RESUMEN
Sediment fluxes at the estuary-sea interface strongly impact particle matter exchanges between marine and continental sources along the land-sea continuum. However, human activities drive pressures on estuary physical functioning, hence threatening estuarine habitats and their ecosystem services. This study explores a 22-year numerical hindcast of the macrotidal Seine Estuary (France), experiencing contrasted meteorological conditions and anthropogenic changes (i.e., estuary deepening and narrowing). The hindcast was thoroughly validated for both water column and sediment bed dynamics and showed good capacities to simulate annual sediment budgets observed from 1990 to 2015. We aim at disentangling the relative contributions of meteorological and human-induced morphological changes on net sediment fluxes between the estuary and its adjacent coastal sea. Our results highlight that intense wave events induce fine sediment (≤ 100 µm) export to the sea but coarser sediment (≥ 210 µm) import within the estuary. Although intense river discharges induce mud export to the sea, moderate to large river discharges prove to support mud import within the estuary. Wave and river discharge events were less intense in 2005-2015 than in 1990-2000, reducing fine sediment export to the sea. The estuary deepening and narrowing due to human activities increased fine sediment import within the estuary, shifting the estuary from an exporting to importing system. We propose a conceptualization of mud flux response to river discharge and wave forcing, as well as anthropogenic pressures. It provides valuable insights into particle transfers along the land-sea continuum, contributing to a better understanding of estuarine ecosystem trajectories under global changes.
RESUMEN
The scientific teams from the interdisciplinary Seine-Aval (SA) research program and the SA's operational pole, GIPSA (Groupement d'Intérêt Public Seine-Aval) have worked together to create a report card designed to help the Estuary Council (Conseil de l'Estuaire) revitalize its original functions: maintaining functional links between the various estuarine ecosystems, comprehending and managing the estuary's natural habitats and biological populations, and monitoring and improving the physical-chemical quality of the estuarine waters. The report card will be able to synthesize the information obtained from several system performance variables and available operational indicators. This approach, intended to guide the estuary managers, is the oeuvre of several scientific teams; it is particularly important in the context of the Water Framework Directive because it facilitates the elaboration of a group of relevant indicators, which can then be used as operational tools. A report card will provide decision-makers (e.g., political authorities; national, regional and local institutions and industries) with the key indicators for evaluating the system and predicting changes in terms of selected objectives, such as the preservation and restoration of the estuary's environmental functionalities. The final objective of the research is to choose among the available indicators to approximate potential ecological risks. Integrating the socio-economical data will perhaps lead to setting risk acceptability thresholds for the different uses of the Seine estuary. In the end, collaboration between the scientists, the managers, and the GIPSA operational pole will be essential to produce a viable report card about the environmental status of the Seine estuary. To illustrate the research now under way, this article presents the results for three actions undertaken, concerning: (i) physical indicators (i.e., an inventory of the estuary first as a whole, and then section by section); (ii) benthic indicators (i.e., seven indices which show a moderate EcoQ for the lower part of the estuary); and (iii) a eutrophication indicator (i.e., an indicator for coastal eutrophication potential (ICEP), which helps to limit the nutrient fluxes (N or P) that exceed the silica flux delivered by the Seine network, based on the Redfield ratios for algal propagation).