Dependence of evolution of Cyanobacteria superiority on temperature and nutrient use efficiency in a meso-eutrophic plateau lake.

Algal blooms in lakes have been a challenging environmental issue globally under the dual influence of human activity and climate change. Considerable progress has been made in the study of phytoplankton dynamics in lakes; The long-term in situ evolution of dominant bloom-forming cyanobacteria in meso-eutrophic plateau lakes, however, lacks systematic research. Here, the monthly parameters from 12 sampling sites during the period of 1997-2022 were utilized to investigate the underlying mechanisms driving the superiority of bloom-forming cyanobacteria in Erhai, a representative meso-eutrophic plateau lake. The findings indicate that global warming will intensify the risk of cynaobacteria blooms, prolong Microcystis blooms in autumn to winter or even into the following year, and increase the superiority of filamentous Planktothrix and Cylindrospermum in summer and autumn. High RUETN (1.52 Biomass/TN, 0.95-3.04 times higher than other species) under N limitation (TN < 0.5 mg/L, TN/TP < 22.6) in the meso-eutrophic Lake Erhai facilitates the superiority of Dolichospermum. High RUETP (43.8 Biomass/TP, 2.1-10.2 times higher than others) in TP of 0.03-0.05 mg/L promotes the superiority of Planktothrix and Cylindrospermum. We provided a novel insight into the formation of Planktothrix and Cylindrospermum superiority in meso-eutrophic plateau lake with low TP (0.005-0.07 mg/L), which is mainly influenced by warming, high RUETP and their vertical migration characteristics. Therefore, we posit that although the obvious improvement of lake water quality is not directly proportional to the control efficacy of cyanobacterial blooms, the evolutionary shift in cyanobacteria population structure from Microcystis, which thrives under high nitrogen and phosphorus conditions, to filamentous cyanobacteria adapted to low nitrogen and phosphorus levels may serve as a significant indicator of water quality amelioration. Therefore, we suggest that the risk of filamentous cyanobacteria blooms in the meso-eutrophic plateau lake should be given attention, particularly in light of improving water quality and global warming, to ensure drinking water safety.

Spatiotemporal dynamics of phytoplankton biomass and community succession for driving factors in a meso-eutrophic lake.

Effects of climate change and nutrient load caused by human activities on lake phytoplankton blooms have attracted much attention globally. However, their roles and synergistic effects on phytoplankton biomass and community historical succession are not well understood, especially for meso-eutrophic plateau lakes. In this study, a multi-year (1997-2022) monthly dataset including hydro-chemical and meteorological indicators of the meso-eutrophic plateau lake Erhai in China, was used to explore the contributions of climate change and nutrients on phytoplankton biomass variation and community succession. Phytoplankton biomass increased from 1997 to 2006, slowly decreased from 2006 to 2015, then increased again from 2015 to 2022, according to a generalised additive model (GAM). Alongside warming, nitrogen, phosphorus and organic matter are key drivers of long-term interannual variation in phytoplankton biomass and historical succession of the phytoplankton community. The extensive blooms in recent years were strongly associated with both organic matter accumulation and global warming. Phytoplankton biomass in northern and southern districts was greater than in central areas, with Cyanophyta and Pyrrophyta dominating in the north and Chlorophyta prevalent in the south. Since 2015, phytoplankton diversity has increased significantly, and biomass has declined in the southern district but increased markedly in the northern district. Spatial heterogeneity was caused by the spatial distribution of nutrients and the buoyancy regulation capacity of cyanobacteria. The results demonstrate that bloom mitigation responds strongly to nitrogen and phosphorus control in meso-eutrophic lakes, therefore preventing and controlling blooms through nitrogen and phosphorus reduction is still an effective measure. Given the accumulation of organic matter in recent years, synergistic control of organic matter and total nitrogen and phosphorus could effectively reduce the risk of cyanobacterial and dinoflagellate blooms.

