Spatiotemporal variation of net anthropogenic phosphorus input and its delayed effect on lake phosphorus in a plateau lake of southwestern China

Plateau lakes are highly sensitive to anthropogenic phosphorus (P) enrichment. Despite recent management strategies reducing watershed P inputs, lake eutrophication persists, indicating a delayed recovery of water P level in response to external P reduction. This study focuses on Xingyun Lake, a typical shallow plateau lake in central Yunnan Province (Yuxi City), China, and applies the net anthropogenic P input (NAPI) model to evaluate the influences of different P sources in the watershed of Xingyun Lake from 1989 to 2020. The convergent cross mapping (CCM) approach was utilized to detect nonlinear and time-lagged causal relationships between NAPI and total phosphorus (TP) dynamics. Segmented regression analysis was further used to identify thresholds of P inputs leading to abrupt changes in lake TP concentrations. NAPI ranged from 1,148 ± 304 to 6,984 ± 2,206 kg km −2 yr −1 during the study period and was primarily driven by food/feed net P input (P im ), ore mining P input (P ore ), and fertilizer P input (P fer ). From 2000 to 2020, P ore was the dominant contributor to NAPI changes in the northern and eastern region, particularly in Jiangcheng and Luju towns. CCM analysis identified a significant nonlinear and unidirectional causal link from NAPI to TP with a 4-year time lag. Threshold analysis demonstrated that lake TP concentrations exhibited a nonlinear upward trend when P fer and P ore exceeded 806 and 1,305 kg km −2 yr −1 , respectively. This study provides the first quantitative evidence for the time-lagged response of lake P dynamics to anthropogenic P inputs in plateau lake systems, offering theoretical support and technical guidance for setting watershed water quality improvement goals and optimizing policy intervention timing.