The potential of large floodplains to remove nitrate in river basins - The Danube case

Increasing anthropogenic stressors influenced the water quality and shifted trophic status of northern Lake Tana Gulf, Ethiopia

Nutrients in an aquatic system determine productivity, integrity and ecological status of the aquatic system. However, the excessive enrichment of these nutrients emanating from severe anthropogenic activity has substantially impacted water quality and biodiversity. There is diminutive information available on the water quality and trophic status of the northern Gulf of Lake Tana, Ethiopia due to accessibility difficulties. Therefore, this study aimed to assess the spatio-temporal changes in water quality and trophic status of Lake Tana, specifically the Gulf of Gorgora. Samples were collected from five sampling points between May 2023 to April 2024, and sampling sites were selected based on their proximity to human interference. The physico-chemical parameters were measured in situ or determined in water samples using standard methods. There were significant (p < 0.05) variations in temperature, dissolved oxygen (DO), electrical conductivity (EC) and turbidity between the dry and rainy seasons. The mean concentrations of nutrients such as NO3-N (0.48 mg/l), soluble reactive phosphorus (SRP) (1.2 mg/l) total nitrogen (TN) (1.27 mg/l), and total phosphorus (TP) (2.1 mg/l), showed significant spatio-temporal variations, with higher concentrations recorded at the Megech and Dablo sites during the rainy seasons. The depth-wise distributions of the parameters exhibited a consistent pattern, with a slight increase in the levels of EC, turbidity, TP, and TN, and a decrease in temperature, DO and NO3-N. The concentrations of SRP, TP, and TN, were slightly higher than the USEPA's threshold concentrations for causing eutrophication. The shift in the trophic status of Lake Tana from oligo-mesotrophic to eutrophic is likely due to nutrient enrichment associated with anthropogenic stressors. Therefore, continuous monitoring is essential to avert future ecological imbalances in the lake.

Process-based modeling to reveal spatio-temporal variations of coastal wetland-mediated nutrient removal

Nutrient pollution intensifies the strain on coastal ecosystems globally. Despite wetlands' significant nutrient removal potential, process-based assessments of nutrient removal in large-scale coastal wetlands face limitations that hinder accurate quantification of water quality improvements. This study proposes a novel quantitative approach by developing a coupled hydrodynamic-water quality-wetland biogeochemical model. The spatio-temporal distributions of water-sediment-vegetation nutrients in a coastal wetland and bay were simulated over an annual cycle, with model parameters and results calibrated and validated through field investigations and laboratory experiments. The findings show distinct spatio-temporal characteristics of nutrient pollutant reduction in coastal wetlands. In the Liaohe estuarine tidal wetland, a large-scale coastal wetland in China, nitrogen and phosphorus reduction fluxes reached 1184 t·yr-1 and 53 t·yr-1, accounting for 19 % and 12 % of the river's fluxes to the sea, respectively. These results underscore the critical function of coastal wetlands in mitigating coastal nutrient pollution. This study presents a novel framework for quantifying coastal water quality improvements by wetlands on a larger scale.

Quantitatively unveiling the role of coastal wetlands in regulating eutrophication and enhancing water environmental capacity

