NITROGEN±ALKALINITY INTERACTIONS IN THE HIGHLY
POLLUTED SCHELDT BASIN (BELGIUM)


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Abstract

We present results of one year observations in highly heterotrophic and oxygen-depleted rivers
of the polluted Scheldt basin. Monthly measurements revealed a high variability for dissolved inorganic
carbon and nitrogen, with the following strong parallelism: highest alkalinity and NH4
+ were associated
with lowest NO3ÿ and oxygen and vice-versa. In river water incubations, nitri®cation lowered the alkalinity
whereas denitri®cation raised it; in an anoxic, NO3ÿ-free incubation an increase of alkalinity was observed,
partially due to ammoni®cation.

INTRODUCTION

Dissolved inorganic carbon (DIC) in rivers exists in
majority in the dissolved gaseous forms of carbon
dioxide (CO2), and in the anionic forms of bicarbo-
nates (HCO3ÿ) and carbonates (CO3
2 ÿ). At usual pH
values, HCO3ÿ represents more than 80% of the DIC
and is considered to result from chemical weathering
of rocks on drainage basins (Meybeck, 1987; Amiotte
Suchet and Probst, 1993).

MATERIALS AND METHODS

The study site was the hypoxic area of the polluted
Scheldt basin (Fig. 1). During one year, we have sampled
the ®ve major tributaries of the Scheldt estuary, at the
upstream limit of the tidal in¯uence (Stations A±E) and in
the Scheldt tidal river, where the waters ¯owing from the
®ve rivers get mixed by the tides (Station F). Residence time
of waters in this area are between 10 and 30 days (Wollast,
1998). Stations were visited every 4±5 weeks during one
year. Subsurface water was sampled with a 1.5 l Niskin
bottle, oxygen was immediately ®xed with Winkler reagents
and samples were directly ®ltered through Whatman GF/F
®lters. The ®ltrate was partly stored in 250 ml plastic bottles
for total alkalinity (TAlk) and partly frozen in 10 ml
propylene vials for inorganic nitrogen. The day after, TAlk
was obtained from the Gran titration and oxygen by the
Winkler technique. Inorganic nitrogen species were deter-
mined with a Technicon Autoanalyser within 3 months.