Nutrient biogeochemistry in the water column (N, P, Si) and porewater (N) of sandy sediment of the Scheldt estuary (SW-Netherlands)

Abstract

The Scheldt river drains a densely populated and industrialized area in northern France, western Belgium and the south-west Netherlands. Mineralization of the high organic load carried by the river leads to oxygen depletion in the water column and high concentrations of dissolved nitrogen and phosphorus compounds. Upon estuarine mixing, dissolved oxygen concentrations are gradually restored due to reaeration and dilution with sea water. The longitudinal redox gradient present in the Scheldt estuary strongly affects the geochemistry of nutrients.Dissolved nutrients in the water column and dissolved nitrogen species in sediment porewaters were determined for a typical summer and winter situation. Water column concentration-salinity plots showed conservative behaviour of dissolved Si during winter. During summer (and spring) dissolved Si may be completely removed from solution due to uptake by diatoms. The geochemistry of phosphorus was governed by inorganic and biological processes. The behaviour of nitrogen was controlled by denitrification in the anoxic fluvial estuary, followed by nitrification in the upper estuary (prior to oxygen regeneration). In addition, nitrogen was taken up during phytoplankton blooms in the lower estuary.Dissolved inorganic nitrogen species in porewaters from the upper 20 cm of sediments were obtained from a subtidal site in the middle of the lower estuary. Dissolved nutrient concentrations were low in the upper 10-15 cm of the sandy and organic poor (<1% POC) sediments mainly as a result of strong sediment mixing.The porewater profiles of ammonium and nitrate were evaluated quantitatively, using a one-dimensional steady-state diagenetic model. This coupled ammonium-nitrate model showed ammonification of organic matter to be restricted to the upper 4 to 7 cm of the sediments. Total nitrification ranged from 3.7-18.1 mmol m -2 d -1, converting all ammonium produced by ammonification. The net balance between nitrification and denitrification depended on the season. Nitrate was released from the sediments during winter but is taken up from the water column during summer. These results are in good agreement with data obtained from the independently calibrated water column model for the Scheldt Estuary (VAN GILS et aL, 1993).

Biogenic silica in tidal freshwater marsh sediments and vegetation (Schelde estuary, Belgium)

“…The Schelde estuary (Fig. 1), in northern Belgium (Flanders) and SW Netherlands, has a history of extensive anthropogenic pollution (Wollast 1988, Boderie et al 1993, Baeyens 1998. It is extremely eutrophic, and receives large inputs of inorganic nutrients from non-point as well as point sources (Heip 1988 January, March, May, July and October 2003.…”

Spatial and temporal patterns of water quality along the estuarine salinity gradient of the Scheldt estuary (Belgium and The Netherlands): results of an integrated monitoring approach

“…The Elbe estuary showed an similar distribution of P species (Van Beusekom & Brockmann, 1998). In the freshwater part, the more complex seasonal pattern of P suggests interaction between a physicochemical buffering mechanism, biological processes and factors controlling input from the river basin (Wollast, 1982;Boderie et al, 1993;Zwolsman, 1994).…”

“…At the mouth of the estuary limitation of DSi for diatoms was evidencec from several approaches: (1) summer, N:Si nutrient ratios <1 (Van Spaendonk et al, 1993;Kromkamp et al, 1995); (2) modeling indicates a DSi limitation of maximum 15% ; (3) low spring DSi concentrations compared to a half-saturation constant, estimated at 1-5 lM (Van Spaendonk et al, 1993). While large amounts of DSi are biologically removed during summer, this element behaves conservatively in wintertime (Boderie et al, 1993;Zwolsman, 1994). Silica loads at the mouth of the estuary were related with discharge (Struyf et al, 2004).…”

“…The Scheldt estuary is not an exception. It is characterized by a notorious history of pollution and eutrophication (Wollast, 1988;Boderie et al, 1993), and the estuarine intertidal habitats have suffered from important area reduction and quality degradation (Meire et al, 2005). The Schelde estuary nevertheless has some high ecological values, being internationally important for several bird species (Van den Bergh et al, this volume), and with large parts of the estuary being designated under the Ramsar Convention and European Birds and Habitat Directive.…”

Dissolved and particulate trace metal geochemistry in the scheldt estuary, S. W. Netherlands (water column and sediments)

“…Related studies supporting the model concern a cohesive sediment balance ( VAN MALDEGEM et aL, 1993), the behaviour of nutrients (BODERIE et aL, 1993) and organic micropollutants (VAN ZOEST and , and the effects of trace metals and organic micropollutants on the biota of the Scheldt estuary (STRONKHORST, 1993).…”