The hyporheic zone is the connection between surface water and groundwater that often plays an important function in nutrient transport and transformation, and acts as an active source of or sink for nutrients to the surface water, depending on its potential water flow patterns. Bottom surface water and sediments in the shallow hyporheic zone (approximately 100 cm depth) were sampled at 12 sites near the shoreline and two sites at the center of Lake Taihu (China) during spring and winter of 2016. Concentrations of total nitrogen, ammonium, nitrate, and nitrite in the bottom surface water and porewater (obtained from sediments using a frozen centrifugation method) were analyzed in a laboratory to establish the nitrogen distribution and potential drivers. The results show that, in general, the quality of bottom water and porewater near the shoreline was poor compared to that at the center, and it gradually improved from the northwestern to the southeastern zones of Lake Taihu. No significant relationship in nitrogen concentration was found between the bottom water and porewater in surface sediments. Nitrogen concentrations in porewater differed between sampling sites and sediment depths in Lake Taihu. Vertical profiles of nitrogen in porewater and differences in nitrogen between the winter and spring seasons indicated that potential upwelling water flow occurred in the hyporheic zone in the south, west, north, and center zones of Lake Taihu, but potentially weak water flow in variable directions likely occurred in the east zone. A strong reducing environment dominated the deep parts of the hyporheic zone (i.e., below 40 cm depth), while a weak oxidizing environment dominated the shallow parts. Furthermore, the decreasing total nitrogen and ammonium nitrogen from the deep to shallow depths in the hyporheic zones in the south, west, north, and center zones indicated that potential anammox and/or denitrification processes occurred. In the east zone, potential weak nitrification processes occurred in the hyporheic zone, and plant fixation and sedimentation of nitrogen also contributed to the surface sediments. In conclusion, the hyporheic zone near the shoreline in the south, west, and north sites of Lake Taihu acts as an active source of nitrogen for the lake water due to potential upwelling water flows, whereas the east site acts as an active source or sink due to seasonally variable directions in water flow. Water flow and biogeochemistry in the hyporheic zone jointly influence nutrient distribution in the hyporheic zone and even switch or alternate the source/sink function of sediment in surface water.
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