryota. Nonmetric multidimensional scaling implied that season and succession strongly influenced species composition on the ATS and a minimum turnover of 0.28 indicates a development towards a more stable community at the end of experiments. We measured maximum biomass production of ~250 g m−2 in June and August and during a period of 5 months nearly 19 kg ha−1 phosphorus could be removed. ATS systems proved to retain nutrients and produce algae biomass in an environmentally friendly and cost effective way and thus support restoration of highly eutrophicated water bodies.

Reference scenarios for exposure to plant protection products and invertebrate communities in stream mesocosms

Higher tier aquatic risk assessment for plant protection products (PPPs) is often based on pond-like mesocosm studies in which transient and dynamic PPP exposure scenarios as observed in lotic systems are hardly achievable. Thus, the present study presents dynamic PPP exposure scenarios at different time scales under flow-through conditions as typical for streams in agricultural landscapes. The stream mesocosm setup allows testing the influence of spatial gradients of exposure over the length of the mesocosms. The use of the fluorescent tracer uranine revealed the hydraulic processes generally underlying peak- and hour-scale exposure scenarios and demonstrated an optimized application technique to achieve stable day-scale exposures. Furthermore, to account for potential reactions of invertebrates to PPP exposures in streams (e.g. avoidance behavior and drift), the present study thus aimed at a comprehensive evaluation on how PPP exposure and the establishment of invertebrates can be advanced within stream mesocosm testing. For both, peak- and hour-scale exposure as well as the experiments considering the establishment of invertebrates, the presented compilation of experiments was able to highlight the influence of aquatic macrophytes within stream mesocosms. Since the field relevance of the higher tier aquatic risk assessment for PPPs relies qualitatively on the presence of potentially sensitive or vulnerable species, those species were especially considered. Thus, the establishment of aquatic invertebrates in non-dosed streams was evaluated with respect to (i) the presence of different aquatic macrophytes and (ii) the duration of the pre-experimental period. The present study highlights the beneficial influence of complex-structured macrophytes and prolonged pre-experimental periods on the abundance of invertebrate taxa. Furthermore, population dynamics were evaluated statistically by simulating PPP-related declines of 30, 50 and 70%. Thereby, minimum detectable difference (MDD) classes of mostly ≥III were found for 12 out of 15 taxa for at least two consecutive sampling dates.

Ecological Effects of Biochar on the Structure and Function of Stream Benthic Communities

The introduction of biochar, activated carbon, and other carbonaceous materials to aquatic ecosystems significantly reduces the toxicity and bioavailability of contaminants. However, previous studies have shown that these materials can have negative effects on aquatic organisms. We conducted field and mesocosm experiments to test the hypothesis that biochar altered the structure and function of stream benthic communities. After 30 d in the field, colonization by stoneflies (Plecoptera) was significantly lower in trays containing biochar compared to the results from the controls. In stream mesocosms, biochar increased macroinvertebrate drift and significantly reduced community metabolism. However, most measures of community composition showed little variation among biochar treatments, and significant responses were limited to a single stonefly species (Capnia confusa). When benthic communities were simultaneously exposed to biochar and Cu, effects were primarily associated with metal exposure. Because it is unlikely that biochar treatments would be employed in uncontaminated areas, these moderately negative effects should be considered within the context of the positive benefits associated with reduced contaminant bioavailability and toxicity. Additional research is necessary to improve our understanding of the mechanisms responsible for biochar effects on benthic communities and to identify the optimal application rates and size fractions that will maximize contaminant sorption but minimize potential negative effects.

Effect of thiram and of a hydrocarbon mixture on freshwater macroinvertebrate communities in outdoor stream and pond mesocosms: I. Study design, chemicals fate and structural responses.

