Study On The Temporal And Spatial Distribution Characteristics And Monitoring Of Red Tide In Zhejiang Coastal Area

Red tide is a common marine disaster,which will cause great damage to our marine ecological environment every year,cause huge losses to marine economy and even threaten human health.Zhejiang Province is located in the southeast coast of China.Due to human factors such as industry,aquaculture and natural factors such as seafloor topography and upwelling,the offshore waters of Zhejiang Province are greatly affected by red tides.The research on spatial-temporal distribution and monitoring of red tides in Zhejiang waters plays a guiding role in prevention and control of red tides,and is very important for protection of marine ecological environment,construction of marine economy and social development in Zhejiang.The study of this paper mainly includes the spatial-temporal distribution characteristics of red tides and red tides monitoring of Zhejiang ocean area.The interannual,seasonal and monthly variation characteristics of red tides in Zhejiang offshore waters were analyzed by collecting red tides data in Zhejiang coastal waters.Meanwhile,the adaptive bandwidth kernel density analysis method was applied to the research,it could analysis red tides impacts’ degree in different region of Zhejiang ocean area.In addition,this study has combined the hydrodynamic environment with Zhejiang coastal area’s environmental factors to discuss their impact on red tide qualitively.Ocean color satellite can obtain spectral information of sea water to derive the marine environmental parameters,and it’s an efficient technology for marine environmental monitoring,and has been widely applied in red tide monitoring research.GOCI,the first geostationary orbit ocean color satellite,can provide remote sensing images of Zhejiang waters with high spatialtemporal resolution.For the process of red tide monitoring and research in Zhejiang ocean area,the L1 B data of GOCI was processed by ultraviolet atmospheric correction method to derive GOCI remote sensing reflectance data,and then Total Suspended Particulate Matter(TPM)and Red Tide Index(RI)of Zhejiang coastal area are retrieved based on remote sensing reflectance data.Through the comparison and analysis of red tides extracted results with different thresholds and bulletins’ red tides information,the optimal thresholds of TPM and RI for extraction of red tides information along Zhejiang coast are determined,and the best combination thresholds of TPM and RI are used to extract red tides information in Zhejiang ocean area for red tides monitoring.Finally,the diurnal variation characteristics of red tides in Zhejiang waters were further studied based on the GOCI which can provide high temporal resolution data.The main research results of this paper include the following five aspects:(1)By collecting the data of red tide in Zhejiang waters and analyzing the annual cumulative area and times of red tide from 2006 to 2017,it is found that the annual cumulative area and times of red tide from 2006 to 2012 show a downward trend,while the annual cumulative area and times of red tide from 2012 to 2017 show an upward trend;by analyzing the monthly cumulative area,times and seasonal cumulative area and times of red tide in Zhejiang sea area from 2011 to 2017 It is found that the inter monthly characteristics of the red tide in Zhejiang are as follows: red tide appeared in February firstly,May is the peak month of the occurrence of the red tide,and red tide ended in September;the seasonal characteristics of the red tide in Zhejiang are high occurred in late spring and summer,less red tide in autumn,generally no red tide in winter.(2)Based on the occurrence area and area of red tides in Zhejiang waters from 2011 to 2017,the scatter diagram of red tides distribution in Zhejiang waters and the distribution map of influence degree were drawn by using adaptive bandwidth kernel density analysis method.From the scatter map of red tides in Zhejiang ocean area,it can be found that red tides mainly occur in the 5 sea areas,which are Shensi Island,Zhoushan Island,Ningbo,Taizhou and Wenzhou.It can be found from the map of impacted degree of red tides that the five sea areas are Zhoushan,Shensi,Wenzhou,Ningbo and Taizhou in order of impacted degree from high to low.(3)The hydrodynamic environment of Zhejiang coastal area is complex.At the same time,it is affected by the Changjiang diluted water,Taiwan warm current,coastal current,upwelling,etc.these different water masses and currents affect the nutrient,salt,temperature,light conditions of Zhejiang waters,and they also affect the temporal and spatial distribution of the red tide and the population structure of the red tide in Zhejiang.(4)L1B data of GOCI were processed by ultraviolet atmospheric correction to obtain GOCI remote sensing reflectance data.TPM and RI data were further derived by TPM inversion algorithm and RI algorithm,and red tides in Zhejiang were extracted by the optimal combination of TPM and RI thresholds.Through statistical analysis of the errors between red-tide extraction results and red-tide information in the bulletin,the optimum thresholds of TPM and RI are determined to be 17 mg/L and 0.23 respectively,i.e.the water is red tide water when TPM is not more than 17 mg/L and RI is not less than 0.23.The results of red tides extraction on June 16,2015 and May 28,2016 were taken as examples to verify the accuracy of the results.The results show that the extraction results of these two red tides events are consistent with the red tides information in Zhejiang Marine Disaster Bulletin(area error is 20.45% and 67.00%,respectively).This shows that the method is suitable for monitoring red tides along the coast of Zhejiang Province.(5)GOCI can provide 8 remote sensing data of Zhejiang ocean area every day,which can provide effective and reliable data for the study of daily changes of red tides in Zhejiang.Taking Wenzhou sea area on May 29,2011 and Zhoushan sea area on May 23,2014 as examples to study the daily variation regularity of red tides in Zhejiang waters,the results show that red tides in Zhejiang ocean area increase first and then decrease in a day,and reach peak around noon,the red tide in Zhejiang coastal area has a significant diurnal variation characteristic which is consistent with the diurnal migration of phytoplankton.