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Agricultural land uses can impact stream ecosystems by reducing suitable habitat, altering flows, and increasing inputs of diffuse pollutants including fine inorganic sediment (<2 mm). These changes have been linked to altered community composition and declines in biodiversity. Determining the mechanisms driving stream biotic responses, particularly threshold impacts, has, however, proved elusive. To investigate a sediment threshold response by benthic invertebrates, an intensive survey of 30 agricultural streams was conducted along gradients of deposited sediment and dissolved nutrients. Partial redundancy analysis showed that invertebrate community composition changed significantly along the gradient of deposited fine sediment, whereas the effect of dissolved nitrate was weak. Pollution-sensitive invertebrates (%EPT, Ephemeroptera, Plecoptera, Trichoptera) demonstrated a strong nonlinear response to sediment, and change-point analysis indicated marked declines beyond a threshold of ∼20% fine sediment covering the streambed. Structural equation modeling indicated that decreased habitat availability (i.e., coarse substrate and associated interstices) was the key driver affecting pollution-sensitive invertebrates, with degraded riparian condition controlling resources through direct (e.g., inputs) and indirect (e.g., flow-mediated) effects on deposited sediment. The identification of specific effects thresholds and the underlying mechanisms (e.g., loss of habitat) driving these changes will assist managers in setting sediment criteria and standards to better guide stream monitoring and rehabilitation.

Water quality agencies and scientists are increasingly adopting standardized sampling methodologies because of the challenges associated with interpreting data derived from dissimilar protocols. Here, we compare 13 protocols for monitoring streams from different regions and countries around the globe. Despite the spatially diverse range of countries assessed, many aspects of bioassessment structure and protocols were similar, thereby providing evidence of key characteristics that might be incorporated in a global sampling methodology. Similarities were found regarding sampler type, mesh size, sampling period, subsampling methods, and taxonomic resolution. Consistent field and laboratory methods are essential for merging data sets collected by multiple institutions to enable large-scale comparisons. We discuss the similarities and differences among protocols and present current trends and future recommendations for monitoring programs, especially for regions where large-scale protocols do not yet exist. We summarize the current state in one of these regions, Latin America, and comment on the possible development path for these techniques in this region. We conclude that several aspects of stream biomonitoring need additional performance evaluation (accuracy, precision, discriminatory power, relative costs), particularly when comparing targeted habitat (only the commonest habitat type) versus site-wide sampling (multiple habitat types), appropriate levels of sampling and processing effort, and standardized indicators to resolve dissimilarities among biomonitoring methods. Global issues such as climate change are creating an environment where there is an increasing need to have universally consistent data collection, processing and storage to enable large-scale trend analysis. Biomonitoring programs following standardized methods could aid international data sharing and interpretation.

Didymosphenia geminata is a benthic bloom-forming diatom that is invasive in many temperate, oligotrophic freshwater ecosystems. D. geminata blooms are unusual, resulting from prolific basal stalk production stimulated by phosphorus limitation. The adaptive value of stalk production and bloom development is disputed. We examined blooms in relation to stalk biomass, biovolume and phosphatase activity. An austral summer survey of 15 sites within the Waitaki River of New Zealand compared reference communities (no detectable D. geminata ), with those impacted by high and low D. geminata biomass. Sites were compared for differences in phosphatase location and activity using chromogenic substrates, community composition using morphological identifications, and overlying water and pore-water chemistry. Experimental microcosms subjected live proliferations to varied phosphate concentrations, and phosphatase rates and location were examined. Survey results identified phosphatase activity increased with D. geminata biomass, with lowest rates in reference communities. Pools of labile nutrients were detected in D. geminata mats, and in vitro hydrolysis rates were rapid in replete phosphoester conditions (~0.2 mmol l −1 h −1 cm −2 at 16°C), with activity concentrated on stalks. Our results suggest D. geminata bloom development is an adaptation to maximise supply of phosphate under chronic phosphorus limitation.

