Coastal lagoons as a transitional zone between land and sea: Insight from trace element pollution in southern China

Despite the importance of the status, pattern and source of trace element (TE) pollution in China's coastal lagoons to the sustainable development of coastal zones, little attention has been given to this in previous research. In this study, TE analyses were conducted of surface sediments from 39 coastal lagoons along the coast of southern China. Multivariate statistical analysis revealed the lithologic sources of V, Tl, Cr, Ni, Co, and Mn and the anthropogenic contribution to Zn, Cu, As, Pb, and Cd. The results showed that the current contamination of the lagoons is generally light, except for some lagoons showing median to high pollution levels of Cd, Cu, and Zn. These polluted lagoons are commonly characterized by higher geological background concentrations coupled with aquaculture and vessel activities contributions, but tidal exchange affords some dilution of the pollutants. Spatial analysis revealed an obvious seaward declining gradient of pollution risk in half of the researched lagoons, highlighting the terrestrial sources of TEs and the role of sea water exchange in pollutant migration. Systematic comparison with other water bodies in southern China revealed the following contamination gradient: urban lakes > reservoirs > rivers > coastal lagoons > continental shelf. The two gradients indicate that coastal lagoons form a transitional zone of TE pollution between land and sea, suggesting that coastal lagoons sensitively record interactions between human activities and sea-land interactions. Our findings provide essential insights into the pollution patterns of coastal lagoons and are also significant for environmental management and ecological restoration of coastal zones.