Water Quality Response Research Articles

A numerical study on the responses of coastal water quality to river runoff after heavy rainfall in the case of a complex coastline with two artificial islands

Heavy rainfall causes large volumes of freshwater and nutrient salts to flow from rivers into the sea. This leads to major changes in the ecological environment of estuarine waters in a short period of time. The responses of the estuarine hydro-environment become more complex with the concentrated construction of artificial structures, which is a major cause of marine ecological disasters. This paper considers the Tang Estuary in Qinhuangdao, where artificial structures (e.g., ports, artificial islands, and bridges) are concentrated, as a prototype with the weak tidal dynamic environment. A nested prototype of hydrodynamics and water quality was established using a Delft3D model, with coupled waves and flows. The model was used to simulate the spatial and ephemeral characteristics of the short-term responses of ecological environmental factors, such as dissolved inorganic nitrogen (DIN), nitrate-nitrogen (i.e., NO3-N), ammonium-nitrogen (NH4-N), and orthophosphate (i.e., PO4-P), to an episode of heavy rainfall in August 2022. The results showed that concentrations of DIN and NO3-N in the source areas remain high after the flooding process. The concentration recovered to normal values approximately 5.5 days after the flooding process. In contrast, NH4-N and PO4-P showed a response with ephemeral correspondence with the flooding process, and concentration recovery took only approximately 1 day. This paper proposes two short-term response modes of pollutant diffusion, which provide help in exploring the role of hydro-environmental changes in offshore algal hazards and the effect of permeable buildings on pollutant dispersion. This additionally provides possibilities with the forecasting of red tide and green tide, and for their prevention in the future.

Spatially distributed water quality responses to freshwater discharge in a tropical estuary, Hilo Bay, Hawai'i.

Hilo Bay, Hawai'i, is an estuary of great importance to its neighboring coastal community, but it is threatened by impaired water quality indicated by excessive turbidity and chlorophyll a associated with river discharges of sediments and nutrients. The Wailuku River in the western half of the bay is the primary source of freshwater discharge, hypothesized here to form a surface water-dominant half of the bay with different water quality traits than the groundwater-dominant, eastern half of the bay where the spring-fed Wailoa River discharges. The water quality of both halves of the bay over different flow conditions of the Wailuku River is examined in this study using spatially distributed water quality sampling which collects hundreds of samples in either half of the bay at a distance of about every 40m. The dense sample shows significant differences between the two halves of the bay, with greater salinity dilution and turbidity in the surface water-dominant area. Both salinity and turbidity have a predictable relation to discharge, with salinity decreasing and turbidity increasing in higher flow conditions. Chlorophyll a, however, has a more complex relation to discharge, as chlorophyll a concentrations are greatest in high-flow conditions, but this may be because the water quality samples were collected in different seasons. Furthermore, significantly greater chlorophyll a concentrations in the groundwater-dominant half of the bay in low-flow conditions show that discharge may be spuriously correlated to chlorophyll a, and further studies of the effects of surface water discharge on chlorophyll a concentrations are warranted.

A framework to identify critical dynamics of water quality for diagnosing river basin ecosystem resilience and management

This study proposed a novel framework to identify critical water quality dynamics as early warning signals for diagnosing changes in the resilience of river basin ecosystems. We established empirical linkages between the theoretical background of three resilience capacities (robustness, adaptability and transformability) and water quality dynamics. Then, the processes of resilience degradation and their risk transfer or accumulation have been identified based on the shifts among different states. The methods of time-domain analysis and frequency-domain analysis were integrated into this framework, aiming to identify gradual and transient responses of water quality and its periodic fluctuation characteristics at multiple temporal scales. The time-domain analysis methods obtained the trend, cumulative periodic fluctuation of water quality by extracting the key characteristic parameters from the time-series data. The wavelet transform methods were introduced into the frequency-domain analysis to reveal the water quality fluctuation patterns at specific temporal scales. We tested the proposed framework in a typical agriculture-intensive watershed in eastern China. The results showed that this framework can be effectively used to identify three resilience states of river basin ecosystems. The degraded resilience regions were mainly distributed in the downstream area, which was influenced by their specific land use/cover and different agricultural soil health conditions. Particularly, urban sewage discharge was the main cause of periodic fluctuation in water quality time series at multiple high-frequency scales. The theoretical background of resilience capacities was elaborated in non-equilibrium dynamics before resilience degradation. Thus, this novel framework could reveal the pollution processes and driving mechanisms in different river reaches, and it also can provide adaptive management suggestions according to resilience dynamics traits.

