Spatial distribution of physicochemical properties and metal concentration in mangrove sediments from Gaoqiao in Zhanjiang, Guangdong of Southern China.

Taking Gaoqiao mangrove of Zhanjiang Mangrove National Nature Reserve as the study area, six sediment cores were sampled along two sample lines A and B, which were perpendicular to the coastline, and spatial distribution features of physicochemical properties and enrichment pattern of metal trace in sediment cores were analyzed. The content of metals, including major metals (Al, Fe and Mg) and trace metals (As, Co, Cr, Cu, Mn, Ni, Pb and Zn) and their possible relationships with physicochemical properties (particle size, pH, exchangeable base cations, total nitrogen, total phosphorus, and organic matter) in core sediment samples were evaluated. Analysis of physicochemical properties revealed that an overall predominance of silt and sand in the study area, the concentration of clay, silt and sand of sediment were (134)%,(3912)% and (4816)%, respectively, organic matters varied from 0.2% to 3.7%, 2.5 -7.3 for pH value, 1.4 -16.4 cmol/ kg for exchangeable base cations, 0.01% -0.2% for total nitrogen, and 130 - 435 mg/ kg for total phosphorus. The sediment quality parameters including grain size, pH value, exchangeable base cations, organic matter, total nitrogen and phosphorus were not significant (P 0.05) between A and B sample lines. The metal concentrations in sediment obtained in this study decreased in the following order,Al (4.5% -19.2%), Fe (1.0% -4.7%), Mn (50.4 -163.4 mg/ kg), Cr (24.4 -43.0 mg/ kg), Zn (14.4 -52.1 mg/ kg), Pb (11.3 -34.9 mg/ kg), Ni (3.5 -19.4 mg/ kg), Co (3.0 - 7.7 mg/ kg), Cu (1.4 - 9.6 mg/ kg), As (1.4 - 5.8 mg/ kg), Mg (0.2 - 0.9 mg/ kg). The difference was significant (P 0.05) among contents of metals excluding Co, Cu, As and Fe. The concentrations of Cr, Cu and Mn on line A were much higher than on line B. Spatial distribution varied markedly in physicochemical properties. The sand content and pH value increased with increasing distance from the seawall. Organic matter, total nitrogen and total phosphorus were affected by mangrove community type and hydrodynamics. Physicochemical properties varied obviously in vertical direction, parameters of physicochemical properties ( not including sand content in surface layer) were the highest, and decreased gradually from top to bottom layer. With increasing distance from the seawall, the metal concentration in core sediment decreased gradually with the tidal level but varied with metals; therefore, tidal level was not the solo factor affecting metal distribution in horizontal level. Similar to the trends of physicochemical properties, hierarchical variation in metal concentration was presented in a vertical direction: concentrations of metals were the highest in surface layer, and decreased gradually from top to bottom layer. Correlation analysis showed that organic matters were positively correlated with finer particles (P 0.01), but negatively correlated with pH value and sand content (P 0.01). A significantly negative correlation was detected between metal content and sand content, and metal concentration had no correlation with pH value. The metal concentration was positively correlated with exchangeable base cations, organic matter and fine particles (P 0.01). The associated or homologous features were detected according to the significant correlation among the metals.