The distribution of species in relation to latitude and altitude is of fundamental interest to ecologists and is expected to attain increasing importance as the Earth’s climate continues to change. Species diversity is commonly greater at lower than higher latitudes on a global scale, and the similarity of communities frequently decreases with distance. Nevertheless, reasons for such patterns are not well understood. We investigated species richness and changes in community composition of mayflies (Ephemeroptera) over 13 degrees of latitude at 81 locations throughout New Zealand by light-trapping and the benthic sampling of streams. Mayflies were also sampled along an altitudinal gradient on a prominent inactive volcano in the east of North Island. Sampled streams were predominantly in the native forest, at a wide range of altitudes from sea level to c. 1000 m a. s. l. A total of 47 of the 59 described New Zealand mayflies were recorded during the study, along with five undescribed morphospecies. Species richness declined and the degree of dissimilarity (beta diversity) of mayfly communities increased significantly from north to south but less strongly with increasing altitude. Our results suggest that the southward decline in species richness has historical origins with the north of the country having acted as a major refuge and region of speciation during the Pleistocene. The increasing dissimilarity of the northern and southern communities may reflect an increasingly harsh climate, variable amounts of subsequent southward dispersal of northern species and, in the South Island, the presence of species which may have evolved in the newly uplifted mountains during the Miocene–Pliocene.

Reducing excessive reactive nitrogen (N) in agricultural waterways is a major challenge for freshwater managers and landowners. Effective solutions require the use of multiple and combined N attenuation tools, targeted along small ditches and streams. We present a visual framework to guide novel applications of ‘tool stacking’ that include edge-of-field and waterway-based options targeting N delivery pathways, timing, and impacts in the receiving environment (i.e., changes in concentration or load). Implementing tools at multiple locations and scales using a ‘toolbox’ approach will better leverage key hydrological and biogeochemical processes for N attenuation (e.g., water retention, infiltration and filtering, contact with organic soils and microbes, and denitrification), in addition to enhancing ecological benefits to waterways. Our framework applies primarily to temperate or warmer climates, since cold temperatures and freeze–thaw-related processes limit biologically mediated N attenuation in cold climates. Moreover, we encourage scientists and managers to codevelop N attenuation toolboxes with farmers, since implementation will require tailored fits to local hydrological, social, and productive landscapes. Generating further knowledge around N attenuation tool stacking in different climates and landscape contexts will advance management actions to attenuate agricultural catchment N. Understanding how different tools can be best combined to target key contaminant transport pathways and create activated zones of attenuation along and within small agricultural waterways will be essential.

Natural and anthropogenic disturbances commonly alter patterns of biodiversity and ecosystem functioning. However, how networks of interacting species respond to these changes remains poorly understood. We described aquatic food webs using invertebrate and fish community composition, functional traits and stable isotopes from twelve agricultural streams along a landscape disturbance gradient. We predicted that excessive deposition of fine inorganic sediment (sedimentation) associated with agricultural activities would negatively influence aquatic trophic diversity (e.g. reduced vertical and horizontal trophic niche breadths). We hypothesized that multiple mechanisms might cause trophic niche ‘compression’, as indicated by changes in realized trophic roles. Food‐web properties based on consumer stable isotope data (δ13C and δ15N) showed that increasing sediment disturbance was associated with reduced trophic diversity. In particular, the aquatic invertebrate community occupied a smaller area in isotopic niche space along the sedimentation gradient that was best explained by a narrowing of the invertebrate community δ13C range. Decreased niche partitioning, driven by increasing habitat homogeneity, environmental filtering and resource scarcity all seemingly lead to greater trophic equivalency caused by the collapse of the autochthonous food‐web channel. Bayesian mixing‐model analyses supported this contention with invertebrate consumers increasingly reliant on detritus along the sedimentation gradient, and predatory invertebrates relying more on the prey using these basal resources. The narrowing of the fish community δ13C range along the sedimentation gradient contributed to an apparent ‘trophic shift’ towards terrestrial carbon, further indicating the loss of the autochthonous food‐web channel. On the vertical trophic niche axis, fish became increasingly separated from aquatic invertebrates with an increase in their estimated trophic position. In combination, these responses were most likely mediated through reduced fish densities and a diminished reliance on aquatic prey. Although species losses remain a major threat to ecosystem integrity, the functional roles of biota that persist dictate how food webs and ecosystem functioning respond to environmental change. Sedimentation was associated with nonlinear reductions in trophic diversity which could affect the functioning and stability of aquatic ecosystems. Our study helps explain how multiple mechanisms may radically reshape food‐web properties in response to this type of disturbance. How ecological networks respond to perturbations remains poorly understood. The authors used stable isotopes to describe aquatic food webs along a landscape disturbance gradient. Increasing sedimentation was associated with reduced trophic diversity, seemingly driven by the collapse of the autochthonous food‐web channel. These changes may affect food‐web stability and ecosystem functioning.

