Nitrogen–alkalinity interactions in the highly polluted scheldt basin (belgium)

Abstract

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 NH 4 + were associated with lowest NO 3 À and oxygen and vice-versa. In river water incubations, nitrification lowered the alkalinity whereas denitrification raised it; in an anoxic, NO 3 À -free incubation an increase of alkalinity was observed, partially due to ammonification. A stoichiometric analysis, taking into account the amount of protons produced or consumed by each process involving nitrogen, revealed that monthly variations of NO 3 À and NH 4 + with ammonification, nitrification and denitrification could explain the 28 and 62% alkalinity variations at all stations, except one. The remaining part of the alkalinity variations was attributed to other anaerobic processes (Mn-, Fe-and SO 4 -reductions). This trend seems to be the result of the whole catchment metabolism (riverine waters and sediments, sewage networks and agricultural soils). The observed HCO 3 À concentrations in the Scheldt basin were 2-10 times higher than the representative concentrations reported in pristine basins and used in chemical weathering models. This suggests the existence of an anthropogenic source, originating from organic matter decomposition. We conclude that in highly polluted basins, nitrogen transformations strongly influence the acid-base properties of water. #

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

“…This is fully consistent with previous modelling studies on the Scheldt estuary (i.e., Regnier et al, 1997). Furthermore, these results are coherent with Abril and Frankignoulle (2001), who focussed on alkalinity variations and identified the nitrogen cycle and the process nitrification as important factor governing the acid-base regime of the Scheldt estuary. In addition to confirming this, the explicit pH modelling approach presented here allows to precisely calculate the contribution of nitrification (and other processes) in the overall proton cycling along the estuarine gradient.…”

Excess atmospheric carbon dioxide transported by rivers into the Scheldt estuary

“…L'excès de CO 2 non imputable à un déficit en oxygène est alors dû aux processus anaérobi-ques. Cependant, nous discutons le fait que la majorité des processus anaérobiques (dénitrification, réduction du fer, du manganèse et des sulfates) produisent des ions bicarbonates, plutôt que du CO 2 [3]. Une réoxyda-tion des composés réduits (manganèse, fer et sulfures dissous) est nécessaire pour produire finalement du CO 2 [28].…”

“…This oxidation is done most of the time by oxygen. In a similar way, nitrification produces protons and transforms an amount of bicarbonates into CO 2 equivalent to the quantity of oxygen it consumes [3,12,23]. For these reasons, oxygen consumption and CO 2 production in the aquatic system, by both aerobic and anaerobic processes, are close to each other, except if methanogenesis is important.…”

“…For these reasons, oxygen consumption and CO 2 production in the aquatic system, by both aerobic and anaerobic processes, are close to each other, except if methanogenesis is important. If some reduced compounds escape oxidation, bicarbonates are produced [3]. We can therefore consider that, except in anoxic and highly reduced waters (like the Zenne river), the oxygen depletion is close to the fraction of excess CO 2 produced by heterotrophic organisms in soils and waters, and the fraction that cannot be accounted for by the oxygen depletion originates in majority from root respiration in soils.…”

“…Sulfate reduction, methanogenesis and methane oxidation by sulfates may produce significant amounts of CO 2 without consuming oxygen. However, anaerobic processes produce in majority bicarbonates [3] and this is why the Zenne is also the river where excess CO 2 represent the lower fraction of DIC (7 % compared to more that 10 % in the four other rivers).…”