Study On Distribution Characteristics Of Chlorophyll-a In Bohai Sea Based On MODIS Data

As an inland sea of China,the Bohai Sea faces great environmental pressures and frequent phytoplankton blooms.Chlorophyll-a is an important indicator of phytoplankton and can reflect the distribution characteristics of marine productivity.Studying on the temporal and spatial characteristics of chlorophyll-a is helpful for further understanding on the characteristics of marine ecological environment.In this thesis,the general situation of the study area,the improved atmospheric correction algorithm,the EOF and the data source were introduced firstly.Then,the chlorophyll-a concentration was obtained from the inversion of MODIS remote sensing data from 2003 to 2017.The characteristics of spatial,interannual and quarterly distribution characteristics were analyzed.The residual current field and the flow intensity were simulated numerically.By analyzing the cumulative variance of chlorophyll-a concentrations,the onset were obtained.Characteristic variables were calculated,including times,blooming duration and the increasing rate and magnitude of chlorophyll-a concentrations during blooms.Spatial and interannual distributions in the characteristic variables were analyzed by using the EOF method.Finally,the response of characteristic variables to the wind mixing was studied.The main conclusions are as follows:The concentration of chlorophyll-a in the nearshore area was higher than that in the central sea.The high value was concentrated in the coastal waters of Bohai Bay,Liaodong Bay and Laizhou Bay,while the low value was concentrated in the central Bohai Sea and the Bohai Strait.The concentration of chlorophyll-a in spring near the coastal waters reached the highest in the year.The chlorophyll-a concentration in summer decreased at the surface layer,while it remained high.The chlorophyll-a concentration increased in autumn compared with that in summer,and in the central sea it was higher than that in spring and summer.The chlorophyll-a concentration in winter is generally high.The interannual variation of the spatial distribution characteristics of chlorophyll-a concentration changed little from 2004 to 2009.In 2010,the chlorophylla concentration decreased,while in 2011,it gradually increased.In 2017,the chlorophyll-a concentration was relatively high.There is no obvious linear relationship between chlorophyll-a and flow field intensity.However,other relationships cannot be excluded.By analyzing cumulative variance of chlorophyll-a concentrations,the onset of phytoplankton blooms in the Bohai Sea were obtained.According to the duration of the phytoplankton bloom,the threshold is set to 10%.The onset of phytoplankton blooms in the coastal area was relatively late compared to that in the central sea.The increases rate of chlorophyll-a concentration gradually increased from the nearshore area to the central sea,and was higher in spring than that in summer and autumn.The magnitude of chlorophyll-a concentration in the northern area was lower than that in the southern area.The duration in spring is longer than that in summer and autumn,from 2011 to2017.The above spatial distribution characteristics of phytoplankton blooms in spring are more obvious in 2007,2010 and 2017 than in other years.The above spatial distribution characteristics of phytoplankton blooms in summer and autumn are more obvious in 2004,2007 and 2012 than in other years.The wind mixing in spring is larger than that in summer and autumn.The wind mixing has a strong correlation with the onset.The stronger the wind mixing in spring,the earlier the onset,the stronger the wind mixing in summer and autumn,the later the onset.Regardless of spring or summer and autumn,the enhancement of wind mixing intensity promoted the increase rate of chlorophyll-a concentration in the Bohai Sea to some extent.Wind mixing is uncorrelated with the magnitude of chlorophyll-a.