Analysis of Seasonal Water Characteristics and Water Quality Responses to the Land Use/Land Cover Pattern: A Case Study in Tianjin, China

As the second largest city in northern China, Tianjin has a unique geographical and social status. Following its rapid economic development, Tianjin is experiencing high levels of surface water pollution. The land use/land cover (LULC) pattern has a considerable impact on hydrological cycling and pollutant transmission, and thus on regional water quality. A full understanding of the water quality response to the LULC pattern is critical for water resource management and improvement of the natural environment in Tianjin. In this study, surface water monitoring station data and LULC data from 2021 to 2022 were used to investigate the surface water quality in Tianjin. A cluster analysis was conducted to compare water quality among monitoring stations, a factor analysis was conducted to identify potential pollution sources, and an entropy weight calculation was used to analyze the impact of the land use pattern on water quality. The mean total nitrogen (TN) concentration exceeded the class Ⅴ water quality standard throughout the year, and the correlation coefficient of the relationship between dissolved oxygen (DO) and pH exceeded 0.5 throughout the year, with other water quality parameters showing seasonal changes. On the basis of their good water quality, the water quality monitoring stations near large water source areas were distinguished from those near areas with other LULC patterns via the cluster analysis. The factor analysis results indicated that the surface water in Tianjin suffered from nutrient and organic pollution, with high loadings of ammonia nitrogen (NH3N), TN, and total phosphorus (TP). Water pollution was more serious in areas near built-up land, especially in the central urban area. The entropy weight calculation results revealed that water, built-up land, and cultivated/built-up land had the greatest impact on NH3N, while cultivated land had the greatest impact on electrical conductivity (EC). This study discusses the seasonal changes of surface water and impact of land use/land cover pattern on water quality at a macro scale, and highlighted the need to improve surface water quality in Tianjin. The results provide guidance for the sustainable utilization and management of local water resources.

A climate-water quality assessment framework for quantifying the contributions of climate change and human activities to water quality variations

Water quality safety has attracted global attention and is closely related to the development of the social economy and human health. It is widely recognized that climate change and human activities significantly affect water quality changes. Therefore, quantifying the contributions of factors that drive long-term water quality changes is crucial for effective water quality management. Here, we built a climate-water quality assessment framework (CWQAF) based on climate-water quality response coefficients and trend analysis methods, to achieve this goal. Our results showed that the water quality improved significantly by 4.45%–20.54% from 2011 to 2020 in the Minjiang River basin (MRB). Human activities (including the construction of ecological projects, stricter discharge measures, etc.) were the main driving factors contributing 65%–77% of the improvement effect. Notably, there were differences in the contributions of human activities to water quality parameter changes, such as DO (increase (I): 0.12 mg/L, human contribution (HC): 66.8%), CODMn (decrease (D): 0.71 mg/L, HC: 67.2%), BOD5 (D: 1.10 mg/L, HC: 77.7%), CODCr (D: 4.20 mg/L, HC: 81.2%), TP (D: 0.13 mg/L,HC: 72.8%) and NH3–N (D: 0.40 mg/L, HC: 63.0%). Climate change explained 23%–35% of the variation in water quality. The water quality response to climate change was relatively significant with precipitation. For example, the downstream region was more susceptible to climate change than was the upstream region, as the downstream movement of precipitation centers strengthened the process of climatic factors affecting water quality changes in the MRB. Generally, although human activities were the main driving factor of water quality changes at the basin scale, the contribution of climate change could not be ignored. This study provided a manageable framework for the quantitative analysis of the influence of human activities and climate change on water quality to enable more precise and effective water quality management.

Response of water quality to climate warming and atmospheric deposition in an alpine lake of Tianshan Mountains, Central Asia

