Stream Mesocosm Experiment Research Articles

Multiple stressors affecting microbial decomposer and litter decomposition in restored urban streams: Assessing effects of salinization, increased temperature, and reduced flow velocity in a field mesocosm experiment

A multitude of anthropogenic stressors impact biological communities and ecosystem processes in urban streams. Prominent among them are salinization, increased temperature, and altered flow regimes, all of which can affect microbial decomposer communities and litter decomposition, a fundamental ecosystem process in streams. Impairments caused by these stressors individually or in combination and recovery of communities and ecosystem processes after release from these stressors are not well understood. To improve our understanding of multiple stressors impacts we performed an outdoor stream mesocosm experiment with 64 experimental units to assess the response of microbial litter decomposers and decomposition. The three stressors we applied in a full-factorial design were increased salinity (NaCl addition, 0.53 mS cm−1 above ambient), elevated temperature (3.5 °C above ambient), and reduced flow velocity (3.5 vs 14.2 cm s−1). After two weeks of stressor exposure (first sampling) and two subsequent weeks of recovery (second sampling), we determined leaf-associated microbial respiration, fungal biomass, and the sporulation activity and community composition of aquatic hyphomycetes in addition to decomposition rates of black alder (Alnus glutinosa) leaves confined in fine-mesh litter bags. Microbial colonization of the litter was accompanied by significant mass loss in all mesocosms. However, there was little indication that mass loss, microbial respiration, fungal biomass, sporulation rate or community composition of aquatic hyphomycetes was strongly affected by either single stressors or their interactions. Two exceptions were temperature effects on sporulation and decomposition rate. Similarly, no notable differences among mesocosms were observed after the recovery phase. These results suggest that microbial decomposers and leaf litter decomposition are either barely impaired by exposure to the tested stressors at the levels applied in our experiment, or that communities in restored urban streams are well adapted to cope with these stressor levels.

Fine sediment and the insecticide chlorantraniliprole inhibit organic‐matter decomposition in streams through different pathways

Abstract Intensive agriculture drives an ongoing deterioration of stream biodiversity and ecosystem functioning across the planet. Key agricultural stressors include increased deposited fine sediment and insecticides flushed from adjacent land into streams. The individual and combined effects on aquatic biota are increasingly studied, but the functional consequences of biodiversity loss associated with agricultural stressors remain poorly understood. We addressed this knowledge gap by examining the effects of fine sediment and different concentrations of the insecticide chlorantraniliprole on organic‐matter decomposition. We conducted an outdoor stream mesocosm experiment. Mesocosms contained a standardised organic‐matter assay (the cotton‐strip assay), which was used to assess organic‐matter decomposition rates (as tensile‐strength loss of the fabric) and microbial respiration of the cotton strips. The decomposition rate of strips buried under fine sediment was inhibited, a result we attribute to the limited accessibility for invertebrate feeding and microbial activities, as well as the limited nutrient and dissolved oxygen exchange. The insecticide also inhibited decay rates, a finding we relate to reduced invertebrate grazing and associated excessive algal growth. In contrast to decomposition rates, we did not observe stressor effects on microbial respiration. An interaction between fine sediment and chlorantraniliprole was not identified. Our results suggest that stressors induced by agriculture affect functions of stream ecosystems through a variety of pathways and operate by modifying habitats and biotic interactions. By examining a combination of stressors and responses that have not been addressed before, this study gives important insights into the effects of agricultural practices on streams. Understanding the effects of chlorantraniliprole is especially important since it is likely to become more widely used in future agricultural practice due to the increasing ban on neonicotinoid insecticides. Furthermore, most experimental studies address multiple stressor effects on biota. For a comprehensive understanding of complex stressor effects on ecosystems, ecosystem functions also need to be studied, such as the organic‐matter decomposition within streams.

