| The south area of the middle and lower reaches of the Yangtze River is one of the strong acid rain centers in the world and there are many lakes in the area.The study of the impact of acid rain on freshwater ecosystems will help safeguard the ecological security of the middle and lower reaches of the Yangtze River.Therefore,this study first collected acid rain data and analyzed the characteristics of spatio-temporal changes of acid rain in China during the period from 2007 to 2016.Then,field investigation were carried out to analyze the difference of pH between the acid rain area and the nonacid rain area at the same nutrient level.Finally,a further simulation experiment was conducted to study the effect of different concentrations of acid rain on the pH and zooplankton of water bodies at different nutritional levels.(1)The acid rain data for the past ten years in China have been collected from the China Environmental Status Bulletin and used as a basis to analyze the temporal and spatial changes of acid rain in China.In the past ten years,the frequency and acidity of acid rain in China have showen a downward trend,and the frequency of different types of acid rain also showed a downward trend.In 2016,the acid rain area in China was significantly reduced compared with 2007,and the acid rain in the western region has weakened obviously,but the region in the south of the middle and lower reaches of the Yangtze River is still the most serious area of acid rain in China.(2)Based on the spatial analysis of acid rain,the lakes in the middle and lower reaches of the Yangtze River were divided into acid rain area lakes and non-acid rain area lakes and lakes were investigated.The results of field investigation showed that the average pH values of the lakes in the acid rain area were lower than the non-acid rain areas at different nutrient levels in spring,summer,and autumn,and the lake pH values were significantly positively correlated with water temperature.(3)Three acid rain concentrations(pH=2.5,4.0,and 5.0)and two nutrient levels(low and high nutrient levels)were set up to simulate the effects of acid rain on lake pH and zooplankton at different nutrient levels.The results of simulation experiments show that under the effect of acid rain with different pH concentrations,although the daily pH trends of the treatment groups are the same,the pH values of the low nutrition group are lower than the high nutrition group and the rate of pH increased more slowly than the latter.The differences in the pH values of different acid rain treatments in the low and high nutrient level groups gradually decreased over time.At the end of the experiment,there was no significant difference in the pH value between the experimental group and the control group,and the water had a buffer capacity for the precipitation of single acid rain.(4)The planktonic crustaceans in the experiment included: Diaphanosoma sp.,Moina sp.,Ceriodaphnia sp.,Macrothrix sp.,Chydorus sp.,Cyclopoida and Calanoida,among which Diaphanosoma sp.,Moina sp.and Cyclopoida are dominant.After adding different concentrations of acid rain,the density of copepods increased from the initial state to the end of the experiment,which was higher than that of Cladocera.The growth in density of Diaphanosoma sp.of different pH treatments in high and low nutrition groups showed 2.5>4.0>5.0 characteristics.Under different pH treatments,the increase of density of Cyclopoida was 2.5<4.0<5.0 in the high nutrition group and 2.5>4.0>5.0 in the low nutrition group.Acid rain at different pH concentrations did not have a significant effect on copepoda nauplii and Moina sp. |
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