Growth And Elimination Of Skeletonema Costatum Bloom Of And Its Impact On Plankton Community Structure

With the development of industrial and agricultural production and the increase of population,people’s dependence on the ocean is increasing,leading to increased marine pollution,and the frequency of bloom is greatly increased,causing serious damage to the ecological environment and human health.Blooms have become a global marine disaster,seriously disrupting the economic development of more than 30 coastal countries.The occurrence of a bloom is often accompanied by changes in the structure of plankton.In order to explore the impact of red tide on plankton community structure,an 11-day on-site investigation was carried out on the red tide of Skeletonema costatum in Gouqi Island,Zhoushan City,Zhejiang Province from April 17 to April 29,2018.During the investigation,the seawater physical and chemical indicators such as water temperature,salinity,dissolved oxygen（DO）,pH（pH）and phosphate(PO₄^3--P)were monitored,and the species composition,abundance,dominant species and community structure index of plankton in the blooms station and non-bloom stations were analyzed.The effects of bloom on the phytoplankton community structure and zooplankton community structure were compared.Meanwhile,combined with remote sensing data to retrieve the trend of Chlorophyll a（Chla）,the generation and elimination of bloom were studied from various angles.The main results are as follows:（1）During the survey period,the water temperature in the sea area of Gouqi Island varied by 14.38^°C-17.47°C,the average temperature was 16.25°C,the lowest temperature appeared on April 17,the highest temperature appeared on April 27,basically rose trend,the range of variation of temperature was not large.The change of salinity in the sea area was 25.54-28.54,the average salinity was 26.78,the highest salinity appeared on April 17,and the lowest salinity appeared on April 27.The overall change was small.During the survey,the variation range of DO in the sea area of the Gouqi Island was 10.51-11.78 mg/L,with an average of 10.93 mg/L.The minimum appeared on April 22,and the maximum appeared on April 29,showed an overall upward trend.The pH of the sea area ranged from8.11 to 8.26,with an average of 8.19.The change range was small and the change was not obvious during the investigation.The concentration of PO₄^3--P in the survey area from April 17 to April 29 ranged from 0.29μmol/L to7.71μmol/L,with an average of 1.08μmol/L.On April 17,the concentration of PO₄^3--P phosphate was the highest in the surveyed area,and on April 29,the concentration of PO₄^3--P phosphate was the lowest.（2）Among the 99 phytoplankton samples were collected from the sea area of Gouqi Island,71 samples were detected by S.costatum,and the abundance of S.costatum in 12samples exceeded 10⁷ cells/m³.On April 17,April 25,and April 29,at the peak of three blooms,the average cell abundance of S.costatum was 1.90×10⁷cells/m³,5.29×10⁸cells/m³,and 7.19×10⁷cells/m³,respectively.Through the inversion of Chla by the geostationary ocean colour imager（GOCI）image data,the effective Chla image was analysised in the middle and late April of 2018 was basically consistent with the on-site investigation of the total abundance of phytoplankton.In addition,the daily variation analysis of Chla image quality was carried out on April 18.It was found that the concentration of Chla in the bloom region was higher in the morning and evening,lower in noon,and there was obvious photoinhibition in phytoplankton.The result of daily variation of Chla was a combination of vertical migration and tidal action.The bloom of S.costatum was very fast,and GOCI can effectively monitor the bloom in the short and long term.（3）A total of 86 species of phytoplankton belonging to 42 genus and 4 phylums were identified in the coast of Gouqi Island by the net collection method,of which 74 species were bacillariophyta,10 species were pyrrophyta,1 species were cyanobacteria and 1species were chlorophyta,and diatoms were dominant.There were 3 phylums,48 species phytoplankton were identified in the coast of Gouqi Island by the net collection method include 41 bacillariophyta and 6 pyrrophyta,and 1 chlorophyta in bloom stations,and diatoms were the dominant species.There were 4 phylums,67 species phytoplankton were identified in the coast of Gouqi Island by the net collection method include 58bacillariophyta and 7 pyrrophyta,1 cyanobacteria and 1 chlorophyta in non-bloom stations.The number of dinoflagellates in bloom and non-bloom stations were similar,and the number of diatoms was quite different.In addition,there were 49 species of 28 genus and 3phylums,including 42 bacillariophyta,6 pyrrophyta and 1 chrysophyta were identified in the coast of Gouqi Island by the water collection method.A total of 28 species of 2 genus,including 23 diatoms,5 dinoflagellate were identified in bloom stations by the water collection method,and and diatoms were the dominant species.A total of 39 species of 3genus,including 32 bacillariophyta,6 pyrrophyta and 1chrysophyta were identified in non-bloom stations by the water collection method.Quantitative analysis of phytoplankton results showed that there were 3 dominant species in bloom stations and 15 species in non-bloom stations.The first dominant species were S.costatum.The average abundance of phytoplankton in bloom stations in the coast of Gouqi Island was 2.13×108cell/m³,and the average abundance of phytoplankton in non-bloom stations was 2.18×106cell/m³.The average Simpson index index（C）,Shannon index index（H’）,Pielou evenness index（J’）,and Margalef index（d）of phytoplankton in bloom stations in the coast of Gouqi Island were 0.75,0.80,0.30 and 0.29,respectively.The average Simpson index index（C）,Shannon index index（H’）,Pielou evenness index（J’）,and Margalef index（d）of phytoplankton in non-bloom stations were 0.41,1.64,0.81,and 0.18,respectively.There are Significant difference between bloom stations and non-bloom stations about these index（t test,P<0.05）.（4）A total of 37 species of zooplankton belonging to 34 genus and 5 phylums were identified in the coast of Gouqi Island,including 21 species in bloom stations and 33species in non-bloom stations.The number of zooplankton species in bloom stations was lower than that of the non-bloom stations.There were 6 dominant species of zooplankton in bloom stations and non-bloom stations.Among the bloom stations,Calanus sinicus had the highest dominance,which was 0.66,followed by Sugiura chengshanense,Muggiaea atlantica,Sagitta nagae,and Macrura larvae.Among the non-bloom stations,C.sinicus has the highest dominance,which is 0.64,followed by S.chengshanense,Sagitta nagae,M.atlantica,Macrura larvae,and Euchaeta concinna.During the survey,the average abundance of zooplankton in bloom stations in the coast of Gouqi Island was 120.78ind./m³,and the average abundance of zooplankton in non-bloom stations was 150.31ind./m³.The average Simpson index（C）,Shannon index（H’）,Pielou evenness index（J’）,and Margalef index（d）of zooplankton in bloom stations were 0.47,1.63,0.46 and 1.82,respectively.The average Simpson index（C）,Shannon index（H’）,Pielou evenness index（J’）,and Margalef index（d）of zooplankton in non-bloom stations were 0.44,1.79,0.49and 1.85,respectively.The zooplankton community structure was relatively stable during the bloom,and the difference between bloom station and non-bloom station was not significant（t test,P>0.05）.Zooplankton is less affected by the bloom of Skeletonema costatum.In conclusion,the bloom of S.costatum in the northern of Zhejiang Province occurred frequently in the spring,and the generation and elimination of bloom was very fast,which had a significant effect on the phytoplankton community structure,and the zooplankton community structure changed little.In addition,the GOCI satellite data can be used to quickly and easily monitor the bloom of S.costatum.It not only helps us to understand more deeply the trend of the bloom of S.costatum,but also helps to understand the temporal and spatial variation characteristics and regularity of the bloom in the East China Sea,which is of great significance for the future monitoring of bloom.