| People now are taking more and more attention on diet structures because of the improvement of living standards.With high nutritional values,aquatic products like fish has been an important part of balanced diets.In 2015,the per capita consumption of aquatic products in urban and rural areas of China reached 14.7 kg and 7.2 kg,2.3 times and 10.3 times of that in 1979,respectively.Consumption demand has been the continuous driving force for the development of China’s aquaculture,making China be the top-ranking producer and exporting country in terms of the quantity of aquatic products.Such growth of aquaculture means the great increase of inputs and more waste accumulated in environment.Phosphorus,the essential nutrient element of aquatic organism,enters into aquaculture system in the form of feeds and fertilizers,and then accumulates in water and bottom sediment in the form of urine,excretion of aquatic products and unconsumed feeds.However,excess phosphorus dissolved in water has caused eutrophication which has negative effects on aquaculture in turn,and more and more solid phosphorus accumulated in bottom sediment has made the available parts reduce dramatically.Doubtlessly,phosphorus cycle has been significantly changed by aquaculture.It is essential to make clear that how phosphorus flows within China’s aquaculture system,to understand temporal and spatial phosphorus patterns,and then optimize phosphorus flows of this system.Based on the theory of substance flow analysis(SFA),this research established a process-mass-balance model to quantify the phosphorus pathway of freshwater aquaculture and mariculture,and then the model was used to examine the phosphorus flows in Mainland China from 1979 to 2013.The results were shown as following:(1)From the perspective of temporal changes,the inputs and outputs of China’ s aquaculture wavelike increased,which are drove by aquatic yields.Most phosphorus enters into the aquaculture system as the form of artificial feeds and fertilizers,and it turned into phosphorus-containing waste which is the main output form.In freshwater aquaculture,the amount of artificial feeds and fertilizers reached 452 Gg-P and 460.6 Gg-P in 2013,23.8 times and 3.5 times of 1979.The output of phosphorus-containing waste increased to 775.2 Gg-P in 2013,and the phosphorus-containing sediment produced in the process of pond culture was the biggest part,reaching 445 Gg-P in 2013,5.4 times more than that in 1979.In mariculture,the amount of artificial feeds increased from 0.7 Gg-P in 1979 to 117 Gg-P in 2013.The output of phosphorus-containing waste of this system increased by 172.2 times during past 35 years.To be specific,the amount of dissolved phosphorus-containing waste increased from 0.2 Gg-P in 1979 to 37.9 GgP in 2013,and the solid phosphorus-containing waste increased from 0.4 Gg-P to 66 Gg-P.Compared with the freshwater aquaculture,the quantity of phosphorus flows were much less in mariculture,while the growth rate of which were much bigger.(2)From the perspective of spatial changes,the inputs and outputs of China’s aquaculture system presented progressive increased from the western regions to the eastern regions of Mainland China,while the geographic distribution of the annual percent change showed the opposite trend.In freshwater aquaculture,the culture areas,like Jiangsu,Guangdong,and Hubei,located in the eastern coastal area or Yangtze River Basin,shared top input of artificial feeds and fertilizers which reached 47.4 GgP to 105.9 Gg-P and top output of phosphorus-containing waste which reached 63.6 Gg-P to 111.4 Gg-P in 2013.The culture areas,like Shanxi,Ningxia,Qinghai,Xinjiang,and Xizang,located in the western or the central of Mainland China,shared the biggest annual percent change of phosphorus flows.However,there was no obvious temporal trend of annual percent change.In mariculture,the phosphorus flows of the southern culture areas were bigger than that of the northern areas,and Guangdong was the topranking area.(3)From perspective of phosphorus use efficiency and phosphorus accumulation,both of them were increasing rapidly.The phosphorus use efficiency in freshwater aquaculture had a straight line growth trend,increasing from 2.4%in 1979 to 15.0%in 2013.While the efficiency in mariculture stabilized at around 11.0%.Over the past 35 years,there were 20626.6 Gg-P and 1127.0 Gg-P phosphorus-containing waste entering into environment because of freshwater aquaculture and mariculture,respectively.The phosphorus accumulation in mariculture had a J growth curve,even which is much less than freshwater aquaculture’s.Based on the results of this research,we provided some suggestions in terms of source control and end of pipe control,which include:(1)accounting carrying capacity of culture areas to avoid the negative effect on environment caused by the blind increase of aquatic yields;(2)improving the artificial feed formulation to decrease the feed conversion ratio and increasing the input of bycatch which could be the substitute for artificial feeds;(3)decreasing the use of organic fertilizers in pond culture,and strengthening supervision on the aquatic growth and water quality;(4)exploring the treatment methods of phosphorus-containing waste,improving the construction of waste disposal facilities,and making corresponding discharge standards;(5)encouraging the change of the culture manners to recirculating aquaculture system. |
