Importance of nitrate reduction in benthic carbon mineralization in two eutrophic estuaries: Modeling, observations and laboratory experiments

Abstract

Estuaries are important nutrient filters between rivers and coastal zones. However, the quantification of the nutrient mitigation capacity related to benthic diagenesis is still poorly quantified. In this paper, we investigated carbon mineralization and the contribution of benthic nitrate reduction in two macrotidal eutrophic estuaries (Elorn and Aulne, Brittany, France) during winter and spring. These anthropized estuaries exhibited large variations of bottom water nitrate concentrations from very high values in upstream waters (up to 500 µM) to low values downstream (less than 10 µM). Bottom water oxygen concentrations presented small gradients compared to nitrate concentrations gradients resulting in large variation in nitrate to oxygen ratios NO x-/O 2 between downstream and upstream (0.03-1.6). We combined the use of diagenesis modeling with field data (porosity, organic carbon and nitrogen, pore water profiles of dissolved oxygen, nitrogen, iron, manganese and sulfide concentrations, published in Khalil et al. 2013) and experimentally-determined nitrate reduction rates, in order to investigate the different organic carbon mineralization pathways in these estuarine sediments including denitrification and the contribution of benthic nitrate reduction to the estuarine N budget. Overall a good agreement between pore water data (organic carbon, oxygen, nitrate and ammonium) and model simulations was observed. The modeled organic matter mineralization rates were high in the upstream estuary and low in the saline estuary for the two estuaries Aulne and Elorn. This decrease may be related to the dilution and the trapping of allochtonous organic matter in estuarine sediments and its subsequent recycling in the upper estuary. Organic carbon mineralization rates were higher in the Elorn than in the Aulne estuary, which is most likely related to the labile character of the organic matter from urban origin exported from the Elorn watershed. The contribution of nitrate reduction to the total mineralization was generally high in upstream sediments (15-35%) of both Aulne and Elorn estuaries and decreased consistently downstream to 5-10%. The relative large contribution of nitrate reduction to organic matter degradation was to a large extent related to high bottom water nitrate concentrations that fueled 37-78% of total nitrate reduction in the upstream part of the estuaries.

Are benthic nutrient fluxes from intertidal mudflats driven by surface sediment characteristics?

“…the TOC content (Table 2). Indeed, an increase in the deposition of OM in the sediment, supported by the river export of fine mud particles (with higher porosity, Table 2), can improve the mineralization rates of SOM, as observed in the Elorn and Aulne estuaries in Brittany [Khalil et al, 2018], and thus lead to a larger NH + 4 regeneration in porewaters of sediment. Thereby, higher NH + 4 recycling rates combined with higher sediment-water exchanges could enhance the diffusion of NH + 4 to the overlying water.…”

“…To a large extent, they depend on (1) the redox conditions, (2) the deposition rate and the composition of the sedimentary organic matter (SOM), and (3) the physical properties of the sediment [e.g. Ait Ballagh et al, 2020, Arndt et al, 2013, Dauwe et al, 2001Khalil et al, 2018, Middelburg et al, 1996. Redox conditions, driven by electron acceptor availability in the porewater, control the microbially mediated processes in N and P cycling [Capone et al, 2008 and references therein;Paytan andMcLaughlin, 2007, Sundby et al, 1992] and the adsorption-desorption processes of PO 4 onto iron oxides [Andrieux-Loyer et al, 2008, Krom and Berner, 1980, Rozan et al, 2002.…”

Nitrogen mineralization and immobilization in surface sediments of coastal reclaimed aquaculture ecosystems

“…anoxic conditions (Khalil et al, 2018). Under anaerobic conditions, NO 3 − , Fe 3+ , SO 4 2− as well as iron and manganese ions often work as active electron acceptors for coupling redox reactions with sediment organic matter (equation 4 and 5) (Pena et al, 2010).…”

Offshore Windfarm Footprint of Sediment Organic Matter Mineralization Processes

“…Furthermore, this approach is not limited to the chosen model domain. Both COAWST and OMEXDIA are flexible models that have been applied to a variety of marine settings [OMEXDIA: (e.g., Soetaert et al, 1996a;Laurent et al, 2016;Khalil et al, 2018;De Borger et al, 2021), COAWST: Warner et al, 2010;Fennel et al, 2013;Bolaños et al, 2014;Zang et al, 2020], including other regions where OWF development may take place. So for now, we have provided a simplification of a complex ecological chain of events, that may be used as a basis in future model developments.…”