Improving water quality is not only about installation of more waste water treatment plants

Date: 7 May 2017

Introduction: Lindsay Geerts (UAntwerpen) investigated why oxygen depletion persists in the Elbe estuary (Germany) and not in the Schelde estuary (Belgium), despite similar water purification efforts.

Estuaries are nutrient-rich and productive systems by nature. Yet, excess input of nutrients and organic matter through human activity (industry, agriculture and sewage disposal) can still lead to severe oxygen deficiencies. Low oxygen concentrations develop when the respiration of organic matter by bacteria uses more oxygen than the oxygen supplied through photosynthesis and air-water exchange. Oxygen depletion has strong implications: invertebrate and fish communities can collapse.

Fig. 1 Discrepancy between the improvement of oxygen concentrations in the Schelde (left) and the persistence of oxygen sags in the Elbe estuary (right). Low oxygen is blue color, oldest data on top.

Since the implementation of the European Water Framework Directive in national legislation, nutrient and organic load reduction have been widely applied to reverse eutrophication and associated oxygen sags. Despite similar efforts for waste water treatment in both the Schelde and Elbe estuary, oxygen concentrations increased strongly in the Schelde estuary, while local oxygen deficiency persists in the Elbe. Why both estuaries respond differently to measures of nutrient reduction has not been documented before. In a new study, Lindsay Geerts (Global Change Ecology Excellence Centre, research group Ecosystem Management) and her co-authors demonstrate that the origin of the organic matter input is important to explain the difference in oxygen conditions between both estuaries. Additionally, estuarine morphology (the shape of the zone under tidal influence) plays a crucial role.

Fig. 2 Both Elbe and Schelde estuary are under strong human influence, with e.g. large harbors in Hamburg and Antwerp. Picture: Elbe near Hamburg (picture taken by Olaf Slader).

Lindsay: “Oxygen minima in the Schelde are mostly related to variability in sewage input from the upper boundary and the tributaries. Minima can periodically arise all year round, and they are not confined to a single location. The organic matter in the Elbe originates mostly from phytoplankton blooms in the Elbe river catchment. The phytoplankton biomass is flushed into the estuary after snow melt in the upper Elbe River.”

The external origin of the organic matter in the Schelde makes it much easier to clean it from the water. This explains why water treatment facilities are much more efficient in the Schelde. To make matters even more complex, the location of the oxygen minimum zone in the Elbe is strongly confined to the zone where the estuary deepens: water here stays shorter in the estuary. Lindsay: “The residence time in the Elbe estuary is much lower than in the Schelde estuary. This seems to be critical in the persistence of oxygen minima. The creation of shallow water areas (with high water residence time) would be an excellent measure to increase oxygen concentrations, as it may provide a source of reaeration and an inoculum for increased primary production.”

Several measures to increase the shallow water areas (by widening the region under tidal influence) have already been performed or planned in the Schelde and Elbe estuary (e.g. Lippenbroek in the Schelde, Kreetsand/Spadenlander Busch in the Elbe). Considering the high load that needs to be reaerated in the Elbe, a shallow water zone ten times larger than in the Schelde, would need to be connected to the estuary. These first crude estimates reveal large scale investments would be needed to prevent any oxygen deficiency situation to recur in the Elbe estuary.

The results were published in the scientific journal Estuarine, Coastal and Shelf Science.