Stream microbial communities and ecosystem functioning show complex responses to multiple stressors in wastewater

Recovery capacity of constructed wetlands in response to multiple disturbances: Microbial interaction perspective

Previous studies have predominantly explored the response mechanisms of constructed wetlands (CWs) to singular disturbances. In practical applications, CWs are frequently subject to multiple disturbances, resulting in complex interference mechanisms. Therefore, this study aims to select harmful algal blooms and microalga ZM-5 as disturbances to investigate the response mechanisms of CWs. Results revealed a dynamic pattern in COD removal efficiency of CWs, with fluctuations at 39.0 ± 6.2 % and 80.1 ± 4.7 % during the disturbances, followed by a recovery to approximately 65.7 ± 3.2 %. Additionally, the CWs exhibited a capacity for self-recovery and enhanced stability by selectively promoting specific microbial communities through the regulation of the genes responsible for indole-3-acetic acid (IAA) and vitamin production. Importantly, this study underscored the establishment of a resilient microbial community structure within CWs following multiple disturbances, characterized by a more interconnected microbial network. These findings shed light on the adaptive mechanisms of CWs in the face of complex environmental challenges.

Response of stream habitat and microbiomes to spruce budworm defoliation: New considerations for outbreak management

Defoliation by eastern spruce budworm is one of the most important natural disturbances in Canadian boreal and hemi-boreal forests with annual area affected surpassing that of fire and harvest combined, and its impacts are projected to increase in frequency, severity, and range under future climate scenarios. Deciding on an active management strategy to control outbreaks and minimize broader economic, ecological, and social impacts is becoming increasingly important. These strategies differ in the degree to which defoliation is suppressed, but little is known about the downstream consequences of defoliation and, thus, the implications of management. Given the disproportionate role of headwater streams and their microbiomes on net riverine productivity across forested landscapes, we investigated the effects of defoliation by spruce budworm on headwater stream habitat and microbiome structure and function to inform management decisions. We experimentally manipulated a gradient of defoliation among 12 watersheds during a spruce budworm outbreak in the Gaspésie Peninsula, Québec, Canada. From May through October of 2019-2021, stream habitat (flow rates, dissolved organic matter [DOM], water chemistry, and nutrients), algal biomass, and water temperatures were assessed. Bacterial and fungal biofilm communities were examined by incubating six leaf packs for five weeks (mid-August to late September) in one stream reach per watershed. Microbiome community structure was determined using metabarcoding of 16S and ITS rRNA genes, and community functions were examined using extracellular enzyme assays, leaf litter decomposition rates, and taxonomic functional assignments. We found that cumulative defoliation was correlated with increased streamflow rates and temperatures, and more aromatic DOM (measured as specific ultraviolet absorbance at 254 nm), but was not correlated to nutrient concentrations. Cumulative defoliation was also associated with altered microbial community composition, an increase in carbohydrate biosynthesis, and a reduction in aromatic compound degradation, suggesting that microbes are shifting to the preferential use of simple carbohydrates rather than more complex aromatic compounds. These results demonstrate that high levels of defoliation can affect headwater stream microbiomes to the point of altering stream ecosystem productivity and carbon cycling potential, highlighting the importance of incorporating broader ecological processes into spruce budworm management decisions.

Substrate promiscuity of xenobiotic-transforming hydrolases from stream biofilms impacted by treated wastewater

Organic contaminants enter aquatic ecosystems from various sources, including wastewater treatment plant effluent. Freshwater biofilms play a major role in the removal of organic contaminants from receiving water bodies, but knowledge of the molecular mechanisms driving contaminant biotransformations in complex stream biofilm (periphyton) communities remains limited. Previously, we demonstrated that biofilms in experimental flume systems grown at higher ratios of treated wastewater (WW) to stream water displayed an increased biotransformation potential for a number of organic contaminants. We identified a positive correlation between WW percentage and biofilm biotransformation rates for the widely-used insect repellent, N,N-diethyl-meta-toluamide (DEET) and a number of other wastewater-borne contaminants with hydrolyzable moieties. Here, we conducted deep shotgun sequencing of flume biofilms and identified a positive correlation between WW percentage and metagenomic read abundances of DEET hydrolase (DH) homologs. To test the causality of this association, we constructed a targeted metagenomic library of DH homologs from flume biofilms. We screened our complete metagenomic library for activity with four different substrates, including DEET, and a subset thereof with 183 WW-related organic compounds. The majority of active hydrolases in the metagenomic library preferred aliphatic and aromatic ester substrates while, remarkably, only a single reference enzyme was capable of DEET hydrolysis. Of the 626 total enzyme-substrate combinations tested, approximately 5% were active enzyme-substrate pairs. Metagenomic DH family homologs revealed a broad substrate promiscuity spanning 22 different compounds when summed across all enzymes tested. We biochemically characterized the most promiscuous and active enzymes identified based on metagenomic analysis from uncultivated Rhodospirillaceae and Planctomycetaceae. In addition to characterizing new DH family enzymes, we exemplified a framework for linking metagenome-guided hypothesis generation with experimental validation. Overall, this study expands the scope of known enzymatic contaminant biotransformations for metagenomic hydrolases from WW-receiving stream biofilm communities.

Sludge degradation, nutrient removal and reduction of greenhouse gas emission by a Chironomus-Azolla wastewater treatment cascade

Wastewater treatment plants (WWTPs) are a point source of nutrients, emit greenhouse gases (GHGs), and produce large volumes of excess sludge. The use of aquatic organisms may be an alternative to the technical post-treatment of WWTP effluent, as they play an important role in nutrient dynamics and carbon balance in natural ecosystems. The aim of this study was therefore to assess the performance of an experimental wastewater-treatment cascade of bioturbating macroinvertebrates and floating plants in terms of sludge degradation, nutrient removal and lowering GHG emission. To this end, a full-factorial experiment was designed, using a recirculating cascade with a WWTP sludge compartment with or without bioturbating Chironomus riparius larvae, and an effluent container with or without the floating plant Azolla filiculoides, resulting in four treatments. To calculate the nitrogen (N), phosphorus (P) and carbon (C) mass balance of this system, the N, P and C concentrations in the effluent, biomass production, and sludge degradation, as well as the N, P and C content of all compartments in the cascade were measured during the 26-day experiment. The presence of Chironomus led to an increased sludge degradation of 44% compared to 25% in the control, a 1.4 times decreased transport of P from the sludge and a 2.4 times increased transport of N out of the sludge, either into Chironomus biomass or into the water column. Furthermore, Chironomus activity decreased methane emissions by 92%. The presence of Azolla resulted in a 15% lower P concentration in the effluent than in the control treatment, and a CO2 uptake of 1.13 kg ha-1 day-1. These additive effects of Chironomus and Azolla resulted in an almost two times higher sludge degradation, and an almost two times lower P concentration in the effluent. This is the first study that shows that a bio-based cascade can strongly reduce GHG and P emissions simultaneously during the combined polishing of wastewater sludge and effluent, benefitting from the additive effects of the presence of both macrophytes and invertebrates. In addition to the microbial based treatment steps already employed on WWTPs, the integration of higher organisms in the treatment process expands the WWTP based ecosystem and allows for the inclusion of macroinvertebrate and macrophyte mediated processes. Applying macroinvertebrate-plant cascades may therefore be a promising tool to tackle the present and future challenges of WWTPs.