Higher-tier ecological risk assessment (ERA) in mesocosms is commonly performed in lotic or lentic experimental systems. These systems differ in their physico-chemical and hydrological properties, leading to differences in chemical fate, community characteristics and potential recovery. This raises the issue of the relevance and sensitivity of community-level endpoints in different types of mesocosms. In this study, macroinvertebrate abundance and biomass estimates were used to assess the effects of a dithiocarbamate fungicide, thiram (35 and 170 µg l(-1)), and a petroleum middle distillate (PMD; 0.01, 0.4, 2 and 20 mg l(-1)) in outdoor stream and pond mesocosms. Streams were continuously treated during 3 weeks followed by a 2-month long post-treatment period. Ponds were treated weekly for 4 weeks, followed by a 10-month long post-treatment period. Taxonomic structure of macroinvertebrate communities was characterized using the α, β and γ components of taxa richness, Shannon and Gini-Simpson indices. Computations were based either on abundance or biomass data. Results clearly highlighted that the effects of chemicals depended on the exposure regime (for thiram) and type of system (for the PMD). Causes of the differences between streams and ponds in the magnitude and nature of effects include differential sensitivity of taxa dwelling in lentic and lotic systems and the influence of hydrology (e.g., drift from upstream) and mesocosm connectivity on recovery dynamics. This study also showed complementarities in the use of both types of mesocosms to improve the characterization of chemical effects on communities in ERA.

Effect of thiram and of a hydrocarbon mixture on freshwater macroinvertebrate communities in outdoor stream and pond mesocosms: II. Biological and ecological trait responses and leaf litter breakdown.

Higher-tier ecological risk assessment of chemicals often relies upon studies in dynamic and/or static mesocosms. Physico-chemical and hydrological properties of each type of mesocosm result in specific chemicals fate, community functioning, and potential recovery. In the present study, macroinvertebrate abundance- and biomass-weighted biological and ecological trait matrices were used to assess the effects of a dithiocarbamate fungicide, thiram (35 and 170 µg l(-1)), and of a petroleum middle distillate (0.01, 0.4, 2 and 20 mg l(-1)) in outdoor stream and pond mesocosms. Trait sensitivity was characterized using functional diversity indices and trait modality distributions to assess the influence of the type of experimental systems and the ability of traits to disentangle chemical-induced effects from temporal and stochastic variations. In addition, leaf litter breakdown was used as an integrative functional endpoint. Regardless to the substance, treatments had a direct effect on the functional structure of benthic macroinvertebrate communities in streams but not in ponds, suggesting that global functional responses to chemicals are system-specific. Although both substances had an effect in streams, differences were noticed in the nature of the affected traits suggesting that chemical mode of action plays a role in functional alterations. This was illustrated by the link between negative effects of chemical exposure on detritivorous taxa and reduced litter breakdown rate in streams. Therefore, characterisation of macroinvertebrate biological traits associated with the measurement of a functional process such as litter breakdown may provide a comprehensive understanding of the effects occurring in mesocosms exposed to organic chemicals.

One-Time Addition of Nano-TiO2 Triggers Short-Term Responses in Benthic Bacterial Communities in Artificial Streams.

Nano-TiO2 is an engineered nanomaterial whose production and use are increasing rapidly. Hence, aquatic habitats are at risk for nano-TiO2 contamination due to potential inputs from urban and suburban runoff and domestic wastewater. Nano-TiO2 has been shown to be toxic to a wide range of aquatic organisms, but little is known about the effects of nano-TiO2 on benthic microbial communities. This study used artificial stream mesocosms to assess the effects of a single addition of nano-TiO2 (P25 at a final concentration of 1 mg l(-1)) on the abundance, activity, and community composition of sediment-associated bacterial communities. The addition of nano-TiO2 resulted in a rapid (within 1 day) decrease in bacterial abundance in artificial stream sediments, but bacterial abundance returned to control levels within 3 weeks. Pyrosequencing of partial 16S rRNA genes did not indicate any significant changes in the relative abundance of any bacterial taxa with nano-TiO2 treatment, indicating that nano-TiO2 was toxic to a broad range of bacterial taxa and that recovery of the bacterial communities was not driven by changes in community composition. Addition of nano-TiO2 also resulted in short-term increases in respiration rates and denitrification enzyme activity, with both returning to control levels within 3 weeks. The results of this study demonstrate that single-pulse additions of nano-TiO2 to aquatic habitats have the potential to significantly affect the abundance and activity of benthic microbial communities and suggest that interactions of TiO2 nanoparticles with environmental matrices may limit the duration of their toxicity.

