Spatial And Temporal Variation Of Sedimentary Heavy Metals By Cage Aquaculture In Sansha Bay,Fujian

The area of offshore aquaculture in China has increased from 6,000 km² in 1990to 22,000 km² in 2020.Although the environmental impacts of aquaculture have been widely studied,quantitative assessment of the effects of aquaculture on sediment heavy metal pollution has been a research challenge because of the complex sources of heavy metals,the difficulty of identification and the complex conservative-nonconservative behavior with salinity gradients.In this study,the world’s largest rhododendron cage aquaculture base,Sansha Bay,Fujian Province,was considered as the typical area for offshore aquaculture.Based on Google Earth Engine cloud platform and Landsat and Sentinel series satellite images,the data of aquaculture area and aquaculture density from 2002 to 2020 were collected.Combined with 2 sediment columns and 38 surface sediments in the bay,the particle size and the content of heavy metal of the obtained samples were systematically analyzed and tested.By comprehensively applying the methods,such as ²¹⁰Pb dating,GIS spatial analysis,multivariate statistics,risk assessment and so on,the effects of cage aquaculture on heavy metal pollution of sediments in the bay were quantitatively explored.The main research findings were as follows:（1）From 2002 to 2020,the cage aquaculture has gradually developed from the small-scale fragmented patches to the large areas of stripes,and its area has increased from 5.3 km² to 33.4 km² with an average annual growth of 1.56 km²,and the largest cage density per square kilometer has increased from 3%to 22%.（2）Since 2000,the contents of Cr,Cu,Zn,As,Cd,Pb and Hg in the sediment column of the cage aquaculture area（Z06）have significantly increased,while in the sediment column of the non-cage aquaculture area（Z30）they were basically stable or slightly decreased.By conducting the regression analysis of the content of heavy metal and aquaculture parameters with the basis of background values,the results demonstrated that for every 1 km²increased in cage aquaculture area,the output of rhubarb production increased by 4,500 tons,and the amount of chilled miscellaneous fish fed increased by 32,400 tons,the contents of Zn,As,Cd,Pb and Hg increased simultaneously by 0.80,0.63,0.54,0.29,0.001,0.43 and 0.002 mg/kg.（3）Compared with the non-cage aquaculture area,the average contents of Cr,Mn,Ni,Cu,Zn,Cd and Pb in the surface sediments of the cage aquaculture area increased by 18%,11%,29%,51%,11%,15%and 17%,respectively.Meanwhile,sediment pro-Fe heavy metals（Cr,Mn,Ni）were non-conservative with salinity,while pro-Cu heavy metals（Cu,Zn,Cd,Pb）were significantly conservative with salinity,and in-bay aquaculture resulted in a 51%,11%,15%,17%decrease in the slope of the regression line of Cu,Zn,Cd,Pb with salinity.The positive matrix factorization analysis further demonstrated that heavy metals in the non-aquaculture area consisted mainly of 33.2%natural sources and 66.8%terrestrial human emission sources,while heavy metals in the aquaculture area consisted mainly of 41.7%natural sources,24.3%terrestrial human emission sources and 34.0%farming activity sources.（4）Large-scale and high-intensity cage aquaculture significantly increased the ecological risk of heavy metal pollution in the aquaculture area.The potential ecological risk coefficients of Cr,Mn,Ni,Cu,Zn,Cd and Pb in the surface sediments of the aquaculture area were 17%,4%,30%,56%,8%,9%and 7%higher than those in the non-aquaculture area,respectively,and all elements were at low ecological risk levels except for Cd,which belonged to the medium risk level.This study quantitatively reveals the effects of offshore cage aquaculture on heavy metal pollution by establishing scientific relationships between cage aquaculture and heavy metals in sediment columns,and between heavy metals and salinity in surface sediments of aquaculture and non-aquaculture areas,which provides scientific instructions for ecological protection and restoration,resource development and sustainable development of coastal zones in areas with intense aquaculture activities.