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Urban streams display many similar symptoms around the world, but awareness is growing that the mechanisms and severity of the symptoms differ among regions. Climate is a key contributor to these differences via: 1) variation in the initial conditions of streams across climate (and their sensitivity to urbanization), 2) interactions between climate and urbanization, and 3) indirect effects of climate on the form of urbanization or management via effects on human behaviors. We discuss the direct and indirect pathways by which climate shapes the structure and function of urban streams and how differences in climate affect how the urban stream syndrome is manifested. For some symptoms, such as stream flashiness, climate can affect the direction and magnitude of response, whereas for others, such as losses of sensitive taxa, climate can affect the magnitude of the response. Mechanisms linking urbanization with ecological degradation, even when responses are similar, can differ among climates. Limited research has been done on the role of climate in stream response to urbanization. Controlled, large-scale studies are needed to address this issue, particularly because climate may covary with types of infrastructure and technological capabilities across the globe. Understanding the linkages between climate and urban stream ecosystems will improve our theoretical understanding of urban streams and our ability to implement tailored and regionally sensitive management strategies.

Indicators based on nutrient-biota relationships in streams can inform water quality restoration and protection programs. Bacterial assemblages could be particularly useful indicators of nutrient effects because they are species-rich, important contributors to ecosystem processes in streams, and responsive to rapidly changing conditions. Here, we sampled 25 streams weekly (12–14 times each) and used 16S rRNA gene metabarcoding of periphyton-associated bacteria to quantify the effects of total phosphorus (TP) and total nitrogen (TN). Threshold indicator taxa analysis identified assemblage-level changes and amplicon sequence variants (ASVs) that increased or decreased with increasing TP and TN concentrations (i.e., low P, high P, low N, and high N ASVs). Boosted regression trees confirmed that relative abundances of gene sequence reads for these four indicator groups were associated with nutrient concentrations. Gradient forest analysis complemented these results by using multiple predictors and random forest models for each ASV to identify portions of TP and TN gradients at which the greatest changes in assemblage structure occurred. Synthesized statistical results showed bacterial assemblage structure began changing at 24 µg TP/L with the greatest changes occurring from 110 to 195 µg/L. Changes in the bacterial assemblages associated with TN gradually occurred from 275 to 855 µg/L. Taxonomic and phylogenetic analyses showed that low nutrient ASVs were commonly Firmicutes, Verrucomicrobiota, Flavobacteriales, and Caulobacterales, Pseudomonadales, and Rhodobacterales of Proteobacteria, whereas other groups, such as Chitinophagales of Bacteroidota, and Burkholderiales, Rhizobiales, Sphingomonadales, and Steroidobacterales of Proteobacteria comprised the high nutrient ASVs. Overall, the responses of bacterial ASV indicators in this study highlight the utility of metabarcoding periphyton-associated bacteria for quantifying biotic responses to nutrient inputs in streams.

Excessive inputs of nitrogen from anthropogenic activities in watersheds can cause detrimental effects to aquatic ecosystems, but these effects can be difficult to determine based solely on nitrogen concentrations because of their temporal variability and the need to link human activities to ecological responses. Here, we (1) tested the use of stable isotopes of nitrogen (δ15N) and carbon (δ13C) in benthic organic matter (BOM) as proxies for isotope ratios of filter feeding bivalves in lakes and estuaries, which can be used as indicators but are harder to sample and often spatially sparse, and (2) evaluated if stable isotope ratios in benthic organic matter could be used to assess impacts from anthropogenic land development of watersheds. The δ15N in BOM isolated from surficial sediment (δ15NBOM) was significantly correlated with δ15N in filter feeding unionid mussels (Elliptio complanata, δ15NUN) from lakes and with hard-shell clams (Mercenaria mercenaria, δ15NMM) from estuaries. In lakes, δ13CBOM was significantly correlated with δ13CUN, but δ13CBOM was not significantly correlated with δ13CMM in estuaries. Values of δ15NBOM and δ15NUN were significantly and positively correlated with increasing amounts of impervious surface, urban land cover, and human populations in watersheds surrounding lakes. In estuaries, δ15NBOM was only significantly and positively correlated with greater percent impervious surface in the watersheds. Correlations of δ13CBOM in lakes and estuaries, δ13CUN, and δ13CMM with land use and human population were mostly non-significant or weak. Overall, these results show that δ15NBOM can serve as a proxy for δ15N of filter feeding bivalves in lakes and estuaries and is useful for assessing anthropogenic impacts on aquatic systems and resources. Our study area was limited in size, but our results support further studies to test the application of this sediment stable isotope-based technique for assessing and ranking aquatic resources across broad geographical areas.