Population‐level responses of stream macroinvertebrates to drying can be density‐independent or density‐dependent

Summary The frequency and intensity of droughts and stream drying are likely to increase with climate change, but how freshwater macroinvertebrate populations respond to such disturbances is poorly understood. Theory predicts that stochastic disturbances should generate density‐independent mortality, but density dependence is possible if mortality arises indirectly through changes in consumer–resource dynamics, or if mortality is mediated by disturbance refugia. Empirical evidence for density‐dependent or density‐independent mortality in freshwater invertebrates is scarce, but important to predict how population sizes will alter with changing climate. In a stream mesocosm experiment, we tested whether short‐term drying events (5–6 days) resulted in population losses of macroinvertebrates, and whether any losses were density‐dependent or density‐independent. On each of 10 replicate drying events over a year and over a naturally occurring range of invertebrate densities, we measured pre‐ and post‐disturbance densities. From these data, we calculated per capita mortality rates, tested statistically for associations between mortality rates and pre‐disturbance densities, and assessed whether mortality differed seasonally. Invertebrate assemblages in the channels were derived from a chalk stream in south‐west England. Analyses focussed on 12 core taxa that were common in the community, which included a taxonomically diverse group of worms, Gastropoda, Gammaridae, Diptera, Ephemeroptera and Trichoptera. Responses to drying were species‐specific and three different response types were observed. Half of the taxa were resistant to drying and there was no apparent mortality, even in the near‐total loss of free water (Potamopyrgus antipodarum, Bezzia sp., Chironomini, Radix balthica and two groups of Naididae). Four taxa showed density‐independent mortality (Polycentropus flavomaculatus, Tanypodinae, Gammarus pulex, Baetis scambus), but differed in vulnerability with per capita loss rates ranging from 57 to 100%. Strong evidence of density‐dependent mortality was observed in Orthocladiinae and suggested also for Tanytarsini. Mortality varied seasonally for only one taxon (Tanytarsini), with higher mortality in warm versus cool months. Particular response types were not necessarily associated with particular taxonomic groups or body shapes. This range of responses to drying by different, naturally co‐occurring, populations suggests that community‐level impacts of drying are underpinned by diverse, taxon‐specific responses. The experiment provides some support for theoretical predictions of density‐independent mortality, but also some of the first evidence that disturbance refugia can result in density‐dependent mortality.

Increases in benthic community production and metabolism in response to marine‐derived nutrients from spawning Atlantic salmon (Salmo salar)

Summary Atlantic salmon (Salmo salar) and other anadromous fishes represent a major vector for transporting marine‐derived nutrients (MDNs) to Atlantic rivers. Marine‐derived nutrient subsidies may be key for maintaining ecological processes and ecosystem function in river basins. Stream channels and mesocosms designed to approximate natural river systems were used to measure the response of stream productivity in two treatments, one with marine‐derived nutrients from spawning Atlantic salmon and one without marine‐nutrient subsidies (control). Biofilm biomass (measured as chlorophyll a) and benthic metabolism (measured as the net change in dissolved oxygen) were evaluated from artificial substrata encompassing pre‐spawning, spawning and post‐spawning periods. We calculated marine‐nutrient contributions from spawning salmon using a simple empirical process model. We found that biofilm accrual was significantly greater with MDN inputs, with mesocosms having higher biomass (34.8 ± 1.2 mg chl a m−2) and growth rates (0.071 ± 0.002 mg m−2 day−1) than the stream channels (24.4 ± 4.8 mg chl a m−2 and 0.052 ± 0.003 mg m−2 day−1, respectively). Both control mesocosms and stream channels had significantly lower biomass (10.4 ± 0.7 mg chl a m−2 and 6.3 ± 2.5 mg chl a m−2, respectively) and growth rates (0.042 ± 0.008 mg m−2 day−1 and 0.029 ± 0.009 mg m−2 day−1, respectively) than treatment channels. Despite having a lower biofilm biomass, stream channels yielded a significantly greater final gross primary production of 2343.5 mg C m−2 day−1. The presence of MDNs in the stream channels shifted biofilm metabolism from heterotrophy (P/R = 0.283) to autotrophy (P/R = 2.422) within 112 days. Spawning Atlantic salmon contributed a total of 20.98 g m−2 of nitrogen and 1.04 g m−2 of phosphorous to each stream channel, through excretion and gametes alone. There was a strong predictable positive relationship between the increase in productivity and the amount of marine‐derived nitrogen and phosphorous delivered. This study highlights the importance of marine‐derived nutrients from Atlantic salmon for driving and maintaining freshwater productivity. Marine‐nutrient subsidies relieve the ‘bottom‐up’ constraints on stream productivity by facilitating the production of enough energy to support the food web, lessoning the reliance on outside energy sources.

