Nitrogen related processes in water and sediment of Hoan Kiem Lake (Hanoi, Vietnam) with respect to sediment dredging

Funding:  German Academic Exchange Service (DAAD)

Project description:

The work was part of the BMBF-funded collaborative project "Development of Strategies and Procedures for Sustainable Rehabilitation, Stabilization and Management of Hoan Kiem Lake in Hanoi, Vietnam" (FKZ 02WT0927).

Although Hoan Kiem Lake (HKL) is of cultural and historical value, has been the habitat of a rare turtle species for centuries, and is a landmark of the city of Hanoi, its ecological situation is extremely unsatisfactory. Eutrophication and silting threaten the last giant softshell turtle (Rafetus swinhoei) living in the lake. For this reason, the project partners developed a hydraulic desilting strategy that gently removes the upper sediment layers without draining the lake. However, dredging of lake sediments regularly leads to the mobilization of nutrients, with nitrogen compounds being of central importance. In the HKL, nitrogen is the growth-limiting factor according to water analyses and experiments with enrichment cultures.

The accompanying studies conducted are of importance in predicting potential effects of sludge removal on sub-processes of nitrogen cycling in the lake such as ammonium release from sediment, nitrification, denitrification, and NH4+ assimilation. Increasing NH4+ concentrations in sediment pore water with increasing sediment depth will result in increased NH4+ release during and after desilting, as demonstrated during incubation of sediment cores blanketed with lake water. As sediment depth increases, organic compound content and bacterial density decrease. Therefore, it could be assumed that nitrification and denitrification are temporarily and locally strongly slowed down when the uppermost sediment layers with their high content of organic material and strong microbial colonization are removed.

Nitrification plays a negligible role in the water of the HKL, occurring only in the uppermost sediment layers. Consequently, immediately after desludging, only a small portion of the ammonium released will be able to be oxidatively converted. Denitrification, as the process that sustainably removes nitrogen from a lake and releases it to the atmosphere, is also greatly reduced at the site of desludging. Thus, the demonstrated slowing of nitrification and denitrification at the sediment surface under conditions of proposed desludging enhances the release of ammonium ions into the lake. Consequently, the study of the assimilation of ammonium ions by phytoplankton is of particular importance.

This was done in enclosure experiments directly in the HKL. The observed comparatively fast disappearance of the added ammonium was, however, not exclusively caused by the assimilation of the phytoplankton, but also by release into the atmosphere, due to the high pH value (pH 9.3-10.1) of the lake water during the investigation period.