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.