| Nutrient enrichment, mainly with nitrogen (N) and phosphorus (P), is a frequently cited cause of eutrophication for rivers. Biologically-derived nutrient thresholds are helpful to regulate nutrient levels in eutrophicated rivers. In this paper, we first selected a typical mountains-plain compound watershed (Western Tiaoxi river) to investigate the spatial and temporal characteristics of chlorophyll-a (Chla) and the relationship with nutrients, then evaluate the level of eutrophication and compare the biological nutrient thresholds in different types of rivers by self-organized mapping analysis (SOM) cluster analysis and forecasting. At last we established a set of evaluation system of nitrogen and phosphorus thresholds which is suitable for Hangjiahu areas in Taihu lake basin, in order to help river managers to implement appropriate management strategies to alleviate trophic status, and hope to provide reference for non-point source pollution control in other similar rivers.Results showed that total N and total P concentrations in the water samples of Western Tiaoxi river system were relatively higher, on average the sestonic chlorophyll-a across the whole sampling points reached 6.23 μg/L, indicating that the water in Western Tiaoxi rivers tends to be eutrophication. Phosphorus may be the potential limiting nutrient for phytoplankton growth. In an algae outbreak cycle, the concentrations of total N, total P and sestonic chlorophyll-a all have obvious seasonal variation, with the maximum of 52.2 μg/L in September, and minimum in November. Overall, the contents in plain urban downstream sites were higher than that in mountain agricultural upstream sites.The SOM analysis performed well in clustering and abstract visualizing their responses and confirmed that the relationships between algae contents and two nutrients are both significantly correlated in this region. Compared to benthic Chla, sestonic Chla is more suitable for determining biological thresholds because the response to sestonic Chla was more direct and close. Moreover, algal responses to nutrients got a better fit in urban downstream than agricultural upstream and the approach of biological thresholds is preferred in the former. In addition, hydrometeorological elements such as temperature, pH, dissolved oxygen and turbidity also had a certain influence on sestonic Chla and benthic TP.Importantly, based on the chlorophyll-a criteria for fresh water and measured variables, we recommend the biological thresholds of total N, total P, ammonium nitrogen (NH4+-N) in water for Western Tiaoxi be set at 2.25 μg/L,0.15 μg/L and 1.06 μg/L, benthic total P and benthic NH4+-N be set at 0.95 g/kg and 63.01 mg/kg. In terms of different sampling time and different types of river, we recommend that the biological thresholds of total N at May, July, September and November should be set at 2.14 mg/L,1.62 mg/L,2.02 mg/L,1.97 mg/L respectively for mountain river system, and 2.55 mg/L,1.82 mg/L,2.26 mg/L,2.14 mg/L respectively for plain river system.The biological thresholds of total P at four sampling time were 0.24 mg/L, 0.13 mg/L,0.20 mg/L,0.16 mg/L respectively for mountain river system and 0.31 mg/L,0.17 mg/L,0.26 mg/L,0.23 mg/L respectively for plain river system. Seasonal management control for different types of river is more conducive to improving the water quality.According to the results of samples in representative rivers in Hangjiahu area, we got the biological thresholds of total N, total P, NH4+-N in the whole Hangjiahu area, which was 2.51 mg/L,0.19 mg/L and 1.78 mg/L respectively. At the same time, the SOM prediction of Chla with TN, TP as input variables were better, the biological threshold assessment method had good applicability and better accuracy in Hangjiahu area. The thresholds should be used as initial nutrient reference values for local river managers to implement appropriate strategies to alleviate nutrient loads and trophic status. According to the research results, a risk evaluation system based on thresholds could be set up. |
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