Reference Streams Research Articles

Multiple Stress Reduces the Advantage of Pesticide Adaptation.

Under global change scenarios, multistress conditions may occur regularly and require adaptation. However, the adaptation to one stressor might be associated with the increased sensitivity to another stressor. Here, we investigated the ecological consequences of such trade-off under multiple stress. We compared the pesticide tolerance of the crustacean Gammarus pulex from agricultural streams with populations from reference streams. Under optimum temperature, G. pulex from agricultural streams were considerably more tolerant to pesticides as compared to the reference populations. Here, we assume that the increased tolerance in agricultural populations is the combination of acclimation, epigenetic effect, and genetic evolution. After experimental pre-exposure to very low concentration (LC50/1000), reference populations showed increased pesticide tolerance. In contrast, pre-exposure did not further increase the tolerance of agricultural populations. Moreover, these populations were more sensitive to elevated temperature alone due to the hypothesized fitness cost of genetic adaptation to pesticides. However, both reference and agricultural populations showed a similar tolerance to the combined stress of pesticides and warming due to stronger synergistic effects in adapted populations. As a result, pesticide adaptation loses its advantage. The combined effect was predicted well using the stress addition model, developed for predicting the synergistic interaction of independent stressors. We conclude that under multistress conditions, adaptation to pesticides reduces the general stress capacity of individuals and trade-off processes increase the sensitivity to additional stressors. This causes strong synergistic effects of additional stressors on pesticide-adapted individuals.

Evaluation of the Impacts of Abandoned Mining Areas: A Case Study with Benthic Macroinvertebrate Assemblages.

Mining activities are among the most long-lasting anthropogenic pressures on streams and rivers. Therefore, detecting different benthic macroinvertebrate assemblages in the areas recovered from mining activities is essential to establish conservation and management plans for improving the freshwater biodiversity in streams located near mining areas. We compared the stability of benthic macroinvertebrate communities between streams affected by mining activities (Hwangjicheon: NHJ and Cheolamcheon: NCA) and the least disturbed stream (Songjeonricheon: NSJ) using network analysis, self-organizing map, and indicator species analysis. Species richness was lowest at sites where stream sediments were reddened or whitened due to mining impacts in NHJ and NCA. Among functional feeding groups, the ratio of scrapers was lower (i.e., NHJ) or not observed (i.e., NCA) in the affected sites by mining. The networks (species interactions) were less connected in NHJ and NCA than in NSJ, indicating that community stability decreased in the area affected by mining activity. We identified five groups based on the similarity of benthic macroinvertebrate communities according to the gradients of mining impacts using a self-organizing map. the samples from the reference stream (clusters 1 and 5), sites located near the mining water inflow area (cluster 4), sites where stream sediments acid-sulfated (cluster 2), and sites that had recovered from mining impacts (cluster 3). Among the 40 taxa selected as indicators defined from the five clusters in self-organizing map, only few (Physa acuta, Tipula KUa, and Nemoura KUb) indicator species were selected in each cluster representing the mining-impacted sites. Our results highlighted that the benthic macroinvertebrate community complexity was lower in streams affected by mining activity. Furthermore, the range of disturbed areas in the streams, where conservation and management plans should be prioritized, can be quantified by examining alterations in the benthic macroinvertebrate community.

Quantitatively Estimating Carbon Sequestration Potential in Soil and Large Wood in the Context of River Restoration

Restoration aimed at rewetting the valley floor has the potential to increase organic carbon stock in the form of floodplain soil carbon, downed wood, and riparian vegetation. The primary goal of stream restoration is typically to restore habitat or maintain balance between natural ecosystem function and human land use. Although many benefits result from stream restoration, the carbon sequestration potential of different restoration approaches in diverse geographic settings has not yet been quantified. We investigate the carbon storage potential of restored stream segments (known as treatment segments) relative to otherwise analogous degraded and reference segments. We develop a conceptual framework to identify the conditions that maximize carbon storage in relation to characteristics of the river corridor and specific restoration practices and propose response surfaces for carbon storage. We illustrate application and quantification of the conceptual framework using data from a pilot study of treatment, degraded, and reference stream segments along two streams in Oregon, United States. The conceptual model is designed to help managers identify levels of hydrologic connectivity, channel and floodplain dynamics, floodplain vegetation, and other variables that may optimize carbon storage at a treatment site.

