Seasonal variation in greenhouse gas concentrations and diffusive fluxes in three river-reservoir systems in the Seine Basin (France).

River and reservoir ecosystems have been considered as hot spots for GHG (greenhouse gas) emissions while their specific hydrological and biogeochemical processes affect GHG concentrations; however, few studies integrated river-reservoir systems to identify the dominant drivers of GHG concentrations and flux changes associated with these systems. In the present study, we examined the seasonal variations in GHG concentrations in the surface water of three river-reservoir systems in the Seine Basin. The levels and seasonal variations of GHG concentrations exhibited distinct patterns among reservoirs, upstream, and downstream rivers. The concentrations of CH4 (methane) in the reservoirs were notably higher than those observed in both upstream and downstream rivers and showed higher values in summer and autumn, which contrasted with CO2 (carbon dioxide) concentrations, while N2O (nitrous oxide) concentrations did not show an obvious seasonal pattern. A high mole ratio of CH4/CO2 was found in these reservoirs, with a value of 0.03 and was more than 30 and 10 times higher than that in the upstream and downstream rivers, respectively. The three river-reservoir systems were oversaturated with GHG during the study period, with the average diffusive fluxes (expressed as CO2eq: CO2 equivalent) of 810 ± 1098 mg CO2eq m-2 d-1, 9920 ± 2413 mg CO2eq m-2 d-1, and 7065 ± 2704 mg CO2eq m-2 d-1 in the reservoirs, upstream and downstream rivers, respectively. CO2 and CH4-CO2 were respectively the dominant contributors to GHG diffusive fluxes in river and reservoir sections, while N2O contributed negligibly to GHG diffusive fluxes in the three river-reservoir systems. Our results showed that GHG concentrations and gas transfer coefficient have varying importance in driving GHG diffusive fluxes among different sections of the river-reservoir systems. In addition, our results also show the combined effect of reservoirs and upstream rivers on the water quality variables and hydrological characteristics of downstream rivers, highlighting the future need for additional investigations of GHG processes in the river-reservoir systems.

Reservoirs change pCO2 and water quality of downstream rivers: Evidence from three reservoirs in the Seine Basin.

The global increase in the construction of reservoirs has drawn attention given its documented hydrological and biogeochemical impacts on downstream rivers; however, the impact of reservoirs on downstream pCO2 (partial pressure of carbon dioxide) is still poorly understood. To evaluate these impacts, the interactions between reservoirs and their corresponding upstream and downstream rivers were analyzed for three reservoirs in the Seine Basin based on monthly measurement during two hydrological years. The seasonal variations of water quality in the reservoirs were mainly driven by the entering water and the biogeochemical processes occurring in the reservoirs. Our results unravel the crucial role of reservoir in downstream water quality, which significantly increased DOC (dissolved organic carbon) and BDOC (biodegradable DOC) concentrations, while lowered DSi (dissolved silica) concentrations during emptying period (p < 0.01). Furthermore, the impacts of reservoirs on the annual fluxes of DOC, BDOC, and DSi were quantified and suggested that the three reservoirs respectively increased 20% and 23% of annual fluxes of DOC and BDOC, while decreased 33% of annual DSi fluxes in their downstream rivers. Additionally, the reservoirs significantly decreased downstream riverine pCO2 (p < 0.01), and enhanced the gas transfer coefficient of CO2 in downstream rivers by 1.3 times during the emptying period, which highlights the necessity to consider the potential impact of reservoirs on riverine CO2 emissions. Overall, our results highlight the importance of combining biogeochemical and hydrological characteristics to understand the impacts of reservoirs on downstream rivers, and emphasize the need of similar studies under the current context of increasing reservoir constructions.

Unravelling nutrient fate and CO2 concentrations in the reservoirs of the Seine Basin using a modelling approach.

