Research On Temporal And Spatial Distribution And Influence Factors Of NPP In Bohai Sea Based On MODIS Data From 2003 To 2015

Organic matter fixed through photosynthesis of phytoplankton in the ocean,subtracts its own consumption of phytoplankton,used for phytoplankton growth and reproduction,is known as net primary productivity(NPP).Half of the photosynthesis of green plants in the world comes from marine phytoplankton,although marine phytoplankton biomass is less than 1% of the global biomass,carbon sequestration accounts for 40% of the globe.Therefore,net primary productivity of oceans is essential for the study of global carbon flux changes.Using MODIS NPP product data,sea surface temperature(SST),PAR and chlorophyll contents data.the temporal and spatial distribution characteristics of NPP,SST,PAR and chlorophyll contents are calculated from 2003 to 2015 in Bohai Sea,and the relationships between NPP and impact factors are analyzed.The main conclusions are as follows:(1)The annual NPP values of Bohai Sea have occasionally dropped but showed a significant increase trend since 2003.NPP reached the maximum value of 2657.08 mgC/(m2·d)in 2015,and the lowest NPP value appeared in 2003,just 2053.40 mgC/(m2·d).The monthly average values of NPP changed greatly in Bohai sea,and NPP began to increase from January,reaching the maximum in August 5205.91 mgC/(m2·d).(2)From 2003 to 2015,the spatial distribution of NPP in Bohai Sea showed the trend of NPP in inner bay higher than that in the outer sea,and NPP values gradually decreased from the coast to the far coast.Overall,there were four obvious high NPP value areas in Bohai Sea,namely,Liaodong Bay,Laizhou Bay and adjacent areas of Qinhuangdao.The average values of NPP in Laizhou Bay were obviously higher than those of other three areas,and the mean of NPP in Bohai Strait was the lowest.(3)Using linear regression method to calculate the annual NPP change rate(Kslope)from 2003 to 2015 in Bohai Sea,then the significance of change trend of NPP was tested through F test.The results showed that the area of NPP increase accounted for 61.89% of the total area,including Liaodong Bay,Qinhuangdao adjacent waters,Laizhou Bay,Bohai Bay and coastal areas in central Bohai Sea.Qinhuangdao Waters had the most obvious NPP growth trend.This may be related to land pollution,human activities and coastal development.The area of NPP decrease accounted for 38.11% of the total area,including Bohai Bay,Laizhou Bay and Bohai Strait.The results of F test showed that in 13 years,the sea area of significant NPP increase accounted for 51.49%,the sea area of significant NPP decrease accounted for 31.12%.(4)The spatial distribution of NPP in Bohai Sea from 2003 to 2015 was mainly influenced by latitude.Sea surface temperature(SST)decreased with the increase of latitude,the SST of Laizhou Bay was the highest,the second was Bohai Bay,the lowest was Liaodong Bay.The highest monthly average value of SST appeared in August,at 25.32?.The lowest SST value appeared in February,at 1.51?.The spatial distribution of PAR was far lower than that of the coast,the PAR in the middle of Bohai Sea is higher,the minimum value was located in the north of Liaodong Bay coastal waters.The monthly average PAR reached the maximum of 49.02 Ein/(m2·d)in June,the minimum of 15.37 Ein/(m2·d)appeared in December.The spatial distribution of chlorophyll concentration showed a decreasing trend from the near coast to the far coast,in which the chlorophyll concentration in Laizhou Bay and Qinhuangdao sea area was the highest,and the lowest was in Bohai strait.The monthly average chlorophyll concentration value was the highest in August,which was 6.11mg/m3.The lowest 4.19mg/m3 was in November.(5)R~2 of the annual NPP and SST,PAR,chlorophyll concentration in summer were 0.5182,0.624 and 0.1948,respectively.Among them,the correlations between NPP with SST and PAR were higher.When SST and PAR values are higher,it is beneficial to the growth of marine phytoplankton,so NPP reaches the peak.(6)NPP in Bohai sea from 2003 to 2015 was positively correlated with SST with low correlation.NPP was mainly negatively correlated with PAR with low correlation.Among them,NPP was moderately related with PAR with medium correlation only by the waters near Qinhuangdao,Huludao and Bohai Strait.NPP in most areas of Bohai Sea was moderately correlated and positively correlated with chlorophyll content,highly positive correlated only in Qinhuangdao sea.In addition,the correlations between NPP and SST,PAR and chlorophyll content in some areas were complementary and opposite.