Predicting environmental stressor levels with machine learning: a comparison between amplicon sequencing, metagenomics, and total RNA sequencing based on taxonomically assigned data

IntroductionMicrobes are increasingly (re)considered for environmental assessments because they are powerful indicators for the health of ecosystems. The complexity of microbial communities necessitates powerful novel tools to derive conclusions for environmental decision-makers, and machine learning is a promising option in that context. While amplicon sequencing is typically applied to assess microbial communities, metagenomics and total RNA sequencing (herein summarized as omics-based methods) can provide a more holistic picture of microbial biodiversity at sufficient sequencing depths. Despite this advantage, amplicon sequencing and omics-based methods have not yet been compared for taxonomy-based environmental assessments with machine learning.MethodsIn this study, we applied 16S and ITS-2 sequencing, metagenomics, and total RNA sequencing to samples from a stream mesocosm experiment that investigated the impacts of two aquatic stressors, insecticide and increased fine sediment deposition, on stream biodiversity. We processed the data using similarity clustering and denoising (only applicable to amplicon sequencing) as well as multiple taxonomic levels, data types, feature selection, and machine learning algorithms and evaluated the stressor prediction performance of each generated model for a total of 1,536 evaluated combinations of taxonomic datasets and data-processing methods.ResultsSequencing and data-processing methods had a substantial impact on stressor prediction. While omics-based methods detected a higher diversity of taxa than amplicon sequencing, 16S sequencing outperformed all other sequencing methods in terms of stressor prediction based on the Matthews Correlation Coefficient. However, even the highest observed performance for 16S sequencing was still only moderate. Omics-based methods performed poorly overall, but this was likely due to insufficient sequencing depth. Data types had no impact on performance while feature selection significantly improved performance for omics-based methods but not for amplicon sequencing.DiscussionWe conclude that amplicon sequencing might be a better candidate for machine-learning-based environmental stressor prediction than omics-based methods, but the latter require further research at higher sequencing depths to confirm this conclusion. More sampling could improve stressor prediction performance, and while this was not possible in the context of our study, thousands of sampling sites are monitored for routine environmental assessments, providing an ideal framework to further refine the approach for possible implementation in environmental diagnostics.

Dark matters: Contrasting responses of stream biofilm to browning and loss of riparian shading.

Concentrations of terrestrial‐derived dissolved organic carbon (DOC) in freshwater ecosystems have increased consistently, causing freshwater browning. The mechanisms behind browning are complex, but in forestry‐intensive regions browning is accelerated by land drainage. Forestry actions in streamside riparian forests alter canopy shading, which together with browning is expected to exert a complex and largely unpredictable control over key ecosystem functions. We conducted a stream mesocosm experiment with three levels of browning (ambient vs. moderate vs. high, with 2.7 and 5.5‐fold increase, respectively, in absorbance) crossed with two levels of riparian shading (70% light reduction vs. open canopy) to explore the individual and combined effects of browning and loss of shading on the quantity (algal biomass) and nutritional quality (polyunsaturated fatty acid and sterol content) of the periphytic biofilm. We also conducted a field survey of differently colored (4.7 to 26.2 mg DOC L−1) streams to provide a ‘reality check’ for our experimental findings. Browning reduced greatly the algal biomass, suppressed the availability of essential polyunsaturated fatty acids, especially eicosapentaenoic acid (EPA), and sterols, but increased the availability of terrestrial‐derived long‐chain saturated fatty acids (LSAFA). In contrast, loss of shading increased primary productivity, which resulted in elevated sterol and EPA contents of the biofilm. The field survey largely repeated the same pattern: biofilm nutritional quality decreased significantly with increasing DOC, as indicated particularly by a decrease of the ω‐3:ω‐6 ratio and increase in LSAFA content. Algal biomass, in contrast, was mainly controlled by dissolved inorganic nitrogen (DIN) concentration, while DOC concentration was of minor importance. The ongoing browning process is inducing a dramatic reduction in the nutritional quality of the stream biofilm. Such degradation of the major high‐quality food source available for stream consumers may reduce the trophic transfer efficiency in stream ecosystems, potentially extending across the stream‐forest ecotone.

Stream Mesocosm Experiments Show no Protective Effects of Calcium on Copper Toxicity to Macroinvertebrates.