Can salinity trigger cascade effects on streams? A mesocosm approach

Human activities have greatly increased the salt concentration of the world's rivers, and this might be amplified by water scarcity in the future. While the lethal effects of salinity have been documented for a wide variety of stream invertebrates, the sub-lethal effects (i.e. changes in biological condition without mortality) are not deeply understood yet. One important sub-lethal effect that has yet to be investigated is changes in predation efficiency, which could trigger cascade effects associated to the abundance of herbivorous invertebrates that control algae biomass. In this study we combined the use of biomarkers with community-level data in a stream mesocosm to evaluate the potential cascade effect of increased salinity on the trophic food web. Both predation and salt treatments had an effect on the aquatic invertebrate abundance, richness and community composition. The presence of predators had a clear cascade effect, it reduced herbivorous invertebrate abundance and richness leading to higher chlorophyll a concentrations. The salt treatment significantly reduced taxa richness, but only in the gravel bed. The predators were significantly stressed by salt addition, as shown by the different analyzed biomarkers. Concordantly, in the presence of predators, Tanytarsini registered higher abundances and chlorophyll a showed a lower concentration when salt was added. However, none of these changes was significant. Therefore, although salt addition significantly stressed Dina lineata, our results suggest that a longer exposure time is needed to fully capture cascading effects (e.g. a decrease in chlorophyll a due to a relaxation of predation on herbivorous invertebrates). We suggest that the potential cascade effects of salinization need to be evaluated when addressing the impacts of water scarcity (as caused by climate change and increasing water demand) on river ecosystems, since flow reductions will lead to higher salt concentrations.

Early life-history consequences of growth-hormone transgenesis in rainbow trout reared in stream ecosystem mesocosms.

There is persistent commercial interest in the use of growth modified fishes for shortening production cycles and increasing overall food production, but there is concern over the potential impact that transgenic fishes might have if ever released into nature. To explore the ecological consequences of transgenic fish, we performed two experiments in which the early growth and survival of growth-hormone transgenic rainbow trout (Oncorhynchus mykiss) were assessed in naturalized stream mesocosms that either contained predators or were predator-free. We paid special attention to the survival bottleneck that occurs during the early life-history of salmonids, and conducted experiments at two age classes (first-feeding fry and 60 days post-first-feeding) that lie on either side of the bottleneck. In the late summer, the first-feeding transgenic trout could not match the growth potential of their wild-type siblings when reared in a hydrodynamically complex and oligotrophic environment, irrespective of predation pressure. Furthermore, overall survival of transgenic fry was lower than in wild-type (transgenic = 30% without predators, 8% with predators; wild-type = 81% without predators, 31% with predators). In the experiment with 60-day old fry, we explored the effects of the transgene in different genetic backgrounds (wild versus domesticated). We found no difference in overwinter survival but significantly higher growth by transgenic trout, irrespective of genetic background. We conclude that the high mortality of GH-transgenic trout during first-feeding reflects an inability to sustain the basic metabolic requirements necessary for life in complex, stream environments. However, when older, GH-transgenic fish display a competitive advantage over wild-type fry, and show greater growth and equal survival as wild-type. These results demonstrate how developmental age and time of year can influence the response of genotypes to environmental conditions. We therefore urge caution when extrapolating the results of GH-transgenesis risk assessment studies across multiple life-history or developmental stages.