Reservoirs are active reactors for the biogeochemical cycling of carbon (C) and nutrients (nitrogen: N, phosphorus: P, and silica: Si), however, our in-depth understanding of C and nutrient cycling in reservoirs is still limited by the fact that it involves a variety of closely linked and coupled biogeochemical and hydrological processes. In this study, the updated process-based Barman model was applied to three reservoirs of the Seine Basin during 2019-2020, considering the variations of carbon dioxide (CO2) concentrations and key water quality variables. The model simulations captured well the observed seasonal variations of water quality variables, although discrepancies remained for some variables. According to the model, we found that: (1) the three reservoirs are autotrophic ecosystems and showed high removal efficiency of dissolved inorganic carbon and nutrients during 2019-2020; (2) phytoplankton assimilation, benthic denitrification, precipitation and dissolution of calcium carbonate, and gas exchange at the water-air interface are the dominant processes for water quality variations in reservoirs; (3) based on scenarios results, trophic state and mean water depth of reservoir would impact the biogeochemical processes and the retention efficiency of nitrate and dissolved silicate. Finally, we expect that the successful application of Barman model in the reservoirs of the Seine Basin could provide a useful tool for simulating reservoir water quality changes and thus evaluating the impacts of reservoirs on downstream water quality.

Long-term assessment of nutrient budgets for the four reservoirs of the Seine Basin (France).

Artificial reservoirs represent one of the most significant human disturbances of water flows and associated water quality, including nutrients and SM (suspended matter). However, most of the previous studies were only focused on few years or even single year, and the long-term dynamics of nutrient retention in reservoir are under explored. In this study, we present the long-term (1998-2018) hydrological characteristics and water quality in four reservoirs (Marne, Aube, Seine, and Pannecière reservoirs) and their related rivers (Marne, Aube, Seine, and Yonne rivers) of the Seine Basin, France. Based on the hydrology and water quality data, the long-term budgets of nutrients and SM were evaluated in these reservoirs according to mass balance calculation. The results indicated that the four reservoirs play important roles in the retention/elimination of nutrients and SM, and the retention/elimination rates may be affected by hydrophysical and biogeochemical processes. The mean annual retention rates accounted for 16-53% of the inputs of DIN (dissolved inorganic nitrogen), 26-48% of PO43--P (orthophosphates), 22-40% of Si (dissolved silicon), and 36-76% of SM in the four reservoirs during the 1998-2018 period. Further analysis suggested that the annual residence time and the percentage of water released from reservoirs during the filling period significantly correlated with DIN retention rates in the four reservoirs (p < 0.01), which highlights the importance of reservoir water management strategies for the DIN concentrations in the downstream rivers. Interestingly, the Wilcoxon test results also revealed that the three diverted reservoirs (Marne, Aube, and Seine reservoirs) indeed lowered the nutrient concentrations in their downstream rivers during the emptying period, thereby modifying the biogeochemical functioning in the downstream river networks. Finally, these results emphasized the importance of hydrological characteristics in better understanding nutrient retention in reservoirs.

Nutrient transfer in three contrasting NW European watersheds: the Seine, Somme, and Scheldt Rivers. A comparative application of the Seneque/Riverstrahler model.

An understanding of the ecological functioning of an aquatic continuum on a multi-regional scale relies on the ability to collect suitable descriptive information. Here, the deterministic Seneque/Riverstrahler model, linking biogeochemistry with the constraints set by geomorphology and anthropogenic activities, was fully implemented to study the Seine, Somme, and Scheldt Rivers. Reasonable agreement was found between calculated and observed nutrient fluxes for both seasonal and inter-annual variations along the networks. Nutrient budgets underline: i) a clear partition of diffuse and point sources with respect to the specific activities of the watersheds, ii) the importance of riparian retention, responsible for 25-50% of nitrogen retention, iii) the role played by benthic processes, resulting in the retention of up to 45% of the phosphorus and 35% of the silica entering the river systems. Nutrient ratios confirmed that fluxes to the Eastern Southern Bight of the North Sea are imbalanced, supporting the potential for undesirable algal blooms.

Long-term changes in greenhouse gas emissions from French agriculture and livestock (1852-2014): From traditional agriculture to conventional intensive systems.

