Reconstructing clear water state and submersed vegetation on behalf of repeated flocculation with modified soil in an in situ mesocosm experiment in Lake Taihu.

The geo-engineering approach of modified soil flocculation has been widely applied to mitigate algal blooms and eutrophication in relatively small lakes. Nevertheless, its potential ecological risks and feasibility should be examined and identified prior to its application in large natural lakes given the multiple functions of these water bodies in human health and welfare. In situ mesocosm experiments on modified soil flocculation were performed in Lake Taihu during summer 2010 and 2011. Chitosan-modified kaolinite (CMK) soil was used to flocculate algal blooms and improve water transparency to facilitate the re-establishment of the submersed macrophyte Vallisneria natans in this shallow eutrophic lake. Moreover, the ecological effects of CMK soil were assessed. Results showed that repeated additions of CMK (0.3g/L for each time) improved water quality in terms of Chl-a, TN, and TP concentrations; TN/TP ratio; turbidity; redox conditions; and nitrification and denitrification activities. These effects lasted for 48days. After the fourth dose of CMK, the biomass of all phytoplankton categories, except for that of Cryptophyta, decreased by >90% (ca. 1-2×106cell/L or 0.38-0.55mg/L of wet weight). Zooplankton biomass markedly decreased after the first CMK addition, and copepods became dominant. These effects, however, did not last for the long term. Most importantly, submersed V. natans was restored successfully when water clarity and quality were improved through repeated CMK flocculation. Nevertheless, the indices of carbohydrate depletion and free amino acid accumulation indicated that the plant experienced physiological stresses. The reestablishment of V. natans reinforced the positive effects of repeated CMK dosing on water quality, and promoted a clear water state. V. natans is recommended for vegetative restoration in shallow eutrophic lakes given its facile transplantation, high stress tolerance, and physiological traits, which can be applied as indices of post-flocculation effects. In summary, the combination of repeated CMK dosing and revegetation of V. natans can feasibly improve water quality and initiate the restoration of a clear water state in shallow eutrophic lakes.