Simulation And Analysis On The Euphotic Zone And Primary Productivity Of Phytoplankton In Wuliangsuhai

The primary productivity of phytoplankton refers to the process that phytoplankton fix inorganic substances into organic substances utilizing sunlight,it is the basis of the biogeochemical cycle of carbon,nitrogen,other biogenic elements,the energy flow and material flow of the ecosystem,which affects the structure and function of ecosystem.The primary productivity of phytoplankton is the basis of the biological productivity of the water and the first link in the food chain.The primary productivity of phytoplankton is of great significance to quantitatively grasp the carbon,nitrogen and phosphorus material cycle of the water,to understand the environmental characteristics and quality of aquatic ecosystems,rationally utilize and manage aquatic biological resources.In this study,the Wuliangsuhai Lake located in the cold and arid area of northern China was taken as the research object,based on field investigation and indoor experimental analysis,the original VGPM model was improved,and finally the calculation models of the euphotic zone and the VGPM of Wuliangsuhai during the frozen period were established,furthermore,a simulation analysis of the euphotic zone and primary phytoplankton productivity from 2014 to 2018 was conducted,its limiting factors and mechanism were discussed,and the eutrophication control of Wuliangsuhai based on primary productivity was explored.The main conclusions were as follows:Spatial and temporal changes of water transparency and influencing factors.The change of water transparency in Wuliangsuhai showed a clear bimodal pattern inside a year,and there was an increasing trend between years.The seasonal variation of transparency in Wuliangsuhai was most obvious in spring,followed by autumn.except 2015,it was smallest in winter.The spatial distribution of the transparency in spring,autumn and winter was that the northern area was lower than the southern area.The spatial distribution in summer was different from other seasons,showing that the northern area was slightly higher than the southern area,except 2014,the spatial distribution of transparency was as follows:Central Lake Area>Southern Lake Area>Northern Lake Area,in 2014,the performance was South Lake Area>Central Lake Area>Northern Lake Area,and the area with high transparency transferred from the southern area to the northern area.Chlorophyll a,suspended solids,water depth and ice thickness were the main factors that affect the transparency,Chlorophyll a,suspended solids and transparency were negatively correlated,water depth was positively related to transparency,the thickness of the ice layer affected the transparency of the lake water in winter by affecting the radiation reaching the water under the ice.Temporal and spatial distribution of chlorophyll a in water and analysis of influencing factors.From 2014 to 2018,the concentration of chlorophyll a generally showed a downward trend,multi-year average is 12.585?g/L.it first increased and then decreased during the non-ice period from April to November,the overall seasonal variation was highest in summer,followed by winter,and the lowest in spring.The spatial distribution was high in the north and low in the south.The chlorophyll a in spring,summer and winter gradually decreased from north to south,in autumn,the minimum of chlorophyll a appeared in the middle of the lake.according to RDA analysis,the explanation rates of the environmental factors selected for the four seasonal changes of chlorophyll a concentration in Wuliangsuhai were 66.45%,77.11%,34.55%,44.96%,water temperature and nutrients affected chlorophyll a by affecting phytoplankton biomass,pH and dissolved oxygen were secondary factors that affect the concentration of chlorophyll a,phosphorus was the limiting factor for the growth of phytoplankton in Wuliangsuhai.The euphotic zone analysis.The ice structure of Wuliangsuhai was columnar with less bubble.During the period without snow cover,the photosynthetically active radiation at the ice-water interface was about 10%of the photosynthetically active radiation on the ice surface,it fall to about 2%during the snow cover period.except 2018,the annual variation of the euphotic zone of the lake had a double-peak characteristic,affected by heavy rain,the annual change of the euphotic zone in 2018 showed a single peak characteristic.In 2014,2015 and 2017,the seasonal variation of euphotic zone was largest in spring,followed by summer and autumn,and the lowest in winter.In 2016 and 2018 it was the largest in summer,followed by spring and autumn,and the lowest in winter.Overall,due to the effect of ice extinction,the euphotic zone in the lake during the frozen period was significantly smaller than that in the non-ice period.Seasonal and inter-annual spatial distribution and change trends of euphotic zone were generally consistent,showing that the northern area<the middle area<the southern area.The spatial and temporal evolution of phytoplankton primary productivity.Combined with the improved VGPM model,the primary productivity of phytoplankton during the frozen period and non-ice period were calculated.the monthly change of phytoplankton primary productivity in a year generally presented double peak distribution,multi-year average is 0.856 gC/m2/d.except 2018,the seasonal changes in primary productivity of phytoplankton were highest in summer,followed by spring and autumn,and lowest in winter.the spatial distribution of primary productivity of phytoplankton decreased from north to south in spring,summer and winter,and a reversely rising area appeared in the south of the lake in autumn.the inter-annual change showed a downward trend,the primary productivity of phytoplankton in 2017 and 2018 was that the northern area was larger than the southern area,in 2014,2015 and 2016,a reversely elevated zone appeared in the southern part.the primary productivity of phytoplankton was small during the ice period,but the primary production capacity was not weak.the concentration and spatial distribution of nutrients,especially total phosphorus,were the controlling factors that affected the primary productivity and spatial distribution of phytoplankton in the lake.water temperature was the main factor that affected the change of primary productivity along time,mainly manifested in the photosynthesis rate of phytoplankton.the inhibition of water temperature was the main reason for the double peak distribution of primary productivity of phytoplankton in Wuliangsuhai.