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Abstract Uptake and regeneration fluxes and concentrations of nutrients, i.e., nitrate (NO3−), ammonium (NH4+), phosphate (PO43−) and dissolved organic carbon (DOC), were evaluated upstream and downstream of a wastewater treatment plant (WWTP) in the River Wandle, UK, from July to October 2019. Using chamber techniques, water-specific nutrient concentrations were measured at two exposures (3 and 10 min) to calculate fluxes. The WWTP effluent contributed to elevated concentrations and modified flux rates, resulting in significant differences at the study sites. Compared with summer, the concentrations of NO3− and DOC increased while NH4+ and PO43− decreased in autumn. Nutrient fluxes varied both temporally and spatially in uptake (i.e., storage in sediments) or regeneration (i.e., release into river water). Under the actions of physical and biological processes, the fluxes of NO3− and NH4+ showed opposite flux directions. Dissolved oxygen (DO) and bioabsorption mainly affected PO43− and DOC fluxes, respectively. Specifically, across all sites, NO3− was −0.01 to +0.02 mg/(m2 s), NH4+ was −29 to +2 μg/(m2 s), PO43− was −2.0 to +0.5 μg/(m2 s), and DOC was −0.01 to +0.05 mg/(m2 s). Further, we did find that these variations were related to nutrient concentrations in the overlying water. Our results provide further evidence to show that reductions in river nutrients are paramount for improving river ecological conditions. Additionally, we suggest that more research is needed to evaluate chamber-based experimental approaches to make them more comparable to in-situ flux methods. Highlights • Sewage effluent resulted in elevated nutrient concentrations and modified fluxes. • Flux was affected by initial nutrient concentrations, DO and microbial activity. • Inexpensive approaches to study nutrient dynamics are needed for river restoration.

We present an approach that uses satellite products to derive models for predicting lake chlorophyll from environmental variables, and for investigating impacts of changing environmental flows. Lake Turkana, Kenya, is the world's largest desert lake, and environmental flows from the Omo River have been modified since 2015 by the Gibe III dam in Ethiopia. Using satellite remote sensing, we have evaluated the influence of these altered hydrological patterns on large-scale lake phytoplankton concentrations for the first time. Prior to dam completion, strong seasonal cycles and large spatial gradients in chlorophyll have been observed, related to natural fluctuations in the Omo River's seasonal discharge. During this period, mean lake chlorophyll showed a strong relationship with both river inflows and lake levels. Empirical models were derived which considered multiple hydro-climatic drivers, but the best model for predicting chlorophyll-a was a simple model based on Omo River discharge. Application of this model to data for 2015–2016 estimated that during the filling of Gibe III annual mean Lake Turkana chlorophyll declined by 30%. Future water management scenarios based on Gibe III operations predict reduced seasonal chlorophyll-a variability, while irrigation scenarios showed marked declines in chlorophyll-a depending on the level of abstraction. These changes demonstrate how infrastructure developments such as dams can significantly alter lake primary production. Our remote sensing approach is easy to adapt to other lakes to understand how their phytoplankton dynamics may be affected by water management scenarios.

Electrofishing is a well‐established and widely used method for surveying fish populations. Nonetheless, its effectiveness is impacted by numerous factors, including water chemistry, habitat type and fish species. Both physiological and behavioural responses make bottom‐dwelling ‘benthic’ fish which lack swim bladders (e.g. European bullhead Cottus gobio) particularly difficult to survey by electrofishing. We compare the performance and practicalities of electrofishing for benthic fish at a rocky northern English headwater stream with two sampling methods originally designed for crayfish surveys; the triple drawdown method which involves repeated dewatering of a site, and the Pritchard Trap method which involves sunken traps filled with natural substrate that samples a small, fixed (0.25 m2) area of river bed. Both the Pritchard trapping and triple drawdown methods provided similar high‐density population density estimates for bullhead which were at least 2.5–5 times higher than predicted from electrofishing derived sweep depletion curves. Electrofishing and the triple drawdown method are both resource‐intensive, requiring expensive equipment and a team of trained operatives. These approaches also pose a risk to fish and non‐target organisms. In contrast, Pritchard Traps provide a cost‐effective passive, low risk survey method requiring minimal training and only one operative. Pritchard traps, therefore, show particular promise for benthic fish surveying and monitoring.

