D 3.1 Acquisition des données pour l’interprétation de la pression associée des prélèvements d’eau souterraine sur l’état et le fonctionnement écologique des cours d’eau

Délivrable du projet ESO-ESUCaractérisation complémentaire des masses d’eau dont le bon état dépend d’interactions entre les eaux de surface et les eaux souterraine

Biogeochemical budgets in three large reservoirs of the Seine basin (Marne, Seine & Aube reservoirs)

International audienc

Phytoplankton pigment analysis as a tool for monitoring a tropical great lake, Lake Kivu (East Africa)

Lake Kivu, East Africa, is a deep oligotrophic and meromictic lake containing high amounts of dissolved methane (∼55–60 km3) and carbon dioxide (∼300 km3) in its deep waters. Methane harvesting for energy production began in 2015, and a monitoring programme was set up to assess possible disturbance on the ecosystem. Phytoplankton biomass and composition was assessed twice per month or monthly at 2 monitoring sites between June 2005 and December 2019, based on HPLC analysis of chlorophyll a (Chl-a) and marker pigments. This long-term series shows that significant changes occurred around 2010 in the lake phytoplankton, with a notable increase of Chl-a and changes in the assemblage toward an increase in non-motile green algae and diatoms. To assess possible changes due to methane harvesting, we compared 2 periods, 2012–2014 and 2018–2019. Chl-a concentration decreased slightly in 2018–2019 compared to the reference period of 2012–2014, and significant changes occurred in composition of the phytoplankton assemblage. In terms of relative contribution to Chl-a, diatoms increased from 26% to 46%, whereas green algae decreased ∼2-fold, from 35% in 2012–2014 to 18% in 2018–2019. Multivariate analyses showed that phytoplankton composition was influenced by seasonal and interannual variations of limnological variables related to changes in meteorological factors. To assess possible future changes due to methane exploitation, we recommend increasing sampling frequency and taxonomic resolution, as well as improving environmental data acquisition

Biogeochemical mass-balances (C, N, P, Si) in three large reservoirs of the Seine Basin (France)

Three major reservoirs (Marne, Seine and Aube), situated in the upstream basin of the river Seine represent a storage capacity of 800 106 m3. In order to quantify the possible role of these reservoirs as a sink or source of nutrients and organic matter for the river system, an input/output mass-balance of suspended matter, organic carbon, inorganic nitrogen forms, phosphorus and reactive silica was established, providing reliable estimates of their retention/elimination and export. The study was carried out over 3 years (1993, 1994 and 1995) in differing hydrological conditions. The retention times varied from 0.3 to 0.8 year, depending on the reservoir and the year, but was longer in 1993 that was a drier year than 1994 and 1995, hydrologically quite similar. Regarding retention (or elimination) and export, the behaviour of the three studied reservoirs was similar. A clear loss or retention of nitrogen, phosphorus and silica was observed in the reservoirs and represented about 40% of the incoming flux of nitrate, 50% of silica, and 60% of phosphate. The retention was lower for total phosphorus than for phosphate. The reservoirs are also sites of suspended matter deposition except during the decennial drawdown, when suspended matter is exported. For inorganic nitrogen, the average amount of nitrate retained in the Seine basin reservoirs upstream from Paris is 5000 tonnes y-1 that is almost equal to the estimated retention by deposition or denitrification in river channel sediments for the whole drainage network. The retention in the reservoirs represents about 12% of the total flux of nitrate at the outlet of the basin upstream from Paris, and 5% at the mouth of the Seine River. We also calculated inlake C, N, P, Si budgets on the basis of direct process measurements. Measurements of planktonic primary and bacterial activity production led to annual net production of 4200 and 580 tonnes of carbon, respectively. A reasonable value (450 tonnes of carbon) of grazing was calculated. Corresponding N, P, Si fluxes were drawn from appropriate C:N:P:Si ratios. Benthic fluxes were measured with bell jars. The retention of P and Si represents a small fraction of important internal fluxes of phytoplanktonic uptake and recycling, while inorganic nitrogen retention depends mostly on benthic denitrification. The behaviour of P and Si differs in that P is mainly recycled in the water column, while Si dissolution occurs at the sediment interface. Nitrogen is recycled in both the planktonic and the benthic phase.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Ecological functioning of the marne reservoir (upper seine basin, France)

The Marne reservoir (Der and Champaubert Lakes, 48 km2, 8 m mean depth) is one of the three major reservoirs constructed on the upper reaches of the Seine river to regulate its flow. The major features of the ecological functioning of the reservoir were analysed on the basis of results obtained in a 3-year routine sampling programme where the main variables of water quality were determined upstream, downstream and within the reservoir. Planktonic and benthic processes were also investigated in 1 year. A model of the reservoir was developed by coupling the RIVE model (which describes the major microbiological processes involved in the ecological functioning of aquatic systems), with a description of the morphology and hydrology of the reservoir. The model was used to simulate the seasonal and year-to-year variations of nutrients (N, P, Si) and phytoplankton biomass, and to compare them with the observed data. The model was then used to explore the effect of different control factors on denitrification, namely residence times and phosphorus loading. The results showed little influence on the absolute amount of nitrate denitrified within a realistic range of residence times and phosphorus loading. The Marne reservoir appears however to be a significant sink for nitrate (40% of the influx is eliminated), but this reduction represents only 10% of the nitrate flux at the outlet of the Marne sub-basin. Copyright © 2000 John Wiley & Sons, Ltd.SCOPUS: ar.jFLWINinfo:eu-repo/semantics/publishe