Delayed water quality response to external nutrient loading reduction: the role of internal nutrient loading in a shallow eutrophic lake over 20 years

Internal nutrient loading is considered the main reason for the delayed response of water quality to a reduction in external nutrient loading in shallow eutrophic lakes. However, how the unobserved internal nutrient cycling changes and contributes to the water quality over a long-term time scale is still an open question. Here, we established a dynamic process-based biochemical model based on a Bayesian hierarchical framework and longterm monitoring data (1999-2018) to explore the temporal dynamics of external nutrient loading, water quality and internal nutrient flux and their relationships in Dianchi Lake, which is a semi-closed lake with relatively deep water depth (mean depth = 5.4 m), long hydraulic retention time (HRT) (mean HRT = 3.5 years) and long history of eutrophication. We found an imbalanced external nutrient loading reduction of which the external phosphorus (P) and nitrogen (N) loading were reduced nearly 90 % and 67 %, respectively, during 1999-2018, and a delayed water quality response to the external nutrient loading reduction. The insufficient reduction of external N and a relative low removal rate induced high nutrient accumulation and sustained sediment N release, further supported the grow of phytoplankton. Meanwhile, we found a shift of water-sediment P exchange from sink to source after 10 years of external P loading reduction. Along with the dynamic change of internal nutrient cycling, there is a shift of nutrient limitation from only N limitation to N + P co-limitation in Dianchi Lake. These findings demonstrated an urgent need for sufficient dual control for the re-oligotrophication of shallow eutrophic lakes.