Seasonal Variation And Anthropogenic Impact On The Transport Of Particulate Organic Carbon In The Lower Yellow River

Rivers are the critical channels that connect terrestrial carbon pool and ocean carbon pool. Both natural processes and human activities in river basins have important impact on the transport of riverine organic carbon (OC). Better understanding transport processe of the riverine OC is essential for the studies of global carbon cycling. Particle size can affect not only the transport of particulate organic carbon (POC) in environment, but also the content and form of OC. In this study, suspended particulate matters are collected per month in 2012 and daily during the water and sediment regulation (WSR) period from the Lijin Station in the lower Yellow River. The samples are physically separated according to hydrodynamic grain size using a water elutriation method. Then various geochemical parameters such as POC contents, stable carbon isotope abundance (δ13C), and lignin composition are analyzed. Further combine with the water and sediment discharges and laser particle size composition of these size-fractionated particles and bulk particles were determined in order to investigate the seasonal variation, sources, composition, fluxes of POC and the effect of WSR on it. The main conclusions are as follows:The WSR in 2012 can be divided into two stages according to the changes of water and sediment discharges:the water-release stage (WRS) and the sediment-release stage (SRS). Variation of the water discharge, sediments, POC and δ13C in these two stages reflects the impacts of WSR on the sources of particulate matters and associated OC. The water discharge in the WRS stage is the highest (4270 m3 s-1), and the sediments scoured from the riverbed in the lower reaches are the major source of suspended particulate matters in the WRS stage, therefore the particles are characterized by relatively coarse grain size (13.9 μm in average of median grain size), low POC content (avg.0.38%) and relatively enriched and constant δ13C (-24.2‰±0.3%o). The SRS stage is characterized by high concentration of suspended particulate matters (up to 17.8 kg m-3). fine particles (5.9μm in average of median grain size), high POC content (avg.0.50%). and depleted and varied δ13C values (-24.8‰±0.6‰). Daily POC flux changes consistent with sediment discharge, and the total POC flux during the water and sediment regulation period is about 1.13×105tons, accounting for 12% of the total POC flux in 2012. Compared with previous years, the total water discharge during the WSR period in 2012 has increased, while the total sediment flux and POC flux have reduced.The water discharges and sediment concentrations in 2012 have significant seasonal variation. Water discharges and concentrations of suspended particulate matters during the flood season are higher. In contrast, water discharges (300 m3 s-1) and sediment concentrations (300 m3 s-1) in spring are relatively lower. Particles in spring is characterized by relatively high POC (avg.0.61%) and negative δ13C (-24.2%o±0.3‰). Variations of parameters in other seasons are more complex with weak regularities. Compared with the WSR period, the particles in non-WSR period is characterized by high POC (avg.0.56%), negative and varied δ13C values (-24.8‰±0.6%o), high ∑8(avg.0.0336 mg gdw-1) and slightly low A8(0.60±0.11 mg 100 mg-1OC). Both the ratios of cinnamyl phenols to vanillyl phenols (C/V) and syringyl phenols to vanillyl phenols (S/V) of lignin phenols indicate that lignin tissues are derived from non-woody and woody angiosperm plants. Lignin decay parameters, such as ratios of vanillic acid to vanillin ((Ad/Al)v), total p-hydroxy phenols to sum of vanillyl and syringyl P/(S+V) and ratios of 3,5-dihydroxybenzoic acid to total vanillyl phenols (3,5-Bd/V) indicate that the particles have certain degree of degradation, but have no apparent seasonal variation. The annual total POC flux in 2012 is 9.3× 105t a-1 tons, which is controlled mainly by the annual total sediment flux.According to the mass distribution results of the water elutriation method, the content of<8 μm fraction in January and the content of>63 μm fraction in February are highest, accounting for 67% and 60% of the mass of total suspended particulate matters, respectively. The content of 16-32 um fraction in other months is highest with a mean of 56%. The particulate matter content of 8-16 μm fraction has the lowest proportion with a mean of 6%. The average content of other fractions is 12%. However, the content of 16-32 μm fraction during the WSR period is highest (avg. 56%). The contributions of 8-16 μm fraction and 32-63 μm fraction to the total suspended particulate matters are 29% and 14% respectively, and the< 8 μm fraction and>63 μm fraction are only 5% and 6% respectively.The parameter variation of the size distributions of particles during the WSR period is the same as particles during the other periods. OC mainly concentrates in fine particles (<32 μm), and the fine particles have low ∧8 and depleted δ13C values (<8 μm, avg.-25.9‰). Coarse particles (>32 (μm) have relatively low POC, depleted 13C values (32-63 μm, avg.-24.9‰) and high ∧8 values (32-63 μm, avg.1.11 mg 100 mg-1 OC). ∑8 values (avg.0.012 mg gdw-1) and ∧8 values(avg.0.62 mg 100 mg-1 OC) in the>63 μm fractions are relatively lower. Compared with other periods, POC is lower during the WSR period, consistent with POC of the bulk particles. Both the C/V and S/V ratios of lignin phenols in different size-fractionated particles indicate that the lignin in the lower Yellow River is a mixture of angiosperms and gymnosperms. The smaller particle size, the relative greater contribution of herbaceous angiosperms particles have, and the larger particle size, the relative greater contribution of woody gymnosperm particles have. Lignin decay parameters, such as (Ad/Al)v, P/(S+V) and 3,5-Bd/V, decrease along with the increase of particle size. The POC whose grain size less than 8 μm,8-16μm and 16-32μm has higher daily flux during the WSR period, 1.2×105 t d-1,1.9×105t d-1 and 1.3×105t d-1 on average, respectively. However, the higher daily POC flux are found when the grain size is<8 μm and in the 16-32 um fractions during the other periods, which are 7.8×104t d-1 and 8.2× 104t d-1 on average, respectively.