Potential effects of the discharge of wastewater treatment plant (WWTP) effluents in benthic communities: evidence from three distinct WWTP systems

Wastewater treatment plant (WWTP) effluents can be sources of environmental contamination. In this study, we aimed to understand whether effluents of three different WWTPs may have ecological effects in riverine recipient ecosystems. To achieve this, we assessed benthic phytobenthos and macroinvertebrate communities at three different locations relative to the effluent discharge: immediately upstream, immediately downstream and 500-m downstream the effluent discharge. Two approaches were employed: the ecological status classification as defined in the Water Framework Directive (WFD) based on biological indicators; constrained multivariate analysis to disentangle the environmental drivers (physicochemical variables and contaminants, namely metals, polycyclic aromatic hydrocarbons, pharmaceuticals, and personal care products) of ecological changes across the study sites. The results showed inconsistencies between the WFD approach and the multivariate approach, as well as between the responses of macroinvertebrates and diatoms. The WWTP effluents impacted benthic communities in a single case: macroinvertebrates were negatively affected by one of the WWTP effluents, likely by the transported pharmaceuticals (other stressors are essentially homogeneous among sites). Given the findings and the scarcity of consistent evidence on ecological impacts that WWTP effluents may have in recipient ecosystems, further research is needed towards more sustainable regulation and linked environmental protection measures.

Multiple stressors alter greenhouse gas concentrations in streams through local and distal processes

Streams are significant contributors of greenhouse gases (GHG) to the atmosphere, and the increasing number of stressors degrading freshwaters may exacerbate this process, posing a threat to climatic stability. However, it is unclear whether the influence of multiple stressors on GHG concentrations in streams results from increases of in-situ metabolism (i.e., local processes) or from changes in upstream and terrestrial GHG production (i.e., distal processes). Here, we hypothesize that the mechanisms controlling multiple stressor effects vary between carbon dioxide (CO2) and methane (CH4), with the latter being more influenced by changes in local stream metabolism, and the former mainly responding to distal processes. To test this hypothesis, we measured stream metabolism and the concentrations of CO2 (pCO2) and CH4 (pCH4) in 50 stream sites that encompass gradients of nutrient enrichment, oxygen depletion, thermal stress, riparian degradation and discharge. Our results indicate that these stressors had additive effects on stream metabolism and GHG concentrations, with stressor interactions explaining limited variance. Nutrient enrichment was associated with higher stream heterotrophy and pCO2, whereas pCH4 increased with oxygen depletion and water temperature. Discharge was positively linked to primary production, respiration and heterotrophy but correlated negatively with pCO2. Our models indicate that CO2-equivalent concentrations can more than double in streams that experience high nutrient enrichment and oxygen depletion, compared to those with oligotrophic and oxic conditions. Structural equation models revealed that the effects of nutrient enrichment and discharge on pCO2 were related to distal processes rather than local metabolism. In contrast, pCH4 responses to nutrient enrichment, discharge and temperature were related to both local metabolism and distal processes. Collectively, our study illustrates potential climatic feedbacks resulting from freshwater degradation and provides insight into the processes mediating stressor impacts on the production of GHG in streams.

Prediction of microbial activity and abundance using interpretable machine learning models in the hyporheic zone of effluent-dominated receiving rivers

Serving as a vital linkage between surface water and groundwater, the hyporheic zone (HZ) plays a fundamental role in improving water quality and maintaining ecological security. In arid or semi-arid areas, effluent discharge from wastewater treatment facilities could occupy a predominant proportion of the total base flow of receiving rivers. Nonetheless the relationship between microbial activity, abundance and environmental factors in the HZ of effluent-receiving rivers appear to be rarely addressed. In this study, a spatiotemporal field study was performed in two representative effluent-dominated receiving rivers in Xi'an, China. Land use data, physical and chemical water quality parameters of surface and subsurface water were used as predictive variables, while the microbial respiratory electron transport system activity (ETSA), the Chao1 and Shannon index of total microbial community, as well as the Chao1 and Shannon index of denitrifying bacteria community were used as response variables, while ETSA was used as response variables indicating ecological processes and Shannon and Chao1 were utilized as parameters indicating microbial diversity. Two machine learning models were utilized to provide evidence-based information on how environmental factors interact and drive microbial activity and abundance in the HZ at variable depths. The models with Chao1 and Shannon as response variables exhibited excellent predictive performances (R2: 0.754-0.81 and 0.783-0.839). Dissolved organic nitrogen (DON) was the most important factor affecting the microbial functions, and an obvious threshold value of ∼2 mg/L was observed. Credible predictions of models with Chao1 and Shannon index of denitrifying bacteria community as response variables were detected (R2: 0.484-0.624 and 0.567-0.638), with soluble reactive phosphorus (SRP) being the key influencing factor. Fe (Ⅱ) was favorable in predicting denitrifying bacteria community. The ESTA model highlighted the importance of total nitrogen in the ecological health monitoring in HZ. These findings provide novel insights in predicting microbial activity and abundance in highly-impacted areas such as the HZ of effluent-dominated receiving rivers.

