Decadal trends in mangrove and pond aquaculture cover on Hainan (China) since 1966: mangrove loss, fragmentation and associated biogeochemical changes

摘要

Mangrove forests suffer from large-scale conversion into pond aquaculture worldwide. However, rarely can the detailed development of these changes and the consequences for coastal biogeochemistry be traced back to baseline conditions. We analyzed decadal changes in mangrove forest and aquaculture pond cover of five estuaries along the east coast of Hainan, northern South China Sea, using aerial photos and satellite images from 1966 to 2009. In addition, we reconstructed historical changes in the biogeochemistry by analyzing three sediment cores from the largest remaining mangrove area in east Hainan (Wenchang/Wenjiao Estuary, WWE). Overall mangrove loss was 72% (from 3697 ha in 1966 to 1041 ha in 2009), ranging from 63% in WWE to virtually 100% loss in Qingge. Land cover of aquaculture ponds in the five estuaries increased from 550 ha in 1966 to 3944 ha in 2009. 55% of the former mangrove area was directly replaced by aquaculture ponds accounting for 76% of the mangrove loss. An increase in the number of individual mangrove area patches from 230 larger to 2134 smaller patches indicates severe fragmentation of the remaining mangrove areas, likely with adverse consequences for ecosystem functioning. The sediment cores from the WWE show that the primary organic matter source changed from mangrove- to aquaculture-derived suspended matter since the 1980s. Moreover, the land cover change likely increased the nutrient export from land due to loss of the mangrove filter and creation of a significant nutrient source by pond effluents with negative impact for adjacent seagrass meadows and coral reefs. This is one of the longest time series documenting massive mangrove decline in recent decades. It highlights the strong and persistent ecological and biogeochemical changes associated with mangrove conversion in tropical estuaries, negatively affecting ecosystem services provided by undisturbed mangrove forests. The speed and magnitude of land conversion and the biogeochemical consequences for adjacent coastal waters observed in Hainan serve as an extreme example of similar activities in Southeast Asia.