Effects of macrophyte development on the oxygen metabolism of an urban river rehabilitation structure

Abstract

To compensate for fairway enlargements and the hydraulic impacts of navigation activity, an artificial rehabilitation structure was constructed in the urban, navigable River Spree in 2004. This wave-protected, shallow littoral zone proved to be highly effective in reducing vessel-induced waves and provided suitable conditions for the development of aquatic plants. However, in time it became less suitable for other aquatic organisms due to hypoxic conditions in late summer. This study aimed to comparatively calculate and analyze the oxygen balance of the rehabilitation structure and the main channel five years after the construction in 2009.In the rehabilitation structure, the production to respiration ratio ranged between 0.10 and 0.34 at the peak of vegetation density, while in the main channel in front of the rehabilitation structure it ranged between 0.67 and 0.86. Dense vegetation limited the water exchange and caused oxygen depletion. Thus, atmospheric oxygen input through the water surface and due to long-term water level changes became the most important supply processes for oxygen in the rehabilitation structure.Enhancing the oxygen supply to improve the suitability of the rehabilitation structure for other aquatic taxa requires an increase in water exchange with the main channel, e.g. by adaptively increasing the lateral connectivity.