Hypoxia And Carbon Dioxide Emission In Shuikou Reservoir,MinJiang River

The construction of dams leads to rivers "being lakes and reservoirs" and being eutrophication.Eutrophic waters(including lakes,reservoirs,estuaries,and oceans)are often accompanied with hypoxia(DO<2.0 mg·L-1),which also produce greenhouse gases such as CO2,contributing to global warming.In this study,Shuikou reservoir,a deep reservoir on the downstream of MinJiang river,was selected as the research obj ect to carry out our survey.We carried out surveys from different time scales including profile measurement and in-situ incubation experiment to analyze the formation of hypoxic zone,the processes of oxygen consuming and carbon dioxide emission in a reservoir.We also want to figure out the coupling mechanism of carbon and oxygen in eutrophicated water.The main findings and conclusions are shown as follows(1)The dual-layered stratification structure of the deep-water reservoir combined with the decomposition of exogenous organic matter caused by storm runoff and endogenous organic matter formed by primary production were the main reasons for the long duration(4 months)and large depth(5 meters or more)of hypoxia in the reservoir.Based on the monthly profile measurement of key site and the multi-parameter synchronous monitoring of surface and bottom water,the dynamic change about formation and disappearance of hypoxic zone in the reservoir was captured.The stratification of water temperature in Shuikou Reservoir began in mid-July and ended in mid-November.The bottom water remained hypoxia for 4 months,and the hypoxia area expanded from the bottom water(60 m)to the surface water(5 m).After the temperature droped in winter,the stratification disappeared,and then the dissolved oxygen returned to normal level within 1 month(5 mg·L-1).In the stratified period,two thermoclines appeard from top to bottom in water column which was the surface thermocline at about 5 m and the middle thermocline of about 40 m.The formation of two thermoclines prevented exchange of low dissolved oxygen bottom water with the high dissolved oxygen surface water.The rainfall in the basin was mainly concentrated in April to September,accounting for 80.6%of the annual rainfall.A large amount of particulate matter carried by storm enters the reservoir,accounting for 62.84%of the annual TSM flux.The high GPP(239.63 μmolO2·L-1·d-1)in the water and the low C/N ratio(9.40)in the surface sediment indicated that there was endogenous organic matter formed by primary production.(2)The oxygen consumption process in the water column of Shuikou Reservoir was dominated by microbial respiration,and the contribution of nitrification to oxygen consumption was small.Based on the seasonal in-situ incubation experiment of surface and bottom water,we found the total oxygen demand(TOD)of Shuikou Reservoir varied from 10.57 to 68.09μmolO2·L-1·d-1 in summer and 2.98 to 30.99 μmolO2·L-1·d-1 in winter.TOD was higher in summer than in winter due to the appropriate water temperature.Nitrification oxygen demand(NOD)was low in Shuikou Reservoir(0.41～1.44 μmolO2·L-1·d-1 in summer and 0.11～0.90μmolO2·L-1·d-1 in winter).The NOD of middle layer was the highest in summer and that of bottom layer was the highest in winter.Microbial respiration oxygen demand(MOD)varies from 10.20 to 68.04 μmolO2·L-1·d-1 in summer;2.01 to 30.40 μmolO2·L-1·d-1 in winter.It can be seen from the data of MOD/TOD that MOD accounts for more than 96%of TOD in summer and more than 67%in winter.(3)The △CO2/△O2 in water reflected that hypoxia in reservoir in summer inhibited the organic carbon cycle,and the bottom water discharge in autumn and winter was the hot spots of atmospheric carbon dioxide source.Assuming that the excess CO2 and O2 depletion of organic matter decomposition is in the Redfield ratio scope of stoichiometric(ACO2/AO2 ratio=0.62～0.90),ACO2/AO2 ratio in summer was lower than the normal range,autumn and winter were higher than the normal range.It is speculated that the main mechanisms was as follows:hypoxia in bottom water in summer,a large number of particulate matter and biomass organic matter brought by runoff deposited to the bottom,and the decomposition was inhibited.The temperature droped in autumn and winter,the stratification weakened,the vertical mixing strengthened,and the inflow of oxygen-rich water from tributaries enhanced the oxygen supply to the bottom water of the lake area,and the decomposition of organic matter accumulated produced a large amount of surplus CO2.The CO2 water-gas exchange flux of the surface water in Shuikou Reservoir varied from-20.10 mmol m-2·d-1 to 346.33 mmol·m-2·d-1.The flux variation of the river downstream of the dam was 278.8～372.2 mmol·m-2·d-1 which was much higher than the flux of surface water in rservoir(202.95 mmol·m-2·d-1).Long-term stratification resulted in a higher CO2 concentration in the bottom water in reservoir.This part of the water discharged to the downstream through the outlet of dam,and the CO2 dissolved in it was released to the atmosphere,which was also an important"source"of CO2 in the atmosphere.