Inferring landscape-scale land-use impacts on rivers using data from mesocosm experiments and artificial neural networks.

Identifying land-use drivers of changes in river condition is complicated by spatial scale, geomorphological context, land management, and correlations among responding variables such as nutrients and sediments. Furthermore, variations in standard metrics, such as substratum composition, do not necessarily relate causally to ecological impacts. Consequently, the absence of a significant relationship between a hypothesised driver and a dependent variable does not necessarily indicate the absence of a causal relationship. We conducted a gradient survey to identify impacts of catchment-scale grazing by domestic livestock on river macroinvertebrate communities. A standard correlative approach showed that community structure was strongly related to the upstream catchment area under grazing. We then used data from a stream mesocosm experiment that independently quantified the impacts of nutrients and fine sediments on macroinvertebrate communities to train artificial neural networks (ANNs) to assess the relative influence of nutrients and fine sediments on the survey sites from their community composition. The ANNs developed to predict nutrient impacts did not find a relationship between nutrients and catchment area under grazing, suggesting that nutrients were not an important factor mediating grazing impacts on community composition, or that these ANNs had no generality or insufficient power at the landscape-scale. In contrast, ANNs trained to predict the impacts of fine sediments indicated a significant relationship between fine sediments and catchment area under grazing. Macroinvertebrate communities at sites with a high proportion of land under grazing were thus more similar to those resulting from high fine sediments in a mesocosm experiment than to those resulting from high nutrients. Our study confirms that 1) fine sediment is an important mediator of land-use impacts on river macroinvertebrate communities, 2) ANNs can successfully identify subtle effects and separate the effects of correlated variables, and 3) data from small-scale experiments can generate relationships that help explain landscape-scale patterns.

The effects of increased flow and fine sediment on hyporheic invertebrates and nutrients in stream mesocosms

Summary River regulation and altered land use are two common anthropogenic disturbances in rivers worldwide. Alteration of the stream bed, through processes such as siltation, or of hydrology through river regulation, are likely to modify hyporheic processes or clog interstitial space and thereby affect both hyporheic invertebrates and nutrient dynamics. We tested the separate and combined effects of increased flow and increased fine sediment on hyporheic water quality and invertebrates in flume mesocosms. Each mesocosm contained two bed sediment types: clean sediment in the upstream section and experimentally colmated (EC) sediment (10% by weight of fine sediment) in the downstream section. Two flow rates were established, a higher flow rate to create turbulent flow in six mesocosms and a lower flow rate to create a transitional flow between turbulent and laminar flows in the remaining six mesocosms. Invertebrates and physicochemistry were sampled after 30 days at three depths (5, 11 and 18 cm), and the flows in six of twelve mesocosms were switched. The experiment was concluded after sampling invertebrates and physicochemistry on day 70. The addition of fine sediment to the mesocosm bed generally increased ammonium and decreased nitrate and soluble reactive phosphorus concentrations, decreased oxygen penetration and altered invertebrate assemblage structure. Increased flow rates generally lowered ammonium concentrations, increased soluble reactive phosphorus concentrations, increased oxygen penetration and altered invertebrate assemblage structure. Our hypothesis that higher flows would ameliorate any effects of added fine sediment was generally supported for oxygen penetration and nitrate concentration. However, we observed no differences in interaction effects of flow regime and sediment type either on other nutrient concentrations or invertebrate assemblage structure. The rates of flow used in our mesocosms did not appear to reach the threshold required to remove fine sediment. It is generally recognised that river hyporheic restoration requires a set of objectives against which the outcomes can be measured yet this is often overlooked. Our research provides preliminary guidelines that small amounts of fine sediment can have deleterious ecological effects. However, further research is required to evaluate whether lower percentages of bed fine sediment result in ecological impairment and to determine what flow rates are required to ameliorate colmation impacts.

