Effects of combined ecological restoration measures on water quality and underwater light environment of Qingshan Lake, an urban eutrophic lake in China

Ecological restoration is vital in the management of eutrophic lakes. However, the effect of combined ecological restoration measures on the improvement of water quality and the light environment of urban eutrophic lakes requires further exploration. This study considered Qingshan Lake, a typical urban eutrophic lake in the middle reaches of the Yangtze River in China and explored the changes in the water quality and light environment before and after the implementation of combined ecological restoration measures from 2021 to 2022. The results showed that: (1) Ecological restoration significantly improved the transparency of Qingshan Lake (P < 0.05), with transparency increased from 33.1 cm to 96.0 cm. Total phosphorus, total nitrogen, chlorophyll a, permanganate index, suspended solids and turbidity decreased by 52.11 %, 65.47 %, 73.84 %, 92.11 %, 83.11 % and 77.52 %; respectively (P < 0.05). (2) Following ecological restoration, Qingshan Lake converted from severe eutrophication to mesotrophication, and the trophic level index decreased from 75.29 to 49.65 (P < 0.05). (3) Ecological restoration had a significant effect on improving underwater light environments. After ecological restoration, the light attenuation coefficient changed between 1.61–3.12 m−1, and the annual mean value decreased by 61.96 % (P < 0.05). The euphotic zone depth changed from 1.85 to 6.19 m, an increase of 167.68 %. (4) Ecological restoration reduced the attenuation of blue light, which reduced the red-blue ratio (R/B) of water. In general, turbidity had the greatest impact on light attenuation and R/B of Qingshan Lake, and ecological restoration combined with multiple measures had a positive effect on the management of eutrophic lakes.