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4c. Oxygen deficiencies

Dissolved oxygen concentrations are observed to be highest in the Weser estuary. Because measurements in the Weser are restricted to the freshwater and oligohaline zones, this can be considered as truly high compared with the other estuaries, since mostly oxygen consuming processes have the tendency to be higher in the upstream part of an estuary (see also ‘4.1 Estuarine patterns’ ). Next, highest concentrations of dissolved oxygen are found in the oligohaline stretch of the Humber estuary. Main oxygen deficit problems are noted in the Elbe and Scheldt estuaries. Mainly in the Scheldt estuary, concentrations drop below 5 mg/l. This is the minimum amount of dissolved oxygen required to sustain a healthy ecological functioning system (Holzhauer et al. 2011). The zone of oxygen deficits in the Scheldt, extends over a large part of the freshwater zone and almost the entire oligohaline zone. In absolute values, most oxygen is lost in the freshwater part of the Elbe estuary. However, considering also the input concentration per zone, it seems that particularly the Scheldt oligohaline zone is affected by oxygen removal. This is in accordance with earlier findings (Van Damme et al. 2005, Soetaert et al. 2006). However, recently oxygen concentrations have improved in the Scheldt estuary and in 2009 oxygen concentrations did not drop below 5 mg/l anymore along the whole estuarine gradient. Nevertheless, a sag in the oligohaline persists and the Scheldt estuary still has the lowest dissolved oxygen concentrations compared to the other estuaries studied. In the Elbe freshwater zone, oxygen concentrations also regularly drop below this dissolved oxygen threshold concentration. Although the Elbe shows most oversaturation events in the most upstream part of the freshwater zone (the shallower part of the estuary), in the more downstream part of this freshwater zone an oxygen deficit zone can be observed, generated near TIDE kilometer 50. In 2006 this oxygen deficit zone temporarily disappeared. This temporary disappearance is likely to be related to the unusual high freshwater discharge during that summer (fig. 9). However, since 2007 the Elbe seems to re-experience oxygen deficits, with this zone apparently even extending over a longer stretch nowadays (a stretch of up to 20 TIDE km). Although dissolved oxygen concentrations are markedly higher in the Elbe compared to the Scheldt estuary, the Elbe estuary appears to persistently experience these oxygen drops below 5 mg/l (see also Kerner 2007, Quiel et al. 2011). Hence, the question rises whether recent improvements observed in the Scheldt estuary will continue, or whether also the Scheldt will re-experience more severe oxygen deficiencies in the near future.

Oxygen dynamics in estuaries are controlled by several oxygen generating and oxygen consuming processes. In summary, the main processes are organic matter mineralization (respiration), nitrification and primary production (Statham et al. 2011). Therefore, biological oxygen demand concentrations, nitrate and ammonium dynamics and chlorophyll a concentrations are considered together with dissolved oxygen concentrations and dynamics (fig. 36). Lower dissolved oxygen concentrations in the Scheldt in general can be explained by the higher biological oxygen demand concentrations observed here compared to the Elbe estuary. Nevertheless, in the Scheldt biological oxygen demand concentrations have improved greatly, while in the Elbe biological oxygen demand concentrations did not change markedly (fig. 46). In the Scheldt input of ammonium strongly reduced. Furthermore, with an increasing water treatment effort since 2007 (Aquiris 2010) there is also less organic matter input in the Scheldt estuary coming from the Rupel tributary. Hence, in the Scheldt the improvement of water quality can be related to a decrease in oxygen consuming processes such as nitrification and mineralization. Also in the Elbe, oxygen deficiencies are likely related to an intense peak of nitrification and mineralization (fig. 36). Although, biological oxygen demand concentrations are much lower in the Elbe estuary, they did not change very much (fig. 46). Contrary to the other estuaries, in the Elbe estuary average depth abruptly increases near kilometer 40 in the freshwater zone, coinciding with a peak of oxygen loss and nitrate and ammonium gain. Increased depth also creates increased residence times and a decreased euphotic depth-mixing depth ratios within this area. As discussed earlier for nitrogen (see ‘4.2.1 Different sink and source functions for nutrients: Dissolved inorganic nitrogen’ ), it is likely that allochthonous (input from upstream) and autochthonous (from algae dying off) organic matter is piling up because of this sudden increase in depth, creating a large zone of intense nitrification and mineralization giving rise to oxygen deficiencies more downstream (Quiel et al. 2011). However, a coinciding peak of biological oxygen demand (like observed near the Rupel) is not observed in the Elbe. This could be explained by increased sinking of the organic matter within this zone, because of the abrupt increase in depth (sampling is performed at the surface). On the other hand, dissolubility for oxygen, nitrate and ammonium makes that these peaks in dynamics are still to be found within this area, contrary to the missing peak of biological oxygen demand.

Thus, in summary, in the Scheldt severe oxygen deficiencies (<5 mg/l) have recently disappeared because of general water quality improvement. In the Elbe these oxygen deficiencies seem to persist due to intensified nitrification related to the particular morphometric characteristics of the freshwater zone. In conclusion, if water quality in the Scheldt does not deteriorate or the Scheldt is not suddenly deepened in a large extent within the freshwater zone, it can be carefully stated that problems of oxygen deficiencies are not expected in the nearby future. In the Elbe, biological oxygen demand concentrations are already much lower compared to the Scheldt estuary (fig. 47, Attachment 2 ) and much lower than what is considered as a healthy threshold (< 6 mg/l) for a good ecological functioning system according to Holzhauer et al. (2011). It might be unrealistic and even not very useful to further reduce the input of degradable organic matter (i.e. phytoplankton). A model has been run to test the effect of a strong reduced algal input at the upper boundary (0.1 µg/l). Severe oxygen deficiencies (<5 mg/l) indeed seem to disappear now in the Elbe freshwater zone. Nevertheless, concentrations still approach 5 mg/l, which is considered the minimum level needed for a good ecological status (Holzhauer et al. 2011). Model runs to test the oxygenation capacity from shallow water zones near the Hamburg port area, showed positive effects within these areas. However, positive effects from shallow water areas upon oxygen levels within the Elbe main stream were restricted (read also Schöl et al. 2012 ). Possibly the positioning of these shallow water zones near the freshwater part of increased depth makes exchange of oxygen with the deeper waters of the partially mixed Elbe very difficult.

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