Amongst a spectrum of benefits, Nature-based Solutions (NBS) are increasingly being advocated as improving the quality of aquatic environments in urban areas. Of these, a widely adopted measure is tree planting. Yet, because of the local complexities and spatial variability of urban hydrological response, it is difficult to predict to what extent improvements in water quality will arise. To overcome this barrier, a standardised approach to processbased model simulation of urban river quality is described (QUESTOR-YARDSTICK (QUESTOR-YS)). The approach eliminates the influence of point sources of pollution and harmonises the way in which river hydrodynamics and contributory catchment size are represented. Thereby, it focuses on differences in water quality between cities due solely to climate, river discharge and urban diffuse nutrient pollution factors. The relative sensitivity to NBS establishment between urban water bodies in different cities anywhere across the world can also potentially be quantified. The method can be readily extended to include wastewater effluents. The validity of the approach is demonstrated for a small river in Birmingham, UK; and thence demonstrated for the case of 10 km of riparian tree planting in Birmingham, Oslo (Norway) and Aarhus (Denmark). Modelling suggests that riparian tree planting can substantially improve water quality in each example city for three key indicators of water quality in sensitive summer conditions (water temperature, chlorophyll-a and dissolved oxygen). Results show the level of benefit achievable in response to a fixed amount of planting will depend on the existing level of riparian tree occupancy.

Riparian tree canopies are key components of river systems, and influence the provision of many essential ecosystem services. Their management provides the potential for substantial control of the downstream persistence of pollutants. The recent advent of new advances in mass spectrometry to detect a large suite of emerging contaminants, high-frequency observations of water quality and gas exchange (e.g., aquatic eddy covariance), and improved spatial resolution in remote sensing (e.g., hyperspectral measurements and high-resolution imagery), presents new opportunities to understand and more comprehensively quantify the role of riparian canopies as Nature-based Solutions. The paper outlines how we may now couple these advances in observational technologies with developments in water quality modelling to integrate simulation of eutrophication impacts with organic matter dynamics and fate of synthetic toxic compounds. In particular regarding solar radiation drivers, this enables us to scale-up new knowledge of canopy-mediated photodegradation processes at a basin level, and integrate it with ongoing improvements in understanding of thermal control, eutrophication, and ecosystem metabolism.

There remains a persistent concern that freshwater biodiversity is in decline and being threatened by pollution. As the UK, and particularly England, is a densely populated nation with rivers of modest dilution capacity, this location is very suitable to examine how freshwater biodiversity has responded to human pressures over the past 30 years. A long-term dataset of 223,325 freshwater macroinvertebrate records from 1989 to 2018 for England was retrieved and examined. A sub-set of approximately 200 sites per English Region (1515 sites in total with 62,514 samples), with the longest and most consistent records were matched with predicted wastewater exposure, upstream land cover and terrain characteristics (latitude, altitude, slope gradient and flow discharge). To understand changes in macroinvertebrate diversity and sensitivity with respect to these parameters, the biotic indices of (i) overall family richness, (ii) Ephemeroptera, Plecoptera, Trichoptera (EPT) family richness, and (iii) the Biological Monitoring Working Party (BMWP) scores of NTAXA (number of scoring taxa) and (iv) ASPT (average score per taxon) were selected. A review of how close the BMWP scores come to those expected at minimally impacted reference sites was included. For all latitudes, altitudes, channel slope, river size, wastewater exposure levels, and differing proportions of upstream woodland, seminatural, arable and urban land cover, all diversity or sensitivity indices examined improved over this period, although this improvement has slowed in some cases post 2003. Mean overall family richness has increased from 15 to 25 family groups, a 66 % improvement. The improvement in mean EPT family richness (3 to 10 families, >300 % improvement), which are considered to be particularly sensitive to pollution, implies macroinvertebrate diversity has benefited from a national improvement in critical components of water quality.