Multidecadal biological monitoring and abatement program\ assessing human impacts on aquatic ecosystems within the Oak Ridge Reservation in eastern Tennessee, USA

AbstractHuman activities can be powerful drivers of ecosystem change within catchments. While most long‐term catchment studies have been conducted at pristine sites, such studies are less common from sites more impacted by human activity. The Oak Ridge National Laboratory's Biological Monitoring and Abatement Program (BMAP) was developed in the mid‐1980s to (1) assess compliance with environmental regulations, (2) identify causes of adverse ecological impacts, (3) provide data for human and ecological risk assessments, and (4) evaluate the effectiveness of remedial actions taken to mitigate the impacts of contaminants in soils, groundwater, and surface water by documenting ecological recovery on the Oak Ridge Reservation (ORR), a federally owned 33 476‐acre site in eastern Tennessee, managed by the US Department of Energy. The ORR is composed of multiple watersheds containing many small to mid‐size streams. BMAP uses an integrated approach for determining stream health; its databases include long‐term seasonal records of contaminant concentrations in water and biota, data from aquatic toxicity testing, and surveys of macroinvertebrate and fish assemblages from impacted and reference streams. These long‐term data provide valuable records of degradation and recovery in catchment ecosystems. Our objective here is to describe our study system and data series in order to increase awareness of the availability of these long‐term data to the catchment science community.

A synthesis of land use impacts on stream biodiversity across metrics and scales

While land use intensification is a major driver of biodiversity change in streams, the nature of such changes, and at which scales they occur, have not been synthesized. To synthesize how land use change has altered multiple components of stream biodiversity across scales, we compiled data from 37 studies where comparative data were available for species' total and relative abundances from multiple locations including reference (less impacted) streams to those surrounded by different land use types (urban, forestry, and agriculture). We found that each type of land use reduced multiple components of within-stream biodiversity across scales, but that urbanization consistently had the strongest effects. However, we found that β-diversity among streams in modified landscapes did not differ from β-diversity observed among reference streams, suggesting little evidence for biotic homogenization. Nevertheless, assemblage composition did experience considerable species turnover between reference and modified streams. Our results emphasize that to understand how anthropogenic factors such as land use alter biodiversity, multiple components of biodiversity within and among sites must be simultaneously considered at multiple scales.

Habitat restoration weakens negative environmental effects on telomere dynamics.

Habitat quality can have far-reaching effects on organismal fitness, an issue of concern given the current scale of habitat degradation. Many temperate upland streams have reduced nutrient levels due to human activity. Nutrient restoration confers benefits in terms of invertebrate food availability and subsequent fish growth rates. Here we test whether these mitigation measures also affect the rate of cellular ageing of the fish, measured in terms of the telomeres that cap the ends of eukaryotic chromosomes. We equally distributed Atlantic salmon eggs from the same 30 focal families into 10 human-impacted oligotrophic streams in northern Scotland. Nutrient levels in five of the streams were restored by simulating the deposition of a small number of adult Atlantic salmon Salmo salar carcasses at the end of the spawning period, while five reference streams were left as controls. Telomere lengths and expression of the telomerase reverse transcriptase (TERT) gene that may act to lengthen telomeres were then measured in the young fish when 15months old. While TERT expression was unrelated to any of the measured variables, telomere lengths were shorter in salmon living at higher densities and in areas with a lower availability of the preferred substrate (cobbles and boulders). However, the adverse effects of these habitat features were much reduced in the streams receiving nutrients. These results suggest that adverse environmental pressures are weakened when nutrients are restored, presumably because the resulting increase in food supply reduces levels of both competition and stress.

A Comparison of Brook Trout Passage at Road Culverts to Broadscale Assessment Criteria in a Pennsylvania Headwater Stream