How redox gradient potentially influences nitrate reduction coupled with sulfur cycling: A new insight into nitrogen cycling in the hyporheic zone of effluent-dominated rivers

The coupled N and S cycling in variable redox gradients in the hyporheic zone (HZ) of the rivers receiving effluents from wastewater treatment plants is unclear. Using two representative effluent-dominated rivers as model systems, a metagenome approach was employed to explore the spatiotemporal redox zonation of the HZ and the N/S cycling processes within the system. The results manifested that nitrate reduction represented the fundamental nitrogen pathway in the HZ. Interestingly, DNRA coupled with sulfur reduction, and denitrification coupled with sulfur oxidation were respectively abundant in the oxic and anoxic zone. Lower nitrate concentration (0-2.72 mg-N/L) and more abundant genes involved in denitrification (napB, NarGHI) and sulfur oxidation (sseA, glpE) were detected in the anoxic zone. Contrarily, the nitrate concentration (0.07-4.87 mg-N/L) and the abundance of genes involved in sulfur reduction (ttrB, sudA) and DNRA (nirBD) were observed more abundant in the oxic zone. Therefore, the results verified the oxygen-limited condition did not suppress but rather facilitated the denitrification process in the presence of active S cycling. The high relative abundances of nosZ gene encoding sequence (3-5 % relative to all nitrogen-cycling processes) in both the effluent-discharging area and downstream area highly confirmed that HZ was capable of alleviating the N2O emission in the region. The functional keystone taxa were revealed through co-occurrence network analysis. The structural equation model shows that the genes of N/S cycling were positively impacted by functional keystone taxa, especially the N cycling genes. Functional keystone taxa were proven driven by the redox gradient, demonstrating their positive roles in mediating N/S cycling processes. The promoting effect on nitrate reduction coupled with sulfur cycling was clarified when redox conditions oscillated, providing a new perspective on mitigating nitrogen pollution and greenhouse gas emissions in effluent-receiving rivers.

Deciphering solute transport, microbiota assembly patterns and metabolic functions in the hyporheic zone of an effluent-dominated river

We lack a clear understanding of how anthropogenic pressures, exemplified by effluent discharge from wastewater treatment plants, destabilize microbial communities in the hyporheic zone (HZ) of receiving rivers. In this study, the spatiotemporal characteristics of hydrological parameters, and the physicochemical properties of surface and subsurface water in a representative effluent-dominated river were monitored. Sequencing of 16S rRNA amplicons and metagenomes revealed the microbial community structure in the HZ of both effluent discharge area and downstream region. The keystone taxa (taxa vital in determining the composition of each microbial cluster) and the keystone functions they controlled were subsequently identified. Effluent discharge amplified the depth of the oxic/suboxic zone and the hyporheic exchange fluxes in the effluent discharge area, which was 50-120% and 40-300% higher than in the downstream region, respectively. Microbial community structure pattern analysis demonstrated an enhancement in the rate of dispersal, an increase in microbial diversity, and an improved community network complexity in the effluent discharge area. By contrast, the number of keystone taxa in the effluent discharge area was 50-70% lower than that of the downstream region, resulting in reduced community network stability and functionality. The keystone taxa controlling metabolic functions in the networks categorized to effluent discharge area were comprised of more genera related to nitrogen and sulfur cycling, e.g., Dechloromonas, Desulfobacter, Flavobacterium, Nitrosomonas, etc., highlighting a research need in monitoring species associated with nutrient element cycling in the HZ of receiving waterbodies. The results showed that the keystone taxa could contribute positively to network stability, which was negatively correlated to hyporheic exchange fluxes and redox gradients. This study provides valuable insights that will improve our understanding of how river ecosystems respond to changes in anthropogenic pressures.

Redox gradients drive microbial community assembly patterns and molecular ecological networks in the hyporheic zone of effluent-dominated rivers

The impacts of effluent discharge on receiving waterbodies have been a research hotspot. Nonetheless, limited information is available on the microbial community assembly patterns in the hyporheic zones (HZ) responding to the changes in the microenvironments, e.g., solute transport and redox gradient variations. Using two representative effluent-dominated rivers as model systems, the spatio-temporal bacterial community dynamics and assembly patterns in oxic and suboxic zones in the shallow riverbed sediments were disentangled via null model- and neutral model-based approaches. Bacterial dynamics in community composition were observed driven by environmental filtering, i.e., impacts of environmental variables, more than geographic distances, i.e., the depths of sediments. The communities in samples collected in summer were largely shaped by stochasticity, in which homogeneous selection occupied a higher proportion in oxic (∼39%) than in suboxic zone (∼23%). Deterministic processes contributed to a more complex community structure for samples from oxic zones, whereas weakened the interspecies interactions in suboxic zones. The richness and abundances of non-neutral community were confirmed governing the deterministic assembly in oxic zones. Key species ascribed to 'connectors' and 'network hubs' dominated the community assembly variations in samples collected in winter, and in oxic zones, respectively. Significant positive relationships between β-nearest taxon index and dissolved organic nitrogen (DON) and nitrate highlighted their vital roles in community assembly via deterministic selective pressures in oxic zones. The significance thresholds of nitrogen species for community transition in winter (ΔDON: 2.81 mg-N/L, ΔNO3-: 1.09 mg-N/L) were lower than in summer, probably implying that stricter effluent quality standards should be established in colder seasons. Combined, our work poses first insights on the roles of redox zonation in driving microbial community assembly in HZ, which is of significance in guiding ecological remediation processes in effluent-dominated rivers.

The combined effects of treated sewage discharge and land use on rivers

Freshwater ecosystems are increasingly threatened by multiple anthropogenic stressors. Release of treated sewage effluent and pollution from agricultural or urban sources can independently reduce water quality with implications for ecological communities. However, our knowledge of the combined effects of these stressors is limited. We performed a field study to quantify the combined effect of treated sewage discharge and land use on nutrient concentrations, sewage fungus presence and communities of macroinvertebrates and benthic algae. Over three seasons in four rivers we found that a model which included an interaction between sewage pollution and time of the year (i.e. months) was the best predictor of nutrient concentrations and the abundance of algae and sewage fungus. Both macroinvertebrate and algae communities shifted downstream of sewage input. Specifically, more tolerant groups, such as cyanobacteria and oligochaetes, were more abundant. The EPT (Ephemeroptera, Plecoptera and Tricoptera) water quality score was best explained by an interaction between month and agriculture in the surrounding landscape. Overall, our results show that sewage discharge has a significant impact on water quality and benthic riverine communities, regardless of the surrounding land uses. Agricultural inputs, however, could be more important than treated sewage discharge in reducing the abundance of sensitive invertebrate taxa. We need both improvements to wastewater treatment processes and reductions in agricultural pollution to reduce threats to vulnerable freshwater communities.

Illuminating plant-microbe interaction: How photoperiod affects rhizosphere and pollutant removal in constructed wetland?

Rhizosphere is a crucial area in comprehending the interaction between plants and microorganisms in constructed wetlands (CWs). However, influence of photoperiod, a key factor that regulates photosynthesis and rhizosphere microbial activity, remains largely unknown. This study investigated the effect of photoperiod (9, 12, 15 h/day) on pollutant removal and underlying mechanisms. Results showed that 15-hour photoperiod treatment exhibited the highest removal efficiencies for COD (87.26%), TN (63.32%), and NO3--N (97.79%). This treatment enhanced photosynthetic pigmentation and root activity, which increased transport of oxygen and soluble organic carbon to rhizosphere, thus promoting microbial nitrification and denitrification. Microbial community analysis revealed a more stable co-occurrence network due to increased complexity and aggregation in the 15-hour photoperiod treatment. Phaselicystis was identified as a key connector, which was responsible for transferring necessary carbon sources, ATP, and electron donors that supported and optimized nitrogen metabolism in the CWs. Structural equation model analysis emphasized the importance of plant-microbe interactions in pollutant removal through increased substance, information, and energy exchange. These findings offer valuable insights for CWs design and operation in various latitudes and rural areas for small-scale decentralized systems.