Human activities have intensified the global challenge of coastal eutrophication. Recently, water resource managers have encountered difficulties in formulating precise pollutant reduction strategies to mitigate coastal eutrophication. Despite the recognized importance of coastal wetlands and pollution sources in influencing coastal nutrient levels, accurately quantifying their impact remains difficult. To address this challenge, this study introduces a novel approach for optimizing water environmental capacity. A coupled model integrating hydrodynamics, water quality, and wetland nutrient mechanisms was developed to simulate the spatio-seasonal distribution of water, sediment, and vegetation nutrients in a semi-enclosed bay (Liaodong Bay, China) and a large-scale coastal wetland (Liaohe estuary wetland, China). Model parameters and simulation results were calibrated and validated using extensive long-term field investigations and laboratory experiments. The average root mean square errors between simulated and observed values for all validation points were as follows: 0.80 mg L-1, 0.53 mg L-1, 0.08 mg L-1, 6.70 μg L-1, and 0.50 μg L-1 for dissolved oxygen, chemical oxygen demand, dissolved inorganic nitrogen, dissolved inorganic phosphorus, and chlorophyll-a, respectively. The total nitrogen (TN) and total phosphorus (TP) in the sediment were 0.10 g kg-1 and 0.05 g kg-1, respectively. For Suaeda salsa, the TN and TP were 2.91 g kg -1 and 0.08 g kg -1, respectively. For Phragmites australis, the TN and TP were 114.22 g kg -1 and 6.21 g kg -1, respectively. The results suggest that excessive river discharge and a stable residual circulation structure contribute to the persistent eutrophication in Liaodong Bay. The Liaohe estuary wetland enhances the environmental capacity of dissolved inorganic nitrogen and dissolved inorganic phosphorus in Liaodong Bay to 271 ± 31 t yr-1 and 8 ± 1 t yr-1, respectively, accounting for 1.8 ± 0.2% and 1.3 ± 0.2% of their respective environmental capacities. The reduction in dissolved inorganic nitrogen concentration is significant, with a maximum decrease of 0.17 mg L-1. The maximum contributions of atmospheric deposition and aquaculture wastewater to dissolved inorganic nitrogen concentration are 0.08 mg L-1 and 0.03 mg L-1, respectively, with higher contributions in spring and summer than in fall and winter. These findings highlight the critical role of coastal wetlands in mitigating eutrophication and underscore the need for spatio-seasonal water management programs. This work serves as a model for effectively reducing global coastal pollution emissions.

Cross-scale and integrative prioritization of multi-functionality in large river floodplains

Floodplains provide an extraordinary quantity and quality of ecosystem services (ES) but are among the most threatened ecosystems worldwide. The uses and transformations of floodplains differ widely within and between regions. In recent decades, the diverse pressures and requirements for flood protection, drinking water resource protection, biodiversity, and adaptation to climate change have shown that multi-functional floodplain management is necessary. Such an integrative approach has been hampered by the various interests of different sectors of society, as represented by multiple stakeholders and legal principles. We present an innovative framework for integrated floodplain management building up on ES multi-functionality and stakeholder involvement, forming a scientifically based decision-support to prioritize adaptive management measures responding at the basin and local scales. To demonstrate its potential and limitations, we applied this cross-scaled approach in the world's most international and culturally diverse basin, the Danube River Basin in Europe. We conducted large-scale evaluations of anthropogenic pressures and ES capacities on the one hand and participatory modelling of the local socio-ecohydrological systems on the other hand. Based on our assessments of 14 ES and 8 pressures, we recommend conservation measures along the lower and middle Danube, restoration measures along the upper-middle Danube and Sava, and mitigation measures in wide parts of the Yantra, Tisza and upper Danube rivers. In three case study areas across the basin, stakeholder perceptions were generally in line with the large-scale evaluations on ES and pressures. The positive outcomes of jointly modelled local measures and large-scale synergistic ES relationships suggest that multi-functionality can be enhanced across scales. Trade-offs were mainly present with terrestrial provisioning ES at the basin scale and locally with recreational activities. Utilizing the commonalities between top-down prioritizations and bottom-up participatory approaches and learning from their discrepancies could make ecosystem-based management more effective and inclusive.

