3. Results

3.1 Water column physicochemical characteristics

The physicochemical characteristics of water column in the Yangtze River are shown in Table 1 . Water temperature in Anhui section, Jiangsu section and estuary ranged from 27.4 to 30.1℃, 27.6 to 30.2℃ and 28.6 to 30.0℃, respectively, and was significantly higher in estuary than in Anhui and Jiangsu section (p< 0.05). Chl-a concentrations were significantly higher in estuary than in river (Anhui and Jiangsu) sections (p< 0.05). COD concentrations were highest in estuary (6.14 ± 1.74 mg/L) and lowest in Anhui section (4.05 ± 0.75 mg/L) (p< 0.05). Similar to the distribution of COD, DOC concentrations were highest in estuary (14.5 ± 5.3 mg/L) and lowest in Anhui section (6.6 ± 4.0 mg/L) (p < 0.05). SS concentrations ranged from 7 to 1315 mg/L, and the highest values were observed in estuary.
In all sites, N concentrations, including TN, PN, NH4+, NOx and urea, ranged from 131 to 307, 1 to 158, < 1.0 to 13.0, 41 to 214, < 0.04 to 19.7 μmol N L−1 with average values of 196 ± 38, 56 ± 43, 4.5 ± 2.8, 112 ± 34, 5.5 ± 5.7 μmol N L−1, respectively (Table 1 ). Along the river-estuary continuum, TN, PN, and NH4+ showed similar increasing trends with the direction flow, while no significant trends were observed in NOx concentrations. TP concentrations ranged from 0.32 to 7.03 μmol P L−1, with the highest values in Anhui section (3.35 ± 1.39 μmol P L−1) and lowest in Jiangsu section (1.69 ± 1.23 μmol P L−1) (p < 0.05).

3.2 NH4+regeneration and potential uptake rates

REGs in all water samples collected from Yangtze River ranged from 0.05 to 1.19 μmol N L−1h−1 with an average of 0.26 μmol N L−1 h−1, showing an increased trend along the river-estuary continuum(Fig. 2) . The REGs were significantly lower in Anhui section (0.14 ± 0.09 μmol N L−1 h−1) than in Jiangsu section (0.31 ± 0.18 μmol N L−1 h−1) and estuary (0.44 ± 0.33 μmol N L−1 h−1) (p < 0.05). Upots in the river-estuary continuum ranged from − 0.22 to 1.99 μmol N L−1h−1. Averaged highest Upots were observed in estuary (0.73 ± 0.56 μmol N L−1h−1), which were about 2 and 5 times higher than that in Jiangsu (0.32 ± 0.21 μmol N L−1h−1) and Anhui section (0.15 ± 0.14 μmol N L−1 h−1) (p < 0.05), respectively. Similar to the spatial characteristics of REGs, Upots also increased along the river-estuary continuum (Fig. 3) , and there was a positive correlation between REGs and Upots (r = 0.89,p < 0.01) (Fig. S1) . In addition, the REGs accounted for 62% ± 18% of the Upots during the study period, this may suggest the processes of NH4+ recycling (NH4+ regeneration and uptake) are critical for regulating the supply of NH4+along the river-estuary continuum (Bruesewitz et al. , 2015).

3.3 Community biological NH4+ demand (CBAD)

Community biological NH4+ demand (CBAD) relates N dynamics to total microbial productivity and NH4+ deprivation in aquatic systems (Gardner et al. , 2017), and CBAD is approximated by observing differences between the potential NH4+uptake rates and actual NH4+regeneration rates (Gardner et al. , 2017). CBAD in the samples collected from Yangtze River ranged from − 0.43 to 0.80 μmol N L−1 h−1, with the highest values found in estuary (0.30 ± 0.24 μmol N L−1h−1) and the relatively lower values in Anhui (0.02 ± 0.16 μmol N L−1 h−1) and Jiangsu (0.01 ± 0.09 μmol N L−1 h−1) section (p < 0.05) (Fig. 4) . The positive and negative values of CBAD indicate the relative magnitude of the NH4+potential uptake and regeneration rates. There were 6 and 8 sites in Anhui and Jiangsu sections showing negative values of CBAD, accounting for approximately 32% and 53% of the sampling sites, respectively. However, the CBAD rates of all sites in the estuary were positive values.