Restoring turbid eutrophic shallow lakes to a clear-water state by combined biomanipulation and chemical treatment: A 4-hectare in-situ experiment in subtropical China.

While biomanipulation and chemical treatments have been applied to speed up the recovery of shallow eutrophic lakes through top-down and bottom-up effects, the efficacy of a combined approach has received less attention. We conducted a large-scale (4 ha) restoration experiment in an isolated part of Lake Yanglan, a shallow eutrophic lake in subtropical China. Here, lanthanum-modified bentonite and polyaluminium chloride were applied after fish removal, followed by transplantation of submerged macrophytes. Samples were collected from within the experimentally restored area and the unrestored area of the lake throughout the study period (August 2017 to May 2018), and data were compared for three periods: 1) fish removal period, 2) chemical treatment and macrophyte transplantation period, and 3) after full restoration. Compared to the unrestored area, water clarity (Secchi depth, SD) increased, total suspended solids and total nitrogen (TN) concentrations decreased significantly in the restored area across the whole study period, while total phosphorus (TP), particulate phosphorus, and soluble reactive phosphorus concentrations declined significantly in the second and third period. Phytoplankton biomass (chlorophyll α concentrations, Chl a) decreased significantly during fish removal in period 1 and after full restoration in period 3, but not in the intervening chemical treatment in period 2. After full restoration, mean SD had increased by a factor of 6.2 (from 29 to 181 cm), mean TN had decreased by 26 % (from 1.68 to 1.25 mg/L), TP by 72 % (from 0.18 to 0.05 mg/L), and Chl a by 78 % (from 49 to 11 μg/L) in the restored area compared to the control. The mobile phosphorus content of surface sediments significantly decreased after full restoration. The zooplankton to phytoplankton biomass ratio in the restored area increased after full restoration, peaking in March when Daphnia abundance was high, indicating enhanced grazing control on phytoplankton. However, the ratio was low in the warm months, likely due to fish recruitment that led to stronger predation on zooplankton. Our eight-month experiment showed that a clear-water state can be successfully restored using a combined approach of biomanipulation and chemical in a subtropical shallow lake. Yet, given that external and internal nutrient loading may increase and the zooplankton to phytoplankton biomass ratio decrease in summer due to the region's subtropical monsoon climate, maintaining a stable clear-water state in the long term may require additional measures such as periodic thinning of the fish stock and/or chemical treatment.