| In this research,the exchange fluxes of NO2-N,NO3-N,NH4-N,PO4-P and SiO3-Si across the sediment-water interface were measured by a laboratory incubation method on the surface sediment samples collected from 6 sites in the Muping Marine Ranch and its adjacent area in summer?August?and autumn?November?2017.The key environmental factors that could influence the nutrient fluxes across the sediment-water interface were analyzed and discussed.The results and understanding were as follows:?1?NO2-N?except for Station S8?,NH4-N,PO4-P and SiO3-Si were released from sediments to overlying water in summer,while NO3-N was the opposite.In autumn,sediments acted as a sink of NO3-N?except for Station S4?,NH4-N and PO4-P,but a source of SiO3-Si.For NO2-N,some sediments acted as a sink while the others were a source,but as a whole they acted as a source of NO2-N.In summer,the exchange fluxes of NO2-N,NO3-N,NH4-N,PO4-P and SiO3-Si across the sediment-water interface were-7.013.0,-173.8-318.6,215.6347.0,20.897.4,922.81435.1?mol/?m2 d?with the average values of 5.7±7.0,-236.0±44.6,265.2±45.7,59.7±26.5,1154.3±177.1?mol/?m2 d?,respectively.In autumn,the corresponding exchange fluxes were-13.715.6,-303.634.3,-172.3-356.0,-21.5-51.4,802.71219.0?mol/?m2 d?with the average values of 1.0±13.2,-182.0±111.8,-268.4±67.5,-45.7±10.1,1013.6±142.9?mol/?m2 d?,respectively.In summer,the exchange fluxes of dissolved inorganic nitrogen?DIN;[DIN]=[NO2-N]+[NO3-N]+[NH4-N]?could provide only?0.11.0?%of primary productivity of overlying water,with the average value of?0.4±0.3?%.PO4-P and SiO3-Si could provide?3.315.6?%and?9.214.4?%of primary productivity,with the average value of?9.5±4.2?%and?11.5±1.8?%,respectively.In autumn,sediments acted as sink of DIN and PO4-P,so theirs fluxes could not contribute to the primary productivity of overlying water.Fluxes of SiO3-Si could provide?11.617.7?%of primary productivity,with the average value of?14.9±2.1?%.?2?The exchange fluxes of inorganic nitrogen were mainly controlled by diffusion process,nitrification and denitrification process.The influences of each environmental factors on the exchange fluxes of NO2-N were not obvious.The tinier sediment particles,the more TOC contents,and the higher moisture contents in sediments resulted in higher exchange fluxes of NO3-N and NH4-N.Salinity and pH of overlying water and C/N ratio of sediments had little effect on the fluxes of DIN.In Summer,the more TN contents,the higher NH4-N concentrations in interstitial water brought about higher exchange fluxes of NH4-N.In summer,there was a negative correlation between NO3-N exchange fluxes and NO3-N concentrations in interstitial water.In autumn,it also had a negative correlation between NO3-N exchange fluxes and NH4-N concentrations in overlying water.These phenomena indicated that the concentration gradients of inorganic nitrogen in interstitial and overlying water may be the key factor to the diffusion of DIN.The geochemical forms of P in surface sediments were extracted by a sequential extraction method.And the contents of varied forms of P changed with the season:De-P>ACa-P>OP>Fe-P>Ex-P in summer,and De-P>ACa-P>Fe-P>OP>Ex-P in autumn.De-P and ACa-P were the most abundant forms of P in surface sediments.Ex-P and Fe-P affected the exchange fluxes of PO4-P,especially for Fe-P,which affected the fluxes through the adsorption-desorption process.The higher net OP contents in summer may be related to higher primary productivity,and the smaller proportions may be due to the relatively vigorous mineralization and disslotion rates.Tinier sediment particles at individual stations,moisture contents,organic matter contents,PO4-P concentrations in interstitial water had a certain positive correlation with the fluxes of PO4-P,but the effects of salinity and pH on the fluxes were not obvious.The diffusion process may have less effect than the adsorption-desorption process on the exchange fluxes of PO4-P.The influences of each environmental factors on the exchange fluxes of SiO3-Si in this area were not obvious.The correlations were only found with the contents of clay and the concentrations of SiO3-Si in overlying and interstitial water.There was a worse correlation between another factors?TOC,salinity,and pH?and fluxes of SiO3-Si.It indicated that the change of single factor has less effect on the exchange fluxes of SiO3-Si,and the precipitation-dissolution process of biogenic silica may play an important role.?3?The effects of temperature and dissolved oxygen concentrations on exchange fluxes were discussed by setting the temperature to T=10 and 20°C,the dissolved oxygen?DO?concentrations to anoxic?DO<2 mg/L?,and aerobic?DO>8 mg/L?conditions.The influences of temperature change on fluxes of NO2-N were complex.High temperature led to high exchange fluxes of NO3-N and NH4-N.The variation of DO concentrations changed the oxidation-reduction potential across the sediment-water interface,which affected the nitrification and denitrification process.The effects of DO concentrations changes on NO2-N fluxes did not reflect certain rules.In anoxic conditions,the concentrations of NO3-N in the interstitial water decreased,and the concentrations of NH4-N in the interstitial water increased,which were beneficial to the migration of NO3-N and NH4-N in summer,conducive to the migration of NO3-N but not to the exchange of NH4-N in autumn.In summer,high temperature promoted the release of PO4-P from sediment to water.However,the high temperature was not conducive to the migration of PO4-P in autumn.Its migration was controlled by both molecular thermal movements and adsorption-desorption of metal-bound P.In summer,the release rates of PO4-P were faster in anoxic conditions.While in autumn was the opposite,the migration direction even changed under anoxic conditions.The phenomena demonstrated that the concentrations of DO may affectd the adsorption-desorption process of iron-manganese oxides with P.Compared with low temperature?T=10°C?,high temperature?T=20°C?promoted to the exchange of SiO3-Si.It not only increased the diffusion rate of dissolved Si from interstitial water to overlying water,but also accelerated the dissolution process of particulate Si.Those processes would produce more SiO3-Si.The effect of DO changes on fluxes of SiO3-Si were not obvious in anoxic and aerobic conditions,and the changes of SiO3-Si fluxes across the sediment-water interface were tiny. |
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