Using stable isotope analysis in stream mesocosms to study potential effects of environmental chemicals on aquatic-terrestrial subsidies.

While recent research has provided evidence that the emergence of merolimnic insects (species with an aquatic larval stage) provides a considerable energy subsidy to riparian food webs, it has also shown that merolimnic insects may serve as a vector for contaminants. Therefore, riparian food webs may be at risk from either an aquatic-terrestrial transfer of contaminants or from the contaminant-driven reductions of emerging merolimnic insects. The objective of the present study was to develop an integrated stream mesocosms test design capable of identifying these inter-ecosystem boundary effects and to provide a comprehensive approach as a basis for ecotoxicological testing. We chose the widely distributed web-building spider Tetragnatha extensa as a representative species for riparian predators. Trophic aspects of riparian food webs were investigated by stable isotope analysis of carbon (δ(13)C) and nitrogen (δ(15)N). Utilization of stable isotope ratios provided detailed information on the riparian food web structure and the dietary composition of T. extensa. Merolimnic invertebrates (mainly Cloeon spp. and Chironomidae) were found to contribute up to 71% of T. extensa's diet, demonstrating their importance in riparian food webs in ecotoxicological mesocosm testing. This study provides a conceptual and methodological basis for assessing aquatic insect emergence-related pollutant transfer or effect translation from aquatic to adjacent terrestrial systems.

Directional movement in response to altered flow in six lowland stream Trichoptera

Understanding the trait adaptations associated with mobility in Trichoptera larvae under different flow conditions would enhance the understanding of survival mechanisms under flow stress induced by spates. In stream mesocosms, we mimicked a lowland stream spate by suddenly increasing current velocity above an organic habitat patch from 10 to 30 or 50 cm/s. Subsequently, we investigated whether short-term, small-scale movements in six Trichoptera species were not random but directional and whether the type of movement was related to the magnitude of flow increase. Main types of response distinguished were as follows: (1) resistance, in which the species remained in the habitat patch, (2) upstream or downstream crawling, and (3) being dislodged from the streambed and drift downstream (vulnerability). The type of response observed was related to the species’ ecological preferences and morphological traits. The experiment showed that movement in Trichoptera larvae was directional and flow-dependent. Drift was the main mechanism observed with an increase in current velocity, but upstream crawling and aggregation in the habitat patch were observed as well. The type and magnitude of the response were highly species specific. It appeared that each combination of morphological and behavioral adaptations developed individually for each species under niche-specific conditions.

Effects of consumers and nitrogen availability on heterotrophic microbial activity during leaf decomposition in headwater streams

Summary Microbial processes in streams may be influenced by bottom‐up factors, such as nutrient availability, and by top‐down factors, such as the activity of upper trophic levels. The latter pathway has been well described between autotrophs and grazers, but the effects of consumers on heterotrophic basal trophic levels may also be important. Consumers may influence microbial activity through consumption of microbial biomass and by altering nutrient availability via consumer nutrient recycling (CNR). We investigated the effects of two shredder taxa, Gammarus spp. and Tipula spp., on nitrogen (N) uptake during organic matter decomposition by leaf‐associated heterotrophic microbial assemblages under conditions of ambient and enriched N availability in stream mesocosms. Colonised red maple leaves were placed in mesocosms with low (<20 μg L−1) or high (>800 μg L−1) nitrate concentration in the presence of amphipods, tipulids or no shredders. To test the importance of consumption and CNR, leaf packs were constructed from both coarse and fine mesh, which respectively allowed or prevented shredders from consuming detritus. Nitrogen enrichment generally increased red maple breakdown rates, while shredder influence on leaf breakdown was taxon specific. Feeding by tipulids increased red maple breakdown rate compared with amphipods, although not significantly. While feeding by amphipods did not directly influence breakdown rates, amphipod nutrient recycling stimulated leaf breakdown rate by the same magnitude as the N‐enrichment treatment. Fungal biomass increased with N enrichment. However, this did not lead to changes in N uptake, which was extremely variable and did not respond significantly to either treatment. In general, N uptake (per leaf mass) was greatest under enriched N conditions when tipulids were present. Our results indicate that both top‐down and bottom‐up factors influence microbial activity in heterotrophic systems and suggest that consumers may play important roles in detrital processing, not only through direct consumption but also through nutrient recycling.