Although the concept and modeling of metal bioavailability and toxicity have been well developed based largely on laboratory experiments with standard test species, additional evidence is required to demonstrate their applicability for macroinvertebrates typically found in natural lotic ecosystems. We conducted 10‐day stream mesocosm experiments to test the hypothesis that increased water hardness (in the present study, the calcium [Ca] concentration was increased by adding CaCl2) would mitigate the effects of copper (Cu) on natural benthic macroinvertebrate communities. Exposure of macroinvertebrate communities to 25 μg/L Cu for 10 days in stream mesocosm experiments resulted in significant decreases in total abundance, in number of taxa, and in abundance of many macroinvertebrate taxa. However, the addition of Ca to stream mesocosms and the associated increase in water hardness up to 250 mg/L CaCO3 did not mitigate these effects of Cu on macroinvertebrate communities. The results showed that the hardness‐based water quality criteria for Cu of the US Environmental Protection Agency were not protective under the conditions of relatively high hardness, low alkalinity, and circumneutral pH. In contrast, the water quality criteria based on the biotic ligand model predicted little protective effects of Ca on Cu toxicity, which is consistent with our results. Additional experiments are required to understand the influence of modifying factors on the toxicity of metals to macroinvertebrate communities. Environ Toxicol Chem 2022;41:1304–1310. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

No evidence of biodiversity effects on stream ecosystem functioning across green and brown food web pathways

Abstract Biodiversity loss is known to affect the two fundamental and opposite processes controlling carbon and nutrient cycles globally, that is, primary production and decomposition, which are driven by green and brown food web compartments, respectively. However, biodiversity in these two food web compartments has been mostly studied independently, and potential reciprocal effects of biodiversity loss on ecosystem processes remain unclear. We conducted a 35‐day stream mesocosm experiment with two levels of algal diversity (natural and diluted periphyton communities) and three levels of litter diversity (no litter, monocultures of poplar, maple, and oak, and the three‐spp. mixture) to simulate changes in biodiversity in both the green and brown pathways of an aquatic food web. We then measured multiple ecosystem processes pertaining to carbon cycling. We predicted that algal diversity would enhance decomposition and sporulation of fungal decomposers, while litter diversity would enhance algal growth and net primary production, due to the more diverse algal exudates or litter nutrients being released from more diverse mixtures. In contrast to this hypothesis, we only found biodiversity effects on an ecosystem process within the green pathway: there was a relationship between algal diversity and biofilm carrying capacity. Nevertheless, we found that this relationship was affected by the presence or absence of litter (algal diversity increased the carrying capacity in presence of litter and decreased it in its absence), which also influenced the algal community structure. Our mesocosm experiment did not evidence relationships between biodiversity and ecosystem processes across different food web compartments, but further studies in more realistic conditions would be necessary to confirm this result. If supported, the lack of biodiversity–ecosystem functioning relationships across compartments would facilitate the prediction of the impacts of biodiversity loss on ecosystems.

Low flow and heatwaves alter ecosystem functioning in a stream mesocosm experiment.

Climate change is expected to intensify the effect of environmental stressors on riverine ecosystems. Extreme events, such as low flow and heatwaves, could have profound consequences for stream ecosystem functioning, but research on the impact of these stressors and their interaction across multiple processes, remains scarce. Here, we report the results of a two-month stream mesocosm experiment testing the effect of low flow (66% water level reduction, without gravel exposure) and heatwaves (three 8-d episodes of +5°C above ambient with 10-15days recovery between each episode) on a suite of ecosystem processes (i.e. detrital decomposition, biofilm accrual, ecosystem metabolism and DOC quantity and quality). Low flow reduced whole system metabolism, suppressing the rates of gross primary production (GPP) and ecosystem respiration (ER), but elevated DOC concentration. Overall, habitat contraction was the main driver of reduced ecosystem functioning in the low flow treatment. By contrast, heatwaves increased decomposition, algal accrual, and humic-like DOC, but reduced leaf decomposition efficiency. Net ecosystem production (NEP) generally decreased across the experiment but was most pronounced for low flow and heatwaves when occurring independently. Assessment of NEP responses to the three successive heatwave events revealed that responses later in the sequence were more reduced (i.e. more similar to controls), suggesting biofilm communities may acclimate to autumn heatwaves. However, when heatwaves co-occurred with low flow, a strong reduction in both ER and GPP was observed, suggesting increased microbial mortality and reduced acclimation. Our study reveals autumn heatwaves potentially elongate the growth season for primary producers and stimulate decomposers. With climate change, river ecosystems may become more heterotrophic, with faster processing of recalcitrant carbon. Further research is required to identify the impacts on higher trophic levels, meta-community dynamics and the potential for legacy effects generated by successive low flows and heatwaves.

