Evolution Characteristics And Carbon Sink Analysis Of Primary Productivity In The Pearl River Estuary

The Pearl River is the 13th largest river in the world that located in southern China.The Pearl River Estuary is the main channel for the Pearl River to flow into the South China Sea and is the only estuary in the world with eight estuaries,breeding a large number of biological species and is located in the core area of the Bay Area.The primary productivity of water body can reflect the supply of plankton community resources,estimate carbon sequestration and fishery production potential,and guide fishery production.This study takes the primary productivity of the water bodies at the eight mouths of the Pearl River as the research object,based on two on-site investigations in spring and summer（May and August）of 2021 and historical data since 2007,the spatiotemporal distribution characteristics,evolution trends and trends of primary productivity were analyzed.Its influencing factors,and the primary productivity was used to estimate the total amount of organic carbon that phytoplankton in the Pearl River Estuary could fix through photosynthesis,that is,carbon sink.The main conclusions are as follows:（1）The spatiotemporal distribution characteristics of primary productivity in the Pearl River EstuaryThe chlorophyll a and primary productivity of the Pearl River Estuary in 2021were significantly higher in summer than in spring（P<0.05）;the highest values of chlorophyll a and primary productivity appeared in Modaomen in summer,and the lowest values appeared in Humen in spring.The main reason may be controlled by river runoff discharge and water temperature.During the rainy season,the input of river runoff is large,and the nutrient input of high temperature and freshwater runoff leads to the proliferation of phytoplankton biomass in summer.Modaomen is the main outlet gate of Xijiang runoff.In summer,the Xijiang River transports a large amount of nutrients to Modaomen through runoff.The rich nutrients and the high,water temperature provides good growth conditions for phytoplankton.Humen is a gate with strong tidal action,and its tidal throughput ranks among the eight gates.The large amount of suspended solids and sediment input results in extremely low transparency,which is not conducive to the phytoplankton accumulation.（2）The spatiotemporal evolution trend of primary productivity in the Pearl River EstuaryFrom 2007 to 2021,the primary productivity of the Pearl River Estuary showed a fluctuating downward trend as a whole,and the decline ranged from 8.21 to 23.46mg·m^-2·d^-1.In the past 15 years,Hong Qili had the highest decline,and Jiaomen had the lowest decline.On the whole,the decline of Dongsikoumen was higher than that of Xisikoumen.The primary productivity of the Pearl River Estuary can be divided into three categories by using the system clustering method.The first category was Modaomen;the second category were Hengmen,Hongqili and Jiaomen;the third category were Jitimen,Yamen,Hutiaomen and Humen.The overall spatial distribution of primary productivity in the Pearl River Estuary from 2007 to 2021 was the highest in Modaomen(236.99 mg·m^-2·d^-1)and the lowest in Jiaomen(162.58mg·m^-2·d^-1).（3）Influencing factors of the primary productivity of the Pearl River EstuaryPerson correlation analysis showed that the main influencing factors of primary productivity from 2007 to 2021 were phosphate,total nitrogen and total phosphorus as well as chlorophyll a,transparency and water temperature（P<0.05）.Transparency was a key factor affecting primary productivity in the Pearl River Estuary;phosphorus（TP）may be responsible for perennial constraints on phytoplankton growth in this area.The primary productivity was positively correlated with industrial wastewater,industrial COD and industrial ammonia nitrogen（P<0.05）;It indicated that the increase and decrease of primary productivity may be related to the nutrients brought in by industrial emissions.（4）Assessment of carbon sequestration capacity of phytoplankton in the Pearl River EstuaryAccording to the results of primary productivity from January to December 2015,the total amount of carbon sequestered in the Pearl River Estuary in 2015 was estimated to be 26.18’10⁴ t a^-1.Among them,the carbon sequestration level in the spring of 2013 was the highest（3051 t）,and the carbon sequestration level in the spring of 2016 was the lowest（130 t）.