Spatial Variation of Phosphorus in Sediments of Baiyangdian Lake and Their Regulatory Role on Phytoplankton Communities

As a key freshwater wetland in the Beijing–Tianjin–Hebei core area, Baiyangdian Lake’s ecological health is strategically significant for regional ecological security, prompting this study to explore how sediment phosphorus forms drive its phytoplankton communities. The research adopted sequential extraction technology, morphological identification, and multivariate statistics in Baiyangdian Lake. Results showed sediment phosphorus was dominated by highly active exchangeable phosphorus (Ex-P, ~60%, with higher levels around villages of lake center and western areas), with occluded phosphorus (Oc-P, ~23%) as the second most abundant form. Ex-P was the core factor shaping phytoplankton communities, directly increasing biomass density ( r = 0.38, p < 0.05) and explaining 17.92% of community variation. Bacillariophyta was the dominant group (43.3%), while calcium-bound phosphorus (Ca-P) maintained diversity and aluminum-bound phosphorus (Al-P) inhibited evenness ( r = −0.35, p < 0.05). Active phosphorus directly affected, and inactive phosphorus indirectly regulated, phytoplankton patterns, clarifying the unique phosphorus structure of northern carbonate-type lakes and filling research gaps. It is suggested to include Ex-P and Ca-P in aquatic ecological monitoring and prioritize sediment passivation and riparian restoration in high-Ex-P areas to mitigate algal bloom risks.