The effect of two types of grid transplantation on coral growth and the in-situ ecological restoration in a fragmented reef of the South China Sea

Abstract

Due to climate change and human activities, coral reef ecosystems are facing a crisis of degradation globally. Some coral reefs in the northern part of Wuzhizhou Island (Southeastern Hainan Island, the South China Sea) have been fragmented because of continuous disturbance, and we systematically conducted in-situ restoration experiments to accelerate the ecological restoration in this area. In September 2019, 40 reefs with hollow structures were placed in the experimental area, and a control area was selected at the same depth. Twenty of the 40 reefs were covered by a cylindrical grid with a diameter of 0.5 cm (GFn group), and the remaining 20 were covered by a flat grid with a width of 1 cm (BFn group). A total of 1140 coral colonies, composed of Acropora hyacinthus, Acropora microphthalma, Acropora florida, Montipora truncata, and Porites lutea, were transplanted in this experiment, with an overall survival rate of 94.27% due to the coral transplant base of the carrying reefs being of sufficient weight, hollow structure, and dense grid. The survival rate and annual growth rate of Acropora in the GFn group with a narrow but large mesh and cylindrical design were significantly higher, and the fastest growth rate was found in A. hyacinthus, growing at 27.33 ± 10.37 cm2·month−1. Montipora truncata and P. lutea in the BFn group with a wide mesh and flat structure had higher survival rates and significantly greater growth rates. In the ecology of the coral community, coral coverage in the GFn group was significantly higher compared with the BFn group, which was mainly attributed to the difference in the growth of Acropora. Compared with the reef fragmentation area, the three-dimensional structure of the hollow reef and its radiation effect significantly attracted the accumulation of large invertebrates and reef fishes. Sea cucumbers and sea urchins gathered faster, forming a stable community structure. The dominant fish species gradually transformed from the large algae-eating fish Siganus fuscessens to the territorial algae-eating fish Dascyllus reticulatus due to changes in the three-dimensional structure of the grid surface caused by coral growth. Studies have shown that the three-dimensional structure of a reef can significantly affect the aggregation of benthic organisms. Among the selected corals, Acropora grew more rapidly, which established more complex three-dimensional structures to achieve a better ecological restoration effect in the reef area. The combination of tiled Montipora and lumpy Porites could increase the base coverage and reduce the impact of algae on the corals. Our results suggest that when transplanting different types of corals, we should consider the use of multiple comprehensive factors such as the type of the reef, the structure of the grid, the characteristics of the transplanted corals, and the influence of environmental factors.