AbstractRestoring stream connectivity by replacing road culverts is a high conservation priority, yet long‐term fish passage data aimed to “ground‐truth” culvert prioritization methods are lacking. One common tool for prioritizing culvert removals is the protocol developed by the North Atlantic Aquatic Connectivity Collaborative (NAACC), which uses culvert characteristics to estimate barrier severity and fish passage. To determine the degree to which NAACC scoring reflects trout passage, we monitored Brook Trout Salvelinus fontinalis movements through three culverts and a reference site (no culvert) in a central Pennsylvania watershed. Passage was nearly continuously monitored for 16 months using remote stationary PIT tag readers. We found that NAACC ranks underestimated Brook Trout passage through two of three culverts. Specifically, the culvert with a rank of “reduced aquatic organism passage (AOP)” experienced higher rates of upstream passage (up to 10 upstream movements per day) than all other sites combined, including the reference site. The reduced AOP site also had a similar proportion of the tagged population that moved upstream (20.9%) compared to the reference site (16.9%). Of the two remaining culverts, both box culverts rated as “no AOP,” one had almost no upstream passage (only two upstream movements over the entire study) and the other had passage rates similar to the reference stream (up to four upstream movements per day). Brook Trout tended to move during high flow and the fall spawning period. We suggest that, for the streams considered in our study, the NAACC protocol may better predict upstream passage if it emphasizes culvert type and adjusts measurement thresholds in the AOP coarse screening. If these patterns are supported by studies at other locations, a more nuanced treatment of culvert type and outlet characteristics in NAACC protocols may more accurately predict Brook Trout passage through culverts.

Short communication: Effects of acute copper exposure on ionic regulation of the freshwater crab Aegla castro

Aeglids are unique freshwater decapods whose habitats are being impacted by metallic compounds, such as copper (Cu). Thus, we investigated the effects of acute Cu exposure on ionic regulation of Aegla castro. For this, male specimens in intermolt were collected from a reference stream and acclimated for 5 days in laboratory. After which, crabs were exposed to 11 μg L−1 Cu (Cu11) or only to water (CTR) for 24 h. Hemolymph samples were withdrawn for the determination of Na+, K+, Ca2+, and Mg2+ concentrations and the posterior gills removed for the analysis of Na+/K+-ATPase, Ca2+-ATPase, H+-ATPase, and carbonic anhydrase (CA) activities. Increased Ca2+ and Mg2+ hemolymph concentrations were observed in animals from Cu11, when compared with CTR group. In addition, decreased activity of CA was observed in animals exposed to Cu. In the current study, alterations in Ca2+ and Mg2+concentrations probably indicate that animals activated exoskeleton reabsorption mechanisms, characteristic of the premolt. Therefore, increased Ca2+ and Mg2+ concentrations in hemolymph may indicate that a biochemical signal associated with the molting cycle was triggered by Cu exposure. Despite the known harmful effects of Cu on osmoregulatory enzymes, here we observed decreased activity only in CA. However, decreased activity of CA could trigger both acid-base imbalance and ionic disruption, since CA provides H+ and HCO3− for intracellular pH maintenance, and underpins Na+ and Cl− for ionic regulation. Therefore, understanding how aeglids respond to metal contamination in laboratory conditions is crucial to assess their potential as an alternative biological model for aquatic ecotoxicology.

Functional trait sorting increases over succession in metacommunity mosaics of fish assemblages.

Metacommunity theory predicts that the relative importance of regional and local processes structuring communities will change over time since initiation of community assembly. Determining effects of these processes on species and trait diversity over succession remains largely unaddressed in metacommunity ecology to date, yet could confer an improved mechanistic understanding of community assembly. To test theoretical predictions of the increasing importance of local processes in structuring communities over successional stages in metacommunities, we evaluated fish species and trait diversity in three pond metacommunities undergoing secondary succession from beaver (Castor canadensis) disturbance. Processes influencing taxonomic and trait diversity were contrasted across pond communities of different ages and in reference streams. Counter to predictions, the local environment became less important in structuring communities over succession but did exert a stronger effect on trait sorting. Beta diversity and trait richness declined over succession while there was no influence on species richness or trait dispersion. The trait filtering in older habitats was likely a response to the larger and deeper pond ecosystems characteristic of late succession. In contrast to these observed effects in ponds, the local environment primarily structured species and trait diversity in streams. Analyses of the relative importance of regional and local processes in structuring fish assemblages within each pond metacommunity suggests that habitat age and connectivity were more important than the environment in structuring communities but contributions were region and scale-dependent. Together, these findings highlight that regional and local processes can differentially influence taxonomic and trait diversity in successional metacommunity mosaics.

