Uptake Characteristics Of Phytoplankton Populations In Offshore Waters And Their Inversion Studies

Marine phytoplankton absorption coefficient plays an important role in the oceanic biological productivity and ecological environment dynamics.Accurate inversion of absorption coefficients of different phytoplankton taxonomic groups can enhance the understanding of phytoplankton communities,and lay the foundation to study many marine ecological and biogeochemical processes.Some studies have been carried out on the absorption of phytoplankton pigments and PSC(phytoplankton size class),while it is still challenging to understand the optical absorption variations associated with phytoplankton taxonomic groups in coastal water area.The phytoplankton can be decomposed into phytoplankton specific absorption coefficients and concentration corresponding.We obtain a_phof phytoplankton taxonomic groups in two aspects,by structuring the inversion model of specific absorption coefficients and remote sensing model of phytoplankton biomass,combined with the in situ dataset collected in BSYS(Bohai Sea and Yellow Sea)cruises,including summer in 201606,winter in 201606,spring in 2018,summer in 2018,and ECS(East China Sea)cruises,including autumn in 2016 and summer in 2017.The spatial distribution of absorption contribution and the vertical distribution of absorption for phytoplankton taxonomic groups were obtained by using the measured and vertical concentrations;the spatiotemporal products were manufactured combined with the remote sensing data of MODIS and Sea Wi FS,then the spatiotemporal distributions of different phytoplankton taxonomic groups were explored,and the correlations between absorption coefficients and environment factors were analyzed.The main research results are as follows:(1)Based on the CHEMTAX software,the phytoplankton pigment concentrations were transformed into the biomass of phytoplankton taxonomic groups,and the phytoplankton absorption coefficients data were combined to construct the phytoplankton specific absorption coefficients quantitative model.Derivative analysis and specific characteristics of groups in previous studies were used to extract Gaussian central wavelength information,optimized the numbers and positions of Gaussian function parameters to obtain the specific absorption spectra of phytoplankton taxonomic groups.The measured concentrations were used to reconstruct the total absorption coefficients.The determination coefficient of the whole band was above 0.75,and MAPE was below 30%in most bands.Compared with the specific absorption of phytoplankton taxonomic groups in the existing studies,the spectra obtained in this study are closer to the specific absorption spectra measured in the laboratory after the normalizations,laying a foundation for the acquisition of absorption coefficients for phytoplankton communities.(2)Based on the optimized Gaussian parameters,the phytoplankton absorption spectra were decomposed to obtain the Gaussian peaks at the corresponding central bands,and then the concentration inversion model of phytoplankton taxonomic groups was constructed.By analyzing the correlation between each Gaussian peak and the concentration,the Gaussian peak with the highest correlation was selected.Based on a variety of mathematical functions,the relationship model between Gaussian peaks and concentrations was constructed and studied,and the accuracy index(R²,ME)was used to determine the inversion accuracy of each mathematical function model,so as to propose the optimal concentration inversion model.It was found that the inversion accuracy of diatoms,chlorophytes,prasinophytes,cryptophytes and chrysophytes were all within the acceptable range,R²>0.6 ME<50%;the inversion effect of cyanobacteria,dinoflagellates and cyanobacteria is poor,MAPE is high and the error is large.Combined with the measured phytoplankton absorption coefficients and remote sensing reflectance data,the correlations between band combination of the remote sensing reflectance and absorption coefficients were analyzed,and the optimal band is selected to construct the absorption coefficients inversion model.(3)Combined with the measured spatial interpolation biomass of phytoplankton taxonomic groups,vertical biomass of phytoplankton taxonomic groups,MODIS and SEAWIFS satellite data,we obtained the spatial distribution of absorption contribution for phytoplankton taxonomic groups,vertical absorption and the long-term series changes of five groups in the coastal area during 23 years,respectively.Results showed:1)In terms of spatial distribution,the absorption distribution of each group was similar.The absorption value was higher in the nearshore area,while the absorption value was lower in the offshore area.The absorption value of chlorophytes was slightly higher than that of diatoms in the central waters of BSYS and ECS.2)In terms of time variation,the monthly average diatom absorption in the Bohai Sea had little variation.Weak peaks appeared in the North Yellow Sea and the central part of the South Yellow Sea,and double strong peaks appeared in the Yangtze River Estuary and its adjacent waters in spring and summer.The absorption of other algae was similar to that of diatoms,but the absorption of green algae in the North Yellow Sea and the central part of the South Yellow Sea was slightly higher than that of diatoms;The age change of absorption for taxonomic groups was stable.3)In different sections,the vertical absorption of phytoplankton taxonomic groups showed an obvious stratification trend,the absorption of the surface was higher than that of the bottom,and the absorption of the surface was high in the Jiangsu shoal and the Yangtze River estuary.4)Based on the data of various environmental state parameters,the correlation analysis shows that K_d(490)and SSS have a greater effect on the absorption of the population,SST has a greater inhibition on the absorption of the population in the coastal area of Bohai Sea and the Yangtze River Estuary,and PAR has a smaller effect on the absorption of the population as a whole.