Nitrogen dynamics in the Westerschelde estuary (SW Netherlands) estimated by means of the ecosystem model MOSES

Abstract

A tentative nitrogen budget for the Westerschelde (SW Netherlands) is constructed by means of a simulation model with thirteen spatial compartments. Biochemical and chemical processes in the water column are dynamically modeled; fluxes of dissolved constituents across the water-bottom interface are expressed by means of diagenetic equations.The model is calibrated on a large amount of observed variables in the estuary (1980)(1981)(1982)(1983)(1984)(1985)(1986) with relatively fine temporal and spatial detail. Additional constraints are imposed by the stoichiometric coupling of carbon, nitrogen and oxygen flows and the required conservation of mass. The model is able to reproduce rather well the observed distributions of nitrate, ammonium, oxygen and Kjeldahl nitrogen both in time and space. Also, model output of biochemical oxygen demand and total organic carbon falls within observed ranges.By far the most pervasive process in the nitrogen cycle of the estuary is nitrification which mainly takes place in the water column of the upper estuarine part. On average about three times as much nitrate is leaving the estuary at the sea side compared to what enters from the river and from waste discharges. Ammonium on the other hand is consumed much faster (nitrification) than it is regenerated and only about one third of the total import leaves the estuary at the sea side. The budget for detrital nitrogen reveals import from the river, from wastes and from the sea. Phytoplankton uptake of inorganic nitrogen is negligible in the model. About 21% of total nitrogen, 33% of inorganic nitrogen, is removed from the estuary (mainly to the atmosphere through denitrification) and the load of nitrogen net exported to the sea amounts to about 51 000 tonnes per year. Total denitrification in our model is lower than what was estimated in the literature from the late seventies, where a nitrogen removal up to 40-50% of the total inorganic load was reported. Part of the differences could be methodological, but inspection of the nutrienf profiles hat led to these conclusions show them to be different to the ones used in our study. The oxygen deficient zone has moved upstream since the late seventies, entrailing the zone of denitrification into the riverine part of the Schelde. The nitrification process now starts immediately upon entering the estuary.

Excess atmospheric carbon dioxide transported by rivers into the Scheldt estuary

“…The total organic carbon input (dissolved and particulate: 83·10 3 TC·yr -1 for an average total river discharge of 70 m 3 ·s -1 ) is consistent with previous estimates that give values around 100·10 3 TC·yr -1 for an average discharge of 90 m 3 ·s -1 [27,31]. Excess CO 2 represented 7 % of the DIC in the Zenne, 13 % in the Nete and 10 % in the Scheldt, Dender and Dijle.…”

“…For an average river flow of 90 m 3 ·s -1 , the total input of excess CO 2 from rivers into the estuary is estimated to 18·10 3 TC·yr -1 . The flows of carbon through the Scheldt estuary are now well documented, both by field and model approaches [8,12,13,23,24,27,31]. In contrast to highly turbid French estuaries (Gironde and Loire [1,2]), there is no evidence for carbonate dissolution or precipitation in the Scheldt estuary [23].…”

Modeling increased riverine nitrogen export: Source tracking and integrated watershed-coast management

“…It has been reported that the value of L N ranges from 0.20 to 0.76 and is strongly influenced by river watershed size (Behrendt and Opitz, 1999;Dumont et al, 2005;Seitzinger et al, 2002;Soetaert and Herman, 1995). According to an empirical formulation for denitrification loss as a function of watershed area (Dumont et al, 2005), L N was estimated to be 0.51 for the North River and 0.45 for the West River.…”

Tuning the tide: creating ecological conditions for tidal marsh development in a flood control area

“…The Schelde has a long tradition of extensive anthropogenic pollution and is characterised as eutrophic (Soetaert & Herman, 1995;Struyf et al, 2004;Van Damme et al, 2005). Notwithstanding large efforts for industrial and municipal wastewater treatment during the last decade in Flanders and other parts of the catchment, water quality is still bad.…”

“…A small rise in the oxygen concentration can have a big influence on the nitrogen cycling. In the very polluted Schelde estuary, nitrogen removal is completely dominated by denitrification in the oxygen poor water column (Soetaert & Herman, 1995). An increase in oxygen concentration will decrease the pelagic denitrification rate.…”