Stream Mesocosms Research Articles

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.

Particulate organic matter quality influences nitrate retention and denitrification in stream sediments: evidence from a carbon burial experiment

Organic carbon supply is linked to nitrogen transformation in ecosystems. However, the role of organic carbon quality in nitrogen processing is not as well understood. We determined how the quality of particulate organic carbon (POC) influenced nitrogen transformation in stream sediments by burying identical quantities of varying quality POC (northern red oak (Quercus rubra) leaves, red maple (Acer rubrum) leaves, red maple wood) in stream mesocosms and measuring the effects on nitrogen retention and denitrification compared to a control of combusted sand. We also determined how POC quality affected the quantity and quality of dissolved organic carbon (DOC) and dissolved oxygen concentration in groundwater. Nitrate and total dissolved nitrogen (TDN) retention were assessed by comparing solute concentrations and fluxes along groundwater flow paths in the mesocosms. Denitrification was measured by in situ changes in N2 concentrations (using MIMS) and by acetylene block incubations. POC quality was measured by C:N and lignin:N ratios and DOC quality was assessed by fluorescence excitation emission matrix spectroscopy. POC quality had strong effects on nitrogen processing. Leaf treatments had much higher nitrate retention, TDN retention and denitrification rates than the wood and control treatments and red maple leaf burial resulted in higher nitrate and TDN retention rates than burial of red oak leaves. Leaf, but not wood, burial drove pore water to severe hypoxia and leaf treatments had higher DOC production and different DOC chemical composition than the wood and control treatments. We think that POC quality affected nitrogen processing in the sediments by influencing the quantity and quality of DOC and redox conditions. Our results suggest that the type of organic carbon inputs can affect the rates of nitrogen transformation in stream ecosystems.

Effects of repeated salt pulses on ecosystem structure and functions in a stream mesocosm

Rivers and streams affected by mining activities often receive short-term sharp salinity increases due to water-soluble stockpiled materials being washed into receiving water bodies. We conducted a mesocosm study to explore the response of structural (diatom and stream invertebrate communities) and functional descriptors (chlorophyll a concentration, fungal biomass and leaf decomposition) to repeated short salinity pulses (3h of duration, with nominal electrical conductivities of 5, 10 and 15mScm−1), mimicking the exposure pattern occurring at salt-mine affected rivers. The experiment was conducted in 12 artificial flow-through stream systems over 16days. The effect of the salt pulses on the ecosystem structure and functioning did not fully match most of our initial hypotheses, with the community response being weaker than predicted. The diatom community was, however, dominated by salt-tolerant species throughout the experiment, showing no consistent response to the treatment. The invertebrate response was associated with statistically significant changes in community structure (i.e. abundance of the different taxa) but no statistically significant changes in taxa richness. The salt pulses affected some functional descriptors of the ecosystem: fungal biomass exhibited a unimodal response to treatment magnitude, algal growth (i.e. chl a biomass) was hampered with increasing conductivity and leaf decomposition was significantly reduced in the high treatment.

Use of Artificial Stream Mesocosms to Investigate Mercury Uptake in the South River, Virginia, USA

Mercury is a globally distributed pollutant that biomagnifies in aquatic food webs. In the United States, 4,769 water bodies fail to meet criteria for safe fish consumption due to mercury bioaccumulation. Although the majority of these water bodies are affected primarily by atmospheric deposition of mercury, legacy contamination from mining or industrial activities also contribute to fish consumption advisories for mercury. The largest mercury impairment in Virginia, a 130-mile stretch of the South and South Fork Shenandoah rivers, is posted with a fish-consumption advisory for mercury contamination that originated from mercuric sulfate discharges from a textile facility in Waynesboro, Virginia, between 1929 and 1950. Although discharges of mercury to the river ceased >60 years ago, mercury levels in fish remain greater than levels safe for human consumption. This is due to the continued cycling of historic mercury in the river and its eventual uptake and biomagnification through aquatic food webs. This study investigated the relative importance of waterborne versus sediment-borne mercury in controlling biological uptake of mercury into the aquatic food web. Twelve artificial stream channels were constructed along the contaminated South River in Crimora, Virginia, and the uncontaminated North River in nearby Port Republic, Virginia, to provide four experimental treatments: a control with no Hg exposure, a Hg in sediment exposure, a Hg in water exposure, and a Hg in sediment and water exposure. After 6 weeks of colonization and growth, algae in each treatment was collected and measured for mercury accumulation. Mercury accumulation in water-only exposures was four times greater than in sediment-only exposures and was equivalent to accumulation in treatments with combined water and sediment exposure. This indicates that mercury in the water column is much more important in controlling biological uptake than mercury in near-field sediments. As a result, future remediation efforts need to focus on strategies that either remove mercury from the water column or decrease flux to the water column.

