Distributions Of Size-fractioned Phytoplankton And Its Controling Factors In The Coastal Seawaters Of Beibu Gulf,Guangxi

As the major contributor of marine primary productivity and the basis of structure and function for the marine food web,phytoplankton plays an important role in the material circulation and energy flow of marine ecosystem.Many of the physio-ecological functions of phytoplankton are determined by the particle size,and changes in its grain size structure affect and reflect the marine environment in the area.Based on the data from the comprehensive investigation of Guangxi Beibu gulf coastal seawaters in August(summer)and December(winter)2017,the spatial distribution and seasonal variation of phytoplankton biomass and its size-fractioned structure were analyzed,and the relationship between the phytoplankton size-fractioned structure and the main environmental factors of nutrient,temperature and salinity was discussed.The main results are as follows:(1)The seasonal variation of phytoplankton biomass was obvious,the average concentration of Ch1 a in summer(6.68 ?g/L)was much higher than that in winter(2.23 ?g/L),which showing a decreasing trend from nearshore to offshore.Nano-phytoplankton(2-20 pm)was the most dominant species of the study area in summer and winter.?with average percentage of 42.5%and 44.7%respectively,Micro-phytoplankton(>20 ?m)came second,with average percentage of 41.0%and 36.9%,Pico-phytoplankton(<2 ?m)accounted for the least,with average percentage of 16.5%and 18.4%.The spatial distribution of nearshore marine areas was dominated by Micro-phytoplankton and Nano-phytoplankton population,and with the increase of offshore distance,the proportion of Pico-phytoplankton would increase.In addition,Micro-phytoplankton and Nano-phytoplankton were dominant when the phytoplankton biomass was high,the proportion of Pico-phytoplankton was significantly improved when the biomass was low.(2)The determination results of nutrients indicated that the average concentration of DIN,PO4-P and SiO3-Si were 20.39 ?mol/L,0.43 ?mol/L,37.38 ?mol/L in summer,and 11.33 ?mol/L,0.60 ?mol/L,12.50 ?mol/L in winter,respectively.The variation of different nutrients forms showed a decreasing trend from the nearshore to offshore,inner bay to the outer bay.Influenced by coastal runoff which input large amount of nutrients,the growth of phytoplankton in the survey area was not restricted by absolute nutrient limiting.Phosphorus(P)was the potential limiting factor of the investigation area.The value of N/P,Si/P,Si/N were all deviated from Redfield ratio in two periods,and the discreteness of datasets was higher in summer.(3)The Pearson correlation analysis between size-fractioned Ch1 a and environment factors showed that there were differences in correlation between various size-fractioned phytoplankton and different nutrients forms,phytoplankton had selective absorption of nutrients.When phytoplankton of different size-fractioned were significantly associated with the same nutrient,but the signs are reversed,there might be competition among phytoplankton.There was a significant positive correlation between Pico-phytoplankton and temperature in the study area in winter,while the Nano-phytoplankton was opposite.Micro-phytoplankton was more adaptable to low salinity environments than Nano-phytoplankton in summer.The principal component analysis method was used to screen the environmental factors affecting the growth of phytoplankton in the study area.The results showed that nutrient was the most important factor affecting the growth of phytoplankton.