[Spatial-temporal Variations and Driving of Nitrogen and Phosphorus Ratios in Lakes in the Middle and Lower Reaches of Yangtze River].

In spring and summer of 2018, 26 lakes in the middle and lower reaches of the Yangtze River were studied to determine the temporal and spatial characteristics of nitrogen and phosphorus ratios (TN/TP) and their influencing factors. The differences in nitrogen and phosphorus ratios in different types of lakes (including water-psaaing lakes, deep reservoirs and eutrophic lakes) and in different seasons were analyzed in terms of the sources of the lakes, lake depth, suspended particulate matter concentrations, and phytoplankton levels. The average TN/TP was 21.52±14.28 in spring and 21.73±23.78 in summer. The TN/TP varied significantly in different types of lakes. The TN/TP ratios in water-passing lakes, deep reservoirs and eutrophic lakes were 20.41±9.25, 40.97±33.37, and 14.38±7.40 during spring, and were 22.62±6.48, 96.38±45.91, and 10.91±4.44 during summer, respectively. The TN/TP of the water-passing lakes and deep reservoirs increased significantly in summer, while that of the eutrophic lakes decreased significantly, which indicates that TN/TP changes and lake nutritional status are closely related. The source of nutrients in lakes and reservoirs affects the TN/TP. The TN/TP of lakes and reservoirs had a significant correlation with the lake depth in both spring and summer, indicating that lake depth is a key factor affecting the ratio of nitrogen and phosphorus. In addition, in eutrophic lakes with higher absolute nutrient concentrations, TN/TP has less effect on phytoplankton, while in deep-water lakes with lower absolute nutrient concentrations, TN/TP can determine the growth of phytoplankton limited by phosphorus. Therefore, the governance strategy of lakes in the middle and lower reaches of the Yangtze River should prioritize phosphorus control. Local digging, controlling non-point source pollution, sediment dredging, and changing fishery production methods can be applied to improve the ecological quality of the eutrophic lakes.

[Influence of Potamogeton crispus on Lake Water Environment and Phytoplankton Community Structure].

The decomposition of submerged macrophytes is generally associated with dramatic changes in the water environment, such as the large release of nutrients (e.g., nitrogen and phosphorus) and organic carbon to the surrounding waters, which may result in significant changes in phytoplankton community structure. In this study, Potamogeton crispus, physicochemical variables, and phytoplankton samples were collected in 14 shallow lakes in the middle and lower Jianghuai Plain in spring (growing period of P. crispus) and summer (decomposition phase of P. crispus) of 2018. The effects of the decline of P. crispus on water quality and phytoplankton community structure were quantified. The results showed that water transparency increased significantly in spring because the growth of P. crispus inhibits sediment resuspension and macrophytes can transport the nutrients from the water column to the sediment. The values of dissolved oxygen and pH also increased significantly due the photosynthesis by macrophytes. In contrast, the decomposition of P. crispus during summer months caused a significant increase in water turbidity and organic matter. There were considerable differences in phytoplankton biomass and cyanobacterial biomass in the sites with or without P. crispus, and the corresponding ratios of cyanobacterial biomass to the total algal biomass were 18.96% and 34.05%, respectively. Higher values of cyanobacterial biomass were observed with the decomposition of P. crispus than its counterpart in summer because ① the decomposition of macrophytes provided sufficient organic matter and nutrient (nitrogen and phosphorus) resources for cyanobacterial growth; ② P. crispus decline in summer significantly increased water turbidity, which makes cyanobacteria occupy a better ecological niche and more efficiently utilize nitrogen and phosphorus.

[Applicability of Bioavailable Phosphorus in Sediments to Indicating Trophic Levels of Lakes and Reservoirs].

Twelve lakes and reservoirs with different water depths and different water residence times were studied to identify the applicability of bioavailable phosphorus of sediments in indicating trophic levels. Water and sediment samples were collected in these 12 lakes and reservoirs to analyze the relationship of nutrient levels between the sediment and the water column. Sodium hydroxide extracted phosphorus (NaOH-P) determined using the SMT classification method is defined as the bioavailable phosphorus of sediment. The results showed that total phosphorus levels in sediments in different lakes and reservoirs ranged from 225 to 760 mg·kg-1 (mean value 502 mg·kg-1); the NaOH-P levels in sediments ranged from 86 to 584 mg·kg-1 (mean value 263 mg·kg-1); the total phosphorus concentrations in the water was 0.02-0.35 mg·L-1 (mean value 0.11 mg·L-1), and the chlorophyll a concentrations in the water were 3-349 µg·L-1 (mean value 51 µg·L-1). It was found that NaOH-P was more effective than total phosphorus in indicating the trophic status of the lakes and reservoirs. However, the NaOH-P levels were significantly related to the phosphorus concentrations in the water column only in shallow water with a long residence time. It was revealed that water residence time and water depth are two key factors that affect the relationship of the phosphorus content between the sediment and the water column. In deep waters or waters with short residence time, the NaOH-P content in the sediment hardly influenced the phosphorus concentration in the water columns, even at high levels. However, in shallow waters with long residence time, the sediment acted as both sources and sinks and frequently exchanged nutrients with the overlying water, especially during bloom periods in summer. Thus NaOH-P could be a potential risk of eutrophication in such waters.

[Chemical properties and enzyme activities of rhizosphere and non-rhizosphere soils under six Chinese herbal medicines on Mt. Taibai of Qinling Mountains, Northwest China].

This paper studied the chemical properties and enzyme activities of rhizosphere and non-rhizosphere soils in different habitats of six Chinese herbal medicines, including Pyrola decorata, Cephalotaxus fortunei, Polygonatum odoratum, Potentilla glabra, Polygonum viviparum, and Potentilla fruticosa, on the Mt. Taibai of Qinling Mountains. In the rhizosphere soils of the herbs, the contents of soil organic matter, total nitrogen, available nitrogen, and available phosphorus and the soil cation exchange capacity (CEC) were higher, presenting an obvious rhizosphere aggregation, and the soil enzyme activities also showed an overall stronger characteristics, compared with those in non-rhizosphere soils. The soil organic matter, total nitrogen, and total phosphorus contents in the rhizosphere soils had significant positive correlations with soil neutral phosphatase activity, and the soil CEC had significant positive correlations with the activities of soil neutral phosphatase and acid phosphatase. In the non-rhizosphere soils, the soil organic matter and total nitrogen contents had significant positive correlations with the activities of soil urease, catalase and neutral phosphatase, and the soil CEC showed a significant positive correlation with the activities of soil urease, catalase, neutral phosphatase and acid phosphatase. The comprehensive fertility level of the rhizosphere soils was higher than that of the non-rhizosphere soils, and the rhizosphere and non-rhizosphere soils of P. fruticosa, P. viviparum, and P. glabra had higher comprehensive fertility level than those of P. decorata, P. odoratum and C. fortunei. In the evaluation of the fertility levels of rhizosphere and non-rhizosphere soils under the six Chinese herbal medicines, soil organic matter content and CEC played important roles, and soil neutral phosphatase could be the preferred soil enzyme indicator.