Mitigation of biocide and fungicide concentrations in flow-through vegetated stream mesocosms.

Organic chemicals entering surface waters may interact with aquatic macrophytes, which in turn may reduce potential negative effects on aquatic organisms. The overall objective of the present study was to determine the significance of aquatic macrophytes to the retention of organic chemicals in slow-flowing streams and thus their contribution to the mitigation of the risks that these compounds may pose to aquatic ecosystems. Hence, we conducted a study on the mitigation of the biocides triclosan and triclocarban and the fungicides imazalil, propiconazole and thiabendazole, which were experimentally spiked to five flow-through stream mesocosms (45 m length, 0.4 m width, 0.26 m water depth, discharge 1 L/s), four of which were planted with the submerged macrophyte (Planch.). Chemical analyses were performed using liquid chromatography-tandem mass spectrometry following solid-phase extraction for water samples and accelerated solvent extraction for macrophyte and sediment samples. The peak reductions of biocide and fungicide concentrations from the inlet to the outlet sampling sites were ≥48% in all stream mesocosms, and the peak reductions in the vegetated stream mesocosms were 20 to 25% greater than in the unvegetated mesocosm. On average, 7 ± 3 to 10 ± 3% and 28 ± 8 to 34 ± 14% of the initially applied amount of fungicides and biocides, respectively, were retained by macrophytes. There was a significant correlation between retention by macrophytes and the lipophility of the compounds.

Effects of a chronic lower range of triclosan exposure on a stream mesocosm community

Triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol) is an antimicrobial found in consumer soaps and toothpaste. It is in treated wastewater effluents at low parts-per-billion concentrations, representing a potentially chronic exposure condition for biota inhabiting receiving streams. For the present study, a naturally colonized benthos was created using flow-through indoor mesocosms; then, the benthic communities were dosed to achieve different in-stream triclosan concentrations (control, 0.1 μg/L, 0.5 μg/L, 1.0 μg/L, 5.0 μg/L, and 10 μg/L) for 56 d. Water quality parameters and endpoints from bacteria to macroinvertebrates, as well as interacting abiotic components, were measured. Effects of triclosan on specific microbial endpoints were observed at all doses, including an effect on litter decomposition dynamics at doses of 1.0 μg/L and higher. Resistance of periphytic bacteria to triclosan significantly increased at doses of 0.5 μg/L and above. By the end of dosing, the antimicrobial appeared to stimulate the stream periphyton at the 3 lowest doses, while the 2 highest doses exhibited decreased stocks of periphyton, including significantly lower bacteria cell densities and cyanobacteria abundance compared with the control. Other than an effect on benthic ostracods, the changes that occurred in the periphyton did not translate to significant change in the colonizing nematodes, the macroinvertebrate community as a whole, or other measurements of stream function. The results shed light on the role a low, chronic exposure to triclosan may play in effluent-dominated streams.

