Spatial Variability of Organic Matter and Phosphorus Cycling in Rhône River Prodelta Sediments (NW Mediterranean Sea, France): a Model-Data Approach

Abstract

The Mediterranean Sea (MS) is a large oligotrophic sea whose productivity is sensitive to riverine nutrient inputs. More specifically, phosphorus (P) river supply is crucial for the MS, with an important role of the estuarine/deltaic filter especially for the storage and recycling in sediments. A benthic dataset from the Rhône River prodelta was used to derive P budgets, by means of an early diagenetic model including the benthic P cycle. The model was fitted to pore water profiles of oxygen, nitrate, sulfate, dissolved inorganic carbon, ammonium, oxygen demand units, dissolved inorganic phosphorus (DIP) and solid data (organic carbon (OC), Fe-bound P, Ca-bound P and organic P). Results indicated that the intensity of biogeochemical processes occurring below the sediment-water interface decreased from the river mouth to the adjacent continental shelf with decreasing integrated rates of OC mineralization (160-10 mmol m−2 day−1). The organic P mineralization was intense near the river mouth and decreased offshore (1196-80 μmol m−2 day−1). Its contribution to DIP release was large (> 90%). Fe-bound P had a key role in transferring P to deeper layers. These deltaic sediments played an important role as a source of regenerated DIP. A significant part of DIP was recycled to the overlying waters (72-94%), representing 25% of the riverine DIP discharge. Simultaneously, 6-28% of DIP produced in sediments was buried as Ca-bound P. Overall, this study highlighted the importance of deltaic sediments as an additional source of DIP to the coastal sea, and a minor but permanent sink of phosphorus as solid P burial.

FESDIA (v1.0): exploring temporal variations of sediment biogeochemistry under the influence of flood events using numerical modelling

“…Our model combines the development in the OMEXDIA model (Soetaert et al, 1996a), applied in the Rhône prodelta area (Ait Ballagh et al, 2021;Pastor et al, 2011) and which has recently been equipped with event-driven processes (De Borger et al, 2021). In De Borger et al (2021, the authors specifically addressed the issue of bottom trawling as a mixing and an erosional process that removes an upper layer of sediment and mixes a certain layer below.…”

“…Although, this assumption is a restrictive as the site of flood deposition can undergo variation in grain size which might affect their porosity (Cathalot et al, 2010), we proceed noting that the fixed parameters which define the porosity curve can be changed when necessary. Thus, using optimized parameters fitted with data in the proximal sites of Rhône prodelta (Ait Ballagh et al, 2021), porosity (φ (z)) in Eqs. ( 10)-( 8) is prescribed as an exponential decay,…”

“…For the solid species, following the flux of organic carbon via the boundary condition (see Sect. 2.2.7), the portion of organic carbon is split between the fast and slow decaying component using a proportionality constant (pfast) as in Ait Ballagh et al (2021). pfast varies from 0 to 1 and it is expressed in percentage of carbon flux deposited associated to either fraction (fast and slow).…”

“…The model parameters in Table 2 (for full model parameter, see Table S1 in Supplement) were derived from previously published model in the Rhône Delta (Pastor et al, 2011;Ait Ballagh et al, 2021). The organic matter stoichiometry for both fractions is represented here by the NC ratio (NCrFdet and NCrSdet) with values of 0.14 and 0.1, respectively.…”

“…The decay rate constant for the labile and semi-labile detritus matter is set as 0.1 and 0.0031 d −1 , respectively, with both fractions split equally with a proportionality constant (pfast) of 0.5. Using parameters fitted by the model of Ait Ballagh et al (2021) to data observed in the Rhône prodelta area, the rate of bioturbation and bioirrigation is fixed as 0.01 cm 2 d −1 and 0.23 d −1 with these fauna-induced activities occurring down to a depth of 5 and 7 cm, respectively. The bottom water temperature was fixed at 20 • C. The bottom water salinity is nearly constant below the Rhône River plume, ranging from 37.8 to 38.2.…”