Bottom-up And Top-down Controls On The Global Marine Nitrogen Fixation

Nitrogen is the basic nutrient element for all organisms.In most regions of the global ocean,the availability and supply of nitrogen often limit productivity and accumulation of the plankton's biomass.Marine nitrogen fixation by marine cyanobacteria provides nearly half of the bioavailable nitrogen source to the global ocean,strongly affecting marine productivity and CO2 storage.Therefore,it is scientifically significant to study the control mechanisms for marine nitrogen fixation and forecast its response to the global change.Previous studies mainly focused on the"bottom-up control" of marine nitrogen fixation,i.e.,the control of marine nitrogen fixation by the environmental factors such as light,temperature,nutrients,etc.However,the "top-down control",the control of marine nitrogen fixation by the zooplankton predation,may also impact marine nitrogen fixation.In this thesis,I combine the bottom-up and the top-down controls,systematically illustrate the control mechanism of marine nitrogen fixation from a new perspective,aiming to improve the predictive ability for the change of marine nitrogen fixation in the future.Firstly,I test the relationship among the observed abundance of nitrogen-fixing cyanobacteria,nifH gene copies,nitrogen-fixing rates and the environmental factors and zooplankton abundance using univariate linear regression.Quadraticly,I use the structural equation model(SEM)to study the relative importance of bottom-up and top-down controls for marine nitrogen fixation and their interactions.Lastly,I establish an ideal mechanism model of marine nitrogen fixation to study the effects of predation on nitrogen fixation rate and the recycle efficiency of newly fixed nitrogen.The main findings are:1.Through univariate linear regression analysis,we find that the main environmental factors on the controls of marine nitrogen fixation are sea surface temperature,subsurface minimum dissolved oxygen,dust deposition,and light radiation.There are correlations between the zooplankton biomass and the abundance of nitrogen-fixing cyanobacteria,nifH gene copy numbers and the nitrogen fixation rates,but most of the correlations are weak or even statistically insignificant.It is still a complex problem on whether zooplankton predation can effectively controls marine nitrogen fixation.2.1 study the relative importance of bottom-up control and top-down control for marine nitrogen fixation using the structural equation model(SEM).Based on the available data,the analysis only shows that the mesozooplankton have influence on the abundance of trichodesmium in the surface ocean.But the effects predation on marine nitrogen fixation under other circumstances are not seen.The main control factors of the global marine nitrogen fixation are energy and nutrient(bottom-up control).However,due to the lack of available data and not able to include the abundance of microzooplankton and unicellular diazotrophs in the SEM model,this study cannot make an exclusive conclusion.3.I establish an ideal mechanism model for marine nitrogen fixation,and find that in an ecosystem with strong predation,newly fixed nitrogen is mostly recycled.The main contributors to the new primary productivity(and export productivity)and total primary productivity are non-nitrogen-fixing phytoplankton.Only the change in predation on non-nitrogen-fixing phytoplankton growth will greatly mediate the f-ratio,i.e.,the ratio of new productivity to total primary productivity.The change of the predation on diazotrophs only has little effect on f-ratio.In the ecosystem with strong predation,nutrients are mostly recycled in the ecosystem,so that the same amount of nitrogen fixation support more primary productivity,while in the ecosystem with weak predation the efficiency of nitrogen fixation in supporting primary productivity is low.The contribution of this thesis is that,the study of the bottom-up and top-down controls of marine nitrogen fixation can improve our understandings in how marine nitrogen fixation will respond to global change.Through contributing to other methods like numerical modeling,we can quantitatively predict how the marine nitrogen fixation will change in the future,and its consequences in biological and chemical changes.Nevertheless,this thesis is just a tentative trial,and the availability of observational data also limits its applicability.However,the prelimilary results show that it is possible that the predation from zooplankton can influence the global distribution of marine nitrogen fixation.In summary,this research preliminarily finds that energy and nutrients(bottom-up control)are the main mechanism for the global marine nitrogen fixation.The top-down control from zooplankton predation also affects the marine nitrogen fixation to a certain extent.However,due to the lack of observation data,it should be explored further with more field and laboratory data,particularly the abudance data of microzooplankton and unicellular diazotrophic bacteria.