Spatial Distribution Characteristics Of Phytoplankton Chlorophyll And Its Relationship With Environmental Factors In The Pearl River Estuary In Summer

Phytoplankton is the main contributor to marine primary productivity and the foundation of supporting marine ecosystems.Chlorophyll concentration(Chl a)is an important parameter indicating the biomass of phytoplankton,and its spatiotemporal variation characteristics are closely related to many environmental factors.Based on the hydrological and biogeochemical observation data of the Pearl River Estuary in the summer of 2019,this study preliminarily discussed the distribution characteristics,influencing factors and environmental effects of chlorophyll a and nutrients of phytoplankton in this period.Two typical red tide organisms representing different groups were selected for indoor culture experiments.The growth of common red tide algae in the northern South China Sea under different environmental factors(N,P concentration and proportion,light,salinity)was analyzed.The response of algae to environmental factors and the mechanism of environmental regulation were further discussed.In order to provide scientific basis for accurate assessment of primary productivity and marine ecological environment protection in the coastal waters of northern South China Sea.The results of field investigation show that the temperature and salinity of the surface layer in the investigated area are 28.30?30.88°C and 0.81?29.01 psu,respectively.The temperature and salinity of the surface layer gradually increase from the inner to the outer sea of the Pearl River Estuary,and there is an obvious thermohaline layer.The concentrations of Chl a and dissolved oxygen in the surface water were 4.19?88.21?g/L and 4.61?10.59 mg/L,respectively.The horizontal distribution of Chl a concentration was patchy,and the west side of the estuary was lower than the east side.The vertical distribution decreased rapidly with the increase of depth.The high value area was distributed outside the Pearl River Estuary,which was located in the southeast of Hong Kong and near the S21 station.The concentration of Chl a was high in the surface waters of the Pearl River Estuary during the survey period.According to the NEEA-ASSETS standard,44.4%of the stations were highly eutrophic(20?g/L<Chl a<60?g/L),25%of stations are hypereutrophic(Chl a>60?g/L).The concentrations of dissolved inorganic nitrogen(DIN),phosphate(PO₄-P)and silicate(Si O₃-Si)were 0.17?1.25 mg/L,0.01?0.04 mg/L and 0.69?6.92 mg/L,respectively.DIN and Si O₃-Si were mainly regulated by land source and runoff input,and negatively correlated with salinity.The concentration of Chl a was low in the Pearl River Estuary,and the growth of phytoplankton was affected by light limitation and runoff dilution.The Pearl River runoff was the main driving factor for the spatial distribution of phytoplankton near the Pearl River Estuary in summer.Due to the consumption of a large amount of nutrients by phytoplankton growth,the concentration of PO₄-P was low,and Chla was significantly negatively correlated with PO₄-P.PO₄-P became a limiting factor for phytoplankton growth in the offshore area of the Pearl River Estuary.According to the nutrient limiting factor standard,86.11%of the stations in the investigated area had P limitation.Studies have shown that light(transparency)and PO₄-P are the main limiting factors for the growth of phytoplankton in the internal and external waters of the Pearl River Estuary,respectively.Freshwater from estuary has a significant impact on the growth of phytoplankton.The results of indoor culture experiments showed that the responses of different algae to nutritional conditions and environmental changes were different.When the P source was sufficient,the biomass of Heterosigma akashiwo and Chaetoceros mulleri increased with the increase of N concentration.However,the two algae had different nitrogen utilization abilities,and Chaetoceros mulleri had faster nitrogen absorption rate than Heterosigma akashiwo.The biomass of the two algae decreased significantly with the increase of N/P ratio in a moderate range.When N/P=18,the two algae grew well.In the experimental setting gradient,the suitable nutrient ratio of Heterosigma akashiwo was N/P=9,and the suitable nutrient ratio of Chaetoceros muelleri was N/P=18.The results of laboratory experiment and field investigation showed that the Chl a content of phytoplankton in the sea area near the Pearl River estuary was low due to the limitation of high N/P ratio.The results of laboratory experiments showed that the biomass of the two algae increased first and then decreased with the increase of light intensity.In the set gradient of the experiment,the optimal light intensity was about 4000 lx,but Chaetoceros muelleri was more tolerant to low light intensity than Heterosigma akashiwo.The results of laboratory experiments and field investigation show that the turbidity of the water in the Pearl River Estuary is large,and the algae are easily limited by insufficient light,resulting in low Chl a content.The results of laboratory experiments showed that Heterosigma akashiwo and Chaetoceros muelleri were halophilic algae,but the biomass of algae in high salinity group and low salinity group decreased.In the experimental setting gradient,the optimum salinity was about 20 psu,and Chaetoceros mulleri had stronger tolerance to low salinity than Heterosigma akashiwo.The results of laboratory experiment and field investigation show that the salinity changes greatly in the sea area near the coastal estuary,which is easy to limit the growth of algae,but the low salinity has relatively small limitation on the growth of Heterosigma akauroshio and Chaetoceros muelleri.