France was a traditionally agricultural country until the first half of the 20th century. Today, it is the first European cereal producer, with cereal crops accounting for 40% of the agricultural surface area used, and is also a major country for livestock breeding with 25% of the European cattle livestock. This major socioecological transition, with rapid intensification and specialisation in an open global market, has been accompanied by deep environmental changes. To explore the changes in agricultural GHG emissions over the long term (1852-2014), we analysed the emission factors of N2O from field experiments covering major land uses, in a gradient of fertilisation and within a range of temperature and rainfall, and used CH4 emission coefficients for livestock categories, in terms of enteric and manure management, considering the historical changes in animal excretion rates. We also estimated indirect CO2 emissions, rarely accounted for in agricultural emissions, using coefficients found in the literature for the dominant energy consumption items (fertiliser production, field work and machinery, and feed import). From GHG emissions of ~30,000 ktons CO2 Eq yr-1 in 1852, reaching 54,000 ktons CO2 Eq yr-1 in 1955, emissions more than doubled during the 'Glorious thirties' (1950-1980), and peaked around 120,000 ktons CO2 Eq yr-1 in the early 2000s. For the 2010-2014 period, French agriculture GHG emissions stabilised at ~114,000 ktons CO2 Eq yr-1, distributed into 49% methane (CH4), 22% carbon dioxide (CO2) and 29% nitrous oxide (N2O). A regional approach through 33 regions in France shows a diversity of agriculture reflecting the hydro-ecoregion distribution and the agricultural specialisation of local areas. Exploring contrasting scenarios at the 2040 horizon suggests that only deep changes in the structure of the agro-food system would double the reduction of GHG emissions by the agricultural sector.

Carbon dioxide, methane and nitrous oxide emissions from the human-impacted Seine watershed in France.

Greenhouse gas (GHG) emissions from rivers and lakes have been shown to contribute significantly to global carbon and nitrogen cycling. In temperate and human-impacted regions, simultaneous carbon dioxide, methane and nitrous oxide emissions from aquatic systems are poorly documented. We estimated carbon dioxide (CO2) concentrations in the Seine hydrosystem (71,730 km2, France) using direct measurements, and calculations of CO2 partial pressures from 14 field campaigns conducted between 2010 and 2017, and compared them to methane (CH4) and nitrous oxide (N2O) concentrations. In the main stem of the Seine River, CO2 showed the same spatial gradient as N2O and CH4 with peaks in concentration downstream from the arrival of effluents from wastewater treatment plants enriched in organic matter, thus favoring mineralization. It is likely that high CO2 concentrations upstream were due to organic carbon inputs from soils and enriched CO2 groundwater discharges, whereas high N2O and CH4 upstream values were likely due to denitrification in riparian wet areas and anoxic decomposition of organic matter-rich wetlands, respectively. In addition, seasonal variations in all three GHGs were observed with higher concentrations in summer when higher temperatures promote mineralization and low water reduces the dilution of organic matter mainly originating from WWTP effluents. GHG emissions were calculated and compared with agricultural and nonagricultural (urban, transport) fluxes in the basin. In the Seine River network, CO2 emissions dominated riverine GHG emissions, reaching 95.3%, while N2O and CH4 emissions accounted for 4.4% and 0.3%, respectively. These indirect emissions from the hydrosystem were estimated to account for 3.7% of the total GHG emissions from the basin that amounted to 61,284 Gg CO2eq yr-1. Comparatively, direct agricultural and nonagricultural GHG emissions were estimated at 23.3% and 73.0%., respectively.

Seasonal and spatial variability of the partial pressure of carbon dioxide in the human-impacted Seine River in France.

Carbon evasion from rivers is an important component of the global carbon cycle. The intensification of anthropogenic pressures on hydrosystems requires studies of human-impacted rivers to identify and quantify the main drivers of carbon evasion. In 2016 and 2017, four field campaigns were conducted in the Seine River network characterized by an intensively cropped and highly populated basin. We measured partial pressures of carbon dioxide (pCO2) in streams or rivers draining land under different uses at different seasons. We also computed pCO2 from an existing data set (pH, water temperature and total alkalinity) going back until 1970. Here we report factors controlling pCO2 that operate at different time and space scales. In our study, the Seine River was shown to be supersaturated in CO2 with respect to the atmospheric equilibrium, as well as a source of CO2. Our results suggest an increase in pCO2 from winter to summer in small streams draining forests (from 1670 to 2480 ppm), croplands (from 1010 to 1550 ppm), and at the outlet of the basin (from 2490 to 3630 ppm). The main driver of pCO2 was shown to be dissolved organic carbon (DOC) concentrations (R2 = 0.56, n = 119, p < 0.05) that are modulated by hydro-climatic conditions and groundwater discharges. DOC sources were linked to land use and soil, mainly leaching into small upstream streams, but also to organic pollution, mainly found downstream in larger rivers. Our long-term analysis of the main stream suggests that pCO2 closely mirrors the pattern of urban water pollution over time. These results suggest that factors controlling pCO2 operate differently upstream and downstream depending on the physical characteristics of the river basin and on the intensity and location of the main anthropogenic pressures. The influence of these controlling factors may also differ over time, according to the seasons, and mirror long term changes in these anthropogenic pressures.