Alpine lakes are usually unaffected by anthropogenic disturbance, yet these aquatic ecosystems are highly sensitive to climate warming and atmospheric deposition. It is unclear how water quality of alpine lakes has responded to these environmental changes. Unprecedented degradation in water quality (characterized by nutrient enrichment and phytoplankton blooms) had occurred in the Tianchi Lake, an alpine lake in the Tianshan Mountains. Here we measured the dust and precipitation chemistry to assess the source, magnitude, and impact of atmospheric deposition on nutrient stoichiometry of Tianchi Lake. Long-term (2015–2021) limnological data consisted of nutrient and chlorophyll were used to quantify the trends of lake water quality, as well as the association between phytoplankton biomass and air temperature. We proposed a critical nutrient model based on water quality guarantee probability to estimate the maximum allowable nutrient load. The results indicated that air pollutants from urban agglomeration and dust derived from semi-arid region represented the primary sources of atmospheric nutrient, which contributed 6.86 t nitrogen (N) and 0.23 t phosphorus (P) to the lake during 2021. Enhanced fertilization effect on phytoplankton due to high atmospheric deposition was expected to promote phytoplankton blooms, but unbalanced N:P ratio would exacerbate the P limitation of primary productivity. Climate warming seemed equally crucial synergistically to stimulate phytoplankton growth, the first appearance of phytoplankton blooms corresponded to the beginning of the warm period. Further studies had shown that increase in frequency and duration of heat waves would accelerate the eutrophication. To mitigate water quality degradation, the critical nutrient model suggested that 40.63 t N and 0.49 t P should be reduced. We conclude that the synergistic effects of climate warming and atmospheric deposition exacerbate the water quality of Tianchi Lake, even though it was protected well by strict environmental policies. More studies should be conducted to assess the potential impact of climate warming and atmospheric deposition on alpine lakes to ensure water quality safety and water ecological security.

SMAP soil moisture data assimilation impacts on water quality and crop yield predictions in watershed modeling

Model soil moisture predictions are routinely updated with independent observations using data assimilation techniques. While the impacts of these updates on modeled hydrology are well studied, impacts on predicted water quality and crop yield are unresolved. In this study, we evaluated data assimilation effects for a range of predictions in the Soil and Water Assessment Tool (SWAT). We used satellite surface soil moisture data products from the Soil Moisture Active/Passive (SMAP) mission to update SWAT soil moisture for two U.S. experimental watersheds. We addressed possible limitations to the vertical transfer of surface soil moisture updates to deeper layers in the model by additionally testing a modified soil percolation approach that relies on relative saturation rather than the original thresholding behavior of SWAT. Results at both watersheds demonstrated that data assimilation greatly impacted water quality and crop yield predictions. Modifying the soil percolation algorithm, however, did not improve assimilation results. Assimilation increased median soil moisture (+2% to +6 %) which in turn increased total streamflow (+0.43 to +1.70 m3/s daily). Critically, streamflow alone was not a sufficient predictor for the assimilation changes to water quality predictions as flow component contributions and seasonality differed between sites. A varied response in annual water quality predictions to soil moisture updates was identified (+1.70 to +123 kg/ha total nitrogen; −0.11 to −0.57 kg/ha total phosphorous; +8.1 to +50.0 kg/ha sediment) that was dependent upon the timing of the updates, site characteristics, and change to specific transport and transformation processes. Crop yield predictions were similarly impacted by data assimilation from changes to both water and nutrient availability that varied by crop type. The overall strong and diverse response of water quality and crop yield predictions to soil moisture updates provides evidence that assimilation can impact a range of model predictions. Efforts to improve soil moisture simulations, therefore, have potential for meaningful improvements in targeted, non-hydrologic predictions.

Effects of submerged macrophytes (Elodea nuttallii) on water quality and microbial communities of largemouth bass (Micropterus salmoides) ponds.

Traditional aquaculture ponds are one of the most vulnerable ecosystems; thus, ecological aquaculture is increasingly valued for its beneficial ecological properties and ecosystem services. However, little is known about ecological aquaculture of largemouth bass with submerged vegetation. Here, we designed three ecological ponds of cultured largemouth bass with submerged macrophytes (the EM group) and three ponds with traditional aquaculture (the M group) to reveal the response of water quality, and phytoplankton and bacterial communities, to submerged macrophyte bioremediation during a 90-day culture period. We observed that Cyanobacterial outbreak occurred in the M group ponds from day 7 to the end of the experiment; however, there were no Cyanobacterial blooms in the EM group ponds throughout the culture period. Compared with the M group ponds, the EM group ponds, which had submerged hydrophytes, had significantly decreased concentrations of TP, TN, and CODMn, but significantly increased DO concentrations throughout the experimental period. Moreover, ecological aquaculture with submerged macrophytes showed strong effects on the phytoplankton and bacterial community compositions. In particular, the M group ponds had higher phytoplankton density and mainly included Cyanobacteria, whereas the EM group had lower phytoplankton density and mainly included Chlorophyta. Moreover, higher alpha diversity, as determined by Ace and Simpson index values, was detected for bacterial communities in the EM group ponds. Furthermore, PCoA clearly grouped the bacterial communities according to the two culture modes throughout the culture period. These results indicate that ecological aquaculture with submerged macrophytes can improve water quality, control Cyanobacterial blooms, and affect the diversity and composition of bacterial communities. These valuable effects seem to be beneficial and consistent to maintaining aquaculture ecosystem stability.