Long-term monitoring reveals convergent patterns of recovery from mining contamination across 4 western US watersheds

Long-term studies of stream ecosystems are essential for assessing restoration success because they allow researchers to quantify recovery trajectories, gauge the relative influence of episodic events, and determine the time required to achieve clean-up objectives. To quantify responses of benthic macroinvertebrate assemblages to stream remediation, we integrated results of 4 long-term (20–29 y) assessments of mining-impacted watersheds that were broadly distributed across the western US (California, Colorado, Idaho, Montana). Using a before–after control–impact (BACI) study design, we observed substantial reductions in metal concentrations and corresponding improvements of benthic assemblages following remediation. Recovery rates were relatively consistent, and streams typically recovered within 10 to 15 y after remediation was initiated (mean = 10.25 y), although episodic events changed trajectories at some sites. Differences in recovery among watersheds were likely determined by a number of factors, including the severity of contamination, effectiveness of remediation, proximity to upstream sources of colonization, and hydrologic variation. We also observed considerable variation in the rate and extent of recovery among assemblage metrics. For example, total abundance and richness recovered rapidly at most sites, but the composition of benthic macroinvertebrate assemblages remained substantially altered compared with reference sites. Using piecewise linear regression, we estimated a threshold response of Ephemeroptera, Plecoptera, and Trichoptera (EPT) species richness at ~1 cumulative criteria unit (CCU), which is the sum of the fractions of chronic water-quality criteria for metals measured, suggesting this value was protective of benthic assemblages. However, EPT richness was reduced by ~20% at 2× this CCU value, indicating that moderate exceedances of water-quality criteria could substantially affect stream biodiversity. Non-metric multidimensional scaling analyses identified common sets of species trait states across the 4 watersheds that were associated with either metal contamination or with recovering and intact reference stream assemblages. Our study illustrates the importance of long-term studies for quantifying responses to stream restoration and the usefulness of BACI designs for demonstrating cause-and-effect relationships between restoration treatments and community recovery. Because these 4 watersheds were among the most severely polluted sites in the western US, our study demonstrates the value of these investments in watershed restoration and the potential for success under the most extreme conditions.

Dams and Climate Interact to Alter River Flow Regimes Across the United States

AbstractStoring and managing river flows through reservoirs could dampen or increase climate‐induced fluctuations in streamflow, but interactions between the effects of dams and climate are poorly understood. Here, we examined how dam properties control different facets of flow alteration across the coterminous United States (CONUS), and compared alteration trends between dam‐affected and reference stream gages. We quantified departures from the natural flow regime using 730 stations with long‐term daily discharge data. Dam size and purpose explained high variation in flow alteration, and alteration was particularly severe in water‐stressed regions. Importantly, regulation of river flows by dams often dampened climate‐driven alteration (48% of the flow metrics), particularly in watersheds with positive flow trends; while worsening climatic impacts in other cases (44%), or even having dual effects (8%). Our results show that dam and climate impacts on streamflow need to be assessed jointly, and based on a diverse range of flow regime facets (e.g., event magnitude and duration, frequency, and timing) instead of mean annual flows only. By pairing the USGS streamflow records available from upstream and downstream of 209 dams across the CONUS, we advance the notion that dams amplify flow alteration, but also ameliorate some flow alteration metrics. Understanding such potential and limitations is important in light of climate non‐stationarity and a new wave of damming in developing economies, and will be key to further advancing environmental flow science into the future.

Stream Macroinvertebrates and Carbon Cycling in Tangled Food Webs

The annual global loss of organic carbon from terrestrial ecosystems into rivers is similar to the organic carbon stored in soils each year. Dissolved organic matter (DOM) flows through the food web to macroinvertebrates, but little is known about the effect of DOM increase on stream food webs and how much macroinvertebrates may contribute to the regulation of carbon fluxes in rivers. Using a before and after control impact (BACI) experimental design, we increased by 12% (+ 0.52 mg C L−1) the concentration of DOM in a stream for three weeks by adding sucrose, with a distinctive δ13C signature, to simulate a pulse of natural DOM supply from soils. We partitioned the diet of macroinvertebrates from carbon sources according to the green pathway (autotrophs) and detrital pathways (bacteria and terrestrial organic matter). Our flow food web approach based on C fluxes, with bacteria as a key node, showed the dominant contribution of the detrital pathways for macroinvertebrates in the reference stream. DOM addition induced changes in the diets of individual taxa, but did not have any strong effects on the relative overall contribution of the detrital pathways versus the green pathway. Autotrophic uptake of CO2 respired by bacteria was much larger than bacterial C flux to invertebrates (that is, the classic microbial loop) and allowed a significant fraction of natural allochthonous organic carbon to make its way to macroinvertebrates via autotrophs fixing CO2 respired by bacteria. Overall macroinvertebrates did not regulate directly to any great extent the flux of stream DOM towards downstream ecosystems.