How can water quality be improved when the urban waste water directive has been fulfilled? A case study of the Lot river (France).

The Lot river, a major tributary of the downstream Garonne river, the largest river on the Northern side of the Pyrenees Mountains, was intensively studied in the 1970s. A pioneering program called "Lot Rivière Claire" provided a diagnosis of water quality at the scale of the whole watershed and proposed an ambitious program to manage nutrient pollution and eutrophication largely caused by urban wastewater releases. Later on, the implementation of European directives from 1991 to 2000 resulted in the nearly complete treatment of point sources of pollution in spite of a doubling of the basin's population. At the outlet of the Lot river, ammonium and phosphate contamination which respectively peaked to 1 mg N-NH4 L-1 and 0.3 mg P-PO4 L-1 in the 1980s returned to much lower levels in recent years (0.06 mg N-NH4 L-1 and 0.02 mg P-PO4 L-1), a reduction by a factor 15. However, during this time, nitrate contamination has regularly increased since the 1980s, from 0.5 to 1.2 mg N-NO3 L-1 in average, owing to the intensification of agriculture and livestock farming. Application of the Riverstrahler model allowed us to simulate the water quality of the Lot drainage network for the 2002-2014 period. We showed that, with respect to algal requirements, phosphorus and silica are well balanced, but nitrogen remains largely in excess over phosphorus and silica. This imbalance can be problematic for the ecological status of the water bodies. Using the model, for simulating various scenarios of watershed management, we showed that improvement of urban wastewater treatment would not result in any significant change in the river's water quality. Even though arable land occupies a rather limited fraction of the watershed area, only the adoption of better farming practices or more radical changes in the agro-food system could reverse the trend of increasing nitrate contamination.

How to avoid eutrophication in coastal seas? A new approach to derive river-specific combined nitrate and phosphate maximum concentrations.

Since 1950, increase in nitrogen (N) and phosphorus (P) river loadings in the North-East Atlantic (NEA) continental seas has induced a deep change in the marine coastal ecosystems, leading to eutrophication symptoms in some areas. In order to recover a Good Ecological Status (GES) in the NEA, as required by European Water Framework Directive (WFD) and Marine Strategy Framework Directive (MSFD), reductions in N- and P-river loadings are necessary but they need to be minimal due to their economic impact on the farming industry. In the frame of the "EMoSEM" European project, we used two marine 3D ecological models (ECO-MARS3D, MIRO&CO) covering the Bay of Biscay, the English Channel and the southern North Sea to estimate the contributions of various sources (riverine, oceanic and atmospheric) to the winter nitrate and phosphate marine concentrations. The various distributed descriptors provided by the simulations allowed also to find a log-linear relationship between the 90th percentile of satellite-derived chlorophyll concentrations and the "fully bioavailable" nutrients, i.e. simulated nutrient concentrations weighted by light and stoichiometric limitation factors. Any GES threshold on the 90th percentile of marine chlorophyll concentration can then be translated in maximum admissible 'fully bioavailable' DIN and DIP concentrations, from which an iterative linear optimization method can compute river-specific minimal abatements of N and P loadings. The method has been applied to four major river groups, assuming either a conservative (8µgChlL-1) or a more socially acceptable (15µgChlL-1) GES chlorophyll concentration threshold. In the conservative case, maximum admissible winter concentrations for nutrients correspond to marine background values, whereas in the lenient case, they are close to values recommended by the WFD/MSFD. Both models suggest that to reach chlorophyll GES, strong reductions of DIN and DIP are required in the Eastern French and Belgian-Dutch river groups.

Nutrient inputs and hydrology together determine biogeochemical status of the Loire River (France): Current situation and possible future scenarios.

