Reactive-transport modelling of C, N, and O2 in a river–estuarine–coastal zone system: Application to the Scheldt estuary

Abstract

A fully coupled, two-dimensional hydrodynamic and reactive-transport model of C, N, O 2 and Si along a river-estuarinecoastal zone system is presented. It is applied to the Scheldt continuum, a macrotidal environment strongly affected by anthropogenic perturbations. The model extends from the upper tidal river and its tributaries to the southern Bight of the North Sea. Five dynamically linked nested grids are used, with a spatial resolution progressively increasing from 33 m to 2.7 km. The biogeochemical reaction network consists of aerobic degradation, nitrification, denitrification, phytoplankton growth and mortality, as well as reaeration. Diagnostic simulations of a typical summer situation in the early 1990s are compared to field data taken from the OMES database (>300 samples per variable). Results demonstrate that the process rates in the tidal river are very high and far larger than in the saline estuary, with maximum nitrification rates in the water column up to 70 mM N day − 1 , and maximum aerobic respiration and denitrification up to 70 and 40 mM C day − 1 , respectively. Phytoplankton production is about one order of magnitude lower, a result which confirms the dominance of heterotrophic processes in this system. The influence of secondary and tertiary wastewater treatment in the catchment is then assessed. Results show a significant decrease of organic matter and ammonium concentrations above Antwerp, which in turn leads to a partial restoration of oxygen levels. The model also predicts a reduction of denitrification rates, which locally results in a 4-fold increase of the nitrate concentration. Mass budgets for carbon, nitrogen and oxygen are established for the saline estuary (km 0 to 100) and for the tidal river network (km 100 to 160). Three scenarios, corresponding to the situation in the early 1990s, the years 2000 and the situation expected in 2010 are considered. They show that the tidal river and the estuary contribute almost equally to the overall biogeochemical cycling of these elements, despite the very different water volumes involved. For the simulated periods, the large decrease in nitrogen input (> 55%) expected between 1990 and 2010 will not lead to a significant decrease of N export to the coastal zone during the summer period.

pH modelling in aquatic systems with time-variable acid-base dissociation constants applied to the turbid, tidal Scheldt estuary

“…Water movement in the Scheldt estuary is dominated by large tidal displacements. The volume of water that enters during flood tide at the downstream boundary of the estuary is 200 times larger than the freshwater discharge integrated over a whole tidal cycle at the upstream boundary (Vanderborght et al, 2007). Yearly averaged, the freshwater discharge is around 100 m 3 s −1 (Heip, 1988).…”

Modelling phytoplankton succession and nutrient transfer along the Scheldt estuary (Belgium, The Netherlands)

“…Hofmann et al, 2008) or only simulate a limited period of time (e.g. summer time; Arndt et al, 2007;Vanderborght et al, 2007). These models are appropriate to reproduce the annual or decadal evolution of biogeochemical processes and concentration along the Scheldt estuary with sometimes a better result than those obtained with our complex model (for N and O 2 in particular).…”

“…However, nutrient exports to the coastal zone are largely controlled by river inputs and annual flows of nutrients, N in particular, must be carefully considered and will be only discussed in a comparative and qualitative way. Several physical-biogeochemical 1D (Cox et al, 2009;Hofmann et al, 2008;Regnier et al, 1997;Soetaert and Herman, 1995a;Vanderborght et al, 2002) or 2D (Arndt et al, 2007(Arndt et al, , 2009Vanderborght et al, 2007) models were previously implemented in the Scheldt estuary to study C, nutrients and/or O 2 biogeochemical transformations and fluxes. In these estuarine models, phytoplankton process formulations are generally based on total primary production (generally associated to diatoms growth) without distinction between phytoplankton groups (e.g.…”

“…Over the last two decades, several physical-biogeochemical models have been implemented in the Scheldt estuary for estimating the nutrient export to the coastal zone (Arndt et al, 2007;Hofmann et al, 2008;Regnier et al, 1997;Soetaert and Herman, 1995a,b;Vanderborght et al, 2002Vanderborght et al, , 2007. The description of the biological processes in these estuarine models is generally simple (based on total primary production) and the estuary is often considered as a unidirectional land-ocean transition.…”

“…However, global change also affects the functioning of the coastal ocean and subsequently impacts the lower estuary. The adjacent coastal sea has been explicitly considered in a few Scheldt estuary models (Arndt et al, 2007;Vanderborght et al, 2007) but these were constrained by observed concentrations at the upper boundary of the estuary. A step further then consists in implementing high resolution coupled physical-biogeochemical models of the whole river-estuaryocean system for describing the effect of changing climate and human pressure on estuarine communities and related biogeochemical processes.…”