Functional response of benthic macroinvertebrates to fire disturbance in patagonian streams

Disturbances often act as ecological filters through which only certain combinations of biological traits may pass. Here, we aimed to determine if an unprescribed human-started wildfire can act as a filter for stream-dwelling macroinvertebrates. To do this, we sampled three burnt and two reference streams in Los Alerces National Park (Patagonia, Argentine side). Sampling began shortly after the fire was extinguished and comprised four visits during the following seven months. We explored how functional richness, evenness, and dispersion at the community level responded to post-fire environmental changes using generalized linear mixed models. We also used RLQ and fourth-corner analysis to identify which trait modalities were specifically affected, either positively or negatively, by the fire. We found that functional richness and dispersion decreased with increasing disturbance intensity, which in this case, was best represented by increased suspended solids. In addition, all three functional metrics were significantly lower in fire-affected sites. Burnt streams were characterized by bi-multivoltine taxa and collector-gatherers, among others. Overall, our study showed how a wildfire can directly constrain the functional composition of macroinvertebrate communities, and highlighted the importance of incorporating trait-based approaches into the study of large-scale disturbances.

Stream biodiversity and monitoring in North Central, Nigeria: the use of macroinvertebrate indicator species as surrogates.

Indicator species (IS) have been employed in modern aquatic research for monitoring of environmental changes and evaluating the efficiency of environmental management procedures. In this study, we evaluated the possibility of developing surrogate indicator groups as tools for the conservation and management of the biodiversity of Northern Nigeria streams by surveying 15 streams in Niger state for benthic macroinvertebrates and environmental variables as data sets, over a period of 24 months (2016 and 2017). Samples were collected in two locations of reference and impacted sites for each of the streams surveyed. The statistically significant (P < 0.05; based on 1000 permutations) indicator species for each of the status classes (reference versus impacted) was identified using the indicator species analysis/indicator value (Indval) method. Canonical correspondence analysis (CCA) was used to evaluate the IS-environment relationships. Indicator value found fifteen species for the reference streams including Ephemeropteran (Bugilliesia sp., Tricorythus sp., Thraulus sp., Crassabwa sp.) and the Tricopteran (Leptonema sp.). Opposite, the Indval found seven (7) indicator species for the impacted streams, which included the Dipteran (Pentaneura sp., Tabanus sp.). Multivariate analysis revealed that species assemblage had wide dispersal patterns in relation to the sites for both status classes. CCA revealed that the reference and impacted indicator species responded to entirely different environmental factors, indicating their preference to particular environmental variables along the ecological gradients. While the indicator species of reference sites were associated with environmental predictors of good water quality such as high DO, increased flow, low conductivity, and low BOD, the indicator species of impacted sites were strongly related to environmental predictors of anthropogenic pollution, including low DO, high BOD, and increased nutrients concentrations. This study has provided a reference point and effective tool to monitor environmental changes, community, and ecosystem dynamics across the Northern Nigeria streams.

Predicting the Thermal Regime Change of a Regulated Snowmelt River Using a Generalised Additive Model and Analogue Reference Streams

Large reservoirs can alter the natural thermal regime and result in the delivery of water with unseasonal water temperatures downstream. Establishing a detailed understanding how a river’s thermal regime has changed from its natural state is a critical step required before mitigation options can be investigated. Our study focussed on one of Australia’s most iconic rivers, the Snowy River which is regulated by multiple headwater regulating structures. We used mean daily pre-dam water temperature (1962–1967) data from the Snowy River to develop generalized additive models (GAMs) to predict the natural (daily mean, max and min) water temperatures in the Snowy River. Daily modelled natural water temperatures were compared with daily observed water temperatures in the regulated Snowy River and two nearby analogue reference streams. Using the GAMs, we identified a reduced thermal range (max and min) and water temperatures ≥ 2 °C warmer than natural between the cooler months of April and July for at least 50 % of the study period. Temperatures rarely ( 2 °C) requires further investigation.

