Total nitrogen (TN) is a critical water quality indicator in reservoirs, significantly influenced by hydrological changes, reservoir operations, and meteorological forcing. However, the specific contributions of various forcings to TN concentration in complex lake and river reservoir systems remain unclear. To address this issue, this study employs the Shapley value method to evaluate the contributions of different forcings to TN concentration, integrating the Ensemble Kalman Filter (EnKF) with the Vollenweider model or Logistic regression model to simulate TN concentration variations across different reservoir regions under multiple forcing influences. This methodology is applied to the Danjiangkou Reservoir (DJKR), a key water source for the South-to-North Water Diversion Project (SNWDP) in China. Reservoirs were divided into Han (river-type reservoirs) and Dan (lake-type reservoirs) using the trend cluster analysis method. Results indicate that in river-type reservoirs, meteorological forcing accounts for 38.71 % of the contribution, followed by hydrological changes (32.25 %) and reservoir operational forcing (29.03 %). In lake-type reservoirs, reservoir operational forcing contributing 50.68 %, followed by hydrological changes (28.4 %), while meteorological forcing (including temperature and solar radiation) has a relatively low contribution of only 20.88 %. The strength of this study's model lies in its process contribution analysis using Shapley values, effectively separating and identifying dynamic driving factors in complex river and lake systems even with limited data. This research offers a novel method for quantitatively assessing the impacts of direct and indirect forcings on reservoir TN concentration, serving as an effective tool for watershed reservoir system water quality management.