Pigment-based Mid-holocene Cyanobacterial Development And Its Driving Factors In Lake Mayinghai,Northern Shanxi Province,China

Lakes are sensitive indicators of regional climate and environmental changes,and human activities have made algal bloom a major ecological and environmental problem of lakes.Although numerous control strategies and measures have been taken to reduce nutrient concentrations in lakes,the problem of cyanobacteria bloom has not been effectively mitigated,so climate warming may be a potential key driver.However,it is still unclear how climate change and associated natural processes influence cyanobacterial development.As a sensitive proxy of lake eco-environmental change,sedimentary pigment can be used to indicate the source of specific algae and further indicate algal compositional changes and cyanobacterial development in lake ecosystem.Therefore,to foster lake management in the warmer future,it is essential to improve understanding of the influence of climate change and associated natural processes on cyanobacteria without human impact,and paleolimnology provides a valuable way to investigate the impacts of past climate change on cyanobacterial development under natural conditions.In this study,the sedimentary pigments of the Lake Mayinghai in northern Shanxi were selected and the influence of climate and environmental changes on the composition of algae and the development of cyanobacteria in the Middle Holocene was investigated using paleolimnological research methods.The following results were obtained by pigment analysis and identification:(1)The average concentration of chlorophyll a+echinenone was high during the early Holocene(ca.11,000-6,000 cal year BP),reached a maximum during the middle Holocene(ca.7,300-5,450 cal year BP)and was lowest during the late Holocene(since ca.3,000 cal year BP).(2)The production of green algae,diatoms and cryptophytes remained almost unchanged during the middle Holocene,while the production of filamentous cyanobacteria decreased slightly and the ratio of β-carotene to chlorophyll a was significantly lower during the middle Holocene.The results of this study suggest that the significantly lower β-carotene to chlorophyll a ratio during the Middle Holocene can be attributed to an increase in non-nitrogen fixing spherical cyanobacteria.Possible drivers are high temperature,high precipitation,high monsoon intensity and high terrestrial deciduous forest cover,among others.(1)High temperatures may promote the growth of non-nitrogen-fixing spherical cyanobacteria,either directly(by increasing their growth rate)or indirectly(by increasing thermal stability).(2)Strong monsoon intensity exacerbates lake scouring intensity,disrupts thermal stratification of the water column and increases mixing of the water column.At the same time,the high intensity of precipitation runoff results in higher inputs of inorganic elements.This provides suitable conditions for the increase of non-nitrogen fixing spherical cyanobacteria.(3)Deciduous forests with high cover provide a high supply of inorganic nitrogen to the lake,increasing the nitrogen to phosphorus ratio of the lake water column.At this point,the high nitrogen to phosphorus ratio favors the development of non-nitrogen fixing cyanobacteria and discourages the development of nitrogen fixing filamentous cyanobacteria.Future lake management should take into consideration the in-lake processes influenced by higher temperatures and the catchment processes modulated by increased precipitation and terrestrial vegetation.