Study On Biogenic Elements In Sediments Of The Bohai Sea

Biogeochemical observations were carried out to determine biogenic elementsand their distribution in surface sediments and core sediments of the Bohai Sea. Thefocus is to understand distribution characteristics of biogenic elements in sediments ofthe Bohai Sea and their influence factors, to understand the deposition processes ofbiogenic elements in sediments and the status of the ecological environment as well.The results show that, the suspended particulate matter concentration in theBohai Sea ranged from5.81to65.83mg/L with an average of16.95mg/L, and thetotal particulate phosphorus content was0.17-1.47μmol/L with an average of0.39μmol/L. The contents of suspended particulate matter and total particulatephosphorus in the Bohai Sea were positively correlated. The concentrations of totalparticulate phosphorus, particulate inorganic phosphorus and particulate organicphosphorus in the overlying water above sediments varied in the range of12.49²3.71μmol·g^-1,8.13¹4.72μmol·g^-1with averages17.17μmol·g^-1,11.98μmol·g^-1,5.19μmol·g^-1respectively. Particulate inorganic phosphorus was the major form oftotal phosphorus accounting for about70%. The particulate biogenic silica in theoverlying water was in the range0.18%to1.21%with an average of0.57%. Thecontents of TP, IP, OP in sediments of the Bohai varied from8.58to22.70μmol·g^-1,5.45to17.84μmol·g^-1,1.29to5.24μmol·g^-1with averages16.19μmol·g^-1,12.79μmol·g^-1,3.41μmol·g^-1respectively, and IP was the dominant phosphorus formaccounting for79﹪of the TP. The biogenic silica （BSi） content ranged from0.17%to1.12%with an average of0.64%. The concentrations of total nitrogen （TN）, totalorganic nitrogen （TON）, fixed nitrogen （Nfix），the exchangeable NO₃^-and NH₄⁺insediments were177.13～783.33μg·g^-1,70.57～611.49μg·g^-1,54.33～159.56μg·g^-1,0.03～2.69μg·g^-1,2.96～37.73μg·g^-1with averages346.93μg·g^-1,218.62μg·g^-1,106.03μg·g^-1,0.63μg·g^-1,14.98μg·g^-1, respectively. TON was63.0%of TN, Nfixaccounted for82.6%of inorganic nitrogen, exchangeable NH₄⁺accounted for96percent of the exchangeable fractions. The content of organic carbon in sediments wasfrom0.16%to0.79%, with an average of0.36%. The concentrations of suspended particulate matter and particulate phosphorus inthe Bohai Sea had the same distribution trend. Their contents in the Yellow Riverestuary and Laizhou Bay, Bohai Bay were much higher than those in the LiaodongBay and Bohai Strait. The concentrations of total phosphorus and particulateinorganic phosphorus in the overlying water were the highest near the Yellow Rivermouth and Laizhou Bay, and were relatively low in the Liaodong Bay and the easternBohai Sea; the particulate organic phosphorus show the similar trend, with high valuealong the coast, and low value offshore. The particulate biogenic silica content in theoverlying water was high in the central Bohai Sea, followed by the Liaodong Bay, andrelatively low in the Laizhou Bay and Bohai Bay. The highest concentrations of TPand IP in the Bohai Sea sediments were in the Yellow River estuary and Laizhou Bay.The phosphorus content was also high in the western Bohai Sea sediments, and thenthe central Bohai, it was the lowest in the eastern Bohai and Bohai Straits. Theconcentrations of organic phosphorus in sediments were high in the northwest of theBohai Bay and Liaodong Bay, and relatively low off the Yellow River mouth andLaizhou Bay. The distribution of various forms of nitrogen is characterized by highvalue in coastal area, and low value offshore. Terrigenous input and hydrodynamicconditions are the dominant factors affecting the content of phosphorus and nitrogenin the Bohai Sea sediments. The biogenic silica content in the Bohai Sea showed lownear-shore, high offshore, it was high at Liaodong Bay estuary and Bohai central area,then the Liaodong Bay, the Yellow River mouth and Laizhou Bay and the Bohai Baywas relatively low. Organic carbon in surface sediments had a high level at the centralsea area, and then the Liaodong Bay, followed by the Yellow River mouth andLaizhou Bay and Bohai Bay. The trend of distribution of biogenic silica was similarwith organic carbon in the Bohai Sea sediments.The vertical distribution of total phosphorus and inorganic phosphorus insediments shows an increasing trend from the bottom to the surface. Organicphosphorus decreased with depth. The fixed nitrogen content was of the trendincreasing slightly with the depth, while exchangeable NH₄⁺content increased andexchangeable NO₃^-significantly decreased with depth. The concentration of biogenicsilica has two different trends with depth. The organic carbon content was high in thesurface, and slightly reduced with depth.The Bohai sediment type had a significant effect on the nitrogen, phosphorus,organic carbon content and distribution. And it was of low correlation between the sediment type and biogenic silica content.The biogenic elements burial flux has a significant regional characteristic. It wasfound a high correlation between burial flux and deposition rate, and the phosphorusburial efficiency （PBE） was related with terrestrial input. The burial flux of totalphosphorus ranged from0.87to111.89μmol·a^-1·cm^-2, and the burial flux of inorganicand organic phosphorus were0.73～91.61μmol·a^-1·cm^-2，0.14～20.27μmol·a^-1·cm^-2.The burial flux of total nitrogen ranged0.93～104.11μmol·a^-1·cm^-2. The burial flux ofbiogenic silica in the Bohai Sea sediments were17.22～875μmol·a^-1·cm^-2. The burialflux of organic carbon in the Bohai Sea sediments were11.26¹100μmol·a^-1·cm^-2.Terrigenous input and deposition rate are dominant factors affecting the biogenicelements burial flux.The TOC/TN ratios ranged from6.9to14.8with an average10.6in surfacesediments of the Bohai Sea. The δ¹⁵N content in sediments was3.80‰to6.76‰with an average4.97‰, and the δ¹³C content was-21.0‰^-25.2‰with anaverage-22.2‰. The sedimentary organic matter sources included terrestrial inputand marine authigenic sediment. The TOC/TN ratios ranged between4.2and14.4in core sediments, and showed an increasing trend with increase in depth, indicatingnitrogen enrichment in sediments. The molar ratios of ON/BSi, OC/BSi, OP/BSiin surface sediments were0.063，2.41，0.03, which were less than the Redfield ratio.The decomposition of organic matter in sediments led to accumulation of biogenicsilica relative to organic carbon.In the last ten years, with the increase in terrestrial sources load into the Bohai Sea,the concentrations of TP, OP, inorganic nitrogen and organic carbon in surfacesediments of central and southern Bohai sea significantly increased, whilethe concentrations of organic nitrogen and total nitrogen decreased; the contents ofbiogenic silica decreased in coastal sediments, but increased in the central partsediments. It was expected that the eutrophication intensified in the Bohai Sea.