Local eukaryotic and bacterial stream community assembly is shaped by regional land use effects

With anticipated expansion of agricultural areas for food production and increasing intensity of pressures stemming from land-use, it is critical to better understand how species respond to land-use change. This is particularly true for microbial communities which provide key ecosystem functions and display fastest responses to environmental change. However, regional land-use effects on local environmental conditions are often neglected, and, hence, underestimated when investigating community responses. Here we show that the effects stemming from agricultural and forested land use are strongest reflected in water conductivity, pH and phosphorus concentration, shaping microbial communities and their assembly processes. Using a joint species distribution modelling framework with community data based on metabarcoding, we quantify the contribution of land-use types in determining local environmental variables and uncover the impact of both, land-use, and local environment, on microbial stream communities. We found that community assembly is closely linked to land-use type but that the local environment strongly mediates the effects of land-use, resulting in systematic variation of taxon responses to environmental conditions, depending on their domain (bacteria vs. eukaryote) and trophic mode (autotrophy vs. heterotrophy). Given that regional land-use type strongly shapes local environments, it is paramount to consider its key role in shaping local stream communities.

Watershed urbanization dominated the spatiotemporal pattern of riverine methane emissions: Evidence from montanic streams that drain different landscapes in Southwest China

Methane (CH₄) emissions from streams are an important component of the global carbon budget of freshwater ecosystems, but these emissions are highly variable and uncertain at the temporal and spatial scales associated with watershed urbanization. In this study, we conducted investigations of dissolved CH₄ concentrations and fluxes and related environmental parameters at high spatiotemporal resolution in three montanic streams that drain different landscapes in Southwest China. We found that the average CH₄ concentrations and fluxes in the highly urbanized stream (2049 ± 2164 nmol L^-1 and 11.95 ± 11.75 mmol·m^-2·d^-1) were much higher than those in the suburban stream (1021 ± 1183 nmol L^-1 and 3.29 ± 3.66 mmol·m^-2·d^-1) and were approximately 12.3 and 27.8 times those in the rural stream, respectively. It provides powerful evidence that watershed urbanization strongly enhances riverine CH₄ emission potential. Temporal patterns of CH₄ concentrations and fluxes and their controls were not consistent among the three streams. Seasonal CH₄ concentrations in the urbanized streams had negative exponential relationships with monthly precipitation and demonstrated greater sensitivity to rainfall dilution than to the temperature priming effect. Additionally, the CH₄ concentrations in the urban and semiurban streams showed strong, but opposite, longitudinal patterns, which were closely related to urban distribution patterns and the HAILS (human activity intensity of the land surface) within the watersheds. High carbon and nitrogen loads from sewage discharge in urban areas and the spatial arrangement of the sewage drainage contributed to the different spatial patterns of the CH₄ emissions in different urbanized streams. Moreover, CH₄ concentrations in the rural stream were mainly controlled by pH and inorganic nitrogen (NH₄⁺ and NO₃^-), while urban and semiurban streams were dominated by total organic carbon and nitrogen. We highlighted that rapid urban expansion in montanic small catchments will substantially enhance riverine CH₄ concentrations and fluxes and dominate their spatiotemporal pattern and regulatory mechanisms. Future work should consider the spatiotemporal patterns of such urban-disturbed riverine CH₄ emissions and focus on the relationship between urban activities with aquatic carbon emissions.

Organic Matter Decomposition in River Ecosystems: Microbial Interactions Influenced by Total Nitrogen and Temperature in River Water

Microbes contribute to the organic matter decomposition (OMD) in river ecosystems. This study considers two aspects of OMD in river ecosystems which have not been examined in scientific studies previously, and these are the microbial interactions in OMD and the influence of environmental factors on microbial interactions. Cotton strip (CS), as a substitute for organic matter, was introduced to Luanhe River Basin in China. The results of CS assay, microbial sequencing, and redundancy analysis (RDA) showed that CS selectively enriched bacterial and fungal groups related to cellulose decomposition, achieving cotton strip decomposition (CSD). Bacterial phylum Proteobacteria and fungal phyla Rozellomycota and Ascomycota were the dominant groups associated with CSD. Network analysis and Mantel test results indicated that bacteria and fungi on CS cooperatively formed an interaction network to achieve the CSD. In the network, modules 2 and 4 were significantly positively associated with CSD, which were considered as the key modules in this study. The key modules were mainly composed of phyla Proteobacteria and Ascomycota, indicating that microbes in key modules were the effective decomposers of CS. Although keystone taxa were not directly associated with CSD, they may regulate the genera in key modules to achieve the CSD, since some keystone taxa were linked with the microbial genera associated with CSD in the key modules. Total nitrogen (TN) and temperature in water were the dominant environmental factors positively influenced CSD. The key modules 2 and 4 were positively influenced by water temperature and TN in water, respectively, and two keystone taxa were positively associated with TN. This profoundly revealed that water temperature and TN influenced the OMD through acting on the keystone taxa and key modules in microbial interactions. The research findings help us to understand the microbial interactions influenced by environmental factors in OMD in river ecosystems.

Effects of anthropogenic stress on hosts and their microbiomes: Treated wastewater alters performance and gut microbiome of a key detritivore ( Asellus aquaticus )

Human activity is a major driver of ecological and evolutionary change in wild populations and can have diverse effects on eukaryotic organisms as well as on environmental and host-associated microbial communities. Although host-microbiome interactions can be a major determinant of host fitness, few studies consider the joint responses of hosts and their microbiomes to anthropogenic changes. In freshwater ecosystems, wastewater is a widespread anthropogenic stressor that represents a multifarious environmental perturbation. Here, we experimentally tested the impact of treated wastewater on a keystone host (the freshwater isopod Asellus aquaticus) and its gut microbiome. We used a semi-natural flume experiment, in combination with 16S rRNA amplicon sequencing, to assess how different concentrations (0%, 30%, and 80%) of nonfiltered wastewater (i.e. with chemical toxicants, nutrients, organic particles, and microbes) versus ultrafiltered wastewater (i.e. only dissolved pollutants and nutrients) affected host survival, growth, and food consumption as well as mid- and hindgut bacterial community composition and diversity. Our results show that while host survival was not affected by the treatments, host growth increased and host feeding rate decreased with nonfiltered wastewater - potentially indicating that A. aquaticus fed on organic matter and microbes available in nonfiltered wastewater. Furthermore, even though the midgut microbiome (diversity and composition) was not affected by any of our treatments, nonfiltered wastewater influenced bacterial composition (but not diversity) in the hindgut. Ultrafiltered wastewater, on the other hand, affected both community composition and bacterial diversity in the hindgut, an effect that in our system differed between sexes. While the functional consequences of microbiome changes and their sex specificity are yet to be tested, our results indicate that different components of multifactorial stressors (i.e. different constituents of wastewater) can affect hosts and their microbiome in distinct (even opposing) manners and have a substantial impact on eco-evolutionary responses to anthropogenic stressors.

