Carbon Transport And Flux In The Yellow River Estuary

Estuaries are regions of active land-ocean interaction and very important toglobal carbon cycles. Due to strong internal biogeochemical activities and a numberof complex sedimentary dynamic processes, mass transported into estuaries usuallyexhibits nonconservative behavior during estuarine mixing. As a result, riverinecarbon fluxes can be over-or underestimated if these physical and biogeochemicalprocesses are not considered. The Yellow River represents typically rivers which arelocated in arid and semiarid regions and hold high turbidity. Studies on carbon cyclesand influence factors in the Yellow River estuary can provide some meaningfuladvice to other rivers with the same characteristics.Using data from monthly investigation at the Lijin station during July2010toJuly2011, six cruises in the Yellow River estuary during2005-2010, within four ofwhich pCO2data were investigated between2009and2010, we discussed seasonalvariations of carbon in the freshwater end member, long time-scale carbon flux from1950s, behaviors of carbon in the estuarine mixing processes, pCO2distribution andair-sea flux in the Yellow River Estuary. Conclusions are as follows:1. Carbon transported in the Yellow River is mainly in inorganic and particulateforms. DIC, DOC and POC flux amonted to56.2×104t C yr-1,3.97×104t C yr-1and35.0×104t C yr-1respectively, from Auguest2010to July2011. Fluxes ofDIC, DOC and POC in summer were much higher than those in other seasons,accounting for55.6%,58.4%,88.6%of the whole year, respectively. In the longtime-scale (1950-2012), fluxes of discharge, sediment and all kinds of carbon areall showing decreasing trend. Compared with1950s, fluxes of discharge,sediment dropped67%,90%respectively in the first decade in21stcentury. As aresult,fluxes of DIC, POC, PIC are decreasing60%,90%and91%, respectively.2. DIC acts non-conservatively and some of it is removed in the estuarine mixingzone. CaCO3precipitation and biological activities are the main reasons for thisphenomenon. Longer freshwater-seawater mixing distances and times, andhigher DIC concentrations in the freshwater end member promote net biologicalproduction and CaCO3precipitation, thus encouraging DIC removal. To our bestknowledge, the Yellow River Estuary is the only example where CaCO3precipitation has been demonstrated to play a very important role in estuarine DIC removal. DOC concentration in the Yellow River (2.39mg L-1) are muchlower than the world average (5.0mg L-1), but it increases in the estuary mixingprocess. Therefore, DOC flux is under-estimated using DOC concentration in thefreshwater.3. Agreed with other estuaries, pCO2in the Yellow River estuary decreased sharplyin the very low salinity area (S<0.5), due to the severe deposition of TSS.Photosynthesis is the main removal mechanism for pCO2while carbonate system(high DIC) in the freshwater end member acts as the dominant factor sustainingthe high pCO2in the Yellow River Estuary. During the heavy rains, lots oforganic materials flushed into the estuary, which may cause biological respirationenhanced and pCO2increased. The Yellow River acts as a CO2source comparedwith atmosphere, but its flux is much lower than other large river estuaries.