Effects of Water Velocity and Specific Surface Area on Filamentous Periphyton Biomass in an Artificial Stream Mesocosm

To evaluate the effects of water velocity and artificial substratum characteristics on the growth rate and biomass accumulation of periphyton, an artificial stream mesocosm experiment was conducted using alternative water sources collected from the Mangwall Stream (MW), the Han River (HR), and bank filtration water (BFW) from the Han River in the Republic of Korea. The measured concentrations of organic matter and inorganic nutrients in the MW were higher than in the HR and BFW. The surface of tile is relatively smooth and nonporous, whereas the surfaces of concrete and pebble are rough with numerous isolated pores in which filamentous periphyton become immobilized against hydrodynamic shear stress and mat tensile strength. Compared with the periphyton biomass of the HR and BFW, the peak biomass in the MW was significantly higher due to higher nutrient concentrations in the MW. Reasonable linear relationships (R2 ≥ 0.69) between water velocity and total periphyton biomass/growth rate were obtained, indicating that water velocities above critical values can cause a reduction in biomass accrual. In addition, reasonable relationships (R2 ≥ 0.58) between specific surface area and total periphyton biomass were obtained for the HR and BFW, indicating that an increase in the specific surface area of the substratum can lead to an increase in periphyton biomass in a nutrient-poor water body. Principal components analysis (PCA) results indicate that nutrient concentrations were the first dominant limiting factor for the growth and accumulation of periphyton, and water velocity and the specific surface area of the substratum were determined to be potential limiting factors. Consequently, the growth rate and biomass accumulation of periphyton were considered to be a complex function of nutrient concentrations, water velocities, and substratum characteristics.

Biophysical controls on cluster dynamics and architectural differentiation of microbial biofilms in contrasting flow environments

Ecology, with a traditional focus on plants and animals, seeks to understand the mechanisms underlying structure and dynamics of communities. In microbial ecology, the focus is changing from planktonic communities to attached biofilms that dominate microbial life in numerous systems. Therefore, interest in the structure and function of biofilms is on the rise. Biofilms can form reproducible physical structures (i.e. architecture) at the millimetre-scale, which are central to their functioning. However, the spatial dynamics of the clusters conferring physical structure to biofilms remains often elusive. By experimenting with complex microbial communities forming biofilms in contrasting hydrodynamic microenvironments in stream mesocosms, we show that morphogenesis results in ‘ripple-like’ and ‘star-like’ architectures – as they have also been reported from monospecies bacterial biofilms, for instance. To explore the potential contribution of demographic processes to these architectures, we propose a size-structured population model to simulate the dynamics of biofilm growth and cluster size distribution. Our findings establish that basic physical and demographic processes are key forces that shape apparently universal biofilm architectures as they occur in diverse microbial but also in single-species bacterial biofilms.

Hybridization between genetically modified Atlantic salmon and wild brown trout reveals novel ecological interactions

Interspecific hybridization is a route for transgenes from genetically modified (GM) animals to invade wild populations, yet the ecological effects and potential risks that may emerge from such hybridization are unknown. Through experimental crosses, we demonstrate transmission of a growth hormone transgene via hybridization between a candidate for commercial aquaculture production, GM Atlantic salmon (Salmo salar) and closely related wild brown trout (Salmo trutta). Transgenic hybrids were viable and grew more rapidly than transgenic salmon and other non-transgenic crosses in hatchery-like conditions. In stream mesocosms designed to more closely emulate natural conditions, transgenic hybrids appeared to express competitive dominance and suppressed the growth of transgenic and non-transgenic (wild-type) salmon by 82 and 54 per cent, respectively. To the best of our knowledge, this is the first demonstration of environmental impacts of hybridization between a GM animal and a closely related species. These results provide empirical evidence of the first steps towards introgression of foreign transgenes into the genomes of new species and contribute to the growing evidence that transgenic animals have complex and context-specific interactions with wild populations. We suggest that interspecific hybridization be explicitly considered when assessing the environmental consequences should transgenic animals escape to nature.

