Spatial heterogeneity in mechanisms of internal phosphorus loading from sediments in a subtropical plateau lake

Phosphorus (P) release from lake sediments is widely recognized as a critical barrier to mitigate eutrophication, as it continuously provides nutrient sources (i.e. internal loading) to support phytoplankton blooms. However, for large lakes the spatial patterns and mechanisms of internal P loading remain poorly understood due to extensive spatial heterogeneity. To address these knowledge gaps, we selected Lake Dianchi-a large subtropical plateau lake in China that has suffered from decades of eutrophication and frequent cyanobacterial blooms. We integrated a lake-wide vertical profile analysis of P binding forms with assessments of nutrient release potential through high-resolution thin film diffusion gradient technology (DGT). The results revealed distinct spatial differences in sediment P dynamics between the northern and southern regions of the lake. In the northern bay, sediment profiles were characterized by low DGT-labile Fe/P ratios and high DGT-labile P and/or NH₄Cl-P contents; the reductive dissolution of Fe-bound P (BD-P) dominated the short-term (decadal scale, ∼10 years) P release (contributing 84.6 %). In contrast, sediment profiles from the southern region were characterized by high DGT-Fe/P ratios and low DGT-labile P and/or NH₄Cl-P contents; organic P (NaOH-nrP) remineralization dominated (contributing 90.7 %) the short-term P release. We furthermore found a significant increase in the contributions of Al-bound P (NaOH-rP) and Ca-bound P (HCl-P) to P release on a long-term scale (∼50 years). The complex interplay among redox-driven Fe-P coupling, organic P mineralization, and legacy P remobilization creates self-reinforcing feedback loops that challenge lake-wide management.