Before-After Control-Impact field surveys and novel experimental approaches provide valuable insights for characterizing stream recovery from acid mine drainage

Mineral extraction has resulted in widespread stream impairment due to habitat degradation and water quality impacts from acid mine drainage (AMD). The North Fork of Clear Creek (NFCC), Colorado, USA was historically impaired by AMD from two major point-source inputs, with some stream segments devoid of aquatic life prior to remediation. In the summer of 2017, the North Clear Creek Water Treatment Plant (NCCWTP) began AMD water treatment. To predict and characterize the biological recovery of NFCC to improvements in water quality, we conducted stream mesocosm and field experiments, as well as biomonitoring of benthic communities using a Before-After Control-Impact (BACI) study design. The NFCC stream community responded rapidly to improved water quality. Benthic algal biomass increased at impacted sites and macroinvertebrate surveys showed significant increases in abundance, taxa richness, and emerging adult aquatic insects. However, the dominant taxa colonizing downstream segments of NFCC differed considerably from those predicted based on previous field and experimental results. We hypothesize that this discrepancy is the result of differences in metal exposure regimes observed between our field and mesocosm approaches (i.e., fluctuating vs stable), colonization attributes (i.e., open vs closed system), and spatiotemporal differences in metal sensitivity due to macroinvertebrate phenology. We expect continued biological recovery in NFCC, but habitat impairment and residual sources of metals will continue to impair aquatic life until those stressors abate. Applying a combination of controlled experimental and BACI field approaches to predict and evaluate AMD-remediation projects in the future will improve the ability to understand the physical, chemical, and biological mechanisms influencing stream recovery.

Anthropogenic stressors affect fungal more than bacterial communities in decaying leaf litter: A stream mesocosm experiment.

Despite the progress made in environmental microbiology techniques and knowledge, the succession and functional changes of the microbial community under multiple stressors are still poorly understood. This is a substantial knowledge gap as microbial communities regulate the biogeochemistry of stream ecosystems. Our study assessed the structural and temporal changes in stream fungal and bacterial communities associated with decomposing leaf litter under a multiple-stressor scenario. We conducted a fully crossed 4-factor experiment in 64 flow-through mesocosms fed by a pristine montane stream (21 days of colonisation, 21 days of manipulations) and investigated the effects of nutrient enrichment, flow velocity reduction and sedimentation after 2 and 3 weeks of stressor exposure. We used high-throughput sequencing and metabarcoding techniques (16S and 18S rRNA genes) to identify changes in microbial community composition. Our results indicate that (1) shifts in relative abundances of the pre-existing terrestrial microbial community, rather than changes in community identity, drove the observed responses to stressors; (2) changes in relative abundances within the microbial community paralleled decomposition rate patterns with time; (3) both fungal and bacterial communities had a certain resistance to stressors, as indicated by relatively minor changes in alpha diversity or multivariate community structure; (4) overall, stressor interactions were more common than stressor main effects when affecting microbial diversity metrics or abundant individual genera; and (5) stressor effects on microbes often changed from 2 weeks to 3 weeks of stressor exposure, with several response patterns being reversed. Our study suggests that future research should focus more on understanding the temporal dynamics of fungal and bacterial communities and how they relate to ecosystem processes to advance our understanding of the mechanisms associated with multiple-stressor interactions.

Resilience and regime shifts: Do novel communities impede ecological recovery in a historically metal‐contaminated stream?