Understanding the combined influence of fine sediment and glyphosate herbicide on stream periphyton communities

Pesticides and deposited fine sediment have independently been associated with changes in relative abundance and species richness in aquatic ecosystems, but the interplay between these two stressors in agricultural streams is poorly understood. A 28-day experiment in outdoor stream mesocosms examined the effects of four levels each of fine sediment coverage (0, 25, 75, 100%) and glyphosate-based herbicide concentration (0, 50, 200, 370 μg/L) on periphyton communities (algae and bacteria) in a fully factorial, repeated-measures design. Our aims were to determine whether (i) increased levels of sediment and glyphosate had individual and/or additive effects, (ii) increased sediment reduced the toxicity of glyphosate (antagonistic multiple stressor interaction), or (iii) sediment-adsorbed glyphosate prolonged the effects of exposure (synergistic interaction). We also assigned all algal taxa to three ecological guilds (low-profile, high-profile and motile growth forms) and separately determined their responses to the treatments. As individual stressors, sediment addition affected all algal community-level metrics, whereas glyphosate addition only affected algal community evenness. Bacterial taxon richness was unaffected by either stressor. In combination, however, significant overall sediment by glyphosate interactions were detected, demonstrating synergistic (algal evenness, high-profile and motile guilds) or antagonistic effects (low-profile guild). Our experiment underscores the importance of considering both structural and functional indicators, including algal guild representation, when assessing the mechanisms by which periphyton communities respond to multiple stressors.

Patterns of benthic algae and cyanobacteria along twin‐stressor gradients of nutrients and fine sediment: a stream mesocosm experiment

Summary The factors underlying population and community dynamics are almost invariably multivariate, and ecosystems worldwide are affected by many anthropogenic stressors. Inorganic dissolved nutrients and deposited fine sediment are common stressors in agricultural streams and can be expected to influence benthic algae and cyanobacteria. We determined population‐ and community‐level responses of phototrophic periphyton along twin‐stressor gradients of dissolved nutrients and deposited fine sediment after 20 days of exposure in stream mesocosms. Multiple linear regression and an information‐theoretic approach were used to select the best predictive models for each response variable, accounting for potential subsidy‐stress responses for each stressor (where at low stressor levels, an ecological variable responds positively up to an inflection point, beyond which the effect is negative) and for multiple‐stressor interactions. This approach enabled us to test the subsidy‐stress hypothesis for each stressor and to investigate whether the two stressors operated singly or in concert (additively or with an interaction) on periphyton attributes. Focussing further on three ecological guilds of periphyton (low profile, high profile, motile), we also tested the specific hypotheses, that sediment augmentation would produce a proportional increase in motile and a decrease in high‐profile forms, and that nutrient augmentation would produce proportional increases in both motile and high‐profile forms. Unimodal subsidy‐stress patterns along the sediment gradient were found only for densities of two common Nitzschia species. By contrast, unimodal patterns along the nutrient gradient occurred frequently at both population and community levels, supporting the subsidy‐stress hypothesis for nutrient enrichment. Some single‐stressor responses were observed, but the twin stressors sediment and nutrients mainly acted in a simple, additive way, probably because their modes of action are different. As a notable exception from this rule, potentially harmful cyanobacteria (Phormidium and Oscillatoria species) responded synergistically to the stressors. Our hypothesis about the representation of ecological guilds across the sediment gradient was fully supported, whilst the hypothesis regarding nutrients received partial support. Categorisation of periphyton taxa into physiognomic guilds revealed predictable response patterns to the stressors. The prevalence of motile taxa in stream communities may be suitable for detecting sedimentation impacts and early signs of nutrient enrichment. Finally, based on periphyton response shapes along the nutrient gradient, thresholds could be identified at which community variables changed abruptly.

See it with my eyes: artificial stream research communicated by an artist

Abstract Artist Anne Rinn takes the reader on an excursion to an indoor and outdoor artificial pond and stream mesocosm system. This device was constructed and put into operation in 2001 in order to carry out scientific investigations concerning the aquatic environment, with the main focus on fate and effects of pollutants. Both technical features of the facility and the work and ideas for scientists and technicians were translated by her into an art exhibition which also included a film. This report on the project consists of two different text parts, one by the third author, who is an aquatic toxicologist, and one by the second author, an art historian. Their different backgrounds are reflected in different thinking and styles of expression. The text is rounded off by the photos of the first author, the artist.

