Temporal And Spatial Variations Of Physico-chemical Characteristics In Sediments Of Lake Taihu, China

In order to study temporal and spatial variation characteristics of sediment physico-chemical properties and assess the potential releases of two forms pore-water inorganic nitrogen (N-NH4+,N-NO3-) into overlying water in different season, setting Xukou Bay dominated by macrophytes(S1), Central Lake with strong wave and less algal bloom(S2) and Meiliang Bay with severe algal bloom(S3) of Lake Taihu as the study area, we collected the overlying water samples and intact sediment cores of Lake Taihu in each season(spring, summer, autumn and winter). Afterwards, physical and chemical parameters of the overlying water samples were determined in the lab immediately. Meanwhile, the upper sediment cores were divided by slicing them into the following 5 sub-samples:the upper 0～1cm, 1-3cm,3-5cm, 8-10cm and 13-15cm, respectively. Sediment physical and chemical characteristics were also determined as:wet bulk density(WD), water content(WC), pH, oxidation reduction potential(Eh), loss on ignition(OM), chlorophyll a(Chl a), total nitrogen(TN), total phosphorus(TP), total organic carbon(TOC), pore-water inorganic nitrogen (ammonia-NH4+, nitrate-NO3-,nitrite-NO2-).The results showed that wet bulk density(WD) increased with sediment depth at three sampling sites. The content of Meiliang Bay(S3) was generally less than that of Xukou Bay(S1) and Central Lake(S2). As to the seasonal variation, the wet bulk density of sediment was high in summer and autumn but low in winter and spring. Unlikely, water content of sediment(WC) decreased through the sediment and Meiliang Bay(S3) was significantly higher than that of Xukou Bay(S1) and Central Lake(S2) (p<0.05). The water content was low in winter but high in summer and autumn. Vertical changes of sediment pH decreased from the surface but then increased a little down the bottom. There was little change of pH value which ranged from 6.5 to 8.5 among the three sampling sites. Seasonal variation of sediment pH in each site varied individually but generally showed low in winter, high in summer and autumn. Sediment Eh decreased with depth, overlying water and sediment surface within 2mm were both in oxidized state. When it came to 25mm below the sediment, it was deoxidated and made the sediment keep in low concentration of dissolved oxygen. Moreover, the lake difference was followed as: Xukou Bay(S1)>Central Lake(S2)>Meiliang Bay(S3). Of the four seasons, the maximum vertical variation was in autumn when sediment Eh showed significant change with the other three seasons.Loss on ignition(OM) of sediment decreased with the depth and the spatial difference showed as:Meiliang Bay (S3)>Xukou Bay (S1)>Central Lake(S2) and the highest content was in winter. Seasonal vertical changes of Chlorophyll a varied individually among the three sites: Central Lake(S2) showed decreasing trend in each season; Meiliang Bay(S3) and Xukou Bay(S1) decreased with depth in winter and spring but in summer and autumn, they decreased at first and then increased apparently. Sediment total nitrogen(TN), total phosphorus(TP), total organic carbon(TOC) decreased with depth, and the Meiliang Bay(S3) had the highest content. Total nitrogen(TN) showed low in winter and high in summer and autumn. The seasonal and vertical variation of total phosphorus(TP) were not obvious. In Central Lake(S2), the total organic carbon (TOC) was much lower than that of Meiliang Bay(S3) and Xukou Bay(S1), the vertical trend was more obvious in summer and autumn than in winter and spring. From the investigation about sediment carbon, nitrogen and phosphorus, we can conclude that the internal loading of the three sampling sites were shown as:Meiliang Bay (S3)>Xukou Bay (S1)>Central Lake(S2).Pore-water ammonium-NH4+concentration increased with sediment depth and its spatial variation differed in each season as the general performance of Meiliang Bay(S3)>Xukou Bay(S1)>Central Lake(S2), which was rather evident in summer and autumn. As to pore-water nitrate-NO3-, it decreased with sediment depth in each season and apparent in summer and autumn In Central Lake(S2) and Xukou Bay(S1), the pore-water nitrate-NO3- also decreased with sediment depth except for Meiliang Bay(S3). The spatial variation was followed as: Central Lake(S2)>Xukou Bay(S1)>Meiliang Bay(S3). Pore-water nitrite-NO2- whose highest concentration was only 100μg/L in the 0-1 cm sediment of Xukou Bay was the lowest one form of the three inorganic nitrogens. The surface concentration was higher than in the deep depth and its spatial difference was in the general performance of Xukou Bay (S1)>Central Lake(S2)>Meiliang Bay(S3).Compared the pore-water inorganic nitrogen (ammonia-NH4+; nitrate-NO3-) concentration of surface sediment samples (0-5cm) with the concentration of the overlying water, we found that there was a huge difference of ammonia concentration but little of nitrate. It wa suggested that there could be a great potential realease of pore-water ammonia into overlying water, and high in autumn and winter low in spring and summer.The spatial difference was showed as: Meiliang Bay(S3)>Xukou Bay(S1)>Central Lake(S2), which also indicated the nutrition exchange of overlying water with sediment was not strong in Meiliang Bay(S3).