Droughts can reduce the nitrogen retention capacity of catchments

Abstract. In 2018–2019, Central Europe experienced an unprecedented 2-year drought with severe impacts on society and ecosystems. In this study, we analyzed the impact of this drought on water quality by comparing long-term (1997–2017) nitrate export with 2018–2019 export in a heterogeneous mesoscale catchment. We combined data-driven analysis with process-based modeling to analyze nitrogen retention and the underlying mechanisms in the soils and during subsurface transport. We found a drought-induced shift in concentration–discharge relationships, reflecting exceptionally low riverine nitrate concentrations during dry periods and exceptionally high concentrations during subsequent wet periods. Nitrate loads were up to 73 % higher compared to the long-term load–discharge relationship. Model simulations confirmed that this increase was driven by decreased denitrification and plant uptake and subsequent flushing of accumulated nitrogen during rewetting. Fast transit times (<2 months) during wet periods in the upstream sub-catchments enabled a fast water quality response to drought. In contrast, longer transit times downstream (>20 years) inhibited a fast response but potentially contribute to a long-term drought legacy. Overall, our study reveals that severe droughts, which are predicted to become more frequent across Europe, can reduce the nitrogen retention capacity of catchments, thereby intensifying nitrate pollution and threatening water quality.

Oxidative degradation technology of synthetic water pollutants containing sulfamethoxazole in urban suburbs based on section water quality response

Oxidative degradation technology of synthetic water pollutants containing sulfamethoxazole in urban suburbs based on section water quality response

Myrcene as water conditioner, stress-reducing and antioxidant agent in transportation of common carp, Cyprinus carpio, with plastic bags

This study aimed at evaluating the efficacy of myrcene on water quality and stress responses of common carp, Cyprinus carpio, during transportation. A total of 126 fish (45.3 ± 1.65 g) were transported using 0, 10, 20, 30, and 50 µL/L myrcene for 6 h. All transported treatments exhibited significant decrease in water pH, dissolved oxygen, and increase in water temperature and ammonia. Myrcene had no effects on water pH and temperature after transportation, but 10 µL/L myrcene significantly decreased water dissolved oxygen, as 30 µL/L myrcene increased it, compared to the fish transported without myrcene. Moreover, 20–50 µL/L myrcene significantly decreased water ammonia, compared to the fish transported without myrcene. Transportation induced significant increases in water alkalinity in 0–30 µL/L myrcene and the highest alkalinity was related to 10 µL/L myrcene. Plasma ammonia significantly increased in all treatments after transportation and the lowest and highest levels were observed in fish transported with 20 and 50 µL/L myrcene. Plasma lactate significantly decreased in fish transported with 0–30 µL/L myrcene; but increased in fish transported with 50 µL/L myrcene. All transported fish showed similar plasma T4 levels, which were significantly lower than the before transportation. Transportation induced significant increase in plasma cortisol, glucose, superoxide dismutase, catalase, malondialdehyde and significant decrease in plasma T3, which 10 µL/L myrcene failed to mitigate such change, and nevertheless, 20 and 30 µL/L myrcene mitigated such changes. 50 µL/L myrcene led to higher cortisol, glucose, CAT, MDA, and lower T3 and GSH, compared to fish transported without myrcene. Plasma glutathione peroxidase activity significantly decreased in fish transported with 50 µL/L myrcene, compared to fish not transported. Myrcene is recommended as a sedative and stress-reducing agent in common carp transportation procedures. The beneficial concentration is 20–30 µL/L, but lower and higher concentrations have negative effects.

