| Mangrove ecosystem,which located at the junction of ocean and land,has important ecological service functions.Mangrove wetland is an important carbon pool,and has important influence on the global carbon balance.At the same time,through a series of physical,chemical and biological functions,it can also alleviate heavy metal pollution by trapping heavy metals,thus reducing heavy metal release to its surrounding estuarine and ocean ecosystems.However,due to the effects of global climate change and human activities such as population growth,social and economic development,and rapid urbanization,mangroves are disappearing with quick speed.This paper chose mangrove wetlands at Dongzhai Harbor,Hainan Island and Qinzhou Bay,Guangxi as the research sites.With the assistance of remotely sensed data,field investigation and laboratory analysis,we aimed to obtain the spatial distribution and area size of the mangroves;to investigate the mangrove wetland landscape pattern change and its driving factors;to compare the carbon sequestration,distribution and influencing factors of different mangrove communities;to understand the distribution and enrichment capability of heavy metals in mangrove soils,the trans-location and bio-concentration of heavy metal in mangrove plants,and the source of heavy metals in the mangroves;and to explore the relationship between landscape pattern change and the concentration of mangrove soil organic carbon(SOC)and heavy metals.The main conclusions are shown as following:1.Mangrove wetland landscape pattern change and its driving factorsIn the past 20 years,the areas of mangrove forest at Dongzhai Harbor,Hainan Island(1809.4,1738.7 and 1608.2 hm2 in 1988,1998 and 2009,respectively),presented a decrease trend,while the areas of mangrove forest at Qinzhou Bay,Guangxi(1668.6,1857.1 and 1257.6 hm2 in 1990,2000 and 2010,respectively),firstly increased and then decreased.Both landscapes encountered fragmentation in the last decades.The transformations among different landscape types indicated that the mangroves,farmland and forest land were mainly changed into built-up area and aquaculture ponds in both of the two study sites.The statistical results obtained from three different methods of accumulative counting,percentage counting and main transformation route counting showed that the main reason of the changes of mangrove patch numbers at Dongzhai Harbor(the percentages of natural force were 58.6%,72.6%and 72.1%,respectively)and Qinzhou Bay(the percentages of natural force was 63.1%,65.6%and 65.6%,respectively)were natural process,while the primary factors of the changes of mangrove patch areas of Dongzhai Harbor(the percentages of human activities were 82.4%,82.4%and 86.2%,respectively)and Qinzhou Bay(the percentages of human activities was 68.2%,69.2%and 70.6%,respectively)were human activities.2.Distribution and influencing factors of SOC content and density of different mangrove communitiesThe soil organic carbon contents of six mangrove communities of Dongzhai Harbor and Qinzhou Bay were ranged from 0.57 g·kg-1 to 89.51 g·kg-1 with a mean of 15.95 g·kg-1,while the soil organic carbon densities were ranged from 0.16 kg·m-2 to 27.68 kg·m-2 with a mean of 4.09 kg·m-2.The soil organic carbon content and density of different mangrove communities were different.The soil organic carbon content of Bruguiera sexangula community was the highest,while Sonneratia apetala community was the lowest.The soil organic carbon density of Aegiceras corniculatum community was the highest,while Sonneratia apetala community was the lowest.In the vertical direction,the maximum values of soil organic carbon and density of Dongzhai Harbor and A.corniculatum of Qinzhou Bay were found at depth of 0-20 cm and 40-60 cm,respectively,and decreased with the depth from surface to bottom approximately.In the horizontal direction,the soil organic carbon and density of three shrub communities and the soil organic carbon of three arbor communities of Dongzhai Harbor showed the tendency of increase with the increase of elevation,while the soil organic carbon and density of A.corniculatum communities of Qinzhou Bay decreased with the increase of elevation.SOC contents were more related to soil nutrient elements than with soil environmental factors.3.Distribution and accumulation of heavy metals in soils and plants in mangrove wetlandThe concentrations of copper(Cu),zinc(Zn),lead(Pb)and chromium(Cr)of Qinzhou Bay were higher than those of Dongzhai Harbor.The concentrations of Cr,cadmium(Cd),Cu,Zn and Pb in the surface soils,soil columns and different plant parts of A.corniculatum community in Qinzhou Bay were 113.66,0.28,34.95,108.69,37.90 μg·g-1·DW,68.65,0.14,19.16,72.68,20.32 μg·g-1·DW,and 8.75,0.08,3.15,20.59,5.24 μg·g-1·DW,respectively.The concentration of Cr in the surface soils was at moderate pollution level,while the pollution level of the other heavy metals were low.Qinzhou Bay was in the slightly potential ecological risk of heavy metal pollution.In the soil columns,the concentrations of heavy metals in the depth of 0-20 cm were relatively high,showing a decreasing trend with depth.Heavy metals in the soils were mainly from agriculture and aquaculture activities,and the grain size composition of soils significantly affected the spatial partitioning of heavy metals.Among different plant parts of A.corniculatum,Cr,Cu,Pb and Cd were mainly accumulated in the roots,while Zn was mainly accumulated in the stems.Compared with the other mangrove species,A.corniculatum showed the strongest translocation capacity for Zn and Cu,and the strongest bioconcentration ability for Cd.4.The relationship between landscape pattern change and SOC and heavy metalsSoil organic carbon content of mangrove wetland decreased with the decrease of mangrove area,but no significant correlation was found between them.The increased area of construction land and aquaculture ponds has resulted in the shrinking and fragmentation of mangrove area with enhanced pollutant discharge,which weakened the mangrove wetland’s ability of absorption and purification of heavy metals,and caused the accumulation of heavy metals in mangrove soil.In conclusion,mangrove wetland areas decreased,while the fragmentation degrees increased in both Dongzhai Harbor,Hainan Island and Qinzhou Bay,Gaugnxi in the past 20 years.Soil organic carbon content and density among different communities were different,and the native species which has strong carbon sequestration ability such as Rhizophora stylosa,Avicennia marina and A.corniculatum,and thus should be highly prioritized rather than the exotic species S.apetala during mangrove restoration.The concentration of heavy metals in mangrove wetlands showed an increasing trend.A.corniculatum could be chosen as the restoration species in the tropical and subtropical coastal zone polluted by Zn,Cu and Cd.The results can provide a scientific basis for the protection,utilization and carbon sequestration during mangrove ecosystem management. |
