4. Discussion
4.1 NH4+recycling rates and contribution to the N budget in Yangtze
River
REGs
and Upots from Yangtze River were 0.26
(0.05~1.19) μmol N L−1h−1 and 0.34 (− 0.22~1.99) μmol N
L−1 h−1, respectively. Generally,
the REGs and Upots in this study were within the range
of values reported elsewhere (Table 2 ). In detail, they were
comparable to several major polluted inflow rivers of Lake Taihu, China
(Jiang et al. , 2019), but were higher than that from the low
nutrients and low primary production Old Woman River (Mccarthy et
al. , 2007b), Aransas River and Mission River (Bruesewitz et al. ,
2015). Compared to other eutrophic lakes, such as Lake Taihu, Lake Petit
saut and Lake Maracaibo (Gardner et al. , 1998; Collos et
al. , 2001; Jiang et al. , 2019), insignificantly lower REGs and
Upots were observed in the river-estuary continuum of
Yangtze River. In addition, it is noteworthy that compared with lake
ecosystems, river ecosystems tend to be found the greater REGs than
Upots, suggesting a potential release risk of
NH4+ in the river water column.
Sources of NH4+ to Yangtze River
mainly include external nutrient inputs (river input, atmospheric
deposition, and sediment release) and undocumented regenerated
NH4+. Although it has been confirmed
that the internal recycling processes play a vital important role in the
NH4+ budget and ecological effect in
eutrophic lakes (Wu et al. , 2017; Paerl et al. , 2011), the
contribution of regenerated NH4+ to
river-estuary continuum of Yangtze River is still unclear. Here we
provide the first estimation of contribution from regenerated
NH4+ to the N budget. Previous study
has reported that river N input from the upper and middle reaches of
Yangtze River was 57.14 × 108 kg N
yr−1, and N input in the lower reaches of Yangtze
River (study area of this study) was estimated as
4.87 × 108 kg N
yr−1 (Liu et al. , 2018). Atmospheric deposition
in our study area was estimated as 2.76 × 108 kg N
yr−1, which was calculated as the deposition to
emission ratio (Chen et al. , 2016). N input from sediments was
estimated as 2.19 × 108 kg N yr−1(estimated using the total mineralized N in the sediments minus the sum
of denitrification, anammox and microbial N assimilation) (Lin et
al. , 2016). Based on The Changjiang Bulletin issued by the Changjiang
Water Resources Commission of the Ministry of Water Resources in 2018,
the annual runoff of Datong station (Fig.1) in 2018 was 8028 ×
108 m3. Together with average REGs
in the river-estuary continuum, we can estimate riverine regenerated
NH4+ was approximately 21.8 ×
108 kg N yr−1. Compared with other N
sources in the Yangtze River (Fig. 5) , the regenerated
NH4+ is lower than riverine input from
upper and middle reaches, but much higher than N input from lower
reaches, atmospheric deposition, and sediments release. The regenerated
NH4+ accounts for 25% of total N
inputs in the study area, suggesting that regenerated
NH4+ is an important internal N source
for microbes and may influence nutrient dynamics in lower coasts.
Although the summer rates of REGs can be significantly higher than in
other seasons (Bruesewitz et al. , 2015; Jiang et al. ,
2019), which made the annual regenerated
NH4+ be overestimated, this ratio is
high enough and could not be neglected.
4.2 Effects of SS on
NH4+ recycling
rates
No significant correlation between REGs and Chl-a was found in
this study (r =0.220, p > 0.05) (Table 3) ,
indicating that algae was not the major source of regenerated
NH4+. Unlike in eutrophic lakes (e.g.,
Lake Taihu, Jiang et al. , 2019), algae abundance in Yangtze River
was low (2.8 ± 3.2 μg L−1) due to high turbidity and
washout from the high velocity of water flow in the Yangtze River, which
leads to low concentrations of algae-derived organic N. However,
significant correlations between REGs and SS instead of between REGs and
DOC (r =0.197, p > 0.05) or DON (r =0.111, p> 0.05) suggested that REGs were mainly influenced by
allochthonous particulate matters. SS is a complex mixture of organic
detritus, microorganisms, and other organisms (Turner and Millward,
2002; Odman et al. , 1999), and SS in Yangtze River mainly come
from the input of basin and soil erosion (Yang et al. , 2007). In
this study, SS were positively correlated with COD (r =0.535, p< 0.01), and TN (r = 0.304, p < 0.05), and PN
(r =0.478, p < 0.01) (Fig. S2) , suggesting
that SS can act as a vector of nutrient to promote N cycling processes
(Bilotta and Brazier, 2008; Zhang et al. , 2019). Moreover,
significant correlations between REGs and COD (roughly represent the
content of particulate and dissolved organic matter) (r =0.608, p< 0.01) and PN (r =0.455, p < 0.01) provide
further evidence that heterotrophic bacterial (e,g., ammonifying
bacteria) degradation of allochthonous particulate organic matters
influences regeneration rates of NH4+.
Ammonifying bacteria, a major participant for
NH4+ regeneration activities, is
abundant and widespread in river systems and tended to attach on SS (Xiaet al. , 2013). Ammonifying bacteria population in culture with 5
g L−1 SS of river systems were shown two orders of
magnitude higher than that without SS (Xia et al. , 2013). Thus,
high regeneration rates of NH4+ in
Yangtze River were deduced to be mainly contributed by heterotrophic
bacteria attached on high levels of SS and PN in SS provides important
substrate for bacterial metabolism.
