Temporal And Spatial Distribution Characteristics Of Chlorophyll-a And Nutrients And Potential Eutrophication In Zhanjiang Bay In Autumn Of 2017--2020

In the research and management of eutrophication,the temporal and spatial distribution of chlorophyll a(Chl-a)and nutrients has always been the focus of attention and research.Zhanjiang Bay is close to the urban area,it is a semi closed bay with narrow exit and poor water exchange capacity.In recent decades,the water pollution in the Bay has become increasingly obvious and the eutrophication has become more serious.In order to understand the current situation of water quality and explore the mechanism of environmental eutrophication in Zhanjiang Bay in recent years,based on the field survey data of four cruises in Zhanjiang Bay from 2017 to2020(November),principal components analysis and Pearson correlation analysis in combination with the nutrient structure were used to analyze the temporal and spatial distribution characteristics of phytoplankton Chl-a and nutrients in the North and south of Zhanjiang Bay.The results were summarized as follows:(1)The concentration of Chl-a in the surface layer of Zhanjiang Bay in autumn has obvious temporal and spatial differences.The spatial distribution of Chl-a concentration in Zhanjiang Bay was lower in the north of the bay,higher in the south of the bay,lower near the shore,higher offshore,and decreasing from the inside of the bay to the outside.The annual average of Chl-a content in Zhanjiang Bay was from1.94?g/L to 3.16?g/L,and the average value over the years showed a"?"type trend of increasing first and then decreasing.The physical and chemical environmental factors affecting the interannual variation of Chl-a content in Zhanjiang Bay include water temperature,pH,DO and transparency.Chl-a content is positively correlated with water temperature,DO and pH,and negatively correlated with transparency.(2)The spatial and temporal distribution of the main environmental factors in Zhanjiang Bay was obvious,and the average water temperature over the years was from 20.95?to 25.66?.The salinity varies little over the years,and the average salinity ranged from 24.65 to 25.33.The average pH value over the years was from7.59 to 8.23,belonging to weakly alkaline water.The DO concentration and transparency increased significantly over the years;the nutrient concentration in the Bay generally showed a downward trend,and the water quality tended to improve,with the average concentration of DIN ranged from 0.26 mg/L to 0.44 mg/L,the concentration of PO₄-P ranged from 0.008 mg/L to 0.21 mg/L,and the concentration of SiO₃-Si ranged from 0.49 mg/L to 3.47 mg/L.The overall distribution of nutrients in Zhanjiang Bay decreased from north to south of the bay and from inside to outside of the bay,and was higher in the nearshore and lower in the far-shore areas.(3)The correlation between Chl-a content and environmental factors in Zhanjiang Bay also has spatiotemporal differences.In 2017,Chl-a in northern bay was negatively correlated with water temperature,SiO₃-Si and PO₄-P,and positively correlated with pH,while that in southern bay was negatively correlated with SiO₃-Si.In 2018,Chl-a content in northern bay was negatively correlated with PO₄-P,and positively correlated with N/P and pH,while that in southern bay was not significantly correlated with environmental factors.In 2019,Chl-a in the north of the bay was significantly negatively correlated with temperature and transparency,while in the south of the bay,Chl-a was significantly negatively correlated with N/P,and significantly positively correlated with PO₄-P.In 2020,Chl-a in the north of the bay was significantly positively correlated with SiO₃-Si,while Chl-a in the south of the bay was significantly positively correlated with water temperature,and significantly negatively correlated with DO and N/P.(4)Principal component linear regression analysis showed that the main environmental factors affecting Chl-a in the south and north of Zhanjiang Bay were temperature,pH,salinity,DO,transparency,SiO₃-Si,PO₄-P and DIN.The environmental factors affecting Chl-a were different in different sea areas and different monitoring years.The main influencing factors were pH,SiO₃-Si,PO₄-P and DIN.(5)In autumn,the inorganic nitrogen in Zhanjiang Bay reached the thermodynamic equilibrium,and NO₃-N was the main component over the years.The nutrient limitation in the Bay over the years:potential nitrogen limitation in 2017,potential phosphorus limitation in 2018 and 2019,and no obvious limitation in2020.The potential eutrophication assessment of the water quality in the Bay over the years showed that the northern part of the bay is eutrophic at grade III,while the southern part of the bay is eutrophic at grade III except for the V_Nlevel in 2017.The trophic status of the outer bay mouth has changed from grade III to IV_Nlevel and then to grade II.