Individual and combined effects of fine sediment and the herbicide glyphosate on benthic macroinvertebrates and stream ecosystem function

Summary Previous research on the combined action of various paired agricultural stressors in stream ecosystems has shown that non‐additive outcomes (i.e. synergisms or antagonisms) are more common than simple additive effects on communities, but the interplay between deposited fine sediment and widely used herbicides such as glyphosate as stressors is not well understood. A 28‐day experiment in outdoor stream mesocosms simulated effects of varying levels of fine sediment and a glyphosate‐based herbicide on macroinvertebrates and ecosystem function (leaf breakdown) in a fully factorial repeated‐measures design. Our aim was to determine whether (i) increased levels of sediment and glyphosate had individual and/or additive combined effects on invertebrates and leaf breakdown, (ii) increased sediment reduced the toxicity of glyphosate (antagonistic multiple stressor interaction) or (iii) sediment‐adsorbed glyphosate prolonged the effects of exposure (synergistic interaction). Sediment addition positively or negatively affected eight of the 10 most common invertebrate taxa colonising the mesocosms and increased or decreased the representation of 12 of 15 biological trait categories possessed by the invertebrates, while glyphosate addition negatively affected only two taxa and two invertebrate trait categories, indicating that glyphosate entering streams as a result of herbicide addition to farmland is less likely to be problematic for invertebrates than fine sediment from catchment erosion. No significant overall sediment‐by‐glyphosate interaction was detected for invertebrate taxonomic or trait compositions or for any common individual taxon, trait or community metric, indicating that the two stressors were acting additively (i.e. without interaction), rather than synergistically or antagonistically. Leaf breakdown rates (measured as mass and strength loss) were accelerated by sediment and glyphosate as individual stressors. Further, the two stressors interacted for leaf strength loss, with their combined effect being less than additive when both stressors were at their highest levels. Population, community and ecosystem variables responded differently to multiple stressors, highlighting the value of combining structural, functional taxonomic and trait data to assess the effects of anthropogenic stressors.

Effects of low-dosed imidacloprid pulses on the functional role of the caged amphipod Gammarus roeseli in stream mesocosms

Effects of two series of imidacloprid pulses on caged amphipods (Gammarus roeseli) and their shredder efficiency for litter decomposition were studied for 70 days as part of a comprehensive stream mesocosm experiment. The duration of each imidacloprid pulse of 12µgL−1 was 12h. About 250mL cages with an initial stock of 10 adult gammarids together with different conditioned litter substrates were used. Beside alder leaves (Alnus glutinosa), straw (× Triticosecale) was also used in different trials and tested for its suitability to serve as litter substrate. Results from tracer and microprobe measurements approved the suitability of the test system under low-flow condition of 10cms−1 in the surrounding stream water.Population development followed a logistic growth function with a carrying capacity of 200 Ind cage−1 for alder and 161 for straw. In the course of the study, the F1 generation reached sexual maturity and F2 offspring appeared. Increased nitrogen contents of gammarid-free trials compared to stocked ones after 70 days indicated that biofilm on both substrates was an important food source for G. roeseli. However, increased shredding activity by gammarids was only detected for alder during the second pulse series. During the remaining time and also for straw, losses of coarse particular organic matter were quite constant and slow indicating the dominance of transport limited decomposition processes on the litter surfaces.No effect of imidacloprid pulses on population levels and litter decomposition could be detected. However, the number of brood carrying females was reduced in the treatments compared to the control groups in the last 3 weeks of the study. In conclusion, repeated low-level and short-term exposition may have adverse long-term effects on G. roeseli in the field with regard to both the population size and the functional role as key shredder.

The Landau Stream Mesocosm Facility: Pesticide Mitigation in Vegetated Flow-Through Streams

Vegetated treatment systems have the ability to reduce the risk of adverse effects of nonpoint source pesticide pollution in agricultural surface waters, however, flow-through systems have rarely been evaluated. Peak concentrations of a mixture of two insecticides and two fungicides (Indoxacarb, Tebuconazole, Thiacloprid and Trifloxystrobin) were reduced by more than 90% in 45 m experimental stream mesocosms. Plant density and solubility had the highest explanatory power for the response variable reduction of peak concentration (R² = 0.70, p < 0.001). Optimized vegetated streams can be highly effective in reduction of runoff related pesticide peak concentrations.