Destabilized microbial networks with distinct performances of abundant and rare biospheres in maintaining networks under increasing salinity stress

Global changes such as seawater intrusion and freshwater resource salinization increase environmental stress imposed on the aquatic microbiome. A strong predictive understanding of the responses of the aquatic microbiome to environmental stress will help in coping with the "gray rhino" events in the environment, thereby contributing to an ecologically sustainable future. Considering that microbial ecological networks are tied to the stability of ecosystem functioning and that abundant and rare biospheres with different biogeographic patterns are important drivers of ecosystem functioning, the roles of abundant and rare biospheres in maintaining ecological networks need to be clarified. Here we showed that, with the increasing salinity stress induced by the freshwater-to-seawater transition, the microbial diversity reduced significantly and the taxonomic structure experienced a strong succession. The complexity and stability of microbial ecological networks were diminished by the increasing stress. The composition of the microorganisms supporting the networks underwent sharp turnovers during the freshwater-to-seawater transition, with the abundant biosphere behaving more robustly than the rare biosphere. Notably, the abundant biosphere played a much more important role than the rare biosphere in stabilizing ecological networks under low-stress environments, but the difference between their relative importance narrowed significantly with the increasing stress, suggesting that the environmental stress weakened the "Matthew effect" in the microbial world. With in-depth insights into the aquatic microbial ecology under stress, our findings highlight the importance of adjusting conservation strategies for the abundant and rare biospheres to maintain ecosystem functions and services in response to rising environmental stress.

Wastewater microorganisms impact microbial diversity and important ecological functions of stream periphyton

Effluents of wastewater treatment plants can impact microbial communities in the receiving streams. However, little is known about the role of microorganisms in wastewater as opposed to other wastewater constituents, such as nutrients and micropollutants. We aimed therefore at determining the impact of wastewater microorganisms on the microbial diversity and function of periphyton, key microbial communities in streams. We used a flow-through channel system to grow periphyton upon exposure to a mixture of stream water and unfiltered or ultra-filtered wastewater. Impacts were assessed on periphyton biomass, activities and tolerance to micropollutants, as well as on microbial diversity. Our results showed that wastewater microorganisms colonized periphyton and modified its community composition, resulting for instance in an increased abundance of Chloroflexi and a decreased abundance of diatoms and green algae. This led to shifts towards heterotrophy, as suggested by the changes in nutrient stoichiometry and the increased mineralization potential of carbon substrates. An increased tolerance towards micropollutants was only found for periphyton exposed to unfiltered wastewater but not to ultra-filtered wastewater, suggesting that wastewater microorganisms were responsible for this increased tolerance. Overall, our results highlight the need to consider the role of wastewater microorganisms when studying potential impacts of wastewater on the receiving water body.

The Roles of Different Fractions in Freshwater Biofilms in the Photodegradation of Methyl Orange and Bisphenol A in Aqueous Solutions

Freshwater biofilms play an important role in the migration and transformation of organic pollutants, especially under illumination conditions. Nonetheless, the roles of variable fractions in freshwater biofilms, e.g., extracellular polymeric substances (EPS), microbial cells and original biofilms, in promoting the photodegradation of trace organic pollutants remain largely unclear. In this study, two contaminants, i.e., methyl orange (MO) and bisphenol A (BPA), were selected, and the roles of different fractions in freshwater biofilms in their photodegradation performances were investigated. After dosing 696 mg/L SS biofilm harvested from an effluent-receiving river, the direct photodegradation rate of MO and BPA was increased 8.7 times and 5.6 times, respectively. River biofilm EPS contained more aromatic fractions, chromogenic groups and conjugated structures than biofilm harvested from a less eutrophic pond, which might be responsible for the enhanced photodegradation process. The quenching experiments suggested that when EPS fractions derived from river biofilm were dosed, ³EPS* was the major reactive oxygen species during the photodegradation of MO and BPA. Meanwhile, for EPS derived from the pond biofilm, ·OH/¹O₂ was predominantly responsible for the enhanced photodegradation. Batch experimental results suggested that the cells and EPS in river biofilms could collaboratively interact with each other to enhance the preservation of reactive species and protection of microbes, thus facilitating the photoactivity of biofilms. Our results might suggest that biofilms generated from eutrophic waterbodies, such as effluent-receiving rivers, could play a more important role in the photodegradation processes of contaminants.

Bacteria communities and water quality parameters in riverine water and sediments near wastewater discharges

Wastewater treatment plant (WWTP) discharges alter water quality and microbial communities by introducing human-associated bacteria in the environment and by altering microbial communities. To fully understand this impact, it is crucial to study whether WWTP discharges affect water and sediments microbial communities in comparable ways and whether such effects depend on specific environmental variables. Here, we present a dataset investigating the impact of a WWTP on water quality and bacterial communities by comparing samples collected directly from the WWTP outflow to surface waters and sediments at two sites above and two sites below it over a period of five months. When possible, we measured five physicochemical variables (e.g., temperature, turbidity, conductivity, dissolved oxygen, and salinity), four bioindicators (e.g., Escherichia coli, total coliforms, Enterococcus sp., and endotoxins), and two molecular indicators (e.g., intI1’s relative abundance, and 16S rRNA gene profiling). Preliminary results suggest that bioindicators correlate with environmental variables and that bacterial communities present in the water tables, sediments, and treated water differ greatly in composition and structure.

