| Cage culture is one of the main forms of mariculture in the world in recent years.It has played a great role in making up for the shortage of world food production and alleviating the pressure of population growth on food demand,but it also inevitably has a negative impact on the marine environment.The impact of cage culture on the environment has been widely concerned.In order to explore the impact of cage culture on the surrounding water environment,28 stations were set up in the cage area,adjacent area and peripheral area of Maniao Bay in Chengmai County of Hainan Province in 2019 to conduct an annual survey of four quarters（including a culture cycle and non culture period）.Combined with the characteristics of cage culture activities in the study area,the seasonal changes and spatial distribution characteristics of water temperature,salinity,p H,DO,DIN,PO4-P,TN and TP in the water environment at different distances from the edge of the cage area were analyzed,and the effects of intensive cage culture on the spatial and temporal distribution characteristics of nutrients were studied.The single index evaluation method of seawater and potential eutrophication of water quality were used to evaluate the nutrient level of water body.At the same time,single factor pollution index method and ecological hazard index method were used to evaluate the pollution degree and potential ecological risk of heavy metals in surface sediments of Maniao Bay.The purpose of this survey is to evaluate the current situation of marine ecological environment quality in Maniao Bay from the aspects of marine environmental quality and heavy metal pollution in surface sediments,so as to provide scientific guidance for the follow-up sustainable development of cage culture activities.The main results are as follows:1.Assessment of the impact of cage culture on the physical and chemical environment of the water quality and the nutritional level of the water body in the Maniao BayIn 2019,the seasonal mean salinity of Maniao Bay is winter（33.24）>spring（31.10）>summer（30.85）>autumn（29.22）.The precipitation in summer and autumn is high,so the salinity is low.The change trend of water temperature is consistent with that of air temperature,and the seasonal size is summer（31.31℃）>autumn（29.22℃）>spring（23.66℃）>winter（22.48℃）.In the same season,there is little difference in water temperature between different study zones and water layers.Cage culture has little effect on salinity and temperature,but has obvious effect on p H and dissolved oxygen（Do）.In autumn（feeding peak）,the p H of cage area and bottom water was significantly lower than that of other areas and layers（P<0.05）.Do content only meets the first class seawater quality standard in spring,the second class in summer and winter,and the third class in autumn.Except in spring,DO content in cage area was significantly lower than that in adjacent area and peripheral area in other three seasons,and DO content in bottom layer was also significantly lower than that in surface layer and middle layer in autumn.The decomposition products of residual bait,feces and excreta in the cage area enter the water body,which makes the p H and DO of the water body decrease significantly.In 2019,DIN was the highest in spring(168.47μg·L-1),followed by autumn(118.85μg·L-1),winter(117.73μg·L-1),and summer(107.66μg·L-1).PO4-P was the highest in winter(7.89μg·L-1),spring(6.80μg·L-1)and autumn(6.65μg·L-1).The average content of TN was the highest in autumn(6.65μg·L-1),followed by spring(279.49μg·L-1),summer(245.35μμg·L-1)and winter(210.55μg·L-1).The average content of TP was the highest in autumn(13.55μg·L-1),winter(12.60μg·L-1)and spring(11.44μg·L-1),in summer(210.55μg·L-1)was the lowest.From the perspective of different regions,the content of DIN,PO4-P and TN in spring seawater has no significant difference among different regions.The content of TP in the outer area(13.15μg·L-1)is significantly higher than that in the cage area(10.43μg·L-1)and adjacent areas(10.35μg·L-1).There was no significant difference in the content of PO4-P and TN in seawater in summer.The DIN content in the cage area(68.97μg·L-1)was significantly lower than the adjacent area(107.76μg·L-1)and the peripheral area(130.75μg·L-1).The TP content in the cage area(12.36μg·L-1)was significantly higher than the adjacent area(9.34μg·L-1)and the peripheral area(10.29μg·L-1).The contents of DIN,PO4-P,and TP in the seawater in autumn were significantly higher in the cage area and the