Study On Sediment Oxygen Demand And Organic Matter Mineralization Of Lake Baiyangdian Sediments

Water hypoxia has become an important problem in aquatic ecosystems and sediment oxygen demand is an important factor causing water hypoxia.The study of sediment oxygen demand is a great significance to the balance of water oxygen budget and the evaluation of water environment quality.Organic matter in sediments is the cause of sediment oxygen consumption,which comes from the decomposition of organic matter and oxidation of reducing substances in sediments.Therefore,it has great scientific significance and practical value to study the formation mechanism of sediment organic matter mineralization,sediment oxygen demand and the relationship between them for the analysis of hypoxia in overlying water.The core incubation method was used to determine the sediment oxygen demand（SOD）of surface sediments in Lake Baiyangdian for different hydrogeomorphology zones and habitat types.On the basis of standards for surface water environment quality in China,the oxygen deficiency effect of SOD in Lake Baiyangdian was evaluated,and the relationship between SOD and organic matter（OM）in the lake is discussed.Analyzing the temperature dependence coefficient of surface sediments in different habitat types,and evaluating the oxygen deficit effect of sediment oxygen consumption on water body at different temperatures.The cumulative mineralization amount and mineralization rate of organic matter in sediments are determined by submerged culture method.The potential mineralization ability of sediment mineralization,the content and structure of dissolved organic matter in overlying water are analyzed,and the mechanism of material migration,transformation and oxygen consumption in sediment organic matter mineralization is discussed.The specific research results are as follows:（1）Our results showed that the OM content in sediments ranged from 4%to 14%with a mean of 9%.SOD ranged from 0.14 to 0.59 g·m^-2·d^-1 with a mean of 0.28 g·m^-2·d^-1.The SOD values in different hydrogeomorphology zones were（from lowest to highest）:southwest inflow zone<mixed flow zone<north inflow zone<south water channel zone<northwest inflow zone.Moreover,the SOD values for different habitats were（from lowest to highest）:ditch of raised field<fish pond<open water<water channel<mash.A water quality effect analysis showed that the current SOD level in Lake Baiyangdian could lead to a decrease in dissolved oxygen（DO）of overlying water from oxygen saturation to 3 mg·L^-1（Class IV water quality standard）after approximately 11.5～29.1 h and to 2 mg·L^-1（Class V water quality standard）after approximately 13.7～34.6 h,which would have a serious impact on the metabolism of benthic organisms.Finally,there was no significant correlation between SOD and OM in Lake Baiyangdian,nor could the OM content in Lake Baiyangdian sediments be used as an indicator to characterize the oxygen consumption level of the sediments.（2）The sediment oxygen demand is greatly affected by water temperature.There is an exponential correlation between oxygen consumption rate and temperature.The temperature dependence coefficient of SOD is 1.06～1.20,with an average of 1.13.Different habitat types have different temperature dependence coefficient,according to the order from high to low:fish pond 11>ditch of raised field 12>open water 9>water channel 3>mash 1>open water 5.The biological activity of sediment was low at low temperature,and SOD increased 2～5 times at 20～30°C.The water quality effects of SOD at different temperatures were shown as follows:the current SOD level in Lake Baiyangdian could lead to a decrease in dissolved oxygen（DO）of overlying water from oxygen saturation to 3 mg·L^-1（Class IV water quality standard）to 2 mg·L^-1（Class V water quality standard）after approximately 137.44～189.2 h below 15°C,and after approximately 39.09～51.61 h is required when the temperature is 15～20°C.It takes 17.03～31.48 h at 25～30°C.（3）The maximum mineralization rate of organic matter in Lake Baiyangdian sediments is 0.06～2.06 g·m^-1·d^-1,with a mean of 0.68 g·m^-1·d^-1.The mineralization rates of sediments in different types of water bodies from high to low were water channel 3>mash 1>fishpond 11>ditch of raised field 12>open water 5≈open water 9 s.The accumulation of mineralized nitrogen formed by mineralization of sediments in 42 days was between 307.80 and 735.09 mg·kg^-1;The accumulation of ammonium nitrogen ranged from 33.18 mg·kg^-1 to 78.49 mg·kg^-1;The accumulation of nitrate nitrogen is between20.93 and 61.03 mg·kg^-1.Before mineralization,ammonium nitrogen is the main form of mineralized nitrogen,accounting for 81.22%of mineralized nitrogen.After mineralization,the proportion of ammonium nitrogen to mineralized nitrogen decreased to 10.35%.The cumulative mineralized nitrogen（without organic nitrogen）has a good correlation after fitting according to the One-pool model,and the correlation coefficient is above 0.88.The obtained N_0（E）is 334.77～744.83 mg·kg^-1,and K_0（E）is 0.04～0.07,indicating that there is a stable organic nitrogen pool in the sediments of Lake Baiyangdian that could release ammonium nitrogen or soluble organic nitrogen through organic nitrogen mineralization.（4）Fluorescence spectra of organic matter components in overlying water of Lake Baiyangdian showed that aromatic protein I,aromatic protein II,fulvic-like acid,soluble microbial degradation by-products and humic-like acids（humic acid）were released from overlying water during sediment mineralization.The positions of DOM fluorescence peaks in different habitats were similar,and the visible region-like fulvic acid fluorescence peak（Peak C）（310～330 nm/410～450 nm）appeared in the V region.The FI index BIX index of dissolved organic matter in overlying water on the 42nd day showed that the dissolved organic matter in overlying water is newly generated,mainly from the suspension of organic matter in sediments and microbial activities.The organic matter had a large contribution to self-origin,high bioavailability,and was easy to be used by microorganisms to consume oxygen.During mineralization,the proportion of degradable components in dissolved organic matter in overlying water decreased slightly from 59.5%to 51.7%.The proportion of refractory components in dissolved organic matter increased from 40.5%to48.3%.It shows that the organic matter in the sediments will diffuse to the overlying water,become dissolved organic matter and consume oxygen in the water.