In landward coastal zones of most mangrove swamps in China, there are many shrimp ponds with surface sediments rich in nutrients such as N and P. During pond cleaning periods, some of these surface sediments are usually directly discharged into mangrove swamps after being flushed by using high-pressure hydraulic giants. Gradually, these sediments from shrimp ponds will accumulate on the floor of mangrove swamps, resulting in the increase in sedimentation thickness. However, the effects of sedimentation of shrimp pond cleaning discharges on mangrove ecosystems are still unknown. Therefore, to study their responses and endurances in growth and physiological characteristics, four thickness treatments (0, 2, 4, 8 cm) were applied to simulate the sedimentation condition of shrimp pond discharges into the mangrove swamp, and Acanthus ilicifolius seedlings were cultivated under these treatments for 424 days.
In terms of growth indexes, including stem diameter, stem height, leaf number, leaf area, total biomass and relative growth rate, growths of A. ilicifolius seedlings were promoted due to the sedimentation of shrimp pond cleaning discharges. The growth condition of A. ilicifolius seedlings under four treatments followed the order of 8 cm > 4 cm > 2 cm > 0 cm. Results also indicated that a small quantity of cleaning discharges (sedimentation thickness of only 2 cm) could obviously enhance the growth of A. ilicifolius seedlings, and this enhancement would not be stronger comparing with the increasing of the sedimentation thickness. The sedimentation of shrimp pond cleaning discharges enhanced the leaf photosynthetic capacity, especially synthesized more chlorophyll a. With 2 and 4 cm sedimentation thicknesses, contents of MDA, soluble protein and soluble sugar in leaves were significantly higher than normal levels, but there was no significant accumulation of free proline in leaves; 8 cm sedimentation thickness resulted in abundant accumulation of MDA, soluble protein, soluble sugar and free proline in the leaves; only in the 8 cm sedimentation thickness group, activities of antioxidant enzymes in leaves were significantly lower than the control group.
In a conclusion, A. ilicifolius seedlings had strong endurance and adaptability to shrimp pond cleaning discharges with the sedimentation thickness less than 4 cm; a sedimentation thickness of over 8 cm resulted in stress damages to seedling physiology, but the damage to seedling growth was not obvious during this experiment period.
Impacts of sedimentation of shrimp pond cleaning discharges on mangrove ecosystems will be long and profound. Related government departments should attach great importance to it, enhance the management and monitor of mangroves nearby aquaculture areas, and balance the relationship between the development of aquaculture and the ecological restoration of mangroves. In addition, shrimp pond cleaning discharges may contain a variety of residual drugs such as disinfectants and antibiotics used for pond disinfection and disease prevention, which may be toxic to the environment. However, the toxic mechanisms and processes of these substances to mangrove ecosystems are still unclear. Therefore, further studies should be focused on the long-term effects of sedimentation and chemical pollution of shrimp pond discharges on mangrove ecosystems.