| Presently, aquaculture production is increasing to meet the increasing demand for protein, and the total area of aquaculture ponds worldwide was 5.4 million hectares in 2016. However, pond aquaculture causes serious environmental problems. Animals generally use only 20%~30% of the input nutrients in feed, and the majority of the remainder are dispersed in the aquaculture system, leading to an increasing waste load inside the aquaculture pond system. The conventional way to improve the aquaculture pond environment is by changing water, however, wastewater drainage is not in accordance with the water shortage situation in China. Additionally, aquaculture wastewater discharge affects the surrounding environment. It showed that China’s seawater ponds discharge 4.77×104 and 3.75×103 tons of nitrogen and phosphorus, respectively, into surrounding seas annually. According to our previous research results, it is estimated that China’s freshwater aquaculture ponds discharge 2.79×105 and 2.89×104 tons of nitrogen and phosphorus, respectively, into the surrounding waters annually.
To reduce the impact of aquaculture wastewater on the environment, our research team built a zero-water exchange aquaculture mode based on the polyculture system of grass carp (Ctenopharyngodon idella), bighead carp (Aristichthys nobilis), and crucian carp (Carassius carassius). At present, this mode has achieved zero-water exchange for four years, and the annual yield is 96 000 kg/hm2. However, the mechanism of zero-water exchange is unclear.
In aquaculture ponds, the organic carbon, nitrogen, and phosphorus contents can directly reflect changes in the aquaculture environment, and the budget of organic carbon, nitrogen, and phosphorus can reflect the nutrient accumulation and utilization efficiency of nutrients by aquaculture animals. Quantifying the budget of organic carbon, nitrogen, and phosphorus can aid the understanding of the system from the material cycle and energy flow, and is of considerable significance for the management and optimization of the system at the ecosystem level. Therefore, this study used the zero-water exchange pond of grass, bighead, and crucian carp as the experimental group, and a common pond for grass, bighead, and crucian carp as the control group. The carbon, nitrogen, and phosphorus budget characteristics of these two groups were studied to provide a theoretical reference for optimizing the zero-water exchange culture mode. This study was conducted between January, 2017 and November, 2018. The results showed that feed was the main source of organic carbon, nitrogen, and phosphorus in the experimental and control groups, contributing 77.06%, 92.08%, and 94.18% in the experimental group, and 81.00%, 92.77%, and 95.63% in the control group, respectively. The main output of organic carbon, nitrogen, and phosphorus was sediment accumulation, which accounted for 43.32%, 61.40%, and 78.71% of the input nutrients in the experimental group and 22.10%, 52.82%, and 79.58% of that in the control group, respectively. Harvesting the fish in the experimental and control groups accounted for 10.08% and 13.05% of input carbon, 21.00% and 25.57% of input nitrogen, and 15.41% and 18.60% of input phosphorus, respectively. The water accumulation amount and accumulation rates of carbon, nitrogen, and phosphorus in the zero-water exchange ponds were significantly lower than those in the common pond (P<0.05), and the water accumulation rates decreased by 92.91%, 88.52%, and 87.12%, respectively. The sediment accumulation of nitrogen and phosphorus in the zero-water exchange pond was significantly higher than that in the common pond; however, the sediment accumulation rates of carbon and nitrogen in the zero-water exchange pond were significantly lower than those in the common pond (P<0.05), decreasing by 48.99% and 13.97%, respectively. The carbon, nitrogen, and phosphorus utilization rates of the zero-water exchange pond were significantly higher than those of the normal pond (P<0.05), increasing by 29.49%, 21.72%, and 20.65%, respectively. These results indicate that the zero-water exchange mode can effectively reduce nutrient accumulation and improve the material utilization rate, representing a green and efficient aquaculture mode. Considering the nutrient emissions of Chinese aquaculture, the zero-water exchange mode has a good application potential. |
