Spatial And Temporal Variability Of Chlorophyll-α Patterns And Its Mechanisms Off The Yangtze River Estuary

Chlorophyll-a (Chl-a) concentration has long been considered as a very important indicator for the biomass of phytoplankton and marine primary production. Exploring the variability of Chl-a concentration and its mechanism is a necessary aspect of understanding the responses of marine ecosystem to the climate change. Besides that, it’s also meaningful for the study of global carbon cycle and global biogeochemical cycles. In the present study, the data interpolating empirical orthogonal functions (DINEOF) methodology was employed for the reconstruction of MODIS Chl-a data sets which is ranged from October2002to September2013. The subsequent empirical orthogonal functions (EOF) analysis shows the long-term spatial and temporal trends of Chl-a. The second part of this paper focuses on the short-term concentration of Chl-a changes. Chl-a products were derived on the basis of a semi-empirical algorithm for Changjiang River Estuary before and after Typhoon Rananim (2004) and Typhoon Kompasu (2010). Remote sensing data including sea surface temperature, photosyntheticalry active radiation, sea surface height, mixed layer depth, wind and current field were used for the detailed analysis.The variance contribution for the first EOF mode is22.41%, the temporal and spatial variation of this mode shows that the current system in the East China Sea (ECS), coastal upwelling region and the Taiwan Warm Current are the main affecting factors for the Chl-a variability. The second and third EOF modes accounts for9.66%and6.91%of total variance, respectively. The second EOF mode is highly believed to be associated with the depletion and replenishment of surface nutrients during different season. The third EOF mode indicates a possible influence of wind field on the Changjiang River Diluted Water (CRDW) expansion and its seasonal diversion, as well as the coastal upwelling.Typhoon events were used for the analyzing of short-term Chl-a variation. After the passage of typhoon Rananim and Kompasu, the concentration of Chl-a varied in a very complicated way, which also indicates that the response mechanism to typhoon for open sea cannot explain all the changes in coastal water or nearshore area. It seems that after these two typhoon events, the rainfall and vertical mixing did benefit phytoplankton to some extent, however, the results proved that they’re not the main affecting factor. The variation of current system caused by typhoon is likely to be the key reason. In addition, typhoon with different tracks could lead to different influences for the current pattern in the ECS.Thus, Changjiang runoff, which is the primary nutrients source for the ECS, plays a very important role in the variability of Chl-a concentration on both long-and short-term scale.