Remote Sensing Inversion Of Net Primary Productivity In The Bohai Sea

Ocean Net Primary Productivity(NPP)is an essential manifestation of the nutrient index of the marine biological chain,and it is also the energy source for the marine ecosystem to maintain its operation.It has an important impact on the global carbon cycle.The Bohai Sea is a semi-enclosed inland sea in northern China with abundant marine resources.At the same time,there are many industrial ports around it.Industrial production and the discharge of domestic sewage have caused much pollution to the seawater and severe damage to the oceanic environment.Using remote sensing to monitor the Bohai Sea NPP will help people detect changes in the Bohai Sea environment in time,protect the Bohai Sea environment from various aspects,and promote the rational and practical use of ocean resources.Based on an in situ dataset and satellite data of the Bohai Sea,this paper used the Eppley-VGPM model to conduct a remote sensing retrieval study of the NPP in the Bohai Sea in recent years.In order to improve the accuracy and applicability of the Eppley-VGPM model,a remote sensing retrieval algorithm was established for the main input parameters of the model:transparency and chlorophyll-a(Chla)concentration.On this basis,the temporal and spatial distribution products of NPP in the Bohai Sea were generated,and the analysis of temporal and spatial variation characteristics and the research on the main environmental factors were carried out.The main research results are as follows:(1)In this study,the Bohai Sea transparency remote sensing retrieval model was established using the measured transparency and OLCI sensor equivalent remote sensing reflectance data in the Bohai Sea.The accuracy of the various retrieval models established in this paper was verified by using the satellite-ground synchronization data,and it was finally determined that the hybrid band transparency retrieval model with B6(560 nm)and B7(620nm)as the sensitive factors had the best effect,and the transparency of the retrieval model was the best.The determination coefficient(R~2)between the retrieval value and the measured value was 0.68,the mean relative error(MRE)was 15.93%,and the root mean square error(RMSE)was 0.48 m.Combined with satellite remote sensing data,the transparency products in the temporal and spatial scale distribution of the Bohai Sea were generated,and it was found that the overall variation range of the Bohai Sea transparency was 0 to 10 m.In summer,it was higher in July and August,and the highest regional transparency can be higher than 9 m;in winter,the transparency was lower,and the transparency of the whole sea area was less than 2m in January and February.In addition,the transparency of the Bohai Sea was characterized by a spatial distribution of low coastal waters and high offshore waters.The central Bohai Sea and Qinhuangdao coastal waters had relatively high transparency,while the Bohai Bay,Liaodong Bay,and Laizhou Bay had low transparency.(2)In this study,a remote sensing retrieval model of Chla concentration in the Bohai Sea was established based on OLCI data.Through the accuracy verification of the model,it was concluded that the R~2 between the retrieval value and the measured value of the model was 0.93,the RMSE was 1.23 mg/m~3,and the MRE was 35%.Combined with satellite remote sensing data,the remote sensing retrieval of Chla concentration in the Bohai Sea showed that the monthly average Chla concentration in the Bohai Sea varies from 2.4 to 5.9 mg/m~3,which was lower in summer and higher in winter.In terms of spatial distribution,the concentration of Chla was relatively high near the coast,showing a decreasing trend towards the central Bohai Sea;the Chla concentration along the coast of the Bohai Bay and Laizhou Bay was relatively higher and was relatively lower in the Bohai Sea Basin and the Bohai Strait.(3)Using the Eppley-VGPM model to perform remote sensing retrieval of the Bohai Sea NPP from 2018 to 2021 and generate a spatiotemporal distribution map of the Bohai Sea NPP over the years.The monthly average NPP variation in the Bohai Sea ranged from 45 to 1500mg C/m~2d,with an upward trend from February to June and a downward trend from August to January.The NPP was the lowest in January with an annual average of 61 mg C/m~2d and the highest in July with an annual average of 924 mg C/m~2d.The Bohai Sea NPP has the most significant growth rate in summer,and there is no significant change in other seasons.In summer,NPP in the Bohai Sea showed a decreasing trend from the coastal to the offshore,and the distribution trend was the opposite in autumn.(4)The main influencing factors of NPP were studied,and it was concluded that transparency had the most significant influence on the retrieval results of NPP in the Bohai Sea,followed by SST and Chla.In addition,it was also concluded that there are spatial differences in the relationship between different impact factors and NPP.In Bohai Bay,there was a negative correlation between Chla concentration and NPP and a positive correlation between SST and NPP.Whereas in the region of Qinhuangdao,the positive and negative correlations between each factor and NPP are reversed.According to the above results,it can be concluded that the change of NPP in the Bohai Sea was affected by the synergistic effect of various factors,and there were specific differences in the synergistic effect in different sea areas.