Abstract Novel communities that result from exposure to contaminants and other anthropogenic stressors often persist in ecosystems that have experienced regime shifts. Because these systems may not return to pre‐disturbance conditions after removal of a stressor, understanding the ecological consequences of regime shifts has important implications for how restoration success is defined. Long‐term observational studies can potentially identify regime shifts in disturbed ecosystems; however, experimental approaches may be necessary to demonstrate their ecological consequences. We report results of a long‐term (28 years) observational study and a series of stream mesocosm experiments that investigated a regime shift in a mining‐contaminated watershed. We tested the hypothesis that establishment of a novel, metal‐tolerant macroinvertebrate community in a previously contaminated stream impeded recovery of sensitive species, despite significant improvements in water quality over the past two decades. We observed significant improvements in abundance and species richness, but differences in community composition and trophic structure persisted downstream from a former source of metal contamination. These downstream communities were dominated by large, metal‐tolerant caddisflies that likely impeded colonization by metal‐sensitive groups (e.g. grazing mayflies). Mesocosm experiments conducted with reference and downstream communities demonstrated that novel communities retained their tolerance to metals, but were significantly more sensitive to other stressors. We suggest that the failure of downstream communities to recover represented a contaminant‐induced regime shift that resulted from the sustained dominance of metal‐tolerant species. Synthesis and applications. Restoration ecologists generally consider increased species diversity or abundance of recreationally important species as indicators of restoration success. However, few stream restoration projects have quantified ecological resilience, and none have included a significant experimental component. Our research is the first to experimentally demonstrate that novel communities in a stream recovering from historical metal pollution have lower resilience to other (e.g. non‐metal) stressors. These results have important implications for how restoration success is defined. In systems where the return to pre‐disturbance conditions is unlikely, a better understanding of the ecological resilience of novel communities may be critical for assessing restoration success. We suggest that resilience to novel stressors is an important indicator of restoration effectiveness that may be applicable in other aquatic ecosystems.

Stream Mesocosm Experiments Show Significant Differences in Sensitivity of Larval and Emerging Adults to Metals.

Evaluations of aquatic insect responses to contaminants typically use larval life stages to characterize taxa sensitivity, but the effects of contaminants to emerging terrestrial adults have received less attention. We present the results of two stream mesocosm experiments that exposed aquatic insects to mixtures of Cu and Zn. We compared responses of larvae and emerging adults in a single-species experiment with the mayfly Rhithrogena robusta and a benthic community experiment. Results showed that R. robusta larvae and emerging adults were highly tolerant of metals. In the benthic community experiment, larval and emerging adult life stages of the mayfly Baetidae were highly sensitive to metals exposure, with significant alterations in adult sex ratios. In contrast, the emergence of Chironomidae (midge) was unaffected, but larval abundance strongly decreased. Timing of adult emergence was significantly different among treatments and varied among taxa, with emergence stimulation in Chironomidae and delays in emergence in R. robusta and Simuliidae (black fly). Our results demonstrate that metal tolerance in aquatic insects is life stage dependent and that taxa sensitivity is influenced by a combination of physiology and phylogeny. Regulatory frameworks would benefit by including test results that account for effects of contaminants on metamorphosis and adult insect emergence for the development of aquatic life standards.

Drought intensification alters the composition, body size, and trophic structure of invertebrate assemblages in a stream mesocosm experiment

Abstract Predicted trends towards more intense droughts are of particular significance for running water ecosystems, as the loss of critical stream habitat can provoke sudden changes in biodiversity and shifts in community structure. However, analysing ecological responses to the progressive loss of stream habitat requires a continuous disturbance gradient that can only be generated through large‐scale manipulations of streamflow. In the first experiment of its kind, we used large artificial stream channels (mesocosms) as analogues of spring‐fed headwaters and simulated a gradient of drought intensity that encompassed flowing streams, disconnected pools, and dry streambeds. We used breakpoint analysis to analyse macroinvertebrate community responses to intensifying drought, and identify the taxa and compositional metrics sensitive to small changes in drought stress. We detected breakpoints for >60% of taxa, signalling sudden population crashes or irruptions as drought intensified. Abrupt changes were most pronounced where riffle dewatering isolated pools. In the remnant wetted habitat, we observed a shift to larger body sizes across the community, primarily driven by irruptions of predatory midge larvae and coincident population collapses among prey species (worms and smaller midges). Our results suggest that intense predation in confined, fragmented stream habitat can lead to unexpected changes in body sizes, challenging the conventional wisdom that droughts favour the small. Pool fragmentation might thus be the most critical stage of habitat loss during future droughts, as the point at which impacted rivers and streams begin to exhibit major shifts in fundamental food web properties.

