To clarify horizontal variability and regulation of bacterial production (BP), we investigated BP and environmental variables along three east-west transects (Lines 12, 15, and 17) covering inshore/offshore stations in Lake Biwa, Japan, during four seasons. In winter, surface BP along Line 12 (southern transect) was higher than Lines 15 and 17 (central and northern transects) and reflected the water-temperature distribution. Additionally, any nutrients and dissolved organic carbon did not correlate to BP, suggesting water temperature regulated BP in winter. In spring, BP was higher at eastern inshore stations, near large agricultural fields, and was correlated with phosphorus concentration rather than water temperature, suggesting that the limitation shifted to nutrient availability. As well as spring, surface BP in autumn was correlated with phosphorus. Additionally, a negative correlation with water temperature in autumn suggested that nutrient loadings through river and groundwater contributed to enhancing BP. In summer, surface BP at offshore stations along Lines 15 and 17 was notably lower than the other stations. Summer BP was correlated with phosphorus concentration, suggesting that allochthonous nutrient loading determines horizontal BP variations. Moreover, summer depth-integrated BP (DBP) at offshore stations was lower (32-52 mgC m-2 d-1) than inshore stations (43-110 mgC m-2 d-1) regardless of water depth. The average DBP at inshore stations in summer was 2.1 times that offshore stations, and the inshore/offshore DBP ratio was higher than the other seasons (0.58-1.0). The results suggest that inshore BP significantly contributes to whole-lake BP in productive seasons.