2.1. Vertical distribution characteristics of nitrogen nutrients at the SWI
The spatial distribution characteristics of NH4+-N and TN in the SWI are shown in Fig. 1: the upstream of the reservoir (S1), the central area of the reservoir (S2) and the downstream of the reservoir (S3) in the overlying water NH4+-N has the same vertical change trend as TN, and its concentration has no obvious change.Among them, the concentrations of NH4+-N and TN in the upstream of the reservoir (S1) were between 3.20 ~ 90.35 mg·L− 1 and 5.85 ~ 95.75 mg·L− 1, respectively, and the average values were 41.77 ± 23.21 and 44.60 ± 22.86 mg·L− 1; the concentrations of NH4+-N and TN in S2 were between 9.11 ~ 86.35 and 9.32 ~ 86.40 mg·L− 1, and the average values were 53.18 ± 25.22 and 54.02 ± 25.19 mg·L− 1; the concentrations of NH4+-N and TN in S3 were 4.45 ~ 57.45 and 6.83 ~ 77.30 mg·L− 1, respectively, the mean values were 43.09 ± 16.25 and 47.49 ± 17.71 mg·L− 1, respectively. From the SWI downward, the oxygen content in the sediment decreases with the increase of depth, and the anaerobic environment is conducive to the ammonification reaction of organic nitrogen, and the digestion of ammonia consumption is weakened, resulting in NH4+-N accumulates in sediments[10], showing that the concentration of NH4+-N increases with depth.
The spatial distribution characteristics of PO43−-P and TP in the SWI are shown in Fig. 1: In the overlying water the S1, S2 and S3 PO43−-P and TP have the same vertical change trend, and their concentration has no obvious change. However, the concentration of PO43−-P and TP in the interstitial water of the sedimentary column at each sampling point showed a gradually increasing trend with the increase of depth. Among them, the concentrations of PO43−-P and TP in S1 were between 0.15 ~ 6.65 and 1.31 ~ 17.10 mg·L− 1, and the average values were 2.60 ± 2.33 and 10.27 ± 4.42 mg·L− 1; the concentrations of PO43−-P and TP in S2 were between 1.10 ~ 7.35 and 1.48 ~ 17.50 mg·L− 1, respectively, and the average values were 3.19 ± 1.57 and 8.60 ± 4.26 mg·L− 1 of the S3 PO43−-P and TP concentrations were between 0.26 ~ 13.45 and 1.00 ~ 26.90 mg·L− 1, and their mean values were 8.50 ± 3.79 and 17.05 ± 7.19 mg·L− 1, respectively. Related research [11] found that: with the strengthening of the reducing environment inside the sediment, Fe3+ was continuously reduced to Fe2+, and iron-bound phosphorus was also released so that the release of phosphorus at the SWI increased. Anaerobic experiments in related laboratories also further verified the coupling relationship between iron and phosphorus, that is, the reduction of iron leads to the release of phosphorus[12]. At the same time, related scholars found that the secretion of nitrate bacteria can accelerate the dissolution of Fe3+, making the adsorption in Fe(OH)3 The P is released [13]. Therefore, the orthophosphate content at 0–3 cm in the SWI environment showed a gradually decreasing distribution feature.
The release flux of NH4+-N at the SWI is shown in Table 1: the correlation coefficient R > 0.80 in S1, S2 and S3. The exponential fitting curve is ideal. From the release flux F value, it can be concluded that NH4+-N at the SWI is released from the interstitial water in the sediment to the overlying water body, and the sediment is the "source" of NH4+-N. The release flux (F) of NH4+-N ranges from 7.26 to 12.72 (mg·m2·d− 1). NH4+-N in the sediment interstitial water, on the one hand, enters the overlying water body with the concentration gradient under the action of molecular diffusion; Potential for NH4+-N release from sediment interstitial water to overlying water [14]. The reason for the higher release flux of NH4+-N in the upper reaches is that aquatic plants and phytoplankton use NH4+-N first when using nitrogen nutrients[15], and on the other hand, most of the NH4+-N in interstitial water comes from The decomposition of organic matter regenerates NH4+-N [16]. Moreover, human activities and industrial production in the upper reaches also contributed abundant nitrogen nutrients to the upper reaches.
The release flux of PO43−-P at the SWI is different from that of NH4+-N, and the F value of PO43−-P (-0.26 mg·m− 2·d− 1) It can be seen that PO43−-P at the SWI in the upper reaches,S1 is in the state of sediment absorption. From the release flux F values in S2 and S3, it can be concluded that PO43−-P at the SWI is released from the interstitial water in the sediment to the overlying water body. Their release fluxes (F) are 0.03 and 0.85(mg·m− 2·d− 1), respectively. The migration and release process of PO43−-P in the SWI is affected by many factors[17]. Li found that under aerobic conditions, the millimeter-scale aerobic layer on the sediment surface and the diffusion boundary layer below the SWI will prevent the release of phosphorus in the interstitial water to the overlying water[18], and when the dissolved oxygen content in the overlying water decreases When,the diffusion boundary layer or aerobic layer becomes thinner or disappears, and phosphorus in the interstitial water will be released to the overlying water body along with the concentration gradient under the action of molecular diffusion[19].
