Effects Of Marine Environmental Changes On Chlorophyll A In The South China Sea

Phytoplankton play an important role in marine atmospheric carbon cycle and environmental monitoring.In recent years,eutrophication,elevated CO₂concentration and global warming may increase the frequency of blooms.Phytoplankton blooms have negative impacts,causing severe economic losses to aquaculture,fisheries,marine environment and human health.Chlorophyll a（Chl-a）concentration is a key indicator for measuring phytoplankton biomass.It can not only reflect the ecological information of the sea area,but also have the ability to respond to environmental changes.Based on the in situ observation data and remote sensing data,the effects of marine environmental changes on Chl-a in the South China Sea are discussed,which are of great significance for estimating the primary productivity in the sea area and deepening the understanding of the response of regional ecosystems to climate change.The main results are as follows:（1）Based on the biological,nutrients and hydrological data in August 2018,the vertical Chl-a profiles and the relationship among surface Chl-a(Chl-a_（0）),maximum Chl-a(Chl-a_（m）)and depth-integrated Chl-a(Chl-a_（int）)are studied in the northern South China Sea.The results indicate that there are 4 different patterns in the vertical Chl-a profiles in the northern South China Sea:（i）there is no subsurface chlorophyll maximum（SCM）,within a certain depth,Chl-a increases with depth;（ii）there exists SCM,with low Chl-a on the surface and at the bottom layers respectively;（iii）there is no SCM,only with high Chl-a on the surface and in the bottom;（iv）there exists SCM with lower Chl-a in the bottom layer and higher Chl-a_（0）.SCM usually exists in the offshore waters of the northern South China Sea in summer.These patterns are mainly regulated by alternative limitation of nutrients and light from the surface to the bottom of euphotic layer.The significant correlation（R²=0.5206,p<0.01）between the depth of SCM(Depth_（m）)and Chl-a_（0）indicates that the SCM depth is regulated by light masking effect of surface phytoplankton,and the shallow nutriclines and high light attenuation coefficient tend to cause high biomass on the surface.Further research results shows that Chl-a_（int）and Chl-a_（m）have a good correlation（R²=0.6397,p<0.01）.However,the correlation between Chl-a_（int）and Chl-a_（0）is relative weak（R²=0.3202,p<0.01）.That could be attributed to the availability of nutrients playing an important role in growth of phytoplankton.The surface water with high nutrients is conducive to the formation of high Chl-a_（0）.（2）Using multi-source remote sensing data,such as sea surface temperature,sea surface wind,aerosol optical thickness,sea level anomalies,and Chl-a data from 1998to 2020 and in situ observations of analyzed data,we analyze the patterns of spatial variation of summer phytoplankton blooms in the western South China Sea.The partial correlation and multiple stepwise linear regression analyses reveal that Ekman transport and Ekman pumping velocity exert a greater impact on the summer phytoplankton blooms than the other environmental factors.Meanwhile,aerosol optical thickness only weakly correlate with Chl-a in the region.The above results suggest that the phytoplankton blooms in the region may be mainly triggered by the enhanced nutrients by wind-induced upwelling and vertical mixing.A northeastward jet cause the distribution of high Chl-a in the western South China Sea to be plume-shaped.A new finding in this study is that the northward current in this area may cause the northward deviation of phytoplankton blooms from the areasof upwelling.（3）Tropical storm is an important driver of short-term changes in the marine environment.Based on in situ observational data from 4 locations（S1、S2、S3、S4）in the coastal area of the northern Beibu Gulf and satellite data,we analyz the changes in temperature,salinity,and turbidity during the transit of tropical storm"Wipha"in2019 and assess the environmental factors controlling Chl-a increases in the coastal area.Our results mainly show that after the tropical storm,the growth of phytoplankton is mainly controlled by the nutrient and light availability in the coastal area.The increased input of freshwater by tropical storm,including direct inputs from rainfall and increased river discharge,reduce the salinity.The decrease in salinity may indicate an increased input of nutrient-rich freshwater at all 4 stations（nutrients input:S1>S2>S3>S4）.The nutrients at S1,S2,and S3 are rich,but S4 is limited.Tropical storm increases the turbidity of the water body and weakens the light intensity of the water column.The turbidity of S1,S3 and S4 decrease to the original level in a short time,while that of S2 last 10 days[recovery time:S4（2 days）<S1（3 days）<S3（5days）<S2（10 days）].The decrease in light intensity caused by water turbidity is an important factor limiting the growth of phytoplankton at the S2.The rapid recovery of the turbidity to the pre-tropical storm levels at S1,S3,and S4 suggest good light conditions soon after the tropical storm,and probably lead to a significant increase in Chl-a（Chl-a:S1>S3>S4）.The less increase of Chl-a at S4 is not only related to nutrient restriction,but also related to weak mixing of water column,while the least significant decrease in the temperature at S4 imply that the enhancing mixing after tropical storm is limited.