| As an inland sea of China,the Bohai Sea is under great environmental pressure and has frequent algal blooms.Chlorophyll-a concentration is the main index of marine primary productivity.Studying the relationship between chlorophyll-a concentration and multiple environmental factors can provide support theoretical for improving the environment of the Bohai Sea.In this thesis,the DINEOF method is used to the fields of reconstruct chlorophylla concentration,sea surface temperature,photosynthetically available radiation,aerosol optical thickness and wind speed from 2003 to 2018.Then,temporal and spatial distribution of chlorophyll-a concentration and environmental factors are obtained.Pearson correlation is used to calculate the seasonal and spatial correlations between chlorophyll-a concentration and environmental factors,and the empirical mode decomposition is used to obtain the periodic characteristics.Since the previous correlation between chlorophyll-a concentration and environmental factors is mostly calculated in time domain,in this thesis the correlation is analyzed in frequency domain by Fourier transform and cross spectrum analysis.Finally,the turbulent kinetic energy in the Bohai Sea area is calculated using Delft3 D hydrodynamic model,and the relationship between turbulent kinetic energy and chlorophyll-a concentration is discussed.The main results are as follows:The annual and seasonal average chlorophyll-a concentration in the nearshore were higher than that in the offshore.The concentration of chlorophyll-a had seasonal diff characters.In spring,chlorophyll-a concentration was the highest.The overall concentration of chlorophyll-a in summer was lower than that in spring.However,the range of high concentration near the shore was expanded.The concentration of chlorophyll-a in autumn was lower than that in summer,and it was the lowest in winter while there were still high values near the coast of Laizhou Bay and Qinhuangdao.From the perspective of interannual variation,the concentration of chlorophyll-a showed a fluctuating upward trend from 2003 to 2011,and a fluctuating downward trend from2011 to 2018.Spatial correlations between sea surface temperature,photosynthetically active radiation and chlorophyll-a concentration were positively correlated nearshore and negatively correlated offshore.Aerosol optical thickness was positively correlated with chlorophyll-a concentration.In autumn and winter,photosynthetically available radiation and sea surface temperature were significantly correlated with chlorophyll-a concentration.The wind speed was negatively correlated near the coast,and positively correlated in most of the central Bohai Sea.The turbulent kinetic energy and chlorophyll-a concentration were mainly negatively correlated in the coastal waters,and positively correlated in the central Bohai Sea.The correlation coefficients between the factors obtained by Fourier transform in the frequency domain were significantly higher than those calculated directly in the time domain,which showed that the correlation analysis performed directly in the time domain possibly underestimated the correlation between the factors due to the phase difference.The results of empirical mode decomposition and Fourier transform spectrum showed that chlorophyll-a concentration had one-year,half-year,four months and seasonal cycles.There were obvious one-year and half-year cycles in wind speed and aerosol optical thickness.The sea surface temperature and photosynthetically available radiation had an obvious one-year cycle,and the half-year cycle was less obvious than the other factors.The results of cross spectrum analysis showed that chlorophyll-a concentration,selected environmental factors and turbulent kinetic energy had the largest cross amplitude in one-year cycle,indicating that the response was the largest at this frequency;Wind speed,aerosol optical thickness,photosynthetically active radiation and turbulent kinetic energy were about 5.0 months,0.2 months,0.2 months and 5.0months ahead of chlorophyll-a concentration respectively,and the sea surface temperature lagged by 1.9 months.The environmental factors were interrelated and jointly affected the temporal distribution of chlorophyll-a concentration. |
PDF Full Text Request