| Minjiang is the largest mountain river in the southeastern of the China,which has an important influence on the economic development of Fujian province and the"source to sink" study of the Taiwan Straits.But there were little research about material fluxes from Minjiang to sea,the field of basic research of carbon transportation and transformation even remains blank.So in this study,we sampled the water,suspended sediments(SS)and surface sediments in four seasons during 2014?2015 in the Minjiang lower reaches and estuary,and used the water quality data of Minjiang estuary in November 2009.The time-space distribution and fluxes of dissolved carbon,COD and total carbon in SS in the study area were analysed,the source,transformation and environmental significance of carbon in the study area were discussed as well.The results show that:The spatial distribution of dissolved carbon in study area showed an increasing trend from estuary to adjacent sea.The COD showed a "low-high-low" spatial distribution features from the lower reaches to the adjacent sea,and its high value areas were located in north and south tributary.The total carbon of SS decreased from the reservoir segment to adjacent sea;the high values of suspended sediments concentration(SSC)and total particulate carbon all located in the turbidity maximum zone,while the distribution of total carbon in the sediments was similar to that of COD.The total carbon content of SS in the estuary intensive sampling stations was increased from fresh water side to the sea side.There had a significantly seasonal variation of different forms of carbon in the study area.The content of COD was the highest in spring,and lowest in summer;the SSC showed the characteristics that autumn>winter>spring>summer.The total carbon in SS and total particulate carbon showed highest in autumn,the next in winter and showed the lowest in spring.The total carbon content of SS in the intensive sampling stations during winter and spring showed that,high tide was higher than other tidal periods,and the saltwater wedges were observed at flood tide and high tide in spring as well.The sea fluxes of DOC and DIC in Minjiang were 4.89×104 t C and 21.19×104tC every year,respectively.The total fluxes of SS and bedload sediments to the estuary were 770.93 ×104 t every year;and the fluxes of total particulate carbon to the estuary were 11.18×104 t C every year,about 471.95×104 t SS and 8.02×104 t C were deposited in the estuary,while about 224.40×104 t SS and 3.16×104 t C were finally transported into the sea every year.The average pCO2 of Minjiang estuary was 619.20?atm,it released 1.93×104 t C of CO2 to the atmosphere during autumn,which indicated that the Minjiang estuary was the source of the atmosphere CO2.The content of carbon and its fluxes in Minjiang estuary were lower than those in the Yellow River,Changjiang and Pearl River.The dissolved carbon in Minjiang estuary had a significant positive correlation with salinity,DIN,TAlk and pH,while it had a negative correlation with chlorophyll a.The total carbon in SS and surface sediments showed a significant positive correlation with total nitrogen,which indicated they had the same sources.The correlation between total carbon in SS and other environmental factors varied with season and river segments.Total carbon in sediments showed a strongly correlated with most environmental factors,however.The correlation and source analysis showed that,the total carbon in SS and surface sediments were mainly from hydrophytes in reservoir segment to north tributary and adjacent sea,and in some sections of north tributary were obviously affected by the urban sewage.From Dazhangxi segment to south tributary,the total carbon was mainly from domestic sewage,agricultural non-point source pollutants and livestock pollutants,while in estuary it mainly from terrestrial organic matter.The transformation of total carbon among water,SS and surface sediments in the estuary,was controlled by the interaction between runoff with tide and human activities. |
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