Regime Shift Of Trophic Status And Nutrient Criteria In The Bohai Sea

Bohai Sea is a semi-closed inland sea of China,and its natural ecosystem plays an important supporting role in the economic and social development in the Bohai Sea Rim.In recent decades,with the rapid development of industry,agriculture and social economy,the eco-environment of the Bohai Sea has been seriously damaged.The main problems in the Bohai Sea are the deterioration of eutrophication and high frequent occurrence of red tide.In order to recover the ecosystem in the Bohai Sea,the action plan for the comprehensive treatment of the Bohai Sea has been put forward,in which clear requirements for the improvement of the eco-environment of the Bohai Sea.This thesis focuses on regime shift of trophic status and the deduction of nutrient criteria in the Bohai Sea.The research results are helpful to understand the status and evolution of the trophic status in the Bohai Sea and have certain guiding significance for prevention and control of eutrophication and red tide in the Bohai Sea.In this thesis,the historical data are used to analyze the historical trends and causes of nutrients and related ecological factors in the Bohai Sea;a retrospective evaluation of the eutrophication status of Bohai Sea in the past decades is carried out,and the regime shift of trophic status are identified by STARs model.The frequency analysis method was used to determine the nutrient criteria in the Laizhou Bay and the Bohai Bay.The main results are as follows:1.The annual average concentration of DIN in the surface water of Bohai Sea generally shows an "N" type trend,whereas the annual concentration of DIP shows an "M" type trend,and the concentration of DSi shows a "Inverted N" type trend in the past six decades.The trends of nutrient changes in the three bays and the central Bohai Sea are similar to that of the Bohai Sea,and it is mainly controlled by changes in the input fluxes from land sources and atmospheric deposition.Changes in nutrient-related ecological factors are closely related to changes in nutrient concentrations.2.The second-generation of eutrophication assessment method was used to evaluate the eutrophication status in the Bohai Sea since 1960s: the eutrophication status in the Bohai Sea was excellent before the 1980 s but it began to deteriorate gradually in the 1990 s with "Medium" eutrophication status during 1990-1995 and "poor" eutrophication status during 1995-2000 and it has been "bad" since the 2000 s.3.Using the STARs model to identify the regime shift of DIN,DIP,Chl a and red tide,the results show that the regime shift of trophic status in the Bohai Sea occurred during 1997-1999 and 2006-2010.4.Taking the Laizhou Bay and Bohai Bay as examples,the values of nutrient criteria were deduced.The nutrient criteria values for DIN,DIP,and Chl a were determined to be 14.90 ?mol/L,0.34 ?mol/L,and 1.23 mg/m3 in the Laizhou Bay,and 15.00 ?mol/L,0.40 ?mol/L,and 1.13 mg/m3 in the Bohai Bay,respectively.