MEASURING LOTIC ECOSYSTEM RESPONSES TO NUTRIENTS: A Mismatch that Limits the Synthesis and Application of Experimental Studies to Management.

MEASURING LOTIC ECOSYSTEM RESPONSES TO NUTRIENTS: A Mismatch that Limits the Synthesis and Application of Experimental Studies to Management.

Direct and indirect effects of pesticides on a benthic grazer during its life cycle

BackgroundMacroinvertebrates in aquatic ecosystems are repeatedly exposed to pesticides during their life cycle. Effects of consecutive exposure during different life stages and possible synergistic effects are not addressed in the standardized hazard assessment. The present study investigated two environmentally relevant exposure scenarios in batch (microcosm) and artificial indoor stream (mesocosm) experiments using the larvae of the mayfly Rhithrogena semicolorata (grazer) and natural aufwuchs. Grazers were analysed regarding growth, physiological condition, and drift behaviour, while the aufwuchs was analysed in terms of biomass using the particulate organic carbon as well as the chlorophyll a content. The aim was to reveal direct and indirect effects of an herbicide exposure during autumn on juvenile grazers and an insecticide exposure during spring on semi-juvenile grazers.ResultsDirect and indirect effects were found in both exposure scenarios at environmentally relevant concentrations. In the herbicide exposure scenario with terbutryn, clear direct effects on the aufwuchs community with a LOEC of 0.38 µg L−1 were found. Effect levels of grazers due to indirect effects were equal, with the overnight drift being the most sensitive grazer endpoint. In the insecticide exposure scenario, clear lethal and sub lethal effects of lambda-cyhalothrin were evident. Derived LC50 values for the artificial indoor stream and batch experiment were 2.42 µg g−1 OC (69 days) and 1.2 µg g−1 OC (28 days), respectively. Sub lethal effects in terms of increased drift as well-reduced growth and triglyceride levels were found at concentrations of 1.4 and 0.09 µg g−1 OC (LOECs). These results were confirmed by the batch experiment, which revealed effect values in the similar range. Finally, a clear indirect effect of the insecticide on the aufwuchs was evident in the batch experiment with an LOEC at 0.9 µg g−1 OC.ConclusionToxicity Exposure Ratios calculated with the derived effect values indicate a risk for the investigated grazer by both pesticides. Moreover, observed indirect effects during the herbicide exposure seem to be able to affect the grazers during a second exposure with an insecticide, due to reduced physiological conditions. We suggest further research with time-shifted exposure scenarios to gain a better understanding of the complex interactions of pesticides with the life cycle and the food webs of macroinvertebrates.

The effects of velocity and nitrate on Phormidium accrual cycles: a stream mesocosm experiment

AbstractProliferations of benthic cyanobacteria in the genus Phormidium are a global concern because of their increasing prevalence and ability to produce harmful toxins. Most studies have been obs...

Responses of stream microbes to multiple anthropogenic stressors in a mesocosm study

Stream ecosystems are affected by multiple anthropogenic stressors worldwide. Even though effects of many single stressors are comparatively well studied, the effects of multiple stressors are difficult to predict. In particular bacteria and protists, which are responsible for the majority of ecosystem respiration and element flows, are infrequently studied with respect to multiple stressors responses. We conducted a stream mesocosm experiment to characterize the responses of single and multiple stressors on microbiota. Two functionally important stream habitats, leaf litter and benthic phototrophic rock biofilms, were exposed to three stressors in a full factorial design: fine sediment deposition, increased chloride concentration (salinization) and reduced flow velocity. We analyzed the microbial composition in the two habitat types of the mesocosms using an amplicon sequencing approach. Community analysis on different taxonomic levels as well as principle component analyses (PCoAs) based on realtive abundances of operational taxonomic units (OTUs) showed treatment specific shifts in the eukaryotic biofilm community. Analysis of variance (ANOVA) revealed that Bacillariophyta responded positively salinity and sediment increase, while the relative read abundance of chlorophyte taxa decreased. The combined effects of multiple stressors were mainly antagonistic. Therefore, the community composition in multiply stressed environments resembled the composition of the unstressed control community in terms of OTU occurrence and relative abundances.