adjacent area than in the outer area,and there was no significant difference in the TN content among different regions.In winter,the DIN content of seawater in the outer area(144.04μg·L-1)and the adjacent area(107.60μg·L-1)was significantly higher than that in the cage area(94.14μg·L-1),and the PO4-P content was significant in the outer area(9.33μg·L-1).Higher than the adjacent area(6.92μg·L-1)and the cage area(7.44μg·L-1),the content of TN and TP in the cage area was significantly higher than that in the adjacent and peripheral areas.Summer and winter are respectively affected by Typhoon No.4"Muen"and tidal waves.In spring（non-cultivation period）DIN,PO4-P,and TN content have no significant differences among different regions,while in autumn（cage aquaculture investment）.In autumn（During the bait peak period）the contents of DIN,PO4-P,and TP in the seawater in the cage area and adjacent areas were significantly higher than those in the peripheral area,indicating that cage culture activities had a certain impact on the nutrient content of the studied sea area.From different levels,there is no significant difference in the content of DIN,PO4-P,and TN in seawater in spring and summer（P<0.05）.There was no significant difference between the layers（P<0.05）,and the TP content of seawater in spring and summer was significantly higher in the bottom layer than in the middle and bottom layers（P>0.05）.On the whole,there is almost no difference in nutrient content in different water layers in the study area.In the four seasons of 2019,the main form of inorganic nitrogen in the seawater of Maniao Bay is NO3-N.Maniao Bay is an open bay,so the water exchange conditions are better,the chemical purification ability of the water body is strong,and the final oxidation product NO3-N It is the main form of inorganic nitrogen,indicating that inorganic nitrogen has fully reached the thermodynamic equilibrium.To evaluate the overall eutrophication level of the waters of Maniao Bay in 2019,the nutrient level of the waters of Maniao Bay in 2019 is relatively low,all of which belong to the oligotrophic level.2.Heavy metal pollution status and risk assessment of surface sediments in the cage culture area of Maniao BayCompared with the background value of heavy metals in the surface sediments of Qiongzhou Strait,the multiple range of the ratio of the average content of each element to the background value in four quarters is 0.93～2.51,and the ranges of spring,summer,autumn and winter are 1.06～2.20,0.93～2.51,0.96～1.90 and 1.03～1.94,respectively.In spring and winter,all elements of all stations exceeded the background value,Zn,Cr and Pb were slightly lower than the background value in summer,Pb was slightly lower than the background value in autumn,and the average contents of other heavy metals were higher than the background value.According to the comparison with the background value,the accumulation of heavy metals in the surface sediments of Maniao Bay was obvious,and the heavy metals in the sediments had some potential pollution hazards.Compared with the marine sediment quality standard（GB 18668-2002）,only the average content of Cr in spring slightly exceeded the first class marine sediment standard,and the average content of other elements and heavy metals in other seasons met the first class marine sediment quality standard.Except in spring,the spatial variation coefficients of Cu and Zn were the largest in summer,autumn and winter,which were nearly 50%.The large spatial variation coefficients of heavy metals indicated that the spatial fluctuation was obvious.To a large extent,the fluctuation was caused by human activities.The main human activity in Maniao Bay was cage culture.The results of correlation analysis and principal component analysis show that there are two main sources of heavy metals in the surface sediments of the Maniao Bay in2019,which are the input of cage culture activities and the residues of agriculture（pesticides,fertilizers,etc.）.The average potential ecological hazard index of each season is spring（107.02）>summer（100.46）>winter（87.31）>autumn（79.73）.The annual average value is93.63,which are all less than 150（low level of ecological risk）.In 2019,the ecological risk of heavy metals in the surface sediments of Maniao Bay belongs to the low hazard level.Although the average content of Cd in the four quarters is the lowest,its ecological risk is the highest,so we need to pay attention to it. |
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