Environmental change accelerates biodiversity loss, especially in vulnerable freshwater ecosystems. Aquatic ecosystems are exposed to multiple stressors. Hereby, it is valuable to understand how impacts of multiple stressors are translated into changes of aquatic biodiversity. Phytoplankton as a key biological element in the aquatic ecosystem, its species richness is an important pointer to interpret the ecological respond under environmental alterations. In this study, we evaluate the responses of riverine phytoplankton species richness to multiple stressors by an interdisciplinary modelling approach. Firstly, a random forest model was developed for ranking the stressors’ importance, and then generalized addictive mixed models were established for quantifying and simulating the potential response of phytoplankton biodiversity evoked by the main stressors. The share of forest and pasture areas had an outstanding explanation to the variation of riverine phytoplankton species richness. Hereby, forest share had a positive effect while pasture affected species richness in a negative way. Our result also enlightened the important influence of the flow regime. Specifically, in deriving indicators of hydrological alteration of short time scale, their close linkage with the phytoplankton biodiversity pattern was shown. Further, our findings highlighted the spatial variability of independent effects of land cover and hydrological alteration impacts on the phytoplankton community structures among different subbasins. Our integrated models’ framework was shown to be beneficial to deal with complex environmental condition analysis and could be applied in river basin management. As a recommendation we would emphasize the significance of preservation of forest area in protecting the aquatic algal biodiversity as key point in maintaining the functioning of river ecosystems.

Urban Green Infrastructure (GI) provides multiple benefits to city inhabitants and can be an important component in nature-based solutions (NBS), but the ecosystem services that underpin those benefits are inconsistently quantified in the literature. There remain substantial knowledge gaps about the level of service supported by less studied GI types, e.g. cemeteries, or less-studied ecosystem services, e.g. noise mitigation. Decision-makers and planners in cities often face conflicting or incomplete information on the effectiveness of GI, particularly on their ability to provide a suite of co-benefits. Here, we describe a feature-based typology of GI which combines elements of land cover, land use and both ecological and social function. It is consistent with user requirements on mapping, and with the needs of models which can conduct more detailed ecosystem service assessments which can guide NBS design. We provide an evidence synthesis based on published literature, which scores the ability of each GI type to deliver a suite of ecosystem services. In the multivariate analysis of the typology scores, the main axis of variation differentiates between constructed (or hybrid) GI types designed primarily for water flow management (delivering relatively few services) and more natural green GI with trees, or blue GI such as lakes and the sea, which deliver a more multi-functional set of regulating services. The most multi-functional GI on this axis also score highest for biodiversity. The second element of variation separates those GI which support very few cultural services and those which score highly in enabling physical wellbeing and social interaction and, to a lesser extent, restoring capacities. Together the typology and multi-functionality matrix provide a much needed assessment for less studied GI types, and allow planners and decision-makers to make a-priori assessments of the relative ability of different GI as part of NBS to address urban challenges.

With the growing demand of assessing the ecological status, there is the need to fully understand the relationship between the planktic diversity and the environmental factors. Species richness and Shannon index have been widely used to describe the biodiversity of a community. Besides, we introduced the first ordination value from non-metric multidimensional scaling (NMDS) as a new index to represent the community similarity variance. In this study, we hypothesized that the variation of diatom community in rivers in an agricultural area was influenced by hydro-chemical variables. We collected daily mixed water samples using ISCO auto water samplers for diatoms and for water-chemistry analysis at the outlet of a lowland river for a consecutive year. An integrated modeling was adopted including random forest (RF) to decide the importance of the environmental factors influencing diatoms, generalized linear models (GLMs) combined with 10-folder cross validation to analyze and predict the diatom variation. The hierarchical analysis highlighted antecedent precipitation index (API) as the controlling hydrological variable while water temperature, Si2+ and PO4-P as the main chemical controlling factors in our study area. The generalized linear models performed better prediction for Shannon index (R2 = 0.44) and NMDS (R2 = 0.51) than diatom abundance (R2 = 0.25) and species richness (R2 = 0.25). Our findings confirmed that Shannon index and the NMDS as an index showed good performance in explaining the relationship between stream biota and its environmental factors and in predicting the diatom community development based on the hydro-chemical predictors. Our study showed and highlighted the important hydro-chemical factors in the agricultural rivers, which could contribute to the further understanding of predicting diatom community development and could be implemented in the future water management protocol.

