Disproportionate decline in ultraviolet versus visible light penetration in lakes across land-use gradients, with implication for the deep chlorophyll maximum

Solar radiation regimes, including both ultraviolet radiation (UVR, composed of UVB at 280–320 nm and UVA at 320–400 nm) and photosynthetically active radiation (PAR, 400–700 nm), are fundamental drivers of lake ecosystems. However, how their penetration may become disproportionate under anthropogenic pressures and the potential ecological consequences thereof remain unclear. To address this issue, we investigated the ratio of UVR and PAR penetration depths (Z1%(UVR)/Z1%(PAR)) across 53 freshwater lakes on the Yunnan-Guizhou Plateau, Southwest China. The goal was to assess relationships between Z1%(UVR)/Z1%(PAR) ratios and optically active substances (OASs), including chromophoric dissolved organic matter (CDOM), phytoplankton (represented by chlorophyll-a, Chl.a), and non-algal particles (NAPs), watershed land use, and ultimately, the deep chlorophyll maximum (DCM) as a key ecological responder. Using cluster analysis and the resulting land-use intensity (LUI) breakpoints, we categorized the lakes into “natural lakes” (G1 group) with minimal human disturbance and “anthropogenically-influenced lakes” (G2 group) with significant human disturbance. The results showed that Z1%(UVR)/Z1%(PAR) was greater for UVA than UVB, and significantly greater in G2 lakes. A distinct shift in regulatory drivers was observed: Z1%(UVA)/Z1%(PAR) appeared to be primarily regulated by CDOM, whereas Z1%(UVB)/Z1%(PAR) was co-regulated by CDOM, Chl.a, and NAPs, with differing degrees of influence observed between G1 and G2 lakes. Different land-use types exerted distinct influences on the composition of the three OASs and on Z1%(UVR)/Z1%(PAR), especially with respect to forest, agricultural, and construction lands. Moreover, LUI affected Z1%(UVB)/Z1%(PAR) and Z1%(UVA)/Z1%(PAR) differently, driving disproportionate changes in light penetration depth at different wavelengths. This effect appeared to be further amplified by other indirect drivers. Critically, the increase in Z1%(UVR)/Z1%(PAR) was significantly correlated with greater depth and thickness of DCM, with a more pronounced relationship observed in G1 lakes. Our study suggests a kind of ecological cascade, in which watershed land use contributes to a disproportionate shift in the underwater light climate through the type-specific modulation of OASs, thereby eventually influencing DCM. This cascade provides a process-based framework for lake ecosystem conservation strategies aimed at reconciling the critical balance between UVR protection and PAR provision.