Measurement(s)	temperature of water • conductivity of water • dissolved oxygen in water • salinity of water • Concentration of Escherichia coli in water • Concentration of total coliforms in water • Concentration of Enterococcus sp. • Concentration of endotoxins in water • Relative abundance of integron 1 in water • Bacterial 16S RNA
Technology Type(s)	YSI field probe • Colilert dection system • Enterolert dection system • Charles River Endosafe system • quantitative PCR • Illumina Sequencing
Sample Characteristic - Organism	Bacteria
Sample Characteristic - Environment	Fresh water river
Sample Characteristic - Location	United States

Functional measures as potential indicators of down-the-drain chemical stress in freshwater ecological risk assessment

Conventional ecological risk assessment (ERA) predominately evaluates the impact of individual chemical stressors on a limited range of taxa, which are assumed to act as proxies to predict impacts on freshwater ecosystem function. However, it is recognized that this approach has limited ecological relevance. We reviewed the published literature to identify measures that are potential functional indicators of down-the-drain chemical stress, as an approach to building more ecological relevance into ERA. We found wide variation in the use of the term "ecosystem function," and concluded it is important to distinguish between measures of processes and measures of the capacity for processes (i.e., species' functional traits). Here, we present a classification of potential functional indicators and suggest that including indicators more directly connected with processes will improve the detection of impacts on ecosystem functioning. The rate of leaf litter breakdown, oxygen production, carbon dioxide consumption, and biomass production have great potential to be used as functional indicators. However, the limited supporting evidence means that further study is needed before these measures can be fully implemented and interpreted within an ERA and regulatory context. Sensitivity to chemical stress is likely to vary among functional indicators depending on the stressor and ecosystem context. Therefore, we recommend that ERA incorporates a variety of indicators relevant to each aspect of the function of interest, such as a direct measure of a process (e.g., rate of leaf litter breakdown) and a capacity for a process (e.g., functional composition of macroinvertebrates), alongside structural indicators (e.g., taxonomic diversity of macroinvertebrates). Overall, we believe that the consideration of functional indicators can add value to ERA by providing greater ecological relevance, particularly in relation to indirect effects, functional compensation (Box 1), interactions of multiple stressors, and the importance of ecosystem context. Environ Assess Manag 2022;18:1135-1147. © 2022 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Drought and nutrient pollution produce multiple interactive effects in stream ecosystems

Drought and nutrient pollution can affect the dynamics of stream ecosystems in diverse ways. While the individual effects of both stressors are broadly examined in the literature, we still know relatively little about if and how these stressors interact. Here, we performed a mesocosm experiment that explores the compounded effects of seasonal drought via water withdrawals and nutrient pollution (1.0 mg/L of N and 0.1 mg/L of P) on a subset of Ozark stream community fauna and ecosystem processes. We observed biological responses to individual stressors as well as both synergistic and antagonistic stressor interactions. We found that drying negatively affected periphyton assemblages, macroinvertebrate colonization, and leaf litter decomposition in shallow habitats. However, in deep habitats, drought-based increases in fish density caused trophic cascades that released algal communities from grazing pressures; while nutrient enrichment caused bottom-up cascades that influenced periphyton variables and crayfish growth rates. Finally, the combined effects of drought and nutrient enrichment interacted antagonistically to increase survival in longear sunfish; and stressors acted synergistically on grazers causing a trophic cascade that increased periphyton variables. Because stressors can directly and indirectly impact biota—and that the same stressor pairing can act differentially on various portions of the community simultaneously—our broad understanding of individual stressors might not adequately inform our knowledge of multi-stressor systems.

Stream Hydrology Controls the Longitudinal Bioreactive Footprint of Urban-Sourced Fine Particles

The relevance of wastewater treatment plant (WWTP) effluents in fluvial networks is increasing as urbanization grows in catchments. Urban-sourced fine particles from WWTP effluents deposit and accumulate in the streambed sediment of receiving streams over time and can fuel respiration rates, which can thus potentially increase rates of biogeochemical reactions and CO₂ emissions to the atmosphere. We aimed to provide a quantitative assessment of the influence of WWTP-sourced fine particles deposited in the streambed sediment on stream metabolic activity for 1 year in an intermittent Mediterranean stream. More nutrient-rich and metabolically active fine particle standing stocks were observed downstream of the WWTP, propagating to the end of the 820 m study reach, especially during the dry period (i.e., when the dilution capacity of the stream to WWTP inputs is <40%). Based on the longitudinal patterns of fine particle standing stocks and their metabolic activity, we estimated that the in-stream bioreactive capacity associated with these fine particles could potentially lead to substantial carbon dioxide emissions to the atmosphere (3.1 g C/m²/d). We show the importance of incorporating fine particle standing stocks downstream of point source inputs, particularly WWTPs in intermittent streams, into carbon budgets.

Wastewater microorganisms impact the micropollutant biotransformation potential of natural stream biofilms

Biotransformation is the most important process removing manmade chemicals from the environment, yet mechanisms governing this essential ecosystem function are underexplored. To understand these mechanisms, we conducted experiments in flow-through systems, by colonizing stream biofilms under different conditions of mixing river water with treated (and ultrafiltered) wastewater. We performed biotransformation experiments with those biofilms, using a set of 75 micropollutants, and could disentangle potential mechanisms determining the biotransformation potential of stream biofilms. We showed that the increased biotransformation potential downstream of wastewater treatment plants that we observed for specific micropollutants contained in household wastewaters (downstream effect) is caused by microorganisms released with the treated effluent, rather than by the in-stream exposure to those micropollutants. Complementary data from 16S rRNA amplicon-sequencing revealed 146 amplicon sequence variants (ASVs) that followed the observed biotransformation patterns. Our results align with findings for community tolerance, and provide clear experimental evidence that microorganisms released with treated wastewater integrate into downstream biofilms and impact crucial ecosystem functions.

Deciphering the influence of multiple anthropogenic inputs on taxonomic and functional profiles of the microbial communities in Yitong River, Northeast China

We conducted the analysis of physicochemical parameters, 16S rRNA amplicon sequencing and real-time quantitative polymerase chain reaction to explore the impact of human inputs on the bacterioplankton communities within a tributary of the largest river flowing through a megacity in northeast China. Agriculture largely accounted for the alteration of diversity and functions of the microbial communities. Furthermore, nitrate and total phosphorus declined at the reservoir outlet. The WWTP effluent discharge caused a decrease of the relative abundance of Actinobacteria and Cyanobacteria, while the impact on the variation of alpha diversity of river microbial community was slight. Carbon fixation and nitrogen cycle varied with the change of land use type. The rare taxa contributed with a predominant role in the response to environmental variables and NH₃-N as well as NO₃^--N were the main environmental factors that drove the shift in the bacterial community. The occurrence of the human-specific fecal indicator was mostly derived from agriculture, and its increase in relative abundance was observed in the WWTP effluent. Thus, our study provides guidance for ecological assessment and management of rivers by revealing the response pattern of river bacterioplankton to multiple types of anthropogenic stressors.