Role of submerged vegetation in the retention processes of three plant protection products in flow-through stream mesocosms

Quantitative information on the processes leading to the retention of plant protection products (PPPs) in surface waters is not available, particularly for flow-through systems. The influence of aquatic vegetation on the hydraulic- and sorption-mediated mitigation processes of three PPPs (triflumuron, pencycuron, and penflufen; logKOW 3.3–4.9) in 45-m slow-flowing stream mesocosms was investigated. Peak reductions were 35–38% in an unvegetated stream mesocosm, 60–62% in a sparsely vegetated stream mesocosm (13% coverage with Elodea nuttallii), and in a similar range of 57–69% in a densely vegetated stream mesocosm (100% coverage). Between 89% and 93% of the measured total peak reductions in the sparsely vegetated stream can be explained by an increase of vegetation-induced dispersion (estimated with the one-dimensional solute transport model OTIS), while 7–11% of the peak reduction can be attributed to sorption processes. However, dispersion contributed only 59–71% of the peak reductions in the densely vegetated stream mesocosm, where 29% to 41% of the total peak reductions can be attributed to sorption processes. In the densely vegetated stream, 8–27% of the applied PPPs, depending on the logKOW values of the compounds, were temporarily retained by macrophytes. Increasing PPP recoveries in the aqueous phase were accompanied by a decrease of PPP concentrations in macrophytes indicating kinetic desorption over time. This is the first study to provide quantitative data on how the interaction of dispersion and sorption, driven by aquatic macrophytes, influences the mitigation of PPP concentrations in flowing vegetated stream systems.

IMPACTS OF DROUGHT AND CRAYFISH INVASION ON STREAM ECOSYSTEM STRUCTURE AND FUNCTION

ABSTRACTDrought and seasonal drying can be important disturbance events in many small streams, leading to intermittent or isolated habitats. Many small streams contain crayfish populations that are often keystone or dominant species in these systems. I conducted an experiment in stream mesocosms to examine the effects of drought and potential ecological redundancy of a native and invasive crayfish species. I examined the effects of drought (drought or control) and crayfish presence (none, native crayfish Orconectes eupunctus or invasive crayfish Orconectes neglectus) on stream mesocosm structure and function (leaf breakdown, community metabolism, periphyton, sediment and chironomid densities) in a fully factorial design. Each mesocosm contained a deep and shallow section, and drought treatments had surface water present (5‐cm depth) in deep sections where tiles and leaf packs were placed. Drought and crayfish presence did not interact for any response variable. Drought significantly reduced leaf breakdown, and crayfish presence significantly increased leaf breakdown. However, the native and invasive crayfish species did not differ significantly in their effects on leaf breakdown. Drought significantly reduced primary production and community respiration overall, whereas crayfish presence did not significantly affect primary production and community respiration. Neither drought nor crayfish presence significantly affected periphyton overall. However, drought significantly reduced autotrophic index (AI), and crayfish presence increased AI. Inorganic sediment and chironomid density were not affected by drought, but both were significantly reduced by crayfish presence. O. eupunctus reduced AI and sediment more than O. neglectus did. Neither drought nor crayfish species significantly affected crayfish growth or survival. Drought can have strong effects on ecosystem function, but weaker effects on benthic structure. Crayfish can have strong effects on ecosystem structure and function regardless of drought. In stream mesocosms, native and invasive crayfish species appeared largely ecologically redundant, although subtle differences in crayfish effects could cascade throughout the food web, and further research is needed to address this question. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.