Combined carbon flows through detritus, microbes, and animals in reference and experimentally enriched stream ecosystems.

Tracking carbon (C) flow through ecosystems requires quantification of myriad biophysical processes, including C routing through microbial and metazoan food webs. Yet detailed organic matter budgets are rarely combined with simultaneous measurement of C flows supporting microbial and animal production. Here, we synthesize concurrent data sets on organic matter, microbes, and macroinvertebrates from two detritus-based stream ecosystems, one of which was subject to experimental nitrogen (N) and phosphorus (P) enrichment. Our synthesis provides new insights into C flow through forest stream ecosystems. Over 3 yr, the reference stream showed a striking balance of inputs and outputs, with a mean surplus of only 7g C·m-2 ·yr-1 (~1% of annual inputs), presumably stored in sediments as fine particulate organic matter (FPOM). In contrast, N and P enrichment over 2 yr resulted in severe deficits of C (-576g C·m-2 ·yr-1 or~170% of annual inputs), a shortfall presumably met by stored C. Our data set provides an ecosystem-based estimate of the fate of forest litter C at ambient nutrient concentrations: 6.2% was leached as dissolved organic C, 40.6% and 8.5% flowed to litter-associated fungi and bacteria, respectively, 7.5% was consumed by macroinvertebrates, 1.8% was exported as coarse particles, and the remainder (35.4%) was presumably fragmented by biophysical processes. Our calculations also allowed an estimate of inputs into the heterogeneous FPOM pool, which is otherwise difficult to obtain. At naturally low nutrient concentrations, 50.7% was derived from fragmented litter, 39.1% from microbial biomass (mostly fungal), and 10.2% from macroinvertebrate egesta. Nutrient addition drove large changes in C fluxes in the experimental stream, especially in flows of leaf litter to fungi (×1.7 pretreatment) and macroinvertebrates (×2.7), and of FPOM to hydrologic export (×2.6). Our results underscore the key roles of both microbes and metazoans in controlling C flow through detritus-based ecosystems, as well as how release from persistent nutrient limitation may perturb steady-state conditions of C inputs vs. outputs. Our analysis also suggests areas for future research, including assessing the relative importance of stored vs. recycled C in fueling detrital food webs subject to altered nutrient regimes and other global-change drivers.

Future changes in the Dominant Source Layer of riparian lateral water fluxes in a subhumid Mediterranean catchment

The ‘Dominant Source Layer’ (DSL) is defined as the riparian zone (RZ) depth stratum that contributes the most to water and solute fluxes to streams. The concept can be used to explain timing and amount of matter transferred from RZs to streams in forest headwaters. Here, we investigated the potential impact of future climate changes on the long-term position of the DSL in a subhumid Mediterranean headwater catchment. We used the rainfall-runoff model PERSiST to simulate reference (1981–2000) and future (2081–2100) stream runoff. The latter were simulated using synthetic temperature, precipitation, and inter-event length scenarios in order to simulate possible effects of changes in temperature, rainfall amount, and rainfall event frequency and intensity. Simulated stream runoff was then used to estimate RZ groundwater tables and the proportion of lateral water flux at every depth in the riparian profile; and hence the DSL. Our simulations indicated that future changes in temperature and precipitation will have a similar impact on the long-term DSL position. Nearly all scenarios projected that, together with reductions in stream runoff and water exports, the DSL will move down in the future, by as much as ca. 30 cm. Shallow organic-rich layers in the RZ will only be hydrologically activated during sporadic, large rainfall episodes predicted for the most extreme inter-event length scenarios. Consequently, terrestrial organic matter inputs to streams will decrease, likely reducing catchment organic matter exports and stream dissolved organic carbon concentrations. This study highlights the importance of identifying vertical, hydrologically active layers in the RZ for a better understanding of the potential impact of future climate on lateral water transfer and their relationship with surface water quality and carbon cycling.