Distribution of selenium: A case study of the Drava, Danube and associated aquatic biotopes

This paper presents the results of the research on the overall distribution of selenium (Se) in various aquatic compartments (water, sediment, plankton and macrophytes) at six selected sites of the Croatian part of the Drava and Danube rivers, the connected floodplain lake and the melioration channel system carried out in two sampling periods (flooding in June and the drought period in September). In addition, the physicochemical water properties, plankton composition and biomass were analysed. Our study revealed low mean Se contents in sediments and water, indicating Se deficiency in the studied freshwater systems. The physicochemical environment, including Se distribution, was primarily influenced by hydrology rather than site-specific biogeochemical and morphological characteristics. The flooding period was characterised by higher Se content in water and higher transparency, nitrate and total nitrogen concentrations than drought conditions. At the river sites, sediment Se content was the highest during the flood period, while at all other sites, higher concentrations were found during the drought, reaching the maximum in the lake. Although Se concentrations were below the threshold for aquatic ecotoxicity, they increased in the following order: water (0.021-0.187 μg Se L-1) < sediments (0.005-0.352 mg Se kg-1) < macrophytes (0.010-0.413 mg Se kg-1) < plankton (0.044-0.518 mg Se kg-1) indicating its possible biomagnification at the bottom of the food chain. Species known for high Se accumulation potential dominated the biomass of the main plankton groups and the composition of the macrophyte community, which may provide a more sensitive and accurate steady-state compartment monitor for Se assessment in freshwater biotopes.

Deterministic factors modulating assembly of groundwater microbial community in a nitrogen-contaminated and hydraulically-connected river-lake-floodplain ecosystem

The river-lake-floodplain system (RLFS) undergoes intensive surface-groundwater mass and energy exchanges. Some freshwater lakes are groundwater flow-through systems, serving as sinks for nitrogen (N) entering the lake. Despite the threat of cross-nitrogen contamination, the assembly of the microbial communities in the RLFS was poorly understood. Herein, the distribution, co-occurrence, and assembly pattern of microbial community were investigated in a nitrogen-contaminated and hydraulically-connected RLFS. The results showed that nitrate was widely distributed with greater accumulation on the south than on the north side, and ammonia was accumulated in the groundwater discharge area (estuary and lakeshore). The heterotrophic nitrifying bacteria and aerobic denitrifying bacteria were distributed across the entire area. In estuary and lakeshore with low levels of oxidation-reduction potential (ORP) and high levels of total organic carbon (TOC) and ammonia, dissimilatory nitrate reduction to ammonium (DNRA) bacteria were enriched. The bacterial community had close cooperative relationships, and keystone taxa harbored nitrate reduction potentials. Combined with multivariable statistics and self-organizing map (SOM) results, ammonia, TOC, and ORP acted as drivers in the spatial evolution of the bacterial community, coincidence with the predominant deterministic processes and unique niche breadth for microbial assembly. This study provides novel insight into the traits and assembly of bacterial communities and potential nitrogen cycling capacities in RLFS groundwater.

Achieving the nutrient reduction objective of the Farm to Fork Strategy. An assessment of CAP subsidies for precision fertilization and sustainable agricultural practices in Germany

The Farm to Fork Strategy of the EU aims at sustainable food systems. One objective of the Strategy is to reduce nutrient losses by at least 50% resulting in at least 20% less fertilizer use by 2030. To this end, Member States are expected to extend digital precision fertilization and sustainable agricultural practices through the Common Agricultural Policy. In this context, this article applies a qualitative governance analysis which aims to assess the extent to which the measures proposed by the Farm to Fork Strategy, i.e., digital precision fertilization and sustainable agricultural practices, contribute to the nutrient objective of the Farm to Fork Strategy. The article analyses how these measures are implemented through the Common Agricultural Policy in Germany and Saxony. Results show that the nutrient objective of the Farm to Fork Strategy itself offers shortcomings. Germany offers some, yet overall limited, support for sustainable agricultural practices and digital precision fertilization. Hence, the Common Agricultural Policy will to a limited extend only contribute to the objective of the Strategy. The results furthermore highlight some general shortcomings of digitalization as sustainability strategy in the agricultural sector including typical governance issues (rebound and enforcement problems), and point to the advantages of quantity-based policy instruments.