Geomorphological alterations, hydrological disconnectivity and water pollution are among the dominant pressures affecting ecological integrity in urban streams. River restoration approaches often involve utilising in-stream structures to encourage flow heterogeneity and promote habitat diversity. However, few studies examine the success of such projects. In this study, fish density, biomass and community structure at paired restored and unrestored reaches across five tributaries of the River Thames were examined. Fish density varied among rivers and reaches but was generally higher at restored sites. Restored sites also exhibited higher overall fish biomass, attributed mainly to the presence of brown trout (Salmo trutta L.) at the River Wandle. Despite higher density and biomass values at restored sites, the community structure analysis did not identify strong links between either river or restoration status using either species-specific density or biomass. Our results highlight that although reach-scale restoration can lead to localised increases in species density and biomass, this may chiefly be due to aggregation owing to preferential habitats created through restoration activities at these sites. Over larger spatial scales, significant improvements to species richness and diversity are likely to be limited due to the poor water quality and disconnected nature of these urban streams. Whilst reach-scale restoration clearly has the potential to provide preferential habitats for fish species, future efforts should focus on improving connectivity for fish across the wider Thames basin network by removing barriers to passage, improving water quality, restoring watershed processes and creating well-connected, diverse habitats which can facilitate the survival of a wide array of fish species throughout their life cycle.

The River Thames through central London has seen significant environmental recovery, particularly in regard to water quality, but a substantial barrier to further ecological improvement is the spatial restriction of riparian areas. Flood defence walls represent a potential habitat for ecological improvements to benefit biodiversity. However before restoration actions are considered, an understanding of the current biodiversity and ecological status of these structures is necessary. Physical habitat and macroinvertebrate richness of flood defence walls at 15, evenly spaced sites along a 32 Km reach in central London were evaluated. We found no longitudinal patterns in total macroinvertebrate richness among our 15 sites, but did find that richness was influenced by wall type. Specifically, we found the highest richness on brick walls and lowest richness on concrete walls. Further, wall sections with algal cover supported significantly higher numbers of macroinvertebrates than sections lacking algal cover. However, reaches of the river where the channel was constricted had fewer macroinvertebrates likely due to scouring flows. These results show that macroinvertebrates of the river walls are influenced by habitat availability (i.e., wall type) and localised river flows. This work suggests that appropriate management of river wall habitats has the potential to enhance the biodiversity of highly modified, urban rivers.

The effects of urbanization on hydrology, water quality and macroinvertebrates were examined in 7 headwater tributaries of the St. Johns River in the Atlantic Coastal Plain of northeast Florida. All streams had sandy substrata and drained small catchments (24–231 ha) that ranged from 0 to 51% total impervious area (TIA). Streams unaffected by urbanization had intermittent stream flow and completely dry channels for several weeks in autumn and spring. Urbanized streams always possessed channel water, but 2 streams ceased flowing and became stagnant in autumn and spring. Principle components analysis of chemical and physical measures (i.e. conductivity, nutrients, pH, metals, and stream flow) produced one axis (PCA1) that explained 54% of the total variation among the streams. The variables that loaded negatively on this axis were associated with low flows, while the variables that loaded positively were associated with urban land-use. PCA1 was also positively associated with %TIA. Macroinvertebrate richness ranged from 27 to 45 taxa and was positively associated with %TIA. Macroinvertebrate biomass ranged from 3 to 45 g AFDM/m2 and showed a significant, exponential relationship with PCA1 (r2 = 0.93) with greatest biomass occurring at intermediate %TIA. Invertebrate community structure in the urbanized streams appeared to be mainly influenced by hydrologic factors (perennial vs. intermittent flow regimes). The effects of urbanization on both hydrological and biological variables among the study streams were apparent, but also influenced by site-specific conditions.

Input, storage, export potential, and system-level processing of coarse organic matter were investigated in the intermittent streams that drain the Bear Brook Watershed in Maine (BBWM). BBWM is a paired catchment study investigating ecosystem effects of atmospheric N and S deposition. We predicted that the increased N loading to the treatment catchment would elevate input of organic matter, result in higher levels of coarse organic matter biomass, and increase litter processing rates in the treatment stream relative to the reference stream. We found that the streams draining BBWM did not have statistically different coarse organic matter input, biomass, or processing rates and we found only modest differences in export potential. System-level processing rates for maple (Acer spp.) litter were similar to rates previously quantified using litterbag methods. However, system-level processing rates for American beech (Fagus grandifolia) litter were an order of magnitude faster than rates measured with litterbags. This difference was likely due to movements of these leaves from riffle/runs and pools into debris dams, rather than differences in measurements of leaf tissue processing rates between methods. Organic matter dynamics of the intermittent streams at BBWM were similar to other forested, headwater streams. Our results indicate that the long-term N manipulation experiment at BBWM has not altered input, storage or processing of coarse organic matter in the treatment stream. Physical characteristics of these stream ecosystems appear to regulate organic matter dynamics rather than differences in nutrient chemistry.

