Modeling the biogeochemical functioning of the Seine estuary and its coastal zone: Export, retention, and transformations

Abstract

The model ECO-MARS3D, successfully applied thus far to coastal ecosystems, has been extended to encompass the Seine estuary up to Poses, at the limit between the river and the estuary. We used updated bathymetric data and thoroughly calibrated the hydrodynamics and the sedimentary dynamics in the turbidity maximum zone (TMZ). Biogeochemical processes related to oxygen were newly implemented, and freshwater phytoplankton and zooplankton groups were added to the existing marine groups. The simulations allowed us to evaluate the filter effect of the estuary with regard to the main nutrients (N, P, and Si). Today, this filter role appears quite limited and variable depending on the hydrology. On average, considering three different hydrological years, the estuary was able to retain (at least temporarily) 19 kt N yr −1 , 10 kt Si yr −1 , and 0.7 kt P yr −1 , amounting to −13%, −11%, and −27% of the total N, Si, and P inputs, respectively. Seasonal differences in the filtering capacity (lower in winter than in summer) were similar between wet and dry years. Nutrient retention was higher in the upstream fluvial estuary than in the TMZ, the former being mostly characterized by uptake, the latter by remineralization. At the coastal zone, the dry year showed greater risk of eutrophication. Despite lower discharge and lower nutrient fluxes, inputs were confined into a shallower layer, close to the coastline, where nutrients were concentrated and allowed phytoplankton to thrive. Differences in the N : P : Si ratios (lower P-deficit during dry years) could also underpin the larger proliferations, and notably the larger dinoflagellate blooms during dry years.

Carbon Dynamics Along the Seine River Network: Insight From a Coupled Estuarine/River Modeling Approach

“…In addition, the several tributaries located downstream of Poses, which are also explicitly simulated by Riverstrahler are accounted for as point sources into C-GEM (the Risle, Austerberthe, Cailly, Andelle, and Eure Rivers, Figure 1). Downstream, C-GEM is constrained by modeling results from the ECO-MARS3D model at Le Havre (Romero et al, 2018). Both C-GEM and Riverstrahler include an explicit description of suspended particulate matter (SPM) which accounts for erosion and deposition processes as a function of the flow velocity.…”

“…The genericity of the Riverstrahler model has been demonstrated throughout successful applications to contrasted fluvial basins, from temperate climate including (among other) the Moselle (Garnier et al, 2000), the Danube (Garnier et al, 2002), the Scheldt (Thieu et al, 2009) among other, sub-tropical climate as the Red River (Le et al, 2014) or the Nam Kan river (Causse et al, 2015) and or under nordic climate, such as the Kalix and Lule rivers (Sferratore et al, 2008) and the Seine river (Raimonet et al, 2018). The Riverstrahler model has also been chained with the coastal marine model (ECO-MARS3D, Cugier et al, 2005), extended to the estuarine part in terms of hydrosedimentary representation (Passy et al, 2016), and with nutrient description (Romero et al, 2018). The development of the pyNuts modeling environment has made it possible, with FIGURE 2 | (A) Width and (B) depth profiles of the Seine estuary from its mouth to Poses.…”

“…These include concentrations for all state-variables of the model and time series for water elevation at the marine boundary as well as fresh water discharge at the riverine boundary. Most of these data are provided by outputs of the ECO-MARS3D model (Romero et al, 2018) at the downstream boundary and by Riverstrahler at the upstream boundary ( Table 2). Downstream, all required inputs were available at a 1 h temporal resolution, which allows capturing the tidal cycle.…”

“…The simulations presented in this study are, to our knowledge, the first transient representation of the inorganic carbon biogeochemical processing of an entire river network resulting from the full coupling of a riverine and estuarine RTM. Such an off-line coupling was recently published using Riverstrahler and ECO-MARS3D for nutrients biogeochemical processing only (Romero et al, 2018). The relatively good performances, at least in terms of level of water quality variables, of both C-GEM and Riverstrahler for chlorophyll, nutrients, oxygen as well as organic and inorganic carbon dynamics proves that our approach is able to capture the main biogeochemical dynamics of the Seine river network all along its main axis and provide a realistic representation of the main biogeochemical processing taking place in the different tributaries (i.e., nutrient consumption, organic matter degradation etc.).…”

“…Spatially and temporally resolved numerical models are necessary to capture the CO 2 dynamics at the air-water interface, both in time and space (Regnier et al, 2013b). In this context, reactive transport models (RTM) allow reproducing the changes in chemical species concentrations due to both transport and biogeochemical transformations, thus providing a mechanistic insight into the fate of chemical species of interest in riverine and estuarine environments (Volta et al, 2014(Volta et al, , 2016aMarescaux, 2018;Romero et al, 2018).…”