Given the many threats to freshwater biodiversity, we need to be able to resolve which of the multiple stressors present in rivers are most important in driving change. Phytoplankton are a key component of the aquatic ecosystem, their abundance, species richness and functional richness are important indicators of ecosystem health. In this study, spatial variables, physiochemical conditions, water flow alterations and land use patterns were considered as the joint stressors from a lowland rural catchment. A modeling approach combining an ecohydrological model with machine learning was applied. The results implied that land use and flow regime, rather than nutrients, were most important in explaining differences in the phytoplankton community. In particular, the percentage of water body area and medium level residential urban area were key to driving the rising phytoplankton abundance in this rural catchment. The proportion of forest and pasture area were the leading factors controlling the variations of species richness. In this case deciduous forest cover affected the species richness in a positive way, while, pasture share had a negative effect. Indicators of hydrological alteration were found to be the best predictors for the differences in functional richness. This integrated model framework was found to be suitable for analysis of complex environmental conditions in river basin management. A key message would be the significance of forest area preservation and ecohydrological restoration in maintaining both phytoplankton richness and their functional role in river ecosystems.

Water quality in lowland rivers is sensitive to changes in flow during summer dry periods, when high temperatures and low pollutant dilution are problematic and may reduce oxygen concentrations to levels of ecological concern. A 10-year period of monitoring data was collated for a typical small lowland UK river. Two hourly-resolution applications of a process-based water quality model (QUESTOR) were made, with and without local knowledge, to establish whether specific information on stream channel hydraulics is an essential precursor to successful simulation. Results showed this information to be necessary, with considerably better goodness-of-fit statistics obtained when the local knowledge was used. In this regard, mean improvements in Nash-Sutcliffe Efficiency across all monitoring sites were from -0.33 to 0.18 and from 0.24 to 0.78 for dissolved oxygen and water temperature respectively. Percent bias was within 10% for the local model. The 10-year record also allowed a detailed characterisation of how changes in flow, as described by a comprehensive range of Indicators of Hydrological Alteration, relate to the water quality determinants. Analysis revealed these dynamics were also captured more realistically when the model was driven by local knowledge. The research concludes that river dissolved oxygen simulations driven by national-level information are of some value as screening tools, but model refinement supported by sufficient provision of local information is necessary when detailed simulations are required to support specific decision-making.

Agrochemicals such as pesticides and nutrients are concurrent chemical stressors in freshwater aquatic ecosystems surrounded by agricultural areas. Lentic small water bodies (LSWB) are ecologically significant habitats especially for maintaining biodiversity but highly understudied. Phytoplankton are ideal indicator species for stress responses. Functional features of the phytoplankton are important in revealing the processes that determine the structure of the communities. In this study, we investigated the effects of pesticides, nutrients, and local environmental variables on the species composition and functional features of phytoplankton communities in LSWB. We studied pesticide toxicity of ninety-four pesticides, three nutrients (NH4-N, NO3-N and PO4-P) and local environment variables (precipitation, water level change, temperature, dissolved oxygen concentration, electrical conductivity, pH) in five LSWB over twelve weeks during the spring pesticide application period. We explored respective changes in species composition of phytoplankton community and functional features. Redundancy analysis and variance partitioning analysis were applied to correlate phytoplankton community compositions with the pesticide toxicity (as maximum toxicity in toxic units), nutrients and local environment variables. We used multiple linear regression models to identify the main environmental variables driving the functional features of phytoplankton communities. Pesticide toxicity, nutrients and local environmental variables significantly (p < 0.001) contributed to shaping phytoplankton community composition individually. Local environment variables showed the highest pure contribution for driving phytoplankton composition (12%), followed by nutrients (8%) and pesticide toxicity (2%). Functional features (represented by functional diversity and functional redundancy) of the phytoplankton community were significantly affected by pesticide toxicity and nutrients concentrations. The functional richness and functional evenness were negatively affected by PO4-P concentrations. Pesticide toxicity was positively correlated with functional redundancy indices. Our findings emphasized the relative importance of concurrent multiple stressors (e.g., pesticides and nutrients) on phytoplankton community structure, directing potential effects on metacommunity structures in aquatic ecosystems subjected to agricultural runoff.