Wastewaters and leachates from various inland resource extraction activities contain high ionic concentrations and differ in ionic composition, which complicates the understanding and effective management of their relative risks to stream ecosystems. To this end, we conducted a stream mesocosm dose–response experiment using two dosing recipes prepared from industrial salts. One recipe was designed to generally reflect the major ion composition of deep well brines (DWB) produced from gas wells (primarily Na+, Ca2+, and Cl−) and the other, the major ion composition of mountaintop mining (MTM) leachates from coal extraction operations (using salts dissociating to Ca2+, Mg2+, Na+, SO42− and HCO3−)—both sources being extensive in the Central Appalachians of the USA. The recipes were dosed at environmentally relevant nominal concentrations of total dissolved solids (TDS) spanning 100 to 2000 mg/L for 43 d under continuous flow-through conditions. The colonizing native algal periphyton and benthic invertebrates comprising the mesocosm ecology were assessed with response sensitivity distributions (RSDs) and hazard concentrations (HCs) at the taxa, community (as assemblages), and system (as primary and secondary production) levels. Single-species toxicity tests were run with the same recipes. Dosing the MTM recipe resulted in a significant loss of secondary production and invertebrate taxa assemblages that diverged from the control at all concentrations tested. Comparatively, intermediate doses of the DWB recipe had little consequence or increased secondary production (for emergence only) and had assemblages less different from the control. Only the highest dose of the DWB recipe had a negative impact on certain ecologies. The MTM recipe appeared more toxic, but overall, for both types of resource extraction wastewaters, the mesocosm responses suggested significant changes in stream ecology would not be expected for specific conductivity below 300 µS/cm, a published aquatic life benchmark suggested for the region.

Nutrient pollution from human activities remains a common problem facing stream ecosystems. Identifying ecological responses to phosphorus and nitrogen can inform decisions affecting the protection and management of streams and their watersheds. Diatoms are particularly useful because they are a highly diverse group of unicellular algae found in nearly all aquatic environments and are sensitive responders to increased nutrient concentrations. Here, we used DNA metabarcoding of stream diatoms as an approach to quantifying effects of total phosphorus (TP) and total nitrogen (TN). Threshold indicator taxa analysis (TITAN) identified operational taxonomic units (OTUs) that increased or decreased along TP and TN gradients along with nutrient concentrations at which assemblages had substantial changes in the occurrences and relative abundances of OTUs. Boosted regression trees showed that relative abundances of gene sequence reads for OTUs identified by TITAN as low P, high P, low N, or high N diatoms had strong relationships with nutrient concentrations, which provided support for potentially using these groups of diatoms as metrics in monitoring programs. Gradient forest analysis provided complementary information by characterizing multi-taxa assemblage change using multiple predictors and results from random forest models for each OTU. Collectively, these analyses showed that notable changes in diatom assemblage structure and OTUs began around 20 μg TP/L, low P diatoms decreased substantially and community change points occurred from 75 to 150 μg/L, and high P diatoms became increasingly dominant from 150 to 300 μg/L. Diatoms also responded to TN with large decreases in low N diatoms occurring from 280 to 525 μg TN/L and a transition to dominance by high N diatoms from 525–850 μg/L. These diatom responses to TP and TN could be used to inform protection efforts (i.e., antidegradation) and management goals (i.e., nutrient reduction) in streams and watersheds. Our results add to the growing support for using diatom metabarcoding in monitoring programs.

Stormwater drainage and wastewater disposal are primary pathways through which urbanization degrades streams. These technologies and management practices change over time, reshaping the urban template and affecting the environmental challenges a city will face in the following decades to centuries. Spatial and temporal asynchrony in the implementation and replacement of these technologies and the adoption of new management approaches means that the mechanisms of the urban stream syndrome will be heterogeneous. Thus, promoting ‘one size fits all’ global panaceas for urban streams may be less effective than local solutions based on the unique urban templates and socioeconomic factors governing streams. Understanding the cumulative effects of spatiotemporal changes in urban templates on streams is critical to protecting and managing them because: 1) existing water infrastructure in many countries (especially high-income countries) is aging and requires replacement, 2) urbanization in rapidly developing countries requires ever more infrastructure, and 3) demand for higher levels of environmental quality is transforming technological and management approaches globally. The management and technological decisions made during the current infrastructure replacement or new construction cycle will define the future urban template and select the trajectory and character of the urban stream syndrome during the coming decades.

Watershed development alters hydrology and delivers anthropogenic stressors to streams via pathways affected by impervious cover. We characterized relationships of diatom communities and metrics with upstream watershed % impervious cover (IC) and with riparian % forest and wetland cover in 120-m buffers along each side of upstream networks. Threshold Indicator Taxa ANalysis (TITAN) identified potential threshold responses of diatom communities at 0.6 and 2.9% IC. Boosted regression trees (BRTs) indicated potential thresholds between 0.7 and 4.5% IC at which relative abundances of low-nutrient diatoms decreased and those of high-nutrient, prostrate, and motile diatoms increased. These individual thresholds indicated that multiple stressors or magnitudes of stressors related to increasing watershed % IC differentially affected relative abundances of taxa, and these differential effects probably contributed to a more gradual, but still substantial, change in overall community structure. BRTs showed that near-stream buffers with >65% and ideally >80% forest and wetland cover were associated with a 13 to 34% reduction in the effects of watershed % IC on diatom metrics and community structure and with a 61 to 68% reduction in the effects of watershed % pasture on motile and high-P diatom relative abundances. Watershed % IC and riparian % forest and wetland cover probably affect hydrologic, nutrient, and sediment regimes, which then affect diatom community physiognomy and taxa sensitive to nutrients and conductivity. Our results emphasize the importance of implementing mindful development and protective measures, especially in watersheds near watershed % IC thresholds. Effects of development potentially could be reduced by restoring and conserving near-stream forests and wetlands, but management and restoration strategies that extend beyond near-stream buffers are needed.

