Analysis Of The Distribution Characteristics Of Green Tide In The Yellow Sea And Its Environmental Impact

Large-scale green tides caused by Ulva prolifera have been recurrent in the Yellow sea of China since 2008,causing deleterious effects on the Yellow sea environment and economy of coastal cities.Learning the distribution characteristics of green tide can help us understand its development and evolution process,as well as carry out green tide prediction.The growth and decay process of green tide will change the environment of water,study these process can provide suggestions for the disaster assessment of green tide.In this study,an ecological model(LTRANS-GT)was established base on the LTRANS model,it can predict the growth and decay process of green tide in response to variations in environmental factors such as temperature,light and nutrients.From the simulation results from 2008 to 2017,the drift path of Yellow Sea green tide in different years were analyzed,and the main controlling factors of growth and decay process were discussed,and the impact of the green tide on the nutrient budget of the Yellow Sea was evaluated.The following conclusions were drawn:The drift paths of the green tide exists annual differences,which can be divided into three types according to the initial drift direction: the first type drifts northwestward at the beginning of the outbreak,and then drifts parallel to the shore in the northeast direction,then affect the coastal sea of Shandong at the end of June;In the second type,the main body is more towards to east compared with the other two types at the beginning,then it drifts to the northwest,and then the drift direction is mainly parallel to the shore in the northeast.The hole process has less impact on the sea near Shandong.The third type drifts northward at the beginning of the outbreak,then drifts northeastward parallel to the shore,and reach the sea near Shandong in mid-July.In the growth stage,suitable light provides the necessary conditions for the Ulva prolifera,and the temperature is in the range which is appropriate for the growth of Ulva prolifera,but the nutrient concentration where the Ulva prolifera cover fluctuates greatly,the difference of the nutrient concentration is the main factor which decide the green tide biomass,and the difference of the nutrient concentration near the Ulva prolifera is caused by the annual difference between the wind field and surface flow.In the decay stage,there is no significant change in light intensity around the Ulva prolifera,and the increase of temperature in the sea where the Ulva prolifera is located reduce the growth rate and increase the death rate of the Ulva prolifera,while the nitrogen and phosphorus nutrient concentration is also at low values,and the significant increase of temperature and the decrease of nutrient concentration together affect the decay of the green tide.The seasonal variation of green tide biomass showed double-peaked characteristic,which was caused by the short-term extreme weather phenomenon in the sea near the green tide,resulting in a temporary decrease of temperature and a temporary increase of nutrient concentration in the nearby sea.The annual variation in the maximum biomass of the green tide is mainly due to two factors: the annual difference in the initial biomass of the green tide,and the annual difference in the nutrient concentration in the area where the green tide is located,and the difference of the nutrient concentration was caused by the annual difference in the surface wind and current fields.The impact of green tide outbreak in the annual nitrogen and phosphorus nutrient budget in the Yellow Sea is small,but the impact on the sea area where green tide covers cannot be ignored.The decay of Ulva prolifera increased the concentration of DON and DOP in the nearby sea by about 11μmol/L and 0.13μmol/L,which nearly doubled the concentration,thus changing the original nutrient structure of the nearby sea.