| Dissolved oxygen is an indispensable element for the survival of marine organisms.In recent years,due to the intensified influence of human activities,the nutritional load of the Bohai Sea has been increasing,red tides have been frequent,and the ecological environment has been rapidly deteriorating.In summer,seasonal hypoxia occurs in the bottom layer,which is mainly distributed at the offshore of Qinhuangdao and the Yellow River Mouth.Since the low-oxygen situation is more serious at the offshore of Qinhuangdao and there are a large number of fishery farms in the coastal waters,the sea at the offshore of Qinhuangdao is selected as the research area.In order to improve the ecological environment at the offshore of Qinhuangdao,the seasonal variation of dissolved oxygen was studied by combining the observation data with the ecosystem dynamic model,and the formation mechanism of the low value zone of bottom oxygen was discussed.According to literature reports,the low-oxygen area at the offshore of Qinhuangdao is located in the middle of Liaodong Bay to the east of Qinhuangdao.Therefore,a section across the low-oxygen area at the offshore of Qinhuangdao was designed for continuous observation once a month from May to August in 2017,June to August in 2018 and June to August in 2019.In addition,the data of two stations in the low-oxygen zone were obtained during the Yellow Sea and Bohai Sea Shared Voyage of NSFC in September in 2017.Data analysis of 12 voyages in three consecutive years shows that weak stratification began to appear at the offshore of Qinhuangdao in May,and at this time,the dissolved oxygen concentration at the bottom layer is consistent,which is about 8-10 mg L-1 and the saturation is between 88%and 107%.In June,after the stable stratification of the water column,the concentration of dissolved oxygen in the bottom began to decrease,and the saturation decreased to less than 100%.In August,the concentration of dissolved oxygen in the bottom reached the lowest value,especially in August 2017,when the concentration of dissolved oxygen in the bottom decreased to 2-3 mg L-1,with a minimum of 2.3 mg L-1,and the saturation was only 28.5%.With the uniform mixing of water in September,the dissolved oxygen concentration in the bottom water gradually increased,indicating that the decreased of dissolved oxygen concentration at the offshore of Qinhuangdao is a seasonal phenomenon.The cause of this phenomenon is controlled by the physical biochemistry at the offshore of Qinhuangdao.Studies have shown that the decrease of solubility caused by temperature rise is the main factor leading to the decrease of dissolved oxygen in the surface layer.Water stratification is the prerequisite for the emergence of low-oxygen zone at the offshore of Qinhuangdao,and the strength of the stratification has no significant relationship with the concentration of the bottom dissolved oxygen.When the strength of the pycnocline,namely the square of the buoyancy frequency,reaches more than 10-3 s-2,the bottom dissolved oxygen begins to decline,and at the same time,the bottom water can maintain a low dissolved oxygen level under this pycnocline strength.Apparent oxygen consumption vertical distribution show that the surface of dissolved oxygen in supersaturated state,the dissolved oxygen in oxygen deficit condition.The correlation analysis showed that chlorophyll a and dissolved oxygen above the pycnocline and the dissolved oxygen and pH below the pycnocline were positively correlated,the correlation coefficient of 0.52 and 0.82,respectively(excluding special weather events).Thus,it was proved that oxygen supersaturation in surface water was mainly related to phytoplankton photosynthesis,and oxygen loss in bottom water was mainly caused by the decomposition and oxygen consumption of bottom organic matter,and the apparent oxygen consumption could reach more than 3 mg L-1.The box model was used to calculate the oxygen consumption rates of water and sediment below the pycnocline from June to August in 2017,2018 and 2019 as 975,834 and 708 mg m-2 d-1,respectively.The comprehensive analysis shows that if the growth of phytoplankton at the offshore of Qinhuangdao is not controlled,the oxygen consumption rate of the bottom layer may be further increased,which further aggravates the degree of hypoxia.Compared with other hypoxic areas near the shore,the oxygen consumption rate in the waters and sediments at the offshore of Qinhuangdao is lower than that in other hypoxic areas,which is also the reason why the reduction of dissolved oxygen at the offshore of Qinhuangdao is lower than that in other hypoxic areas near the shore.The change mechanism of dissolved oxygen at the offshore of Qinhuangdao was discussed by using the results of physical ecological coupling model(ROMS-CoSiNE)from May to September in 2017.The results show that the concentration of dissolved oxygen is mainly affected by temperature and the salinity has little effect on the concentration of dissolved oxygen.The results show that the average value of dissolved oxygen reduction caused by temperature rise from June to August is 97.22%(with a certain salinity)of the total dissolved oxygen reduction,and the average value of dissolved oxygen reduction caused by salinity rise is 0.02%(with a certain temperature)of the total dissolved oxygen reduction.In addition,the increase of nutrient in summer stimulates the growth of phytoplankton,whose photosynthesis can supplement the dissolved oxygen in surface water.At the same time,with the increase of phytoplankton,the bottom debris also increased.Correlation analysis showed that the accumulation rate of the two was positively correlated,with a correlation coefficient of 0.52,indicating that the phytoplankton proliferation led to the increase of bottom organic matter.There was a significant negative correlation between the apparent oxygen consumption of the bottom debris and the bottom with a correlation coefficient of-0.86,indicating that aerobic decomposition consumed a large amount of dissolved oxygen in the bottom water and was the main reason for the low value of the bottom oxygen.The increase of wind speed and sudden change of wind direction can enhance water mixing,but the influence depth is limited.In summary,the surface dissolved oxygen in the waters at the offshore of Qinhuangdao is mainly affected by temperature and phytoplankton photosynthesis.The dissolved oxygen in the bottom layer is mainly affected by aerobic respiration.In addition,the wind process will destroy the pycnocline,so that the dissolved oxygen in the bottom layer will rise up,then the pycnocline will recover quickly,and the dissolved oxygen in the bottom layer will continue to fall down. |
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