Quantifying Differences in Responses of Aquatic Insects to Trace Metal Exposure in Field Studies and Short-Term Stream Mesocosm Experiments.

Characterizing macroinvertebrate taxa as either sensitive or tolerant is of critical importance for investigating impacts of anthropogenic stressors in aquatic ecosystems and for inferring causality. However, our understanding of relative sensitivity of aquatic insects to metals in the field and under controlled conditions in the laboratory or mesocosm experiments is limited. In this study, we compared the response of 16 lotic macroinvertebrate families to metals in short-term (10-day) stream mesocosm experiments and in a spatially extensive field study of 154 Colorado streams. Comparisons of field and mesocosm-derived EC20 (effect concentration of 20%) values showed that aquatic insects were generally more sensitive to metals in the field. Although the ranked sensitivity to metals was similar for many families, we observed large differences between field and mesocosm responses for some groups (e.g., Baetidae and Heptageniidae). These differences most likely resulted from the inability of short-term experiments to account for factors such as dietary exposure to metals, rapid recolonization in the field, and effects of metals on sensitive life stages. Understanding mechanisms responsible for differences among field, mesocosm, and laboratory approaches would improve our ability to predict contaminant effects and establish ecologically meaningful water-quality criteria.

Individual and combined effects of fine sediment and glyphosate herbicide on invertebrate drift and insect emergence: a stream mesocosm experiment

Pesticides and deposited fine sediment have been associated independently with changes in relative abundance and number of sensitive species in aquatic ecosystems, but the interplay between these stressors in agricultural streams is poorly understood. We used a 28-d experiment in outdoor streamside mesocosms to examine the effects of varying levels of fine sediment and a glyphosate-based herbicide on 2 important aspects of macroinvertebrate community dynamics, invertebrate drift and adult insect emergence. We applied 4 levels of each stressor in a full factorial, repeated-measures design (4 glyphosate × 4 sediment levels × 2 replicates of each treatment combination). We focused on the propensities of the invertebrates to drift or emerge (number drifting per mesocosm/benthic individuals per m2). Sediment addition affected drift propensities of 4 of the 10 most common invertebrate taxa, community-level measures of drift (total taxon and Ephemeroptera, Plecoptera, Trichoptera [EPT] richness) and adult emergence (total propensity and taxon richness), whereas glyphosate addition affected drift propensity of only 1 taxon. These results suggest that glyphosate entering streams during terrestrial herbicide operations may be less problematic for aquatic invertebrate community dynamics than fine sediment from catchment erosion. No significant sediment × glyphosate interaction was detected for any individual taxon drift propensity or community-level drift or emergence measure, indicating that the 2 stressors acted additively, rather than synergistically or antagonistically. The dynamic measures (invertebrate drift and adult insect emergence propensities) often showed the opposite pattern compared to benthic invertebrate densities in the same experiment, a result highlighting the value of using dynamic aspects of stream invertebrate communities to identify mechanisms underlying stressor effects on benthic standing stocks.

Effects of major ions on natural benthic communities: an experimental assessment of the US Environmental Protection Agency aquatic life benchmark for conductivity

AbstractElevated concentrations of HCO3−, Cl−, SO42−, Mg2+, Na+, and Ca2+ in freshwater ecosystems are often associated with anthropogenic disturbances. The US Environmental Protection Agency developed a field-based specific conductance (SC) benchmark of 300 µS/cm for streams affected by mountain-top mining operations. The benchmark has been criticized because of the potential influence of confounding variables and difficulty in demonstrating a causal relationship between elevated SC and macroinvertebrate responses. We conducted 4 stream mesocosm experiments to quantify the effects of major ions on aquatic insect assemblages. We exposed insects from streams with low (60–72 µS/cm) and moderate (200–250 µS/cm) SC to major ions at values bracketing 300 µS/cm. We measured community metabolism, macroinvertebrate drift, community composition, and survival. Sixty-six taxa were exposed to NaHCO3, MgSO4, and NaCl in 4 mesocosm experiments, and 8 dominant families/subfamilies occurred in sufficient densities to dev...

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.