Deficit mulched drip irrigation improves yield, quality, and water use efficiency of watermelon in a desert oasis region

In the context of global warming, drought is a crucial factor affecting agricultural production, especially for the oasis farming areas of northwestern China, where water resources are extremely scarce. To reveal the response of watermelon water consumption (WC), yield, quality, and water use efficiency (WUE) to water deficit (WD) and to determine the optimal mode of irrigation in the desert oasis areas of Hexi, we applied different WD treatments on a field oasis watermelon in 2020 and 2021. The experiments employed two gradients of light (l) and moderate (m) WD treatments during the seedling (S), vine (V), expansion (E), and maturity (M) stages except for flowering and fruiting (F) stage, with full irrigation during the entire growth period as the control (CK). The results showed that the WC of oasis watermelon at each growth stage was in the order: E > V > S > M > F, in which WC by E stage accounted for 33.15–40.33%. Compared with the CK, WD had an obvious effect on water saving, and WD during the V or E stages inhibited the growth of vine leaves to a certain extent. The Sl treatment (light WD during S stage) had the highest yield and WUE, with significant increases of 5.23–5.38% and 7.10–7.37% (p < 0.05), respectively, in the two years, while the Mm treatment (moderate WD during the M stage) resulted in the best overall nutritional quality of watermelon, with soluble solids, soluble sugars, vitamin C, and soluble protein contents increasing by 11.31%, 33.43%, 24.03%, and 10.31% (p < 0.05), respectively. However, an integrated evaluation that coupled grey relation analysis (GRA) with a technique for determining order preference by its similarity to an ideal solution (TOPSIS) model, considering growth, yield, quality, and efficiency indicators, concluded that the Ml treatment (light WD during the M stage) achieved the highest comprehensive score, followed by the Sl treatment. Therefore, it is recommended that the optimal deficit irrigation mode for oasis watermelon is the Ml treatment, which can balance water savings, yields, and quality improvements, while the Sl treatment can be considered as a backup mode. This study provides theoretical and technical support for achieving efficient water-saving cultivation and scale development of watermelon production in desert oasis areas.

Tilapia aquaculture water quality in response to the addition of fermented herbal extract

This study aims to find a suitable Fermented Herbal Extract (FHE) dose to reduce ammonia levels in tilapia cultivation. This research was carried out for one month, from August to September 2021, at Fish Hatchery Laboratory of Marine Sciences and Fisheries Faculty, Hasanuddin University. The study was conducted using a completely randomized design with six treatments and three replications. The treatments were various doses of FHE added to the cultivation water, including 0 ppm, two ppm, four ppm, six ppm, eight ppm, and ten ppm. The juvenile tilapia measuring about 5 cm body length stocked with five fish for each aquarium. Water quality like Dissolved Oxygen, Temperature, pH, Nitrate, Nitrite, and Ammonia content was measured in this study. The results showed that adding FHE to tilapia culture media affected the ammonia concentration. In this study, six ppm of FHE treatment resulted in the lowest ammonia concentration on the 10th and 12th days of observation.

Turning up the heat: Long‐term water quality responses to wildfires and climate change in a hypereutrophic lake

AbstractClear Lake (Lake County, CA, USA) is hypereutrophic and used for drinking water, tribal use, and supports a significant fishing economy. The Mendocino Complex (2018), one of the largest wildfires in California's post‐settlement history, burned 40% of the Clear Lake watershed, providing a timely opportunity to study the impacts of historical and current wildfires on this valuable aquatic resource. Using long‐term monthly monitoring data from 1968 to 2019, paired with historical watershed fire data, we found that for the three largest fire years in the watershed's history, 3‐year postfire median July–October epilimnetic total phosphorus (TP) concentrations were below or equal to 3‐year prefire TP concentrations. However, both median TP epilimnetic concentrations and deepwater temperature across the lake have increased since the late 1960s. Long‐term TP increases were more strongly correlated with monthly maximum air temperatures than precipitation, suggesting a potential role of warming‐induced water column stratification, dissolved oxygen (DO) depletion, and high potential for internal phosphorus loading. Hypoxic occurrences were correlated with higher hypolimnetic soluble reactive phosphorus and TP concentrations, but additional high‐frequency monitoring of DO will help determine the duration of anoxia and its contribution to internal phosphorus loading. These long‐term data suggest that for this large, hypereutrophic lake, wildfires did not significantly alter in‐lake TP concentrations based on long‐term, monthly monitoring and that other internal or external sources of TP may mask any wildfire effects. Nonetheless, our study underscores the value of synthesizing decades of water quality, watershed wildfire, and climate data to build a more comprehensive, nuanced picture of multiple long‐term threats to aquatic ecosystems under global change. Moreover, monitoring and studying fire effects across a wide range of lake types beyond this study will help promote more effective lake management during changing climates and increasingly frequent large wildfires.