Table 1
Fluxes of NH4+-N and PO43−P
Nutrients | Sampling point | Curve fitting | R | \({D}_{s}\times {10}^{-6}\) \(({cm}^{2}\bullet {s}^{-1})\) | \({\partial }_{c}/{\partial }_{x}\) \(mg\bullet {\left(L\bullet cm\right)}^{-1}\) | \(F\) \(mg\bullet {\left({m}^{2}\bullet d\right)}^{-1}\) |
NH4+-N | S1 | y = 17.57e-0.093x | 0.97 | 11.3 | -1.63 | 12.72 |
S2 | y = 29.578e-0.069x | 0.95 | 11.3 | -0.93 | 7.26 |
S3 | y = 25.622e-0.06x | 0.82 | 11.3 | -1.54 | 11.97 |
PO43--P | S1 | y = 1.9464e0.0502x | 0.33 | 3.92 | 0.1 | -0.26 |
S2 | y = 2.7313e-0.005x | 0.10 | 3.92 | -0.01 | 0.03 |
S3 | y = 3.4611e-0.091x | 0.72 | 3.92 | -0.31 | 0.85 |
The distribution of nutrient content at the SWI mainly shows that the content in the sediment is significantly greater than that in the overlying water body. According to the calculation of the release flux according to Fick's first law, it is found that only the PO43−-P in S1 the value of F is negative (-0.26 mg/m2·d), that is, it is deposited from the overlying water to the sediment. The release rates (12.72 mg/m2·d, 7.26 mg/m2·d, 11.97 mg/m2·d) of NH4+-N in S1, S2, S3 were significantly. It is greater than the release rate of PO43−-P (0.03 mg/m2·d, 0.85 mg/m2·d) in S2 and S3. The reason is that when aquatic plants and phytoplankton use nitrogen nutrients, they will first use NH4+-N, and most of NH4+-N in interstitial water comes from the decomposition of organic matter to regenerate NH4+-N, while for PO43−-P, The diffusion boundary layer and aerobic layer in the lower layer of the interface will prevent the release of phosphorus in the interstitial water to the overlying water, or the peak concentration of PO43−-P in the sediment surface will inhibit the migration and release of PO43−-P in the lower layer to the upper layer, The release flux of PO43−-P was significantly lower than that of NH4+-N.
2.2. Structural characteristics of microbial communities at the SWI
The distribution characteristics of the microbial community structure in the SWI are shown in Fig. 2a. It can be seen from the figure that the similarity of the microbial community structure in the SWI is reflected in the overlying water bodies, and the differences are reflected in the sediments. Methyloparacoccus is a strictly respiratory Gram-negative aerobic genus with oxygen as the terminal electron acceptor, and its average relative abundance in the SWI of each sampling point is the highest (13.88% ± 6.30% ), its relative abundance is the highest (23.42%) at 20cm of the overlying water downstream of the reservoir (S3), and its relative abundance is the lowest (1.58%) at -02cm of the sediment. The genus Methylomonas has a wide distribution and is a kind of chemoorganotrophic bacteria that has no strict requirements on inorganic nutrients and may grow in natural substrates containing methane. Most of them are symbiotic or associated with heterotrophic bacteria that cannot oxidize methane, and it is extremely difficult to isolate, purify and retain pure species. Trace amounts of organic substances such as amino acids and polypeptides can inhibit their growth, and even some methane-oxidizing bacteria are also inhibited by very small amounts of methanol. Strictly aerobic, using oxygen molecules as electron acceptors, gradually oxidizes methane into alcohols, aldehydes and acids until carbon dioxide, and obtains energy through the monophosphate ribose pathway or the serine pathway [20]. The average relative abundance of Methylomonas at the SWI was 13.60% ± 8.55%, second only to Methyloparacoccus.It reaches the highest (28.24%) at 05cm of the overlying water in the core area of the reservoir (S2), and the lowest relative abundance (2.12%) at -02cm of the sediment upstream of the reservoir (S1). The average abundances of Arenimonas and Methylophilus at the SWI of each sampling point were also relatively high, reaching 9.57% ± 3.46% and 7.94% ± 2.26%, respectively.The maximum values appear at 10cm (14.54%) and 15cm (9.66%) of S1, respectively, and the smallest values are respectively distributed in S2 at 05cm of the upper water (5.79%) and the S3 sediment − 02cm (1.96%). The average abundance of Polynucleobacter, Phaeodactylibacter, Sulfurisoma and Subdivision3_genera_incertae_sedis on the SWI of each sampling point was above 3.00%, the maximum value of Phaeodactylibacter and Sulfurisoma all appeared at 20cm above the S1, respectively: 7.20%, 8.84%, and the maximum value of Subdivision3_genera_incertae_sedis was 9.44% at -02cm of the sediment in S3;The minimum values of Polynucleobacter and Phaeodactylibacter both appeared at S3 -02cm, at 0.55%, 1.32%, and the minimum value of Sulfurisoma appeared at 10cm of the overlying water downstream of the reservoir (S3): 0.64%, And the minimum value of Subdivision3_genera _incertae_sedis is 1.69% at 25cm of overlying water downstream of the reservoir (S3).