From legacy effects of acid deposition in boreal streams to future environmental threats

Few environmental issues have resulted in such a heated policy-science controversy in Sweden as the 1990s acidification debate in the north of the country. The belief that exceptionally high stream acidity levels during hydrological events was caused by anthropogenic deposition resulted in a governmentally funded, multi-million dollar surface-water liming program. This program was heavily criticized by a large part of the scientific community arguing that the acidity of northern streams was primarily caused by naturally occurring organic acids. Here, we revisit the acid deposition legacy in northern Sweden two decades after the culmination of the controversy by examining the long-term water chemistry trends in the Svartberget/Krycklan research catchment that became a nexus for the Swedish debate. In this reference stream, trends in acidic episodes do show a modest recovery that matches declines in acid deposition to pre-industrial levels, although stream acidity continues to be overwhelmingly driven by organic acidity. Yet there are legacies of acid deposition related to calcium losses from soils, which are more pronounced than anticipated. Finally, assessment of these trends are becoming increasingly complicated by new changes and threats to water resources that must be recognized to avoid unnecessary, expensive, and potentially counterproductive measures to adapt and mitigate human influences. Here we make the argument that while the acidification era is ending, climate change, land-use transitions, and long-range transport of other contaminants warrant close monitoring in the decades to come.

Predicting high‐frequency variation in stream solute concentrations with water quality sensors and machine learning

AbstractStream solute monitoring has produced many insights into ecosystem and Earth system functions. Although new sensors have provided novel information about the fine‐scale temporal variation of some stream water solutes, we lack adequate sensor technology to gain the same insights for many other solutes. We used two machine learning algorithms – Support Vector Machine and Random Forest – to predict concentrations at 15‐min resolution for 10 solutes, of which eight lack specific sensors. The algorithms were trained with data from intensive stream sensing and manual stream sampling (weekly) for four full years in a hydrologic reference stream within the Hubbard Brook Experimental Forest in New Hampshire, USA. The Random Forest algorithm was slightly better at predicting solute concentrations than the Support Vector Machine algorithm (Nash‐Sutcliffe efficiencies ranged from 0.35 to 0.78 for Random Forest compared to 0.29 to 0.79 for Support Vector Machine). Solute predictions were most sensitive to the removal of fluorescent dissolved organic matter, pH and specific conductance as independent variables for both algorithms, and least sensitive to dissolved oxygen and turbidity. The predicted concentrations of calcium and monomeric aluminium were used to estimate catchment solute yield, which changed most dramatically for aluminium because it concentrates with stream discharge. These results show great promise for using a combined approach of stream sensing and intensive stream discrete sampling to build information about the high‐frequency variation of solutes for which an appropriate sensor or proxy is not available.

Added value of the NemaSPEAR[%]-index to routinely used macrofauna-based indices for assessing the quality of freshwater sediments

Assessments of the ecological status of freshwater ecosystems, such as mandated by the EU Water Framework Directive, are routinely conducted by monitoring macroinvertebrates. However, for the quality assessment of fine sediments, macroinvertebrates are of limited suitability. In such habitats they show a low species diversity and often low densities, whereas a more diverse meiofauna can be found. Among the meiofaunal groups in benthic habitats, nematodes are one of the most abundant and species-rich. Fine, cohesive sediments considerably contribute to many ecosystem services, but they are often hotspots of chemical contamination as well. In the present study, the added value of the recently developed and validated NemaSPEAR[%]-index was evaluated by directly comparing it to routinely used macrofauna-based indices. Macrofaunal and nematode communities were synchronously monitored at seven sites in six different streams. The results of a chemical analysis of sediment pollutants combined with sediment quality guidelines revealed widely diverging toxic potentials at the seven investigated locations. The seasonal robustness of the NemaSPEAR[%]-index compared with macrofauna-based indices was also determined, by additionally obtaining synchronous samples of macrofauna and nematodes over the course of one year at one of the seven sites, a reference stream with very low toxic potential.The NemaSPEAR[%] performed robustly despite seasonal variations in the nematode community in the sediment of the unpolluted stream. At the seven sampling sites, representing a pollution gradient, the NemaSPEAR[%]-index correlated well with the toxic potential of the sediments. By contrast, the macrofauna-based indices did not correlate significantly with either the toxic potential of the sediments or with the results of NemaSPEAR[%] at the seven sites. For many non– endobenthic macroinvertebrates, chemical exposure is mostly through the water phase, such that the toxic potential of the sediments will not necessarily be reflected directly by macrofaunal indices. Accordingly, identifying the stressors that contribute to degrading the ecological status of a water body requires the inclusion of methods that examine different types of stressors, targets, and exposure pathways. Our study shows that the NemaSPEAR[%]-index provides added value to routinely used macrofaunal-based indices.