Dispersed Urban‐Stormwater Control Improved Stream Water Quality in a Catchment‐Scale Experiment

AbstractTraditional urban drainage degrades receiving waters. Alternative approaches have potential to protect downstream waters, but widespread adoption requires robust demonstration of their feasibility and effectiveness. We conducted a catchment‐scale experiment over 19 years to assess the effect of dispersed stormwater control measures (SCMs), measured as a reduction in effective imperviousness (EI) on stream water quality in six sites on two streams. We compared changes in those sites over 7 years as EI decreased, to changes in the 12 preceding years, and in three reference and two control streams. SCMs reduced phosphorus concentrations and summer temperature to reference levels in dry weather where EI was sufficiently reduced, but effects were smaller with increased antecedent rain. SCMs also reduced nitrogen concentrations which were influenced by septic tank seepage in all sites. SCMs had no effect on suspended solids concentrations, which were lower in urban than in reference streams. SCMs increased electrical conductivity: along with reduced temperature this is evidence of increased contribution of groundwater to baseflows. This experiment strengthens inference that urban stormwater drainage increases contaminant concentrations in streams, and demonstrates that such impacts are reversible and likely preventable. Variation in degree of water quality improvement among experimental sites suggests that achieving reference water quality would require SCMs with large retention capacity intercepting runoff from nearly all impervious surfaces, thus requiring more downslope space and water demand. EI is a useful metric for predicting stream water quality responses to SCMs, allowing better catchment prioritization and SCM design standards for stream protection.

High-resolution multi-parameter monitoring of microbial water quality and particles at two alpine karst springs as a basis for an early-warning system

Karst aquifers are important resources for drinking water supply and are very vulnerable to contamination. Microbial concentrations at karst springs, in particular, often vary quickly over a short period of time. In this study, the response of microbial water quality and particle-size distribution of two alpine karst springs to rainfall events was investigated to test and validate parameters that can be used as early-warning systems for fecal contamination. At both investigated karst springs, total organic carbon, particle-size distribution (especially small particle fractions), and particle load show a good correlation to the fecal indicator bacteria E. coli and can therefore be used as a real-time indicator of fecal contamination at the investigated springs. In addition to conventional bacterial determination methods, the β-D-glucuronidase activity, which can be measured in near real-time, was used as a novel indicator parameter for fecal contamination. At the event scale, the β-D-glucuronidase (GLUC) activity shows a good correlation to E. coli and can be used as an additional real-time indicator of fecal contamination. For the studied springs, when they show two peaks in turbidity and small particles, these two parameters are suitable for an early warning system because the bacterial contamination occurs during the secondary peak of these parameters. These results highlight the vulnerability of karst aquifers and demonstrate the applicability of advanced measurement techniques in detecting fecal contamination in real-time, which is especially important given the time-consuming nature of conventional bacterial detection methods.

Exploring the relations between sequential droughts and stream nitrogen dynamics in central Germany through catchment-scale mechanistic modelling

Like many other regions in central Europe, Germany experienced sequential summer droughts from 2015 to 2018. As one of the environmental consequences, river nitrate concentrations have exhibited significant changes in many catchments. However, catchment nitrate responses to the changing weather conditions have not yet been mechanistically explored. Thus, a fully distributed, process-based catchment Nitrate model (mHM-Nitrate) was used to reveal the causal relations in the Bode catchment, of which river nitrate concentrations have experienced contrasting trends from upstream to downstream reaches. The model was evaluated using data from six gauging stations, reflecting different levels of runoff components and their associated nitrate-mixing from upstream to downstream. Results indicated that the mHM-Nitrate model reproduced dynamics of daily discharge and nitrate concentration well, with Nash-Sutcliffe Efficiency ≥ 0.73 for discharge and Kling-Gupta Efficiency ≥0.50 for nitrate concentration at most stations. Particularly, the spatially contrasting trends of nitrate concentration were successfully captured by the model. The decrease of nitrate concentration in the lowland area in drought years (2015–2018) was presumably due to (1) limited terrestrial export loading (ca. 40 % lower than that of normal years 2004–2014), and (2) increased in-stream retention efficiency (20 % higher in summer within the whole river network). From a mechanistic modelling perspective, this study provided insights into spatially heterogeneous flow and nitrate dynamics and effects of sequential droughts, which shed light on water-quality responses to future climate change, as droughts are projected to be more frequent.