Watershed management and policies affecting downstream ecosystems benefit from identifying relationships between land cover and water quality. However, different data sources can create dissimilarities in land cover estimates and models that characterize ecosystem responses. We used a spatially balanced stream study (1) to effectively sample development and urban stressor gradients while representing the extent of a large coastal watershed (>4400 km²), (2) to document differences between estimates of watershed land cover using 30-m resolution national land cover database (NLCD) and <1-m resolution land cover data, and (3) to determine if predictive models and relationships between water quality and land cover differed when using these two land cover datasets. Increased concentrations of nutrients, anions, and cations had similarly significant correlations with increased watershed percent impervious cover (IC), regardless of data resolution. The NLCD underestimated percent forest for 71/76 sites by a mean of 11 % and overestimated percent wetlands for 71/76 sites by a mean of 8 %. The NLCD almost always underestimated IC at low development intensities and overestimated IC at high development intensities. As a result of underestimated IC, regression models using NLCD data predicted mean background concentrations of NO₃ ⁻ and Cl⁻ that were 475 and 177 %, respectively, of those predicted when using finer resolution land cover data. Our sampling design could help states and other agencies seeking to create monitoring programs and indicators responsive to anthropogenic impacts. Differences between land cover datasets could affect resource protection due to misguided management targets, watershed development and conservation practices, or water quality criteria.

Diatoms, microscopic unicellular algae, have been widely sampled from streams around the world because of their use in biological assessments, but as a result they are often only collected from epilithic habitats in riffles, which may lead to a substantial underreporting of their diversity and distributional patterns. The goals of this study were to identify how sampling methods and water chemistry affect the characterization of local and regional diversity and distributions. Species richness, Shannon diversity, and evenness were significantly greater in multiple habitat (MH) than epilithic habitat (EH) samples (paired t-test; P < 0.01). At the local scale, MH sampling yielded 76% of the total species collected (42% unique), EH sampling captured 58% (24% unique), and 34% were shared. From MH samples, 3, 28, and 60 regionally rare species were collected from >75, >50, and >25% of sites, respectively, whereas 1, 14, and 34 were collected from EH samples, respectively. Regional abundance more strongly predicted site frequency than mean local abundance. Smaller drainage basins tended to have fewer species, likely because of species-area relationships, but local factors (% EH and alkalinity) influenced the taxonomic complexity of assemblages. Diversity was greatest at intermediate % EH, but low alkalinity reduced diversity, which is potentially important for ecosystems affected by anthropogenic acidification, as in our study. Multiple habitats need to be sampled for better documenting diatom diversity and distributions, which could improve conservation efforts at local and regional scales along with the characterization of ecological patterns and processes.

Benthic diatoms are important indicators of human impacts on streams. Epilithic diatoms are collected most often for bioassessments, but potentially important ecological information from other habitats could be missed. Within our study region, substrata ranged from 100% rock to 100% sandy silt at 61 sites, leading us to question the appropriateness of sampling solely epilithic surfaces in riffles. We compared two protocols for collecting diatoms: (1) epilithic habitat samples (EHS) within riffles and (2) multi-habitat samples (MHS), which proportionately included different habitats (e.g., riffles and pools) and substrata (e.g., sand, silt, and rock). Three streams were not compared because rocks were absent. Nonmetric multidimensional scaling indicated that diatom communities from EHS and MHS responded similarly to alkalinity and agricultural gradients, and Procrustes analysis showed that ordinations were similar (P < 0.01). Percent motile diatoms and metrics indicating high or low P and N from MHS showed stronger relationships with land use variables than those metrics from EHS. Bray-Curtis (BC) similarity between the protocols increased as diversity and richness decreased in MHS, as agricultural impacts increased, and as motile diatoms became more abundant in EHS, which likely indicated greater habitat homogeneity or an overriding effect of water chemistry. The two protocols classified 78% of sites concordantly as minimally impacted or impacted. Multi-habitat sampling is preferred where stream habitats vary greatly because it assessed land use impacts more effectively than EHS, while indicating water chemistry impacts as effectively. Multiple habitats also contained greater species diversity, which is important to documenting species distributions, and were always available for sampling, whereas epilithic habitats could be sparse or absent.