Rivers and related freshwater ecosystems are facing increasing natural disturbance and anthropogenic stressors. Understanding the key ecological processes that govern the riverine biota in aquatic ecosystems under multiple pressures has crucial importance. However, there is still insufficient knowledge in quantifying of stressors interactions. Moreover, the understanding of the responses of riverine phytoplankton to multiple stressors is still scarce from catchment aspect. As an interdisciplinary study, the catchment hydrological processes were linked to ecological responses in this study, and we chose phytoplankton functional groups (PFGs) instead of taxonomic classifications of algae to examine their responses to land-use pattern (L), hydrological regime (H), and physicochemical condition (P) across two contrasting hydrological periods (dry, wet). The traits-based phytoplankton functional groups are highly suggested as robust bio-indicators for better understanding the current ecological status. The hydrological regime was described by a matrix indices of hydrological alteration based on the outputs of a well-established ecohydrological model (SWAT). The results from variation partitioning analysis showed that P and H dominate during the dry period and P in high flows. Structural equation models (SEM) showed that the skewness of 7 days discharge emerged as a key driver of H, and had always an indirect effect on functional group TB (benthic diatoms) during both hydrological periods. The functional group M (mainly composed by Microcystis) has directly related to phosphorous in both periods, while indirectly to L of urban area in high flow period, and water bodies in low flow period. This study emphasized that climate change and anthropogenic activities such as altering flow regime and land-use pattern affect directly or indirectly riverine phytoplankton via physicochemical conditions. In addition, our findings highlighted that biomonitoring activities require detailed investigation in different hydrological periods. SEM is recommended for improved understanding of phytoplankton responses to the changing environment, and for future studies to fulfill the increasing demand for sustainable watershed management regarding aquatic biota.

To understand the lowland riverine phytoplankton community (of species and functional groups), and to investigate the effects of abiotic factors on phytoplankton variance during the high-flow season, 59 sites of the Treene catchment, northern Germany, were visited in December 2014, where 327 species belonging to six taxonomic groups and 21 phytoplankton functional groups were observed. Bacillariophyta, especially benthic pennales (corresponding to the functional group TB), were dominant in most of the study area. Cyanobacteria (specifically indicated by the functional groups M and S1), which are typically found in lake environments, were also found in the main stream of the River Treene. Furthermore, Euglenophyta (represented by the functional groups W1 and W2) showed a higher percentage in the headwaters. A redundancy analysis indicates that the land-use pattern and physiochemical condition co-contributed to the phytoplankton community variance. The total explained variance (68.4 %) of phytoplankton data was partitioned into three parts: purely physiochemical factors contributed to 24.1 % of the explanation, followed by land-use characteristics (15.6 %), and their shared effects (28.7 %). The functional groups W1 and W2 exhibited a strong positive correlation to the percentage of agricultural land use. The percentages of urban land use and phosphate concentration in the stream were interrelated with the functional groups Lo and M, which indicates mesotrophic to eutrophic water conditions. The unexplained variance (31.6 %) may result from hydrological regimes, which will be taken into account in subsequent studies.

The importance of phytoplankton-based bio-assessment has been recently recognized in lowland rivers which are affected by multi-environmental factors. However, some basic questions remain unclear to date, such as: (i) spatial and temporal variations of phytoplankton, (ii) the impact of upstream lakes on downstream community, (iii) the main drivers for species composition or (iv) the regional biodiversity along a lentic-lotic continuum. To answer these questions, we collected and analyzed the fluvial phytoplankton communities along a lentic-lotic continuum from a German lowland catchment, where a well-established ecohydrological modeling predicted long-term discharges at each sampling site. Our results revealed very high spatial and temporal variations of phytoplankton community. The changes of a lake on downstream phytoplankton assemblages were significant, especially the nearest reach after the lake. However, these influences varied along with seasons and limited in a relatively short distance to the lake. Redundancy analysis and Mantel tests showed that phytoplankton composition and dissimilarities along the lentic-lotic continuum attributed more to local hydrological and physicochemical variables than species dispersal, which confirmed the suitability of lowland phytoplankton-based bioassessment. In addition, our findings highlighted the importance of flow regime in shaping phytoplankton community composition and regional beta diversities. This study emphasized the necessity to include the hydrological variables and their relationship with phytoplankton community in future bio-monitoring investigations.

