3.1. Spatial variations in water quality
In addition to NH4+-N, the other eight water quality parameters, including TN, NO3−-N, DOC, TP, SRP, PP, pH and EC, showed significant differences across the nine subareas (Table 2, Fig. 3). TN concentrations ranged from 0.90–4.10 mg/L (mean 1.86 ± 0.95 mg/L), with highest values in FH, followed by ZZ, NL, DT, CP, QT, ZL, and SC. NO3−-N concentrations ranged from 0.15–3.49 mg/L (mean 1.00 ± 1.07 mg/L), also with highest values in FH. These decreased eastward and southward toward the minimum concentration in ZL. NH4+-N concentrations ranged from 0.32–0.55 mg/L (mean 0.41 ± 0.08 mg/L), with no significant difference between the 9 subareas.
As with the spatial variations of TN and NO3−-N, TP and SRP concentrations were also highest in FH, decreasing eastward and southward toward their minimum concentrations in SC. In contrast to TN and TP, concentrations of DOC, pH and EC were lowest in FH and increased eastward and southward, reaching highest concentrations in the QT subarea.
Table 2
Analysis of Variance (ANOVA) Results for water quality in different subareas
Water quality | ANOVA | Subareas |
p-value | FH | ZZ | NL | SC | DC | DT | CP | QT | ZL |
TN | < 0.001 | a | b | bc | c | bc | bc | bc | bc | bc |
NO3−-N | < 0.001 | a | b | bc | c | bc | c | c | c | c |
NH4+-N | 0.422 | a | a | a | a | a | a | a | a | a |
TP | 0.041 | a | ab | ab | c | bc | ab | ab | bc | bc |
SRP | < 0.001 | a | a | b | bc | bc | bc | bc | bc | c |
PP | 0.031 | a | c | ab | c | bc | ab | ab | bc | bc |
DOC | < 0.001 | c | bc | c | bc | bc | bc | bc | a | ab |
PH | < 0.001 | c | b | ab | ab | ab | a | b | a | ab |
EC | < 0.001 | c | c | c | bc | a | ab | ab | a | ab |
Note: Subareas with the same letter within the same water quality parameters are no significant difference according to Tukey tests (p < 0.05). |
3.2. Influences of water landscape type on water quality
Correlation analysis between 11 water quality parameters (T, pH, EC, DO, DOC, TN, PON, NH4+-N, NO3−-N, DON and TP) and area proportion of landscape type (reed littoral zones, open water and fish ponds) by 100, 300, and 500 m buffers (Fig. 4) showed negative correlations between all but NH4+-N and the area proportion of reed littoral zones for the 300 m and 500 m buffers. Open water generally contributed to the concentrations of DO, DOC, TN, PON, and NH4+-N for the 300 m and 500 m buffers. In contrast, the concentrations of DO, DOC, TN, PON, DON, NO3−-N, and TP were significantly positively correlated with the area proportion of fish ponds for the 100 m buffer.
There were significant differences in some (TN, NO3−-N, TP, PP, SRP, DOC, pH and EC) but not all (NH4+-N) parameters among the four water landscape types of FH, open water, reed littoral zones, and fish ponds (Table 3, Fig. 5). TN concentrations were significantly lower in open water (1.67 ± 0.66 mg/L), reed littoral zones (1.07 ± 0.34 mg/L), and fish ponds (1.66 ± 0.70 mg/L) than in FH (3.90 ± 0.31 mg/L). Similarly, NO3−-N concentrations were significantly lower in open water (0.75 ± 0.72 mg/L), reed littoral zones (0.35 ± 0.20 mg/L), and fish ponds (0.40 ± 0.28 mg/L) than in FH (3.28 ± 0.32 mg/L). In contrast, NH4+-N concentration were no significant difference between the four water landscape types.