As shown in Fig. 2b, the principal component analysis (PCA) of the different bacterial colonies and the sampling sites showed that there were significant differences between the upper, middle and lower reaches of the reservoir (S1, S2 and S3). The microorganisms in different areas showed some variability, which may be caused by the disturbance of hydraulic conditions, different species of submerged plants and benthic fauna. However, the overall microbial community structure of the overlying water bodies is very similar, while the sediments all have different microbial community structure characteristics. Among these microorganisms, Thiobacillus spp. were absolutely dominant, with the highest abundance of Smithella (9.87%±0.73%) in the upper reservoir (S1), and Smithella spp. were essential for the degradation of natural polymers in the anaerobic habitat through the formation of a closely coupled symbiotic metabolism, microbial syntaxin. The central region of the reservoir (S2) had the highest abundance of Sulfurisoma (10.61%±1.11%), a parthenogenic autotroph isolated from the freshwater lake, which is involved in the oxidation of inorganic sulfur in the water column. Downstream of the reservoir (S3), the highest abundance (10.83%±1.25%) was found in Thiobacillus, a colorless sulfur bacterium most commonly found in soil and natural water bodies, which oxidizes sulfides to monomersulfur or sulfate, or thiosulfate to sulfate.
2.3.Relative abundance of pathogenic bacteria at the SWI
The absolute abundance distribution of E.coli and ENT on the SWI is shown in Fig. 3. Since the sediment depth in the central area of the reservoir (S3) is relatively high, the overlying water body is only sampled to the surface sediment 10cm above. It can be seen from the figure that the absolute abundance ranges of E.coli upstream of the reservoir (S1), the central area of the reservoir (S2), and the downstream of the reservoir (S3) are respectively 2.03×105~4.72×106, 2.41×105~7.39×106 and 2.20×105~2.47×107 (copies·g− 1), the absolute abundance of E.coli at the SWI had little difference. It is worth noting that the absolute abundance of E.coli in the surface layer of the SWI − 02cm sediment in the three sedimentary columns was 1 order of magnitude higher than that in the overlying water body. The absolute abundance of E.coli in the overlying water of the SWI has little difference, but the absolute abundance of E.coli at 20cm above the SWI the upstream of the reservoir (S1) and downstream of the reservoir (S3) Slightly higher than other depths of overlying water.
The distribution characteristics of the absolute abundance of ENT and E.coli at the SWI were not much different, both showing a trend that the sediment was higher than the overlying water body, but the absolute abundance of ENT at the SWI was about E.coli was 2 orders of magnitude higher. The absolute abundance ranges of ENT in the upstream of the reservoir (S1), the central region of the reservoir (S2), and the downstream of the reservoir (S3) were 6.30×106~1.23×109, 6.94×106~8.81×108 and 8.28×106 ~ 1.43×109. The abundance of ENT in the downstream of the reservoir (S3) was about 1.60 times that of the sedimentation column in the central area of the reservoir (S2), and 1.37 times that of the sedimentation column in the upstream of the reservoir (S1). The absolute abundance of ENT on the SWI was not significantly different. Similar to E.coli, the absolute abundance of ENT in the three sediment columns was about 1 order of magnitude higher in the surface − 02cm sediment at the SWI than in the overlying water.
Table 2 found that E.coli, ENT, TN, and TP in the SWI in the upstream (S1) and downstream (S3) had extremely significant positive correlations (P < 0.01) or significant positive correlations (P < 0.05). The specific performance is: there is a significant positive correlation between E.coli and TN (P < 0.05) and TP (P < 0.05) in the SWI in the upstream of the reservoir (S1); E.coli and TN (P < 0.05) and TP (P < 0.05) in the SWI had a significant positive correlation, while E.coli and TN (P < 0.01 ), TP (P < 0.01) all had extremely significant positive correlations; ENT in the upstream (S1) SWI had extremely significant positive correlations with TN (P < 0.01), TP (P < 0.01) Positive correlation; but the correlation between ENT and TN and TP in the SWI in the center of the reservoir (S2) was not significant (P > 0.05); the SWI downstream of the reservoir (S3) There were extremely significant positive correlations between enterococcus and TN (P < 0.01) and TP (P < 0.01).
Table 2
The Pearson correlation analysis between pathogenic bacteria and TN、TP in the SWI
Types of pathogens | Pearson correlation | S1 Sedimentary column | S2 Sedimentary column | S3 Sedimentary column |
TN | TP | TN | TP | TN | TP |
E. coli | Correlation coefficient | 0.862* | 0.830* | 0.993* | 0.990* | 0.972** | 0.969** |
Significance | 0.027 | 0.041 | 0.045 | 0.045 | 0.001 | 0.001 |
ENT | Correlation coefficient | 0.996** | 0.999** | 0.981 | 0.977 | 0.981** | 0.983** |
Significance | 0.00003 | 0.000 | 0.123 | 0.136 | 0.001 | 0.0004 |