NH4+ uptake is primarily composed of
nitrification as well as assimilation by phytoplankton (Hampel et
al. , 2017). However, the low biomass of phytoplankton in the turbidity
river may contribute less to NH4+uptake. This is supported by the insignificant correlation between
Upots and Chl-a (r =0.257, p> 0.05). Study reported that nitrification rates increased
with SS as a power function in the Yellow River, China, which was
characterized by high SS concentrations (Xia et al. , 2009).
Combined with high SS concentrations and low algae biomass in the
Yangtze River, we can infer that NH4+uptake is mainly due to nitrification on SS. In this study, the SS
concentrations were found to be significantly correlated with
Upots (r =0.825, p < 0.01), which
evidences the importance of SS in the
NH4+ uptake process. Other factors
such as the concentrations of COD, TN and PN can also affect
NH4+ uptake rates. This may be due to
the significant positive correlations between COD, TN, PN and the REGs,
which provides abundant substrates for the uptake of
NH4+.
4.3 Higher planktonic
NH4+ demand in the estuary than river
section of Yangtze
River
Many studies have reported that Yangtze River estuary is characterized
as P limitation due to sufficient N inputs (Wong et al. , 1998;
Liang and Xian, 2018). However, results in this study suggest that
planktonic NH4+ limitation may also
occur in the river-estuary continuum of Yangtze River. CBAD as the index
of NH4+ limitation in water, addresses
the vital question whether or not the
NH4+ demand of plankton could be met
by NH4+ recycling in the water column
(Jiang et al. , 2019; Gardner et al. , 2017). Our results
showed that CBAD in the estuary of Yangtze River were significantly
higher than in the Anhui and Jiangsu sections (near zero) (Fig.
4) , where NH4+ regeneration and
uptake were nearly balanced.
Significant higher CBAD in the estuary could be due to several possible
reasons. First, high concentrations of SS can not only promote microbial
nitrification, but also accelerate
NH4+ regeneration (Xia et al. ,
2013; Xue et al. , 2019). Our results further indicated that SS
concentrations promoted Upots faster than REGs
(Fig.6 ), resulting in higher CBAD in the estuary of Yangtze
River. Second, higher water temperatures (averaged 29.2 ± 0.5℃) in the
estuary than other sections (averaged 28.4 ± 0.7℃ in Jiangsu section,
averaged 28.1 ± 0.8℃ in Anhui section) of Yangtze River promote the
higher CBAD values and NH4+limitation. Culture incubations in Lake Taihu showed that
Upots increased faster than REGs in response to
increasing water temperature between 5.0 and 32.9℃ (Jiang et al. ,
2019). Another possible explanation for the higher CBAD in estuary is
the higher plankton biomass. Previous studies reported that
approximately 57% of bacterial cells were retained on the 0.7 μm
filters and confirmed the presence of small phytoplankton and archaea
through DNA analysis (Sipler et al. , 2017; Connelly et
al. , 2014). Our results showed that PN was significantly correlated
with Chl-a (r =0.314, p < 0.05) (Fig.
S3) . Thus, PN concentrations can approximately represent biomass N of
plankton intercepted by filters (GF/F). Comparing with the river
sections, the estuary showed higher PN concentrations and CBAD increased
with increasing PN concentrations (r =0.44, p < 0.01),
implying that organisms may be increasing
NH4+-deprived as plankton biomass
increases, which was similar to eutrophic lakes (Gardner et al. ,
2017).
4.4 Implications for
management
Our results suggest that SS is a key factor controlling
NH4+ recycling rates. High
concentrations of SS in the water column of Yangtze River can promote
both NH4+ regeneration and
nitrification rates, resulting in the persistence of algal growth and
potential high levels of NO3−concentrations. This is consistent with a previous study which showed
that nitrification is one of the major sources of
NO3− in the Yangtze River (Li et
al. , 2010). Thus, effective measures to reduce SS concentrations in
this turbid river may reduce the risk of N pollution. For example,
restoration of shoreline vegetation, improvement of land use, and
management of soil erosion are recommended to reduce SS concentrations
in the lower reaches of Yangtze River.
Most previous studies in Yangtze River pay a special attention to
NO3− exports into the East China Sea
due to its high concentrations (Zhou et al. , 2008; Mulleret al. , 2008; Liang and Xian, 2018). As a more preferred N
nutrition for most phytoplankton species,
NH4+ were overlooked in river systems
because of low ambient concentrations. Our study and more evidences have
shown that NH4+ recycling rates can be
high even with low ambient concentrations (Jiang et al. , 2019;
Gardner et al. , 2017), which can still influence estuaries and
its coastal areas profoundly. High
NH4+ recycling rates and demands
reflect rapid microbial metabolism including both algal uptake and
bacterial activities, which closely related to the growth and
persistence of microbes (Hampel et al. , 2019). Compared to
eutrophic lakes, NH4+ recycling in the
estuary of Yangtze River was influenced in a larger part by bacterial
activities. As large amounts of SS and organic matters flow downside
into the coastal areas, rapid internal
NH4+ recycling may also support algal
growth and contribute to bloom persistence. Thus, diverse
countermeasures should be focused on reducing estuarine inputs of SS or
labile organic matters that potentially support high
NH4+ recycling. Future studies are
expected to understand how different properties of SS and organic matter
influence water column NH4+ recycling.
Results of this study is not only
beneficial to the management of this third longest and most economically
valuable river in the world, but also valuable to other large rivers and
river-estuary continuum systems.