In-lake algal bloom removal and submerged vegetation restoration using modified local soils

Abstract

A “modified local soil induced ecological restoration” (MLS-IER) technology was developed for the restoration of degraded shallow lakes. Modified local soils that mixed with macrophyte seeds were used to flocculate the algal blooms and sink them down to the bottom of the lake. The increased water clarity and the improved sediment quality due to the covering of clean modified local soils make it possible for a quick restoration of submerged macrophytes in eutrophic shallow lakes. The MLS-IER technology was tested in the whole bay of Liaoyangyuan (0.1km2) in Lake Tai (Wuxi, China) in August 2006. The whole bay was fully covered by more than 1cm cyanobacterial bloom since June, which caused massive killing of fish and aquatic vegetations. Some 4tons of chitosan-modified local soils were sprayed over the whole bay and the severe bloom was successfully removed within one day. The secchi depth was increased from 0cm to 30cm, and the chlorophyll-a, total-P, and total-N were all reduced by more than 86% within one day's time. Four months after the treatment, submerged macrophytes were successfully restored within the whole bay. Cyanotoxin microcystins RR and LR were reduced by 50% and 40%, respectively, compared to those outside the bay 4 months later. The biodiversity index of zoobenthos and that of phytoplankton inside the bay became higher than that outside the bay, while zooplankton diversity index remained relatively unchanged. This field trial study indicated that restoration of submerged macrophytes in shallow lakes could be significantly accelerated by using MLS-IER technology. The long-term ecological response and the transition mechanism between algal cells and submerged macrophytes in the sediment need to be further studied in controlled whole lake experiments.