Study On Phytoplankton Phenological Changes In Major Fishing Grounds At Home And Abroad Caused By Rising Sea Surface Temperature

Since the industrial revolution,the use of fossil energy to improve social productivity has brought mankind into an unprecedented era of rapid development.However,the rapid development has also brought a huge impact on our ecological environment.According to the fifth assessment report(AR5)of the Intergovernmental Panel on Climate Change(IPCC),a large number of greenhouse gas emissions have increased the global surface temperature by 0.85℃in the past 130 years,and will continue to rise at a rate of about 0.2℃every decade in the future.The oceans,as the largest whole,accounting for 71%of the earth’s surface area,not only play an important role in regulating the global ecosystem and climate system,but also are significantly affected by climate change.Phytoplankton,as the most important producer in the marine ecosystem,can respond well to the impact of climate change on the marine ecosystem,so it is usually used as an indicator to characterize the marine climate change.This paper synthesized the sea surface temperature data and ocean water color data from 1998 to 2020,and statistically analyzed the distribution characteristics and change trend of phytoplankton chlorophyll in the sea surface temperature of the four major fishing grounds at home and abroad(Zhoushan Fishing Ground,Hokkaido Fishing Ground,Beihai Fishing Ground,Peru Fishing Ground).The influence of sea surface temperature change on phytoplankton phenology in various fishing grounds was analyzed.It lays a foundation for us to further understand the relationship between global warming and marine ecosystems.The main research contents and conclusions of this paper are as follows:(1)By analyzing the trend of sea surface temperature changes in various fishing grounds,it was found that there were varying degrees of changes in sea surface temperature in all four fishing grounds.During the time range of 1998 to 2020,the sea surface temperature changes of Zhoushan Fishing Ground,Hokkaido Fishing Ground,Beihai Fishing Ground,and Peru Fishing Ground were+0.39℃,+0.18℃,+0.46℃,and-0.11℃,respectively.During this time range,the global average sea surface temperature of the ocean has changed by about+0.20℃.Relatively speaking,the sea surface temperature of the Zhoushan fishing ground near China and the North Sea fishing ground in Europe have shown a significant upward trend,far exceeding the global average.The sea surface temperature changes in the equatorial eastern Pacific Ocean where Peru’s fishing grounds are located are relatively abnormal,and the sea areas where sea surface temperature decreases are mainly located in the waters flowing through by the Peruvian cold current.(2)Through the analysis of the occurrence time of phytoplankton algal blooms and the monthly average change of the sea surface chlorophyll concentration in various fishing grounds and nearby waters,it was found that the algal blooms in Zhoushan fishing ground,Hokkaido fishing ground and Beihai fishing ground in the northern hemisphere basically occurred from March to May,that is,the spring algal blooms in the middle latitude,and there were autumn algal blooms with lower intensity in September to November.The Peruvian fishing ground is located in the low-latitude waters of the southern hemisphere,and its algal bloom outbreak time is basically from September to April of the next year,which is significantly different from the mid-latitude waters of the northern hemisphere.(3)Based on the daily sea surface chlorophyll concentration data from 1998 to 2020,the change trend of chlorophyll concentration in the sea area of fishing grounds is calculated,and it is found that the change trend of sea surface chlorophyll concentration in different fishing grounds is quite different.According to the change trend and distribution of chlorophyll concentration in various fishing grounds,the sea area with high chlorophyll value showed a downward trend,while the sea area with low chlorophyll value showed an upward trend.Among them,the concentration of chlorophyll in the inshore waters of Zhoushan Fishery is decreasing,while the concentration of chlorophyll in the outer waters is increasing.Zhoushan Fishery is in the transitional sea area between the two.The concentration of chlorophyll in the whole sea area has decreased by 0.090mg/m~3;The concentration of chlorophyll at sea surface in most of the sea areas of Hokkaido fishing ground showed an upward trend,and the concentration of chlorophyll increased by 0.057mg/m~3for the whole fishing ground;The concentration of sea surface chlorophyll in the coastal waters of the North Sea fishing ground showed a downward trend,while the concentration of sea surface chlorophyll in the ocean waters showed an upward trend.Taking the whole sea area into consideration,the concentration of sea surface chlorophyll increased by 0.073mg/m~3;In the Peruvian fishing grounds,only part of the sea area with the decrease of chlorophyll concentration at sea surface along the Peruvian coast showed an upward trend,and the chlorophyll concentration in the whole sea area increased by 0.01mg/m~3.(4)By defining the algal bloom outbreak value determined by the threshold method,the change trend of the annual algal bloom duration(the number of days in a year when the chlorophyll value is greater than the outbreak threshold)of each fishery was analyzed.Among them,the annual duration of algal blooms in Zhoushan fishery and Peru fishery is increasing,with the rising trend of+0.76 days/year and+0.35 days/year respectively;The annual algal bloom duration of Hokkaido fishery and Hokkaido fishery decreased,with a downward trend of-2.8 days/year and-0.72 days/year.Determine the time of algal bloom outbreak each year by defining the threshold method,and analyze the time distribution of the first algal bloom outbreak in each fishery each year.It was found that although the time of the first outbreak of algal blooms in each of the four fishing grounds was different,they all showed a consistent downward trend within the statistical time range of 1998-2020,that is,the time of the first outbreak of algal blooms each year had an early trend.The advance trend of Zhoushan fishery,Hokkaido fishery,Beihai fishery and Peru fishery is 0.46 days/year,0.60 days/year,0.62 days/year and0.51 days/year respectively.In the upper layers of the ocean,primary producers mainly composed of phytoplankton provide about 50%of the global primary productivity,making the ocean a huge carbon sink and playing an important regulatory role in global climate change.Therefore,it is crucial to have a correct understanding of the changing trends of phytoplankton phenology under global warming.