Distribution Of Grazing Pressure By Zooplankton And Their Top-down Controls On Major Diazotrophic Groups In The Global Ocean

Zooplankton are important primary production consumers in marine food webs,and the degree of structural changes and biomass reduction of phytoplankton caused by zooplankton grazing is called zooplankton grazing pressure.Usually,zooplankton grazing pressure is represented by the ratio of zooplankton graze to phytoplankton chlorophyll(or primary productivity)based on zooplankton predation experiment.However,this kind of experimental data too less to reflect the change of grazing pressure by zooplankton in different nutrient gradient areas.Therefore,this study based on the global copepod biomass historical observation database and satellite Chlorophyll-a data,deduced the global marine zooplankton grazing pressure distribution by establishing the analysis framework of grazing pressure.In addition,the framework can also deduce the grazing pressure on zooplankton when zooplankton is preyed by superior predators(grazing pressure on zooplankton for short).It is found that the grazing pressure by zooplankton is low in nearshore and high in ocean.Data from various oceans show that the grazing pressure by zooplankton in the Arctic Ocean is the lowest.The grazing pressure on zooplankton is low in summer and high in winter in all oceans except the Southern Ocean.According to the correlation analysis between environmental factors and grazing pressure,we found that chlorophyll and dissolved oxygen are strongly negatively correlated with zooplankton grazing pressure,showing that the grazing pressure by zooplankton decreased with the increase of primary productivity.Nutrients and zooplankton grazing pressure show a non-linear correlation,and the trend of fitting is to decrease at first and then increase.It is speculated that the increase of grazing pressure by zooplankton in the later period is due to the excessive nutrient concentration inhibiting the growth of phytoplankton,and the grazing pressure by zooplankton in eutrophic regions is still lower than that in oligotrophic regions.Temperature and the grazing pressure on zooplankton have a complex non-linear relationship,which is presumed to be caused by temperature simultaneously affecting the life activities of phytoplankton and zooplankton.As a key functional group in marine planktonic ecosystem,known as diazotrophs,the bacteria fixing N2,can be affected by ’Bottom-up control’ dominated by environmental factors and Top-down control’ controlled by zooplankton grazing.The predation degree of diazotrophs on zooplankton determines the efficiency of nitrogen recycling and total primary productivity,and may also affect the distribution of nitrogen fixation in the ocean.The research objects of this study are Trichodesmium,unicellular nitrogen fixing cyanobacteria(UCYN)and diatom-diazotroph associations(DDAs).Based on the framework model of the relationship between biomass and productivity of prey,the linear regression relationship between log converted biomass(cell number or copy number of nifH gene copy)and nitrogen fixation rate was investigated to determine whether the diazotrophic groups in the world are mainly controlled by the Bottom-up control by resources or the Top-down control by zooplankton predation.The initial results of the study showed that Trichodesmium and UCYN are under weak Topdown control,and DDAs is basically not affected by Top-down control.In the past,scientists tended to think that Trichodesmium is difficult to be preyed on,but this study found that Trichodesmium is controlled by certain predation and shows certain temporal and spatial changes.For example,further analysis of data from the North Atlantic revealed that Trichodesmium is more predatory in the east of this area than in the west.This study also found that DDAs are the weakest predation.It is speculated that the main reason are the short time from formation to death of diatom blooms and the siliceous shell,which leads to its rapid sedimentation,indicating that DDAs may be difficult to support regenerative productivity.Finally,this study collected and updated observational data of nitrogen fixation rates based on the historical nitrogen fixation database,used environmental parameters and zooplankton grazing pressure as predictors to estimate the global ocean nitrogen fixation rate by machine learning.In conclusion,the trend of global ocean grazing pressure by zooplankton and that on zooplankton can be demonstrated,but environmental factors such as light and nutrients can not fully explain the law of grazing pressure,In the future,more exploration is needed in conjunction with experiments.In addition,the three main groups of diazotrophic groups are subject to varying degrees of Top-down control,and the relationship between Trichodesmium and zooplankton predation has shown a closer relationship,which points out that it is necessary for us to further study the control mechanism of marine nitrogen fixation at a higher system level.