Evaluation of the effectiveness and effects of long-term ecological restoration on watershed water quality dynamics in two eutrophic river catchments in Lake Chaohu Basin, China

The ecological restoration projects are important in alleviating the ecological degradation conditions. The evaluation of the long-term effects and effectiveness of ecological restoration projects could provide an empirical foundation for future projects and assessment of watershed water environment restoration. However, empirical research, based upon long-term monitoring data on this topic is relatively lacking. In this study, the effectiveness of restoration projects on water quality improvement of two eutrophic rivers in Lake Chaohu Basin (LCB) was investigated. The spatiotemporal dynamics of water quality from 2000 to 2021 were studied and a delayed water quality response to restoration projects was revealed. In the past two decades, especially since 2014, the water quality in the LCB improved significantly whereas restoration projects have been implemented since late 2012. Additionally, a sharp regression discontinuity (RD) design was constructed to confirm the effectiveness of restoration projects on water quality improvement successfully. Moreover, the positive effects of restoration projects on water quality dynamics were quantified and distinguished from the effects of negative human activities and climate change using the residual trend method. During 2014–2020, ecological restoration projects contributed more than half (64.3 % SWL1 and 58.2 % HB2) of water quality variation in two eutrophic river catchments. Finally, a framework from the perspective of lake eutrophication management with effective measures is proposed for maintaining the sustainable development of ecological restoration projects in watersheds. The empirical study here provides scientific support for achieving better effectiveness of ecological restoration projects in watersheds with water pollution problems worldwide.

Impact of climate change on coastal water quality and its interaction with pollution prevention efforts

The impact of climate change on nearshore coastal water quality and its interaction with pollution prevention efforts (e.g., the development of green and gray water infrastructure) still lack systematic investigation. This study performed a holistic analysis of the impact of climate change on the salinity and concentrations of total nitrogen (TN), total phosphorus (TP) and chlorophyll a (Chl.a) in Shenzhen Bay between Shenzhen and Hong Kong, the two most developed megacities in South China, based on three-dimensional hydrodynamic and water quality modeling. The major study findings were as follows. First, Chl.a was the most sensitive parameter, and its bay-wide average concentration in 2100 was predicted to be approximately 13% and 46% higher than those in 2015 under mild and rapid climate change scenarios, respectively. Second, sea level rise was found to be a major driver of all four water quality parameters, while temperature and radiation mainly influenced Chl.a and precipitation mainly influenced nutrients. Third, water quality responses to climate change were highly heterogeneous over the bay. Even under a mild climate change scenario, the highest location-specific changes (2100 vs. 2015) in salinity and TN, TP and Chl.a concentrations were projected to be approximately 21%, 19%, 25%, and 65%, respectively. Fourth, changes in seasonal variation due to climate change may lead to an enhanced ecological risk of algal blooms. Finally, the effect of reducing TN and TP concentrations by proposed water infrastructure development was found to be significantly weakened (nearly 40% and 20% for TN and TP, respectively, under a mild climate change scenario), while the negative effect (i.e., increase in the Chl.a concentration) was notably accelerated. Regional cooperation is critical for protecting the water quality of the bay, particularly under climate change. The insights obtained in this study are applicable to other coastal water zones around the world with similar socioeconomic backgrounds and climatic conditions.

Worldwide moderate-resolution mapping of lake surface chl-a reveals variable responses to global change (1997–2020)

Anthropogenic activity is leading to widespread changes in lake water quality—a key contributor to socio-ecological health. But, the anthropogenic forces affecting lake water quality (climate change, land use change, and invasive species) are unevenly distributed across lakes, across the seasonal cycle, and across space within lakes, potentially leading to highly variable water quality responses that are poorly documented at the global scale. Here, we used 742 million chlorophyll-a (chl-a) estimates merged over 6 satellite sensors (daily, 1 to 4 km resolution) to quantify water quality changes from 1997 to 2020 in 344 globally-distributed large lakes. Chl-a decreased across 56% of the cumulative total lake area, challenging the putative widespread increase in chl-a that is expected due to human activity. 19% of lakes exhibited both positive and negative chl-a trends (p-value &lt; 0.1) across different locations or times of the year. This spatiotemporal complexity demonstrates the value of moderate resolution mapping of lake chl-a to inform water management decision-making and to determine the local ecological consequences of human activity.