Benthic Exchange Of Dissolved Nutrients And The Impact Of Relevant Environmental Factors At Sediment-water Interface In The Jiaozhou Bay

Intatct sediment cores and the bottom seawater in the corresponding stations in Jiaozhou Bay was collected in July 2015 and Jaunary 2016, and the benthic exchange rates of NO₃-N, NO₂-N, NH₄-N, PO₄-P and SiO₃-Si were measured by intact sediment cores incubation. Further, the benthic fluxes and its contributions to the primary productivity were estimated, and the impacts of relevant environmental factors on the benthic transport were discussed. The main results are as follows: 1、SiO₃-Si was directed out of sediment to the overlying water in Jiaozhou Bay,while the exchange of NO₃-N、NO₂-N、NH₄-N and PO₄-P varied with season. Sediment was an important source of DIN and PO₄-P in summer, DIN and PO₄-P transferred from water column to sediment in winter. Based on the exchange rates and the proportion of different sediments, the exchange fluxes of dissolved nutrient were estimated. The result showed that sediment could provide 39.3% and 14.1% of N and P required by the primary productivity respectively. Sediment was always the source of SiO₃-Si, providing 15.6% SiO₃-Si required for primary productivity in summer, and 25.8% SiO₃-Si in winter.The exchange of dissolved inorganic nitrogen at sediment-water interface in Jiaozhou Bay was mainly in forms of NO₃-N and NH₄-N. The exchange of NO₃-N and NH₄-N was in direction from sediment to overlying water in summer, while it became uncertain in winter. The exchange rates ranged from-714 to 1 560 μmol /（m2·d） for NO₃-N and from-1 334 to 26 064 μmol /（m2·d） for NH₄-N. The exchange rates of NO₂-N and PO₄-P varies in summer and winter, ranging from-117 to 941 μmol /（m2·d） for NO₂-N and from-128 to 861 μmol /（m2·d） for PO₄-P. The exchange rates of SiO₃-Si were in the direction from sediment to overlying water, ranging from 43 to 4 889 μmol/（m2·d）.In summer, sediment was an important source of NO₃-N、NO₂-N and NH₄-N,the fluxes at the sediment-water interface were 2.35×108、6.35×107 and 1.34×109 mmol·d-1 respectively, providing 39.3% N required for primary productivity. In winter, however, NO₃-N、NO₂-N and NH₄-N transferred from water to sediment, and fluxes were-6.39×107、-1.49×107 and-1.33×108 mmol·d-1,respectively. PO₄-P transferred from water to sediment with fluxes of 3.69×107 mmol·d-1 in summer, providing 14.1% P required for primary productivity. In winter, the transferring direction was reversed, and the flux was-2.20×107mmol·d-1. Sediment was always the source of SiO₃-Si, and its fluxes were 6.50×108 mmol·d-1 in summer and 1.32×108 mmol·d-1 in winter,providing 15.6% and 25.8% Si required for primary productivity respectively. 2、The most important environmental factors and processes controlling the nutrient exchange rates varied with season and the type of nutrients. In summer, mineralization was an controlling processes for all kinds of nutrients. The exchange of NH₄-N in summer was also controlled by assimilation and adsorption-desorption, while the exchange of SiO₃-Si was mainly controlled by dissolution and diffusion. In winter, mineralization became weak and the exchange of NO₃-N, PO₄-P and SiO₃-Si were mainly influenced by assimilation and diffusion. The exchange of PO₄-P in winter was also influenced by the process of adsorption-desorption caused by organic matter mattered, and the exchange of SiO₃-Si in winter was also influenced by dissolution process. Same as summer, the exchange of NH₄-N in winter was probably significantly influenced by mineralization and adsorption-desorption at sediment-water interface.The exchange rates of NO₃-N were correlated to water ratio of surface sediment and bottom-water NO₃-N concentration and pore-water NO₃-N concentration, while the exchange rates of NO₂-N and NH₄-N were not correlated to any factor of surface sediment or the concentration of corresponding nutrient. Based on principal component regression analysis, the result revealed that total organic carbon（TOC）, chlorophyll a（Chl a）, total nitrogen（TN）, water ratio and concentration of dissolved nitrogen（DIN） were the the most important factors influencing the exchange of NO₃-N、NO₂-N and NH₄-N. As a result, mineralization and diffusion might be the controlling processes for the exchange of NO₃-N. Mineralization, assimilation, adsorption-desorption and diffusion, however, might be the most controlling processes for the exchange of NH₄-N in summer. Exchange of NO₂-N varied in the same pattern with NH₄-N in summer, so that nitrification may be the key process controlling the exchange of NO₂-N.The exchange rates of PO₄-P in summer were positively related to TOC and negatively correlated to C/N in surface sediment, and the major factors influcing the exchange of PO₄-P were Chl a、TOC and TP. As a result, mineralization might be the most controlling processes for the exchange of PO₄-P in summer.The exchange rates of SiO₃-Si in summer were significantly related to TOC and Chl a in surface sediment, and also positively related to biogenic silicate（BSi）, content of clay, water ratio, SiO₃-Si concentration in pore water and the concentration difference of SiO₃-Si at the interface, and TOC, Chl a, biogenic silicate（BSi）, content of clay, water ratio of surface sediment and SiO₃-Si concentration in pore water were the most important relevant factors. As a result, the exchange of SiO₃-Si at sediment-water interface in summer was a consequence of dissolution-diffusion process which was dominantly controlled by biological activity.The exchange rates of NO₃-N in winter were not correlated to any factor of surface sediment or the concentration of corresponding nutrient. Considering that Chla in the surface sediment and the DIN concentration in pore water were the most important factors, assimilation and diffusion might be the controlling processes for the exchange of NO₃-N. The exchange rates of NO₂-N in winter were only related to Chla in the surface sediment and the mechanism was not clear yet. The exchange rates of NH₄-N in winter were positively related to Chl a, and Chl a in surface sediment, content of clay and medium diameter（D50） had significant effects on the exchange of NH₄-N, so the degradation of marine endogenous organic matter and adsorption- desorption at sediment-water interface might be the controlling processes for the exchange of NH₄-N in winter.The exchange rates of PO₄-P in winter were only related to TOC in surface sediment, and Chl a, TOC, water ratio and PO₄-P concentration in bottom water and pore water were the major relevant factors influencing the exchange of PO₄-P, so assimilation, dilution and adsorption-desorption caused by the organic matter might be the controlling processes for the exchange of PO₄-P in winter.The exchange rates of SiO₃-Si in winter were significantly positively related to BSi in surface sediment and SiO₃-Si concentration in pore water, and the Chl a in surface sediment and pore-water SiO₃-Si concentration had major effects on the exchange of SiO₃-Si at sediment-water interface, so the exchange of SiO₃-Si in winter was controlled by assimilation, diffusion and dissolution.