We assessed the effect of atmospheric nitrogen (N) deposition on secondary production in a first-order intermittent stream at the Bear Brook Watershed in Maine (BBWM). BBWM is a paired-catchment experiment designed to determine the effects of N deposition on a forested ecosystem. Nitrogen as (NH4)2SO4 has been applied to the treatment catchment since 1989, with the adjacent catchment serving as a reference (both are drained by a firstorder intermittent stream). Secondary production and detritus biomass were quantified for 2 yr. Production did not vary between streams (range 5 1.7‐2.3 g ash-free dry mass m 22 yr 21 ) but was ;35% higher for both streams in the second year. The distribution of production among functional feeding groups varied little between streams but differed among taxa. Detritus biomass was similar between streams but was ;25% higher in the second year. The (NH4)2SO4 treatment had no effect on production in the treatment stream. The statistically identical level of production between streams is presumably because of similar habitat, channel drying, and trophic resources. Difference in production between years was positively related to detritus biomass. Differences in the distribution of production among taxa between streams were likely the result of contrasts in the duration of flowing water, with the treatment stream having the longer duration. Our study indicates that patterns of litter input and channel drying, rather than N deposition, control levels of secondary production in these intermittent streams by altering both resource availability and community structure. These variables apparently override the effects of N deposition in regions where nitrogen is not a limiting nutrient.

The Bear Brook Watershed in Maine (BBWM) is a paired catchment study investigating ecosystem effects of N and S deposition. Because of the decade long (NH4)2SO4 addition, the treatment catchment has higher stream NO3− and enriched foliar N concentrations compared to the reference catchment. We investigated how both stream N and foliar N affect stream detritus processing. Differences in litter processing were assessed by measuring mass loss, tissue softness, and shredder biomass. To examine both stream effects and leaf source, N-enriched and reference litter bags were prepared for 3 leaf species and placed in each stream. Red maple leaves were examined in 1997, 1998, and 1999. Sugar maple and American beech were examined in 1999. In all years, the only stream effects were increased mass loss for sugar maple and higher shredder biomass for red maple in 1998 in the treatment stream. Several leaf source effects were observed. N-enriched leaves of sugar maple, American beech, and red maple in 1998 had ...

Secondary production of the burrowing mayfly, Hexagenia limbata, was quantified from four sites differing in seasonal salinity within the Lower Mobile River, Alabama, from October 1995 to September 1996. This population was univoltine, with emergence occurring from late May through early August. Comparisons with other populations of this species showed latitudinal trends suggesting that summer temperatures may exceed an upper thermal threshold for growth. Longitudinal differences in riverine salinity (i.e., upriver sites, 0‰; downriver sites, 5.5‰ maximum salinity) explained most of the differences among sites, both for average density (upriver sites, 75.6 mayflies m 22

The Bear Brook Watershed in Maine (BBWM) is a long-term, paired watershed experiment that addresses the effects of acid and nitrogen (N) deposition on whole watersheds. To examine stream response at BBWM, we synthesized data on organic matter dynamics, including leaf breakdown rates, organic matter inputs and standing stocks, macroinvertebrate secondary production, and nutrient uptake in treated and reference streams at the BBWM. While N concentrations in stream water and leaves have increased, the input, standing stocks, and breakdown rates of leaves, as well as macroinvertebrate production, were not responsive to acid and N deposition. Both chronic and acute increases of N availability have saturated uptake of nitrate in the streams. Recent experimental increases in phosphorus (P) availability enhanced stream capacity to take up nitrate and altered the character of N saturation. These results show how the interactive effects of multiple factors, including environmental flow regime, acidification, and P availability, may constrain stream response to chronic N deposition.

The need for sulfur and nitrogen emission abatement in Europe has been established, and several strategies have been used or conceived in order to accomplish this. Strategies have been based on flat rate reductions (“30 per cent club”), varying percentage reductions for different countries or by the implementation of best available technology. Much effort has recently concentrated on the derivation of ecologically based target deposition levels worked out from critical load estimates for ecosystems. These targets are goals and may be used in cost-effective strategies which abate emissions with reference to the sensitivity of the receptor regions. The relative sensitivity of ecosystems to the acidic deposition has been worked out by dividing a limited number of variables controlling site sensitivity (bedrock lithology, soil type, land use, rainfall) into a small number of categories and combining them using weights. The resulting distribution of five relative sensitivity classes in Europe is shown. With reference to the critical load estimates for specific sites, targets have been applied to the relative sensitivity classes. 20 keq H+ km−2yr−1 is the target applied to the most sensitive regions. Due to complex nitrogen cycling within the ecosystem, modifications need to be carried out in order to incorporate the acidifying effect of nitrogen deposition. It is necessary to work out the degree to which deposited nitrate leaches from sites and a method is suggested using estimates of plant uptake and site factors affecting drainage, mineralization, nitrification and denitrification.