The toxicity of cyanotoxins on plant has been reported. However, in eutrophic waters harmful cyanobacteria are associated with other environmental pollutants, such as persistent organic pollutants (POPs) and metals. Information on the phytotoxicity and bioaccumulation of coexisted cyanotoxins and these environmental pollutants is still lacking. In this study, the combined phytotoxicities of three types of cyanobacteria-associated pollutants, i.e., microcystin-LR (MC-LR), cadmium (Cd), 2, 4, 4′-Trichlorobiphenyl (PCB-28) were systematically investigated. After 7-days exposure, strong synergistic effects can be detected when Arabidopsis thaliana seeds and seedlings exposed to binary mixtures of MC-LR+PCB-28 and PCB-28+Cd. The strongest inhibition occurred when A. thaliana exposed to their ternary mixture under both glasshouse and semi-field conditions. Moreover, bioaccumulation of MC-LR, Cd and PCB-28 was enhanced when seedlings exposed to their binary/ternary mixtures, especially when seedlings were treated with higher concentrations of toxicants (MC-LR, 1mgL−1; Cd, 10mgL−1; PCB-28, 1μgL−1). Additionally, pronounced toxic effects could be determined under 7-days after seedlings were irrigated with raw cyanobloom-containing water (collected from Lake Taihu in China)and its dilutions. Seeds production decreased significantly after the continuous irrigation with cyanoblooms-containing water. Collectively, this work will be an informative implication for risks of cyanoblooms and adequate utilization of freshwater containing cyanoblooms for crop irrigation.

There has been increasing interest in algae-based bioassessment, particularly, trait-based approaches are increasingly suggested. However, the main drivers, especially the contribution of hydrological variables, of species composition, trait composition, and beta diversity of algae communities are less studied. To link species and trait composition to multiple factors (i.e., hydrological variables, local environmental variables, and spatial factors) that potentially control species occurrence/abundance and to determine their relative roles in shaping species composition, trait composition, and beta diversities of pelagic algae communities, samples were collected from a German lowland catchment, where a well-proven ecohydrological modeling enabled to predict long-term discharges at each sampling site. Both trait and species composition showed significant correlations with hydrological, environmental, and spatial variables, and variation partitioning revealed that the hydrological and local environmental variables outperformed spatial variables. A higher variation of trait composition (57.0%) than species composition (37.5%) could be explained by abiotic factors. Mantel tests showed that both species and trait-based beta diversities were mostly related to hydrological and environmental heterogeneity with hydrological contributing more than environmental variables, while purely spatial impact was less important. Our findings revealed the relative importance of hydrological variables in shaping pelagic algae community and their spatial patterns of beta diversities, emphasizing the need to include hydrological variables in long-term biomonitoring campaigns and biodiversity conservation or restoration. A key implication for biodiversity conservation was that maintaining the instream flow regime and keeping various habitats among rivers are of vital importance. However, further investigations at multispatial and temporal scales are greatly needed.

There has been increasing interest in diatom-based bio-assessment but we still lack a comprehensive understanding of how to capture diatoms’ temporal dynamics with an appropriate sampling frequency (ASF). To cover this research gap, we collected and analyzed daily riverine diatom samples over a 1-year period (25 April 2013–30 April 2014) at the outlet of a German lowland river. The samples were classified into five clusters (1–5) by a Kohonen Self-Organizing Map (SOM) method based on similarity between species compositions over time. ASFs were determined to be 25 days at Cluster 2 (June-July 2013) and 13 days at Cluster 5 (February-April 2014), whereas no specific ASFs were found at Cluster 1 (April-May 2013), 3 (August-November 2013) (>30 days) and Cluster 4 (December 2013 - January 2014) (<1 day). ASFs showed dramatic seasonality and were negatively related to hydrological wetness conditions, suggesting that sampling interval should be reduced with increasing catchment wetness. A key implication of our findings for freshwater management is that long-term bio-monitoring protocols should be developed with the knowledge of tracking algal temporal dynamics with an appropriate sampling frequency.

