Characteristics And Influencing Factors Of Sediment Active Organic Carbon In Northern Bohai Bay

The oceans are the largest carbon reservoir on the planet and play a very crucial role in regulating the global carbon cycle.Although the marginal seas make up less than 8%of the world’s oceans,they deposit more than 80%of the total organic carbon in the world’s oceans,and not only that,but the organic carbon in marine sediments is not stable and constant,but is subject to mineralisation over time.In particular,when fresh organic carbon enters the sediment,its level declines rapidly within 10～20years,with the remainder stabilising at a new equilibrium over the following millennia.This organic carbon,which is initially consumed by mineralisation,is often referred to as reactive organic carbon and plays an important role in the cycling of marine matter and the transport and transformation of pollutants.In this study,surface sediment samples were collected from the northern part of Bohai Bay and their basic physicochemical properties were analysed to understand the basic properties and spatial distribution of sediments in Bohai Bay.On this basis,the dissolved organic carbon（DOC）fraction in the sediment,which is easily released into the water column,was obtained by aqueous extraction,and three-dimensional fluorescence spectroscopy combined with parallel factor and ultraviolet spectroscopy was used to reveal the characteristics of DOC and discuss the source of DOC under the influence of special gating in the study area.The composition of each oxidizable organic carbon pool is then explored using infrared spectroscopy to explore the ecological and environmental effects of the different organic carbon components.Considering that extracellular enzymes in sediments are a key factor affecting organic carbon mineralization,the activities of catalase,urease and alkaline phosphatase in sediments were finally analyzed and the relationship between enzyme activities and environmental factors as well as organic carbon pools was discussed,which finally provided a basis for judging the pollution level of sediments and the prevention of eutrophication in water bodies in this sea area.The main conclusions are as follows:（1）Using parallel factor analysis,three fluorescent fractions of dissolved organic matter（DOM）in the sediment were finally resolved:humic-like fraction A（Ex/Em=240（310）/430）,humic-like fraction B（Ex/Em=255（380）/490）and protein-like fraction C（Ex/Em=225（270）/310）.Fraction A is mainly derived from old carbon in marine sediments,while Fraction B is mainly derived from marine humus-like material.Sediment dissolved organic matter is predominantly of terrestrial origin,but overlying water has both terrestrial inputs and endogenous contributions.The tide gates widen the DOM differences between sediment and overlying water,and sediment carbon sources do not vary with changes in riverine input controlled by the gates,but organic carbon distribution and composition vary markedly.（2）The content of elemental carbon in the sediments ranged from 12.45 g/kg to19.14 g/kg,with an average of 16.23±2.78 g/kg,mainly in the form of CaCO₃.The average sediment organic carbon（SOC）content was 12.68±1.14 g/kg.spatially,there was no significant trend in the CaCO₃ content,but the SOC content was significantly higher in the estuary than in other areas.Among the easily oxidized components of SOC,the proportions of reactive organic carbon（LOC）and medium reactive organic carbon（NOC）were low,with their contents accounting for 9.49%and 18.15%of the total organic carbon,respectively.Within the study area,the spatial distribution characteristics of LOC and NOC are similar to those of SOC.This implies that human activities have increased the content of marine SOC and its active components.（3）Catalase was mainly enriched in the estuary area,while urease and alkaline phosphatase were mainly enriched at point BH1,where red tide was frequent.There were good positive correlations between peroxidase activity and sediment organic matter（SOM）,urease activity and TN,and alkaline phosphatase activity and TP.Since P limitation is evident in this study area and alkaline phosphatase can hydrolyse large molecules of organic phosphorus into small molecules of inorganic phosphorus,thus causing rapid blooms of algae and other planktonic organisms,sediment alkaline phosphatase activity is a good indicator of eutrophication in the water column in this area.From the redundancy analysis（RDA）of sediment enzyme activity and the readily oxidizable fraction of sediment carbon,the lower the activity of sediment carbon,the higher the degree of humification,and the sediment enzymes promote microbial secretion of enzymes that lead to the degradation of complex organic carbon.