Inland-to-Island Water Diversion Enhances Nutrient Use Efficiency and Phytoplankton Stability in a Subtropical Lake

Inland-to-island water diversion is a critical engineering solution to mitigate freshwater scarcity on islands, yet its long-term ecological impacts on the core freshwater lake ecosystems remain unclear. Here, we present a 7-year (2012–2018) continuous field monitoring study of Sanshiliujiao Lake, the largest natural freshwater lake on Pingtan Island (southeastern China), to resolve how phytoplankton communities respond to sustained inland-to-island water diversion. We find that water diversion significantly enhanced the nitrogen and phosphorus resource use efficiency (RUEN and RUEP) of the phytoplankton community, with phytoplankton biomass doubling from 1.27×106 cells/L to 2.63×106 cells/L, driven by nutrient inputs from diverted water. Concomitantly, the Shannon diversity index and functional redundancy of phytoplankton increased significantly, alongside marked gains in the complexity and stability of the phytoplankton co-occurrence network. Quantitative analyses further reveal that the improved nutrient use efficiency was strongly and significantly correlated with diversion-induced shifts in environmental factors (e.g., increased nutrient concentrations and dissolved oxygen) and biotic attributes (e.g., higher functional redundancy and network stability). By clarifying this temporal coupling, our study provides critical insights for refining region-specific water diversion strategies to support sustainable island freshwater management.