Traditional classification of phytoplankton assemblages does not adequately reflect their ecological function in aquatic ecosystems. Therefore, we used a functional group approach based on the Q index to analyze the spatial and temporal patterns, environmental factors and the ecological status of phytoplankton in the Danjiangkou Reservoir (DJKR) of China in July 2011 to April 2012. Groups B (Cyclotella species), C (Asterionella formosa and Cyclotella meneghiniana), D (Stephanodiscus hantzschii, Synedra acus and Nitzschia sp.), Lo (mainly Peridiniopsis spp.), P (Fragilaria capucina and Aulacoseira spp.) and Y (Cryptomonas ovata and Cryptomonas erosa) were classified as dominant phytoplankton functional groups. We ran redundancy analysis and Pearson correlation analysis to assess the relationship between functional groups and environmental factors. Total phosphorus, pH and soluble silicate were apparently the key factors driving variation in phytoplankton functional groups. The analyses suggest P-limitation of phytoplankton growth. The Q index based on functional groups indicated the relatively good ecological status of the DJKR which varied as a function of the mixing regime; most samples were scored as medium or high quality according to the Q index evaluation (Q>2). This is the first application of the assemblage index to a water supply reservoir for the Middle Route Project (MRP) for South-to-North Water Transfer (SNWT) in China.

The Danjiangkou reservoir,located in the junction area of Hubei Province,Henan Province,and Shanxi Province,is the only water source of the Central Line Project of South-to-North Water Diversion of China,which serves as an important drinking water source for most cities facing severe water shortage in the North China Plain(e.g.Beijin,Tianjin,Shijiazhuang).Understanding the ecological condition of the Danjiangkou reservoir is a valuable knowledge for the regional water resource protection and management.Euphotic depth is one of the most important factors to assess the condition of the aquatic ecosystem because it represents a zone where almost all of the primary production occurs.Investigating the variation of euphotic depth in Danjiangkou reservoir and assessing the influencing factor can improve our understanding on the ecosystem dynamics of the Danjiangkou reservoir and its relationships with light.Relevant results could provide useful information for the sustainable water resource management of South-to-North Water Diversion.To investigate the spatio-temporal dynamics of euphotic depth and its influencing factors,a total of 25field sampling sites were investigated during the high flow period,median flow period,and low water period in 2011-2012in the Danjiangkou Reservoir.First,we investigated the spatiotemporal variations of the underwater photosynthetic active radiation and the concentration of three main substances(inorganic suspended particulate, algal chlorophyll a,dissolved organic carbon).Then,we used the step-wise multiple regression analysis to determine the most important factors affect the variation of euphotic depth in the Danjiangkou Reservoir.The results showed that,the euphotic depth of the Danjiangkou Reservoir range from 1.04mto 15.35m,with average values of 6.08 m,5.35 m,and 7.65 min the high flow period,median flow period,and low flow period, respectively.In the temporal scale,the highest fluctuation of the euphotic depth was observed in the high flow period(CV=0.70),and the lowest fluctuation of the euphotic depth was observed in the low water period(CV= 0.19).In the spatial scale,an increasing trend of euphotic depth was observed from the head to the tail in both Hanjiang and Danjiang reservoir basins.The result suggested that,the spatiotemporal variation of euphotic depth was mainly based on the hydrological conditions and impoundment regulation in the river-reservoir hybrid system. The stepwise regression analysis revealed that,the euphotic depth was determined by different factors in different hydrological periods.In the high flow period and median flow period,the euphotic depth was mainly determined by the suspended inorganic particulate and phytoplankton;however,in the low flow period,the he euphotic depth was determined by the chlorophyll a and dissolved organic carbon.