Phosphorus speciation and its coupling with organic matter in the water–sediment system of Lake Dian, southwestern China

The distribution characteristics, concentration dynamics, and organic matter (OM) interactions of different phosphorus (P) species in lake sediments are fundamental to understanding P cycling and release mechanisms. This study investigated P speciation, concentrations, and their associations with OM in the sediments and pore water of Lake Dian, with a focus on spatial heterogeneity and centennial-scale evolution. Results showed that sedimentary P was predominantly retained in relatively stable fractions, namely NaOH-rP, HCl-P, and Res P. Pore-water dissolved organic matter (DOM) was dominated by authigenic, protein-like components, whereas sedimentary OM originated from mixed sources, including aquatic macrophytes, algae, and sewage inputs. Centennial sediment records indicated that increases in P and OM concentrations coincide with the intensification of watershed anthropogenic activities. The spatial co-distribution of OM and P suggested that OM enhanced internal P mobilization by regulating redox conditions and promoting the transformation of labile P fractions (e.g., BD-P). The sediment–water P cycle was governed by coupled physical transport and OM-driven biogeochemical processes. Notably, the degradation of algal-derived OM was identified as a key driver of internal P loading, as it stimulated microbial activity and oxygen consumption, thereby facilitating P release. These findings indicate that effective lake management should integrate external nutrient load reduction with strategies targeting OM-mediated internal P cycling to achieve sustainable water quality improvement in Lake Dian.