Exploring the light environment of lakes with different trophic levels in the middle reaches of the Yangtze River: implications for lake regime shifts

• First quantitative exploration of UV-A, blue, green, red, NIR light in the lakes. • The higher the trophic level, the higher the underwater light attenuation and R/B. • Blue light attenuated faster in mildly eutrophic and mesoeutrophic lakes. • Red, blue, and green light dominate underwater in oligotrophic lakes. • Turbidity is the main influence factor in different trophic levels lakes. The underwater light environment serves as an indicator of water trophic levels and affects their regime shifts. However, the effects of different trophic levels on underwater light environments remain unclear, which limits our understanding and prediction of changes in lake ecosystems. The first quantitative exploration of ultraviolet-A (UV-A), blue, green, red, and near-infrared (NIR) light in five lakes with three trophic levels in the middle reaches of the Yangtze River was conducted. In contrast to the specific patterns exhibited by other light in lakes with different trophic levels, UV-A light consistently exhibited the most rapid attenuation across trophic levels. Blue, green, red, and NIR light attenuated the fastest in the mildly eutrophic lake, whereas in the oligotrophic lake, they always attenuated the slowest. Moreover, this study focused closely on the red-to-blue light ratio (R/B) and found that its mean value followed the pattern: mildly eutrophic lake > mesotrophic lake > oligotrophic lake. The different light attenuation rates further led to significant variations in the underwater light environment. The underwater spectral ranges of the mildly eutrophic and mesotrophic lake were wide, with NIR light dominating in the air and at the water’s surface, while red and green light were dominant 0.5 m underwater. However, the spectral range of the oligotrophic lake was narrow, with evident peaks. The water surface below 0.5 m was dominated by green and blue light, and the proportion of UV-A light remained above 4 %. This investigation into the composition of light quality at different underwater depths fills the knowledge gap regarding lake underwater light environments. This study demonstrated that turbidity played a more universal and quantitatively dominant role across trophic levels, likely due to its combined reflection of both suspended solids and organic pollution under anthropogenic influence. In the future, it will be necessary to strengthen the monitoring of these two pollutants to maintain about 30 % of blue light underwater and R/B < 1, which can support the growth of more submerged macrophytes and sustain the clear-water state of lakes.