TP concentrations were lower in open water (0.09 ± 0.03 mg/L) and reed littoral zones (0.09 ± 0.05 mg/L) than in fish ponds (0.13 ± 0.09 mg/L) and FH (0.14 ± 0.05 mg/L). Similarly, PP concentrations were lower in open water (0.04 ± 0.03 mg/L) and reed littoral zones (0.04 ± 0.04 mg/L) than in fish ponds (0.07 ± 0.05 mg/L) and FH (0.05 ± 0.05 mg/L). However, the highest SRP concentrations were found in FH (0.06 ± 0.01 mg/L), with lower concentrations in open water (0.02 ± 0.01 mg/L), reed littoral zones (0.02 ± 0.02 mg/L), and fish ponds (0.02 ± 0.03 mg/L).
Fish ponds had the highest DOC concentrations (12.23 ± 4.38 mg/L), open water (8.54 ± 2.24 mg/L) and reed littoral zones (8.58 ± 2.07 mg/L) were mid-level, and FH (5.70 ± 0.52 mg/L) was lowest. EC and pH followed the same trend as DOC.
Table 3
Analysis of Variance (ANOVA) Results for water quality of different water landscape types
Water quality | ANOVA | water landscape type |
p-value | FH | OW | RLZ | FP |
TN | < 0.001 | a | b | c | b |
NO3−-N | < 0.001 | a | b | c | c |
NH4+-N | 0.487 | a | a | a | a |
TP | 0.043 | a | b | ab | b |
SRP | 0.041 | a | b | b | b |
PP | 0.015 | a | b | b | b |
DOC | < 0.001 | c | b | b | a |
PH | < 0.001 | b | a | a | a |
EC | < 0.001 | b | b | b | a |
Note: Water landscape type with the same letter within the same water quality parameters are no significant difference according to Tukey tests (p < 0.05). |
3.3. Effects of water landscape type on N:P,individual composition of nitrogen and phosphorus and DOM fluorescence components
TN:TP, DN:DP, and NO3−-N:SRP were all significantly affected by water landscape type (Fig. 6). TN:TP was highest for FH (29.5 ± 9.2), followed by open water (19.8 ± 7.8), fish ponds (15.7 ± 6.6), and reed littoral zones (13.2 ± 5.8), and the same trend occurred for DN:DP and NO3−-N:SRP. These results suggested that BYD has the ability to remove incoming nitrogen. In contrast, phosphorus appeared to be relatively enriched compared to nitrogen, while SRP concentrations were reduced in open water, fish ponds, and reed littoral zones.
Figure6. Variations in (a) TN:TP, (b) DN:DP, and (c) NO3−-N:SRP for the four water landscape types
Variations in nitrogen and phosphorus composition were significantly affected by water landscape type (Fig. 7). The proportion of NO3−-N were significantly higer in FH, open water and reed littoral zones than in the fish ponds. NH4+-N accounted for the highest and lowest proportion of TN in the reed littoral zones and FH respectively, and the proportion of NH4+-N was not meaningfully different between open water and fish ponds. DON accounted for the highest proportion of TN in the fish ponds. Meanwhile, PON accounted for the highest proportion of TN in the fish ponds, declining in open water and reed littoral zones. SRP accounted for the highest and lowest proportion of TP in the FH and fish ponds, and the proportion of SRP had no significant difference between the open water and reed littoral zones. The proportion of PP was significantly lower in FH, open water and reed littoral zones than in fish ponds.
PARAFAC analysis separating the different 3D-EEM spectral components detected three main fluorescence components in BYD (Fig. 8). Component 1 and component 2 were indicative of the presence of tyrosine-like proteins (ProⅠ) and tryptophan-like proteins (ProⅡ) respectively, both of which could be produced by plankton death, and decomposition along with microbial metabolism. Component 3 was attributable to humic acid substances (HA), which could be decomposed as plant residues and not easily degraded or utilized by microorganisms (Zhao et al. 2016). The proportions of Pro I in fish ponds accounted for 39 ± 11%, significantly higher than in FH, open water, and reed littoral zones (Fig. 9). In contrast, the proportions of HA in fish ponds were 23 ± 15%, significantly lower than those in FH, reed littoral zones and similar to open water.