Benthic Biofilm Bacterial Communities and Their Linkage with Water-Soluble Organic Matter in Effluent Receivers

Benthic biofilms are pioneering microbial aggregates responding to effluent discharge from wastewater treatment plants (WWTPs). However, knowledge of the characteristics and linkage of bacterial communities and water-soluble organic matter (WSOM) of benthic biofilms in effluent-receiving rivers remains unknown. Here, we investigated the quality of WSOM and the evolution of bacterial communities in benthic biofilm to evaluate the ecological impacts of effluent discharge on a representative receiving water. Tryptophan-like proteins showed an increased proportion in biofilms collected from the discharge area and downstream from the WWTP, especially in summer. Biofilm WSOM showed weak humic character and strong autochthonous components, and species turnover was proven to be the main factor governing biofilm bacteria community diversity patterns. The bacterial community alpha diversity, interspecies interaction, biological index, and humification index were signally altered in the biofilms from the discharge area, while the values were more similar in biofilms collected upstream and downstream from the WWTP, indicating that both biofilm bacterial communities and WSOM characters have resilience capacities. Although effluent discharge simplified the network pattern of the biofilm bacterial community, its metabolic functional abundance was basically stable. The functional abundance of carbohydrate metabolism and amino acid metabolism in the discharge area increased, and the key modules in the non-random co-occurrence network also verified the important ecological role of carbon metabolism in the effluent-receiving river. The study sheds light on how benthic biofilms respond to effluent discharge from both ecological and material points of view, providing new insights on the feasibility of utilizing benthic biofilms as robust indicators reflecting river ecological health.

Wastewater constituents impact biofilm microbial community in receiving streams

Microbial life in natural biofilms is dominated by prokaryotes and microscopic eukaryotes living in dense association. In stream ecosystems, microbial biofilms influence primary production, elemental cycles, food web interactions as well as water quality. Understanding how biofilm communities respond to anthropogenic impacts, such as wastewater treatment plant (WWTP) effluent, is important given the key role of biofilms in stream ecosystem function. Here, we implemented 16S and 18S rRNA gene sequencing of stream biofilms upstream (US) and downstream (DS) of WWTP effluents in four Swiss streams to test how bacterial and eukaryotic communities respond to wastewater constituents. Stream biofilm composition was strongly affected by geographic location - particularly for bacteria. However, the abundance of certain microbial community members was related to micropollutants in the wastewater - among bacteria, micropollutant-associated members were found e.g. in Alphaproteobacteria, and among eukaryotes e.g. in Bacillariophyta (algal diatoms). This study corroborates several previously characterized responses (e.g. as seen in diatoms), but also reveals previously unknown community responses - such as seen in Alphaproteobacteria. This study advances our understanding of the ecological impact of the current wastewater treatment practices and provides information about potential new marker organisms to assess ecological change in stream biofilms.

Implications of bacterial mineralisation in aquatic ecosystem response models

Widespread wastewater pollution is a major barrier to the sustainable management of freshwater and coastal marine ecosystems worldwide. Integrated multi-disciplinary studies are necessary to improve waterway management and protect ecosystem integrity. This study used the Generalised Likelihood Uncertainty Estimation (GLUE) methodology to link microbial community ecotoxicology laboratory data to a mechanistic aquatic ecosystem response model. The generic model provided good predictive skill for major water quality constituents, including heterotrophic bacteria dynamics (r2 = 0.91). The model was validated against observed data across a gradient of effluent concentrations from community whole effluent toxicity (WET) laboratory tests. GLUE analysis revealed that a combined likelihood measure increased confidence in the predictive capability of the model. This study highlights the importance of calibrating aquatic ecosystem response models with net growth rates (i.e., sum of the growth minus loss rate parameter terms) of biological functional groups. The final calibrated net growth rate value of heterotrophic bacteria determined using the GLUE analysis was selected to be 0.58, which was significantly greater than the average literature value of -0.15. This finding demonstrated that use of literature parameter values without a good understanding of the represented processes could create misleading outputs and result in unsatisfactory conclusions. Further, fixed bulk mineralisation rate literature values are typically higher than realistically required in aquatic ecosystem response models. This indicates that explicitly including bacterial mineralisation is crucial to represent microbial ecosystem functioning more accurately. Our study suggests that improved data collection and modelling efforts in real-world management applications are needed to better address nutrients released into the natural environment. Future studies should aim to better understand the sensitivity of aquatic ecosystem response models to bacterial mineralisation rates.

Impact of wastewater on the microbial diversity of periphyton and its tolerance to micropollutants in an engineered flow-through channel system

Wastewater treatment plants (WWTPs) play an important role in retaining organic matter and nutrients but to a lesser extent micropollutants. Therefore, treated wastewater is recognized as a major source of multiple stressors, including complex mixtures of micropollutants. These can potentially affect microbial communities in the receiving water bodies and the ecological functions they provide. In this study, we evaluated in flow-through channels the consequences of an exposure to a mixture of stream water and different percentages of urban WWTP effluent, ranging from 0% to 80%, on the microbial diversity and function of periphyton communities. Assuming that micropollutants exert a selective pressure for tolerant microorganisms within communities, we further examined the periphyton sensitivity to a micropollutant mixture extracted from passive samplers that were immersed in the wastewater effluent. As well, micropollutants in water and in periphyton were comprehensively quantified. Our results show that micropollutants detected in periphyton differed from those found in water, both in term of concentration and composition. Especially photosystem II inhibitors accumulated in periphyton more than other pesticides. Although effects of other substances cannot be excluded, this accumulation may have contributed to the observed higher tolerance of phototrophic communities to micropollutants upon exposure to 30% and 80% of wastewater. On the contrary, no difference in tolerance was observed for heterotrophic communities. Exposure to the gradient of wastewater led to structural differences in both prokaryotic and eukaryotic communities. For instance, the relative abundance of cyanobacteria was higher with increasing percentage of wastewater effluent, whereas the opposite was observed for diatoms. Such results could indicate that differences in community structure do not necessarily lead to higher tolerance. This highlights the need to consider other wastewater constituents such as nutrients and wastewater-derived microorganisms that can modulate community structure and tolerance. By using engineered flow-through channels that mimic to some extent the required field conditions for the development of tolerance in periphyton, our study constitutes a base to investigate the mechanisms underlying the increased tolerance, such as the potential role of microorganisms originating from wastewater effluents, and different treatment options to reduce the micropollutant load in effluents.