The Grafs-Seneque/Riverstrahler model was implemented for the first time on the Loire River for the 2002-2014 period, to explore eutrophication after improvement of wastewater treatments. The model reproduced the interannual levels and seasonal trends of the major water quality variables. Although eutrophication has been impressively reduced in the drainage network, a eutrophication risk still exists at the coast, as shown by the N-ICEP indicator, pointing out an excess of nitrogen over silica and phosphorus. From maximum biomass exceeding 120 µgChla l-1 in the 1980's, we observed decreasing maximum values from 80 to 30 µgChla l-1 during the period studied. Several scenarios were explored. Regarding nutrient point sources, a low wastewater treatment scenario, similar to the situation in the 1980's, was elaborated, representing much greater pollution than the reference period (2002-2014). For diffuse sources, two agricultural scenarios were elaborated for reducing nitrogen, one with a strict application of the agricultural directives and another investigating the impact of radical structural changes in agriculture and the population's diet. Although reduced, a risk of eutrophication would remain, even with the most drastic scenario. In addition, a pristine scenario, with no human activity within the basin, was devised to assess water quality in a natural state. The impact of a change in hydrology on the Loire biogeochemical functioning was also explored according to the effect of climate change by the end of the 21st century. The EROS hydrological model was used to force Riverstrahler, considering the most pessimistic SRES A2 scenario run with the ARPEGE model. Nutrient fluxes all decreased due to a >50% reduction in the average annual discharge, overall reducing the risk of coastal eutrophication, but worsening the water quality status of the river network. The Riverstrahler model could be useful to help water managers contend with future threats in the Loire River, at the scale of its basin and at smaller nested scales.

Cost assessment and ecological effectiveness of nutrient reduction options for mitigating Phaeocystis colony blooms in the Southern North Sea: an integrated modeling approach.

Nutrient reduction measures have been already taken by wealthier countries to decrease nutrient loads to coastal waters, in most cases however, prior to having properly assessed their ecological effectiveness and their economic costs. In this paper we describe an original integrated impact assessment methodology to estimate the direct cost and the ecological performance of realistic nutrient reduction options to be applied in the Southern North Sea watershed to decrease eutrophication, visible as Phaeocystis blooms and foam deposits on the beaches. The mathematical tool couples the idealized biogeochemical GIS-based model of the river system (SENEQUE-RIVERSTRAHLER) implemented in the Eastern Channel/Southern North Sea watershed to the biogeochemical MIRO model describing Phaeocystis blooms in the marine domain. Model simulations explore how nutrient reduction options regarding diffuse and/or point sources in the watershed would affect the Phaeocystis colony spreading in the coastal area. The reference and prospective simulations are performed for the year 2000 characterized by mean meteorological conditions, and nutrient reduction scenarios include and compare upgrading of wastewater treatment plants and changes in agricultural practices including an idealized shift towards organic farming. A direct cost assessment is performed for each realistic nutrient reduction scenario. Further the reduction obtained for Phaeocystis blooms is assessed by comparison with ecological indicators (bloom magnitude and duration) and the cost for reducing foam events on the beaches is estimated. Uncertainty brought by the added effect of meteorological conditions (rainfall) on coastal eutrophication is discussed. It is concluded that the reduction obtained by implementing realistic environmental measures on the short-term is costly and insufficient to restore well-balanced nutrient conditions in the coastal area while the replacement of conventional agriculture by organic farming might be an option to consider in the nearby future.

Assessing the effect of nutrient mitigation measures in the watersheds of the Southern Bight of the North Sea.

The Seine, Somme, and Scheldt Rivers (France, Belgium, and Netherlands) are the major delivering rivers flowing into the continental coastal zone of the Southern Bight of the North Sea, an area regularly affected by eutrophication problems. In the present work, the Seneque-Riverstrahler model was implemented in a multi-regional case study in order to test several planned mitigation measures aimed at limiting stream nutrient contamination and restoring balanced nutrient ratios at the coastal zone. This modeling approach, which is spatially distributed at the basin scale, allows assessing the impact of any change in human activities, which widely differ over the three basins. Here, we define realistic scenarios based on currently proposed measures to reduce point and non-point sources, such as the upgrading of wastewater treatment, the introduction of catch crops, and the development of extensive farming. An analysis of the current situation showed that a 47-72% reduction in P point-source emissions within the three basins could be reached if the intended P treatment was generalized to the largest treatment plants. However, only an overall 14-23% reduction in N could be achieved at the outlet of the three basins, by combining improved wastewater treatment and land use with management measures aimed at regulating agricultural practices. Nonetheless, in spite of these efforts, N will still be exported in large excess with respect to the equilibrium defined by the Redfield ratios, even in the most optimistic hypothesis describing the long-term response of groundwater nitrate concentrations. A comprehensive assessment of these mitigation measures supports the need for additional reductions of nutrient losses from agriculture to control harmful algae development. It also stresses the relevance of this mechanistic approach, in which nutrient transfers from land to sea can be calculated, as an integrated strategy to test policy recommendations.