Recent Patterns Of Algal And Carbon Cycling Changes In Response To Climate Change And Catchment Development In Two Medium-sized Reservoirs In Southeast Yunnan

In the context of global warming,the rapid increase in population and continuous economic development have led to the continuous increase in the intensity of catchment development(agricultural development,hydrological regulation,etc.),resulting in a decline in water quality of lakes and reservoirs and the degradation of ecological functioning.Lake District in Yunnan-Guizhou Plateau is one of the five major lake areas in China,with abundant lake types,wide distribution,obvious water quality differences and various environmental problems.In recent decades,lakes in Yunnan have experienced multiple problems,such as hydrological regulation,water quality deterioration,algal blooms and biodiversity decline.Among these,multiple stressors,such as lake damming,agricultural development and extreme drought events in the basin,exacerbated the ecological risks of lakes and reservoirs.Existing researches on plateau lakes mostly focus on the evaluation of water quality and water resources,while the research on environmental changes in lakes and reservoirs at different stages of hydrological regulation is restricted by insufficient field monitoring data.Lake and reservoir sediments serve as information collectors for the history of environmental changes in catchment and water bodies.A combined analysis of multiple sediment proxies can provide the history of environmental changes in lakes and reservoirs and help identify the response patterns of algae to environmental changes.In order to explore the algal change and carbon cycling over the last century in response to catchment development and climate change,this study selected two medium-sized reservoirs(Taiping Reservoir and Sanjiaohai Reservoir)which are known to experience hydrological regulations in southeastern Yunnan.Multiple indicators of sediment physics,chemistry and biota,in combination with modern monitoring data and documentation records of river basin development,were analyzed for the historical reconstruction of environmental changes in two different types of reservoirs,and futher analyses were applied to identify the temporal patterns of algal response and carbon cycle at different hydrological stages of the reservoirs.Statistical analyses and comparative analysis were conducted to quantitatively identify the characteristics of ecological and environmental changes,their similarities and differences between both reservoirs,and the key environmental factors in driving sediment carbon cycle and algal community succession.The main conclusions are listed as follows:(1)The two studied reservoirs experienced multiple hydrological control events during the 1950s-1990 s,changing from a natural water body to a medium-sized reservoir under anthropogenic regulation,and the limnological characteristics of both water bodies have changed significantly.During the period of water level regulation,the median grain size of the sediments in both reservoirs increased but gradually decreased after the dam was built.This indicates a similar pattern that the hydrodynamic conditions increase during the dam construction period,the water level rises after the dam is built,but the hydrodynamic force gradually weakens more recently.The historical and remote sensing data of both reservoirs show that the population size and the arable land both increased significantly after ?1950,especially after ?2000.Sediment records show that the overall increase of nutrients in reservoirs reflects the direct impact of catchment development on eutrophication.In recent 30 years,the water level of both reservoirs has been significantly affected by precipitation in the basin.In dry years,the water level of both reservoirs has decreased and the water area has decreased significantly.Meanwhile,the decrease of exogenous input caused the obvious fluctuation of nutrients,sediment carbonate content and algae in the deepwater Taiping Reservoir,but showed no obvious effect on the shallow water Sanjiaohai Reservoir.(2)In the past 80 years,the algae communities in Taiping Reservoir has changed significantly.Sediment pigments indicate that algal biomass has changed obviously,and nutrient enrichment obviously promotes the growth of algae after dam construction,as well as the changes in cyanobacteria composition.During different hydrological stages shifting from natural fluctuation to artificial regulation,the succession of diatom community structure in sediments generally showed an alternating dominant pattern of "planktonic-benthic-planktonic taxa".Benthic diatoms generally dominated during dam construction while the percentage of planktonic diatoms increased gradually after the dam construction.During the period of extreme droughts,benthic species such as N.lapidosa,N.cryptocephala and N.brevissima increased significantly,reflecting the driving effect of hydrological regulation and extreme drought events on diatom community succession.Multivariate statistical analyses further showed that the diatom community succession was mainly driven by nutrients and hydrological fluctuations.(3)Since ?1740,the algae communities recorded in the sediments of Sanjiaohai Reservoir have displayed a long-term succession.Sediment pigments indicate that algal biomass has changed significantly,especially since 2000.With the acceleration of agricultural development in the basin,nutrient enrichment has significantly promoted the growth of algae,and also significantly changed the composition of cyanobacteria community.The succession of diatom community structure showed a pattern of "planktonic-benthic taxa".Since 1980,the percentage of benthic diatoms with high tolerance of pollutants has gradually increased(the main benthic species include A.minutissima and F.capucina).Multivariate statistical analysis showed that diatom community succession in this reservoir was also significantly affected by changes in nutrient and hydrology.(4)Time series analyses of both reservoirs for different phases of hydrological regulation showed that with the dam construction hydrological conditions continued to weaken,dam building caused a gradual enrichment of nutrients.The rapid expansion of agricultural development has further aggravated eutrophication,so as to drive algal succession and also led to a significant increase in algal biomass,promoting an overall increase in sediment burial of total organic carbon(TOC).(5)A comparative analysis of the carbon cycle patterns between both reservoirs shows that there exist significant differences in the composition of total carbon(TC)in the sediments.The sediment total carbon(TC)is mainly composed of both carbonate and TOC in Taiping Reservoir,but is mainly composed of TOC in Sanjiaohai Reservoir.The sediment TOC in both reservoirs mainly came from the contribution of endogenous algae.The variation of sediment total inorganic carbon(TIC)in both reservoirs is obviously different.The sediment TIC is mainly related to carbonate in the nutrientpoor reservoir,but is mainly drived from the decomposition and transformation of TOC in the more eutrophic reservoir.At the same time,the sediment carbonate content show an opposite pattern of temporal variations.In the deep-water reservoir,the sediment content displayed an overall rise gradually and both carbonate and TOC content showed a significantly positive correlation,reflecting that an increase in water depth and thermal stratification can decrease bottom-water dissolved oxygen and temperature,thus inhibiting the mineralization and transformation of sediment TOC.The dam construction can also lead to a longer water residence time that enhance the sediment accumulation of carbon.The carbonate content in the shallow reservoir displayed an overall decline over the last decade,and a negative relationship with the sediment TOC.This may be due to that strong water mixing in shallow-water reservoirs can enhance the carbonate dissolution and the produced DIC can be absorbed by aquatic plants which can be translated into organic carbon storage.In conclusion,in the context of climate warming,hydrological regulation,eutrophication and extreme drought events are important factors in driving environmental changes of both reservoirs in Southeast Yunnan.Agricultural development and nutrient enrichment can aggravate eutrophication,thus enhancing algae succession and carbon cycling.Therefore,with the continuous increase of human disturbance and climate warming,the water quality protection of reservoirs in southeast Yunnan should prioritize the sustainable management of catchment development and moderate control of hydrological regulation.