[Effect of Nitrogen on Magnesium Modified Biochar Adsorption to Phosphorus].

The effect of ammonia-nitrogen in water on phosphorus removal by magnesium modified biochar (MBC) was developed to increase the utilization of wetland plants. The crystal structures were measured by X-ray powder diffraction (XRD). MBC was prepared using reed as the biomass feedstock, which was modified with magnesium chloride. The raw biochar (BC) was prepared as a control. The removal of phosphate from solution using four different methods, i.e. MBC, BC, BC, and MgCl2 solutions (BC+Mg2+) and MgCl2 solutions (Mg2+), under different nitrogen to phosphorus molar ratios and initial phosphorus concentrations was investigated in batch experiments. The results demonstrated that the phosphorus removal efficiency of the four treatment methods, which followed the order of MBC>>BC+Mg2+≈Mg2+>BC. NH4+ in the solution, promoted phosphorus removal by MBC. In addition, the larger the ratio of nitrogen to phosphorus and the higher the initial phosphorus concentration, the stronger the phosphorus removal capacity of MBC was. In the three treatments with MBC, BC+Mg2+, and Mg2+, the XRD analysis showed that NH4+ reacted with Mg2+ and PO43- in the solution to form MgNH4PO4·6H2O at N:P=5 or 10, promoting the removal of phosphorus. For recycling purposes, waste biomass from constructed wetlands could be used to produce MBC and treat polluted water rich in ammonium and phosphate. Moreover, the ammonium-nitrogen promotes the phosphate removal by MBC. The results from this study provide a new theoretical basis and data support for the treatment of water eutrophication.

[Response of Inflow Water Quality to Land Use Pattern in Northern Watershed of Lake Erhai].

The inherent relationship between land use pattern and inflow water quality is essential for nonpoint source pollution study. Response of inflow river quality to land use pattern in Northern part of Lake Erhai Watershed was analyzed in aspects of land use composition and configuration using spatial analysis and statistical analysis methods. The results indicated that as land use composition indexes, average slope and vegetation area percentage had significant relationship with TN and TP of inflow rivers. As land use configuration indexes, patch density, patch density of agriculture land and landscape shape index of water had significant relationship with TP and NH4+-N of inflow rivers. Response of inflow river quality to land use pattern was better at class level than at landscape level. Water quality response indicators were TP in rainy season and NH4+-N in dry season, of which regression adjustment coefficient R2 was 0.761 and 0.978, respectively. Response of inflow river quality to land use pattern was better in dry season than in rainy season, and the water quality response indicators were TN, TP and NH4+-N. Improving vegetation coverage and agriculture land intensive degree and avoiding human disturb to natural water area especially in dry season were suggested in nonpoint source pollution control of northern watershed of Lake Erhai Watershed. Later research on relationship between land use configuration and inflow river quality should better be at class level.

[Relationship Between Agricultural Land and Water Quality of Inflow River in Erhai Lake Basin].

We studied the relationship between agricultural land and water quality of inflow river in Erhai Lake Basin, by means of spatial and statistical analysis, from the perspective of comprehensive agricultural land and the area percentage of different types of agricultural land. The obtained results indicated that inflow water quality showed a significant spatial difference, the inflow TP pollution in the western inflow rivers of Erhai Basin was serious. The major pollution indicators in the northern and southern inflow rivers (except for D3) were organic matter and nitrogen. The area percentage of agricultural land had a significantly indicative effect on the water quality of inflow river. The area percentage of comprehensive agricultural land negatively correlated with permanganate index, NH4(+) -N, TN and TP contents in wet season, the correlation coefficients were - 0.859, - 0.565, - 0.693, - 0.181. It negatively correlated with permanganate index and NH4(+) -N content in dry season, the correlation coefficients were - 0.384, - 0.328. It had positive relationships with and TN, TP content in dry season, the correlation coefficients were 0.221 and 0.146. The area percentage of different types of agricultural land had an obviously indicative effect on the inflow water quality. Farmland positively correlated with TN and TP contents both in wet and dry seasons. The correlation coefficients between farmland and TN, TP were 0.252, 0.581 in rainy season and were 0.149, 0.511 in dry season. It had positive and negative relationships with permanganate index, NH4(+) -N content in wet season and dry season, respectively. The correlation coefficients between farmland and permanganate index, NH4(+) -N were 0.388, 0.053 in rainy season and were -0.137, -0.147 in dry season. Forest land exhibited an opposite performance to that of farmland. The correlation coefficients between forest land and TN, TP, permanganate index, NH4(+) -N were - 0.526, - 0.275, - 0.469, -0.155 in rainy season and were -0.012, -0.100, 0.282, 0.151 in dry season. Fishpond had a weakly indicative effect on TN and TP contents. Meadowland and orchard had a similar performance with that of farmland in wet season, but were opposite to that of farmland in dry season. Therefore, management for farmland, meadowland and orchard in the north and south during wet season should be emphasized during the agricultural nonpoint source control in Erhai Lake Basin.