Fish-mediated nutrient cycling and benthic microbial processes: can consumers influence stream nutrient cycling at multiple spatial scales?

Fish-mediated nutrient recycling influences nutrient dynamics in stream ecosystems, but its consequences for smaller-scale microbial processes in benthic habitats are not well understood. We quantified the effect of nutrient recycling by the grazing fish, Campostoma anomalum, on downstream periphyton in 12 flow-through stream mesocosms. We compared periphyton nutrient ratios and algal biomass (as chlorophyll a [chl a]) between tiles upstream and downstream of enclosures with and without fish to measure nutrient-cycling effects in streams with low (11) and high (177) surface-water dissolved N∶P molar ratios. No upstream–downstream changes in periphyton nutrient ratios were observed in low N∶P streams with or without fish. In high N∶P streams, periphyton C∶N decreased and C∶P and N∶P increased on tiles downstream of enclosures. In high N∶P streams, downstream changes in periphyton nutrient ratios were greater in streams with than without fish, and chl a significantly increased downstream of enclosures with fish. We linked nutrient-recycling effects to downstream microbial processes by comparing bacterial biomass production (BBP), photosynthesis (PS) rates, and the degree of coupling between the 2 processes on tiles downstream of enclosures. We estimated the degree of coupled production between algae and bacteria downstream of enclosures as the covariance between PS and BBP among replicates within each stream (COVPS–BBP). In high N∶P streams, areal BBP and PS rates and COVPS–BBP were higher downstream of enclosures with fish. Chl a and COVPS–BBP declined with increasing periphyton C∶N content, resulting in a positive relationship between COVPS–BBP and algal biomass across all treatments. Our results indicate that grazing fish alter stream ecosystem N and P dynamics through consumer-mediated recycling pathways, but downstream responses depend on background nutrient regimes. Fish-mediated changes in nutrient dynamics and algal biomass influence reliance of heterotrophs and autotrophs on nutrients recycled within periphyton communities to support benthic production.

How do stream organisms respond to, and influence, the concentration of titanium dioxide nanoparticles? A mesocosm study with algae and herbivores

The biologically active properties of many nanomaterials, coupled with their rapidly expanding production and use, has generated concern that certain types of nanoparticles could have unintended impacts when released into natural ecosystems. In the present study, the authors report the results of an experiment in which they grew three common species of stream algae as monocultures and together as polycultures in the biofilms of stream mesocosms that were exposed to 0, 0.1, or 1.0 ppm nanoparticle titanium dioxide (nTiO(2) ). The nTiO(2) did not alter the growth trajectory of any algal biofilm over 10+ generations. However, Ti accrual in biofilms not only differed among the algal species but was also higher in polycultures than in the average monoculture. Variation in accrual among species compositions was readily predicted by differences in the total biomass achieved by the different biofilms. When biofilms were fed to the herbivorous snail Physa acuta at the end of the experiment, initial concentrations of nTiO(2) did not alter short-term rates of herbivory. However, because of differences in palatability among the algae, biofilm composition influenced the amount of nTiO(2) that accumulated in the herbivore tissue. The results have important implications for understanding how efficiently nTiO(2) is removed from surface waters and the potential transfer of nanomaterials to higher trophic levels.