Streams affected by acid mine drainage (AMD) are highly stressed ecosystems and occur worldwide. These streams typically have low pH, high concentrations of dissolved metals, and substrata coated with metal hydroxide precipitates. This combination of chemical and physical stressors creates a challenging environment for aquatic biota. We provide a synthesis of the effects of AMD on stream food webs to provide a holistic perspective of these highly stressed ecosystems. First, we reviewed the effects of AMD on the structure and function of algal, microbial, invertebrate, and fish communities. Then, we used this published information to propose generalized food webs and identify areas for future research. In general, AMD-affected streams have depauperate communities that are dominated by a few tolerant species, and ecosystem processes (e.g., decomposition) are often impaired. Biota respond differently to the individual stressors (e.g., pH compared to precipitates), which may complicate remediation efforts that focus primarily on neutralizing acidity and removing metals from mine discharges. Food webs in these streams are substantially altered because basal resources are less productive or inaccessible, microbial processing of organic matter is slow, many grazers and shredders are absent, and fish are replaced by invertebrates as top predators. Structurally, declines in species diversity and the loss of fish shorten and simplify food webs by decreasing the number of interactions among species. Functionally, most energy pathways are weakened by disrupted trophic links, and this problem should to be a key target of restoration efforts. We think research that focuses on species interactions in a foodweb context is needed to provide a better understanding of community organization and functioning in these highly stressed ecosystems.

Understanding what makes food webs stable has long been a goal of ecologists. Topological structure and the distribution and magnitude of interaction strengths in food webs have been shown to confer important stabilizing properties. However, our understanding of how variable species interactions affect food-web structure and stability is still in its infancy. Anthropogenic stress, such as acid mine drainage, is likely to place severe limitations on the food-web structures availabe, due to changes in community composition and body mass distributions. Here, we used mechanistic models to infer food-web structure and quantify stability in streams across a gradient of acid mine drainage. Multiple food webs were iterated for each community based on species pairwise interaction probabilities, in order to incorporate the variability of realistic food-web structure. We found that food-web structure was altered systematically with a 32-fold decrease in the number of links and a twofold increase in connectance across the gradient. Stability generally increased sixfold with increasing acid mine drainage stress, regardless of how interaction strengths were estimated. However, the distribution of the stability measure, s, for some impacted communities separated into clusters of higher and lower magnitude depending on how interaction strengths were estimated. Management and restoration of impacted sites needs to consider their increased stability, as this may have important implications for the recolonization of desirable species. Furthermore, active species introductions may be required to overcome the internal ecological inertia of affected communities.