[Spatial distribution characteristics and correlation analysis of organic matter, total nitrogen and total phosphorus in topsoil around Chaohu Lake].

Sixty topsoil composite samples (0-20 cm) have been collected in 3 258 km2 surrounding area of Chaohu Lake. The concentrations of organic material (OM), total nitrogen (TN) and total phosphorus (TP) were determined, and their spatial distribution characteristics were studied by geo-statistics software named GS 7.0 +, Surfer 8.0 and Mapinfo 8.5. The correlations of these 3 kinds of nutrient were analyzed by SPSS 17.0 at the same time. Results showed that the average omega (OM), omega (TN) and omega (TP) in topsoil around Chaohu Lake were 19 500 mg x kg(-1), 1 027 mg x kg(-1) and 483 mg x kg(-1). The omega (OM) and omega (TN) in topsoil in the east of Chaohu Lake were both higher than those in the west of the lake, while omega (TP) was reverse. The omega (TP) in topsoil around Hangbu-Fengle River and Baishitianhe River which located in the southwest of Chaohu Lake was higher than that in any other rivers and with the serious soil erosion. Hence, to control the TP released by nonpoint pollution, attention should be paid on these two rivers. Furthermore, the semivariogram models of omega (OM), omega (TN) and omega (TP) followed linear model and they all had a strong spatial autocorrelation, the ratios of nugget to sill were 0.015, 0.202 and 0.128, respectively. The characteristics of spatial distribution of omega (OM), omega (TN) and omega (TP) were similar. Moreover, the Pearson correlation analysis showed that omega (OM), omega (TN) and omega (TP) were significantly correlated. The omega (TN) and omega (TP) can be calculated by two recommended formulas, and the results were acceptable in daily non-pint pollution management.

Influence of phosphorus on Microcystis growth and the changes of other environmental factors.

The growth processes of Microcystis aeruginosa (FACHB-41) in simulated Taihu Lake water with different phosphorus concentrations were investigated using laboratory microcosms. The algal biomass increased with the increase of phosphorus concentration when it was lower than 0.445 mg/L, while the dissolved oxygen (DO) and pH increased, dissolved inorganic nitrogen(DIN) and light intensity underwater (I) decreased. Responding to the changes of the "environmental factors", the cellular carbohydrate and its ratio to cellular protein decreased generally as phosphorus increased. However, when phosphorus concentration was higher than 1.645 mg/L, the biomass, the "environmental factors", the cellular carbohydrate and its ratio to cellular protein did not change likewise. Since the environmental factors and the physiological and biochemical responses are important factors, the change of environmental factors and cell physiology and biochemistry induced by phosphorus may become the key factors that steer the growth and dominance of Microcystis under certain conditions. To sum up, phosphorus not only stimulate the growth of Microcystis directly by supplying nutrient element, but also has complex interactions with other "environmental factors" and play important roles in the growth processes of Microcystis.