Nutrient Exchange Fluxes And Influencing Factors At The Sediment-Water Interface Of The Weizhou Island Coral Reef

Under the influence of global climate change,coral reefs have suffered severe degradation,especially from eutrophication of seawater and a skewed nutrient structure caused by human activities.Coral reef ecosystems have high productivity and biodiversity due to the efficient cycling of nutrients.The sediment-water interface can regulate the concentration and structure of nutrients in waters,which plays an important role in the biogeochemical cycle of nutrients in coral reefs.We investigated the spatial variation of nutrients in the offshore area of Weizhou Island,Guangxi in 2019,and clarified the key factors affecting the nutrient concentration and structure by using the nutrient budget model from the Land-Ocean Interactions in the Coastal Zone（LOICZ）research projects.A series of flow-through reactor（FTR）was used for incubation experiments to understand the effect of nutrient exchange at the sediment-water interface on the concentration and structure of seawater nutrients.The effects of temperature,organic matter,nutrient concentration and porewater advection on the dynamics of nitrogen（N）,phosphorus（P）,and silicon（Si）at the sediment-water interface were further evaluated by controlled incubation under different conditions.The main results are obtained as follows.（1）The concentrations of dissolved inorganic nitrogen（DIN）and dissolved inorganic phosphorus（DIP）in the surface seawater around Weizhou Island ranged from 0.39 to 6.11μmol L^-1 and 0.08 to 0.26μmol L^-1,respectively;the concentrations of DIN and DIP in the bottom layer ranged from 5.12 to 9.44μmol L^-1 and 0.11～0.35μmol L^-1,with slightly higher concentrations in the bottom layer than in the surface layer.The concentrations of DIN and DIP outside the bay are higher than those inside the bay.During the survey period,the average value of nitrogen to phosphorus ratio（N/P）around Weizhou Island ranged from 16.2 to44,with the phenomenon of excess nitrogen and phosphorus limitation.（2）The results of applying the LOICZ model to estimate nutrient budget and its key influencing factors in Weizhou Island waters show that,the net budget values of DIN and DIP are-9.45×10⁵ mol yr^-1 and 0.95×10⁵ mol yr^-1,respectively.The DIN and DIP in the bay are mainly attributed to the discharge of groundwater and the nutrient exchange at sediment-water interface,the latter while the DIN output is mainly due to the uptake 91.42%of DIN by sediments;the release of phosphorus from the sediments contributed the 76.5%of DIP input,and the harvest of shellfish removed 96.6%of DIP.（3）The concentrations of DIN,DIP,and DSi in the sediment porewater ranged from 16.64～225.21μmol L^-1,0.46～0.69μmol L^-1,and 36.21～439.03μmol L^-1.The contents of inorganic phosphorus（IP）and organic phosphorus（OP）within the sediments ranged from 0.30～0.52μmol g^-1 and 0.03～0.09μmol g^-1.The content of biological silicon（BSi）ranged from 0.01～0.12%.The content of total nitrogen（TN）is 0.001～0.006%.The exchange fluxes of DIN,DIP and DSi at the sediment-water interface,which were quantified by FTR incubation experiments,ranged from-0.26～0.028 mmol m^-2 h^-1,-0.13～0.05 mmol m^-2 h^-1,and-0.49～2.06 mmol m^-2 h^-1,respectively.During the incubation process,after the seawater flows through the sediment,the N/P of the outflowing seawater is significantly lower than that of the incoming seawater,which is jointly determined by the DIN and DIP exchange fluxes at the sediment-water interface,regulating to some extent the nutrient concentration of the sea and its structure.（4）The regulation mechanism of nutrient fluxes at the sediment-water interface was obtained through controlled FTR incubation experiments on the sediment under different conditions.Due to the excess nitrogen in Weizhou Island waters,the increase in temperature will lead to benthic release of DON and the dissimilatory nitrate reduction to ammonium,thus increasing the nitrogen stress in the sea.The phosphorus in the sediment can be released under the change of environmental factors,which plays a role in regulating the concentration and structure of nutrients in the overlying water.The concentration of phosphate in seawater was reduced to 0.40±0.06μmol L^-1,and DIP shifted from being absorbed by sediments to being released from sediments to seawater.In microzones of the Ca CO₃ grain pores are sites for dissolution,accelerates the dissolution of calcium carbonate and the release of DIP in the sediments,and can alleviate the phosphorus limitation in the sea to a certain extent.However,the extremely low content of calcium-bound phosphorus in the sediments of Weizhou Island may weaken its influence on the DIP concentration and nutrient structure in seawater.Moreover,warming and moderate advection flow rates can promote DSi uptake by sediment benthic diatoms.The results of this study may provide a scientific basis for assessing the potential impact of nutrients on coral reefs in the waters around Weizhou Island and the ecological management of the offshore environment.