Macroinvertebrate responses along broad stressor gradients of deposited fine sediment and dissolved nutrients: a stream mesocosm experiment

Summary1. Degradation of freshwater ecosystems is a major concern worldwide. Agriculture introduces multiple stressors to streams, presenting a challenge for freshwater managers who must understand the relative strengths of individual stressor effects and their combined multiple‐stressor effects.2. Using a replicated regression design, we manipulated levels of fine sediment and nutrients to create a broad eight‐point gradient for each stressor across 128 stream mesocosms to determine the shapes of macroinvertebrate responses after 21 days of exposure.3. Subsidy–stress response patterns occurred along both stressor gradients. Sediment addition provided an initial subsidy for some pollution‐tolerant taxa and total macroinvertebrate density, but produced consistently negative effects for other taxa, total taxon richness and the density and richness of pollution‐sensitive Ephemeroptera, Plecoptera, Trichoptera (EPT) taxa. Nutrient enrichment provided an initial subsidy for densities of two common EPT taxa, total EPT density, macroinvertebrate and EPT taxon richness.4. Few macroinvertebrate variables were affected by a single stressor (always fine sediment). The majority of responses were additive multiple‐stressor patterns, although Chironomidae density and EPT taxon richness followed complex synergistic patterns.5. Fine sediment was a much more pervasive stressor than augmented nutrient concentrations. Overall, macroinvertebrate traits responded to stressors as strongly as community variables or individual taxa.6. Synthesis and applications. Managing both fine sediment and nutrient inputs from agriculture is crucial to achieve good stream condition but priority should be given to minimizing fine sediment, which should be maintained below a threshold of 5% cover and 0·5‐mm depth. Managers also need to be wary of interactive multiple‐stressor effects because ecological outcomes of an increase in stressor load may be worse than predicted based on the knowledge of single‐stressor effects.

Effects of repeated insecticide pulses on macroinvertebrate drift in indoor stream mesocosms

Pesticide contaminations via run-off or spray drift have been reported to result in the mass drift of macroinvertebrates as well as causing structural and functional changes of the corresponding stream sections. However, pesticide pulses in the field are associated with sudden increases in flow velocity, water turbidity, and changes in water temperature, which can also induce drift. Only through replicated community testing under highly controlled conditions can these effects be disentangled. In a stream mesocosm study, 12-h pulses of 12μg/L imidacloprid were set three times at weekly intervals and are considered a “pulse series”. Two pulse series of this neonicotinoid insecticide were run in both spring and summer with 4 treatment and 4 control stream mesocosms used in each pulse series. Prior to the start of the mesocosm experiment, both pulse concentration and duration had been screened for drift responses in larval Baetidae, Chironomidae and adult Gammarus roeseli in laboratory experiments. In the subsequent mesocosm study, each pulse caused a pronounced increase in the drift of insect larvae and gammarids. The drift response was taxon-specific, which was related to preferred habitat and exposure to other stressors like current velocity, in addition to imidacloprid sensitivity. Activity measurements employing a Multispecies Freshwater Biomonitor® revealed that in Baetis sp. the diurnal activity pattern became more pronounced even 12h after the pulse though with slightly decreased mean physical activity. Adult G. roeseli showed a drastic pulse by pulse decrease in physical activity which after the 3rd pulse lasted longer than 24h. In conclusion, drift is a sensitive, ecologically relevant endpoint and should be regarded when a specific risk assessment for lotic surface waters is done, e.g. in the context of a spatially explicit risk assessment.

Grazing minnows increase benthic autotrophy and enhance the response of periphyton elemental composition to experimental phosphorus additions

Abstract. Excessive nutrient inputs and grazers can influence biomass and elemental composition of primary producers in freshwater ecosystems. How interactions between nutrient enrichment and grazing fish alter benthic habitats through effects on periphyton autotrophy, biomass, and elemental composition has been studied rarely. We compared the effects of grazing by central stonerollers (Campostoma anomalum) on autotrophic and total periphyton biomass, sediment mass, and C, N, and P stoichiometry of periphyton in 12 flow-through stream mesocosms randomly assigned to 1 of 3 different PO4-P concentrations (control: 8 µg/L, low: 20 µg/L, high: 100 µg/L). Fish grazing suppressed periphyton ash-free dry mass (AFDM) and sediment accumulation, regardless of P treatment. However, grazing also increased the proportion of algal biomass in the periphyton, evidenced by a reduction in benthic C:chlorophyll a on grazed substrates. The response of periphyton stoichiometry to experimental P enrichment was stronger on gra...