The biological assessment of rivers i.e., their assessment through use of aquatic assemblages, integrates the effects of multiple-stressors on these systems over time and is essential to evaluate ecosystem condition and establish recovery measures. It has been undertaken in many countries since the 1990s, but not globally. And where national or multi-national monitoring networks have gathered large amounts of data, the poor water body classifications have not necessarily resulted in the rehabilitation of rivers. Thus, here we aimed to identify major gaps in the biological assessment and rehabilitation of rivers worldwide by focusing on the best examples in Asia, Europe, Oceania, and North, Central, and South America. Our study showed that it is not possible so far to draw a world map of the ecological quality of rivers. Biological assessment of rivers and streams is only implemented officially nation-wide and regularly in the European Union, Japan, Republic of Korea, South Africa, and the USA. In Australia, Canada, China, New Zealand, and Singapore it has been implemented officially at the state/province level (in some cases using common protocols) or in major catchments or even only once at the national level to define reference conditions (Australia). In other cases, biological monitoring is driven by a specific problem, impact assessments, water licenses, or the need to rehabilitate a river or a river section (as in Brazil, South Korea, China, Canada, Japan, Australia). In some countries monitoring programs have only been explored by research teams mostly at the catchment or local level (e.g., Brazil, Mexico, Chile, China, India, Malaysia, Thailand, Vietnam) or implemented by citizen science groups (e.g., Southern Africa, Gambia, East Africa, Australia, Brazil, Canada). The existing large-extent assessments show a striking loss of biodiversity in the last 2–3 decades in Japanese and New Zealand rivers (e.g., 42% and 70% of fish species threatened or endangered, respectively). A poor condition (below Good condition) exists in 25% of South Korean rivers, half of the European water bodies, and 44% of USA rivers, while in Australia 30% of the reaches sampled were significantly impaired in 2006. Regarding river rehabilitation, the greatest implementation has occurred in North America, Australia, Northern Europe, Japan, Singapore, and the Republic of Korea. Most rehabilitation measures have been related to improving water quality and river connectivity for fish or the improvement of riparian vegetation. The limited extent of most rehabilitation measures (i.e., not considering the entire catchment) often constrains the improvement of biological condition. Yet, many rehabilitation projects also lack pre-and/or post-monitoring of ecological condition, which prevents assessing the success and shortcomings of the recovery measures. Economic constraints are the most cited limitation for implementing monitoring programs and rehabilitation actions, followed by technical limitations, limited knowledge of the fauna and flora and their life-history traits (especially in Africa, South America and Mexico), and poor awareness by decision-makers. On the other hand, citizen involvement is recognized as key to the success and sustainability of rehabilitation projects. Thus, establishing rehabilitation needs, defining clear goals, tracking progress towards achieving them, and involving local populations and stakeholders are key recommendations for rehabilitation projects (Table 1). Large-extent and long-term monitoring programs are also essential to provide a realistic overview of the condition of rivers worldwide. Soon, the use of DNA biological samples and eDNA to investigate aquatic diversity could contribute to reducing costs and thus increase monitoring efforts and a more complete assessment of biodiversity. Finally, we propose developing transcontinental teams to elaborate and improve technical guidelines for implementing biological monitoring programs and river rehabilitation and establishing common Water 2021, 13, 371 3 of 45 financial and technical frameworks for managing international catchments. We also recommend providing such expert teams through the United Nations Environment Program to aid the extension of biomonitoring, bioassessment, and river rehabilitation knowledge globally.

Nitrate pollution is a global issue threatening the health and function of many lowland freshwater ecosystems. Quantifying nitrate loads and instream attenuation associated with land use is a critical requirement for improving freshwater management. One often overlooked nitrate source in catchments is invasive N-fixing trees such as Ulex europaeus (European gorse). This study compared nitrate concentrations in conjunction with stable isotope analyses (nitrate δ¹⁵N and δ¹⁸O) to investigate the effects of catchment gorse cover on stream nitrate export relative to three other land uses. These were regenerating native forest, low-intensity (dry-stock) and high-intensity (dairying) agriculture. We tested two hypotheses: (1) gorse is a regionally significant nitrate source; (2) instream nitrate attenuation is land-use dependent. The study was conducted in 24 reaches across six small, mixed land-use coastal catchments located on Banks Peninsula, New Zealand. Our results demonstrated that gorse-dominated stream reaches had significantly higher nitrate concentrations than all other land uses. Within the gorse-dominated reaches, nitrate concentration was significantly correlated with upstream catchment gorse cover. Furthermore, nitrate oxygen and nitrogen stable isotope compositions demonstrated that elevated nitrate concentrations in gorse streams were associated with decomposition of dead gorse foliage. The isotope data revealed sub-catchment-scale land-use-specific patterns of nitrate attenuation within streams. All three anthropogenic land uses (gorse, dry stock and dairy) showed distinctly different N-cycling from native-forested reaches where nitrate was efficiently cycled with evidence for highly localised nitrification. Stable isotope data demonstrated that overall nitrate attenuation became less efficient with higher nitrate loads. Our research demonstrates the significant impact N-fixing plants have on nitrate concentrations and instream attenuation. Quantifying the effects of N-fixing plants on water quality is an important step in achieving globally significant goals of sustaining ecosystem health and (sub) catchmentscale nutrient management.