The Distribution Of Nutrients And The Affecting Factors In Different Habitats In Nanhui Tidal Flat Of Yangtze River

Nutrients are essential elements for biological growth and are involved in global biogeochemical cycles.Excessive nutrients in water can lead to eutrophication.As a pollutant sink,tidal wetlands play a significant role in water purification.Up to now,only a few studies have investigated the purification function of salt marshes under natural conditions.Systematically studying multi-components in different habitats is of importance to understand the nutrient attenuation role of salt marsh habitats.The study site was in Nanhui tidal flat in the Yangtze Estuary,where the three main habitats are distributed:bare flat,Scirpus mariqueter and Spartina alterniflora.We conducted seasonal samplings of physiological characteristics in water,sediment and salt marshes,and analyzed p H,salinity,dissolved inorganic nitrogen（DIN）and dissolved inorganic phosphorous（DIP）concentrations in the surface water and porewater,as well as p H,conductivity,water content,bulk density,redox potential,carbon,nitrogen,phosphorous and iron contents in sediments and plant biomass.We also conducted an incubation experiment to study changes of DIN and DIP concentrations in the surface water at different depths of sediments in the three habitats.The results revealed the reduction effects of DIN and DIP in different salt marshes,and also contributed to the understanding of the purification function of natural salt marshes.The results were shown as follows:（1）Salt marshes increased the salinity in water and sediment,increased total carbon content,and decreased sediment particle size,which reduced the infiltration rate and enhanced the capacity to retain water and organic matter.The p H of the water was alkalescence in all three habitats,and the bare flat was higher than that the in the vegetated habitats.The sediment p H was alkaline and there were no significant differences among the three habitats.In both water and sediment,the salinity differed significantly in the three habitats,with the order of Spartina alterniflora>Scirpus mariqueter>bare flat.There were significant differences in the sediment moisture of the three habitats,with the order of Spartina alterniflora>Scirpus mariqueter>bare flat.Sediment moisture had a significant positive correlation with median size,TC,TOC,and Fe contents in the sediment（p<0.01）,while bulk density showed a significant negative correlation（p<0.01）with the above factors.The aboveground biomass in the Spartina alterniflora was 10 times higher than that in the Scirpus mariqueter.The total biomass in the Spartina alterniflora was significantly higher than that in the Scirpus mariqueter.（2）The concentrations of DIN and DIP in the surface water and porewater in the vegetated habitats were significantly lower than that in the bare flat.The relative reduction efficiency for DIN in the Spartina alterniflora habitat was 82.5%～97.3%,significantly higher than that in the Scirpus mariqueter habitat with 25.5%～75.8%.The relative reduction for DIP was 54.1%～89.3%in the Spartina alterniflora habitat,followed by 55.7%～76.4%in the Scirpus mariqueter habitat,but no significant differences were observed within vegetated habitats.（3）Nitrate（NO_x^--N）was the main component of DIN in surface water,and the average proportion of NO_x^--N was 67.9%in the three habitats.The main component of DIN in porewater was ammonium（NH₄⁺-N）,and the average proportion of NH₄⁺-N was 88.6%in the three habitats.The main influencing factors of DIN and DIP concentrations in the surface water were p H and salinity in the water,with sediment TC and TN also having an impact on the concentration of DIN in the surface water.The main influencing factors of DIN concentration in porewater were sediment IN,median sediment size and conductivity also having an impact on the concentration of DIN in the porewater in vegetated habitats.No significant factors for DIP concentration in the porewater were detected.（4）The experiment results of the nutrients exchange at the interface of sediment-overlying water indicated that the concentration of DIN in the surface water decreased with incubation time,showing a decline range of 22%to 90%.The concentration of DIP in the surface water showed the opposite trend with incubation time.Hence,the tidal flat sediment acted as a sink of DIN and a weak source of DIP in the surface water.In summary,our findings suggest that salt marshes alter biogeochemical parameters and can strongly reduce the nutrients surplus even if they are under open tidal conditions.Therefore,priority can be given to the restoration of native Scirpus mariqueter,and invasive Spartina alterniflora can be retained temporarily in the section where the native species are difficult to colonize.This study provides insights into pollution mitigation and ecological management of salt marsh restoration in the eutrophic Yangtze Estuary.