Spatial And Temporal Variation And Influencing Factors Of Phytoplankton Biomass In The Yellow Sea Based On Wavelet Analysis

Phytoplankton biomass is one of the important indicators reflecting the eutrophication of marine ecosystem,and it is affected by various environmental factors,including light,temperature,nutrient,and zooplankton predation.In this study,chlorophyll a(Chl-a)was used as biomass indicator.We analyzed the spatio-temporal variations of Chl-a and influence of physical environmental parameters(Sea surface temperature,SST;Photosynthetically active radiation,PAR;Mix layer depth,MLD)and Nino 3.4 on it in the Yellow Sea using wavelet analysis.The phytoplankton phenological indexes were further used to study the occurrence of phytoplankton blooms in typical regions of the Yellow Sea.The response of phytoplankton growth to the changes of physical environmental parameters was systematically revealed.The main conclusions of this study are as follows:(1)Based on the reconstructed MODIS satellite Chl-a data,the spatial distribution of Chl-a and the seasonal and inter-annual variations of Chl-a and physical environmental parameters in five typical regions of the Yellow Sea from 2003 to 2018 were analyzed.Bivariate wavelet coherence(BWC)and Multiple wavelet coherence(MWC)were used to analyze the correlation between phytoplankton biomass and physical environmental parameters in each region.The results showed that the spatial distribution of Chl-a decreased from the coastal to the middle of the Yellow Sea,but the change rate of biomass showed an opposite trend from the coastal to the open sea.Due to the influence of MLD,Chl-a in the Yellow Sea had obvious inter-annual periodicity,and it was positively correlated with MLD,but negatively correlated with SST and PAR.PAR and MLD were the main factors affecting Chl-a in the Estuary of the Yangtze River(EYR)and the North Yellow Sea(NYS).SST and PAR were the main factors affecting Chl-a in the Jiangsu Shoal(JSS).In the Yellow Sea Cold Water Mass(YSCWM)and the Southern coast of Shandong Peninsula(SCSDP),the SST and MLD were the main factors affecting Chl-a.The best combination of three factors in each region included Nino 3.4,indicating that El Nino has a certain influence on the long-term changes of phytoplankton biomass in the Yellow Sea.(2)Spatial variations of Chl-a with the latitude and longitude in the Yellow Sea were analyzed by using the multi-satellited Chl-a product of OC-CCI(Ocean Color Climate Change Initiative)from 2003 to 2020.Furthermore,BWC and MWC were used to explore the heterogeneity of Chl-a with the latitude and longitude and their relationship with physical environmental parameters.The results showed that,Chl-a presented a low-high-low trend from high latitude to low latitude due to the influence of Yangtze River water and cold water mass,and the Chl-a decreased firstly and then increased along the longitude,which indicated that the coastal area was mainly affected by precipitation.The Chl-a was low in summer and high in winter,while the Chl-a was low in the open sea due to the influence of cold water mass or terrestrial water inflow.There was no obvious spatial heterogeneity of Chl-a with the latitude and longitude.The BWC and MWC showed that SST was the main single factor influencing the spatial heterogeneity of Chl-a with the latitude in spring,summer and winter,SST and MLD were the best bivariate combination,MLD was the main factor influencing the spatial heterogeneity of Chl-a in autumn,PAR and MLD were the best bivariate combination.SST and MLD were the best bivariate combination to explain the spatial heterogeneity of Chl-a with the longitude in spring,autumn and winter,while SST and PAR were the best combination in summer.(3)The phytoplankton phenological indexes were gained using the modified Chl-a in the northern and southern cold water mass,based on the improved zonal floating threshold method.The phytoplankton phenological changes were analyzed,and the formation mechanism of its spatial and temporal distribution was further explored using BWC and Partial wavelet coherency(PWC).The results showed that spring phytoplankton bloom(SPB)and fall phytoplankton bloom(FPB)occurred in most areas of the Yellow Sea every year,and the frequency of FPB was higher than that of SPB.Chl-a in two regions both showed significant seasonal cycles,especially in region SYSCWM.The initiation of the SPB and FPB in region NYSCWM was usually earlier than that in region SYSCWM.The ending time of the SPB was similar in two regions.While the FPB in region NYSCWM ended much later than that in region SYSCWM.Generally,there was a strongly positive correlation between the initiation time and PAR for the SPB in the Yellow Sea.Specifically,the SST was the main factor affecting the change of Chl-a in region NYSCWM,and the information of the phytoplankton bloom was mainly affected by large-scale climate events related to SST in this region.MLD was the main factor to explain the fluctuation of Chl-a in the SYSCWM.The influencing of MLD on Chl-a in this region was mainly concentrated at seasonal scale.