Understanding the connectivity of ecosystems in the Anthropocene

In focus: Nash, L. N., Antiqueira, P. A. P., Romero, G. Q., de Omena, P. M., and Kratina, P. (2021). Warming of aquatic ecosystems disrupts aquatic-terrestrial linkages in the tropics. Journal of Animal Ecology. Meta-ecosystem ecology offers a holistic framework linking populations, communities and ecosystems in heterogeneous landscapes. This perspective is particularly relevant as anthropogenic drivers of environmental change proliferate, with the potential for impacts to propagate to spatially connected habitats. In aquatic ecosystems, reciprocal exchanges of energy, materials and organisms that form strong connections with adjacent terrestrial habitats can be disrupted by human impacts. Nash et al. (2021) demonstrate how a warming environment alters aquatic-terrestrial linkages by measuring rates of aquatic insect emergence and decomposition in a tropical context. While decomposition predictably increased with warming, insect emergence was greatly reduced. Using metabolic theory, I highlight how these results deviate from previous studies and help underscore the need for comparative research in different biomes. I conclude by exploring where progress can be made in quantifying, predicting and utilising the connectivity of ecosystems to fully realise the potential of a meta-ecosystem perspective and help address the dual crises of biodiversity loss and climate change.

Warming of aquatic ecosystems disrupts aquatic-terrestrial linkages in the tropics

Aquatic ecosystems are tightly linked to terrestrial ecosystems by exchanges of resources, which influence species interactions, community dynamics and functioning in both ecosystem types. However, our understanding of how this coupling responds to climate warming is restricted to temperate, boreal and arctic regions, with limited knowledge from tropical ecosystems. We investigated how warming aquatic ecosystems impact cross-ecosystem exchanges in the tropics, through the export of aquatic resources into the terrestrial environment and the breakdown of terrestrial resources within the aquatic environment. We experimentally heated 50 naturally assembled aquatic communities, contained within different-sized tank-bromeliads, to a 23.5-32°C gradient of mean water temperatures. The biomass, abundance and richness of aquatic insects emerging into the terrestrial environment all declined with rising temperatures over a 45-day experiment. Structural equation and linear mixed effects modelling suggested that these impacts were driven by deleterious effects of warming on insect development and survival, rather than being mediated by aquatic predation, nutrient availability or reduced body size. Decomposition was primarily driven by microbial activity. However, total decomposition by both microbes and macroinvertebrates increased with temperature in all but the largest ecosystems, where it decreased. Thus, warming decoupled aquatic and terrestrial ecosystems, by reducing the flux of aquatic resources to terrestrial ecosystems but variably enhancing or reducing terrestrial resource breakdown in aquatic ecosystems. In contrast with increased emergence observed in warmed temperate ecosystems, future climate change is likely to reduce connectivity between tropical terrestrial and aquatic habitats, potentially impacting consumers in both ecosystem types. As tropical ectotherms live closer to their thermal tolerance limits compared to temperate species, warming can disrupt cross-ecosystem dynamics in an interconnected tropical landscape and should be considered when investigating ecosystem-level consequences of climate change.

Forested Riparian Zones Provide Important Habitat for Fish in Urban Streams

Riparian zones form a boundary between aquatic and terrestrial ecosystems, with disproportionate influences on food web dynamics and ecosystem functioning in both habitats. However, riparian boundaries are frequently degraded by human activities, including urbanization, leading to direct impacts on terrestrial communities and indirect changes that are mediated through altered connectivity with adjacent aquatic ecosystems. We investigated how riparian habitat influences fish communities in an urban context. We electrofished nine urban site pairs with and without forested riparian buffers, alongside an additional 12 sites that were located throughout the river networks in the Oslo Fjord basin, Norway. Brown trout (Salmo trutta) were the dominant fish species. Riparian buffers had weak positive effects on fish densities at low to moderate levels of catchment urbanization, whereas fish were absent from highly polluted streams. Subtle shifts in fish size distributions suggested that riparian buffers play an important role in metapopulation dynamics. Stable isotopes in fish from buffered reaches indicated dietary shifts, pointing to the potential for a greater reliance on terrestrial-sourced carbon. Combining these results, we postulate that spatially-mediated ontogenetic diet shifts may be important for the persistence of brown trout in urban streams. Our results show that using a food web perspective is essential in understanding how riparian buffers can offset impacts in urban catchments.

Organic Matter Decomposition and Ecosystem Metabolism as Tools to Assess the Functional Integrity of Streams and Rivers–A Systematic Review

Streams and rivers provide important services to humans, and therefore, their ecological integrity should be a societal goal. Although ecological integrity encompasses structural and functional integrity, stream bioassessment rarely considers ecosystem functioning. Organic matter decomposition and ecosystem metabolism are prime candidate indicators of stream functional integrity, and here we review each of these functions, the methods used for their determination, and their strengths and limitations for bioassessment. We also provide a systematic review of studies that have addressed organic matter decomposition (88 studies) and ecosystem metabolism (50 studies) for stream bioassessment since the year 2000. Most studies were conducted in temperate regions. Bioassessment based on organic matter decomposition mostly used leaf litter in coarse-mesh bags, but fine-mesh bags were also common, and cotton strips and wood were frequent in New Zealand. Ecosystem metabolism was most often based on the open-channel method and used a single-station approach. Organic matter decomposition and ecosystem metabolism performed well at detecting environmental change (≈75% studies), with performances varying between 50 and 100% depending on the type of environmental change; both functions were sensitive to restoration practices in 100% of the studies examined. Finally, we provide examples where functional tools are used to complement the assessments of stream ecological integrity. With this review, we hope to facilitate the widespread incorporation of ecosystem processes into bioassessment programs with the broader aim of more effectively managing stream and river ecosystems.

Benthic Diatom Communities in Urban Streams and the Role of Riparian Buffers

Urbanization impacts stream ecosystems globally through degraded water quality, altered hydrology, and landscape disturbances at the catchment and riparian scales, causing biodiversity losses and altered system functioning. Addressing the “urban stream syndrome” requires multiple mitigation tools, and rehabilitation of riparian vegetation may help improve stream ecological status and provide key ecosystem services. However, the extent to which forested riparian buffers can help support stream biodiversity in the face of numerous environmental contingencies remains uncertain. We assessed how a key indicator of stream ecological status, benthic diatoms, respond to riparian habitat conditions using 10 urban site pairs (each comprising of one unbuffered and one buffered reach), and additional urban downstream and forest reference upstream sites in the Oslo Fjord basin. Diatom communities were structured by multiple drivers including spatial location, land use, water quality, and instream habitat. Among these, riparian habitat condition independently explained 16% of variation in community composition among site pairs. Changes in community structure and indicator taxa, along with a reduction in pollution-tolerant diatoms, suggested tangible benefits of forested riparian buffers for stream biodiversity in urban environments. Managing urban impacts requires multiple solutions, with forested riparian zones providing a potential tool to help improve biodiversity and ecosystem services.