Early Warning Study Of Nutrient-induced Red Tide At Sewage Outlet In Coastal Waters

The frequency of red tide in local waters is caused by the excess of nitrogen,phosphorus and other nutrients in seawater caused by land-based sewage.Thus,the collapse of nearshore marine ecosystems is gradually becoming a widespread concern.Although there were many methods and models evaluate the environmental elements of ecological collapse and their thresholds,it was still a hot issue how to predict the collapse so that the red tide blooms could be controlled and prevented in the early age.Therefore,the near-shore sea area of the outfall of Xiaoping Island,Dalian was used as the study area from April 24 to December 16,2018,covering spring,summer,and autumn.126 seawater and 378 biological samples were collected in spring and summer（from April to September）and autumn（from October to December）on 2018,which the Section 1 was near the sewage outlet,Section 2 was 10 km away from the sewage outlet,and section 3 was 20 km away from the sewage outlet.The characteristics of changes in plankton and microbial community structure and biodiversity were analyzed in different sections and different seasonal environments.Procrustes and SEGMENTED methods were applied to assess the environmental elements affecting each community and their ecological thresholds.The susceptible microorganisms of OPI（Organic Pollution Index,Comprehensive index of organics polluting）were analyzed by co-occurrence network method,and the association between susceptible microorganisms and the development pattern of red tide occurrence was analyzed by Pearson method.The results of the study were as follows.The concentrations of inorganic nitrogen（DIN）and phosphate（PO₄-P）in section 1 near the sewage outlet were higher than the Chinese Seawater Quality Standard（GB 3097-1997）and much higher than those in section 2 and section 3 concentrations.Above the 505μm mesh net samples,type 1 net zooplankton abundance analysis in different seasons shows that the highest abundance was observed in spring.There were significant differences between sections 1 and 2 and between sections 2 and 3 in summer（p<0.05）.Zooplankton diversity in different seasons was highest in summer.While the diversity of Section 1 was strongly correlated with Section 2 and there was great difference between Section 2 and 3 in spring（p<0.05）.The diversity of Section 1 was lower than Section 2 and 3 in summer and autumn（p<0.01）.The results showed that Paracalanus parvus in section 1 accounted for 42.22%in summer,but only 0-1.28%in sections 2 and 3 among the dominant species of zooplankton above 505μm.Ophiuroidea ophiopluteus larva and Evadne nordmanni in Section 1accounted for only 0.49%and 3.73%in summer,but 6.10-6.63%and 5.85-14.10%in Sections 2 and 3,respectively.Above the 170μm,the abundance and diversity of zooplankton in type II were analyzed,and the highest abundance was still found in spring.There was no significant difference in the abundance variation between Section 1,2,and 3 in spring and summer,while it showed an increasing trend from section 1 to section 3 in autumn.Analysis of their diversity showed that section 1 was lower than Section 2 and 3 in spring,summer,and autumn.There was a significant difference between Section 2 and Section 3 in summer（p<0.05）.The analysis of their dominant species showed that Section 1 in autumn had a distinctly different composition structure from sections 2 and 3.The analysis of phytoplankton chlorophyll with different particle sizes in each section.The results showed that Section 1 was dominated by micro-phytoplankton.Section 3 was dominated by nano-and pico-phytoplankton.The abundance of phytoplankton was highest in summer and significantly higher in Section 1 than in Section 2 and 3 when in spring and summer（p<0.05）.The diversity of Section 1 was strongly correlated with Section 2 and 3 in summer（p<0.05）.The dominant species in Section 1 were mainly Skeletonema costatum.The dominant species in Sections 2 and 3 were Paralia sulcate,Coscinodiscus wailesii,Protoperidinium sp..The red tide of S.costatum with a density of 2.3×10⁷ cell/L that occurred at station C1 of Section 1on June 8,and the red tide of Ceratium tripos with a density of 2.9×10⁸ cell/L that occurred at station A1 of Section 1 on July 22 were monitored.The structure and diversity of the planktonic bacterial and planktonic eukaryotic microbial communities were further analyzed in Section 1 and Section 3.Among the planktonic bacterial communities,the percentage of Firmicutes and Cyanobacteria in section 1 was 3.80-9.40%and 1.32-5.38%,respectively,while it in section 3 was 1.08-2.50%and 6.14-11.64%,respectively.Among the planktonic eukaryotic microbia,Chlorophyta and Bacillariophyta were 8.31-12.32%and 3.72-18.63%in section 1,but only 1.36-4.83%and 1.52-4.58%in section 3,respectively.Ascomycota accounted for only 0.19-1.05%in section 1,lower than 2.08-7.75%in section 3.Haptophyceae in section 1 accounted for 4.47%and 10.67%in spring and autumn,higher than 2.91%and 1.90%in section 3 in spring and autumn,respectively.The results of species community structure and diversity in different sections indicated that their biodiversity changes also showed significant differences,especially in Sections 2 and 3 where salinity levels were consistent.The results indicated that near shore sea area of the Xiaoping Island in Dalian had been affected by nutritions from land-based discharges for a long time.And the ecosystem community structure of its nearby waters had been reshaped.It might have spread to the sea area at 10 km（Section 2）.Analyze environmental factors that affected community structure of different species.It was shown that water temperature,transparency,DO,NO₃-N,PO₄-P and Si O₃-Si were significantly associated with the composition and diversity of zooplankton communities in type I nets（p<0.05）;Water temperature and NO₃-N had significant effected on phytoplankton and zooplankton communities in type II nets（p<0.05）;The results showed that OPI,TN,DIN and TP were strongly associated with bacterial and eukaryotic communities（p<0.05）.Water temperature,transparency,DO,NO₃-N,PO₄-P and Si O₃-Si had a close association（p<0.05）on the composition and diversity of zooplankton communities above 505 um particle size.Water temperature and NO₃-N were strongly associated with phytoplankton and zooplankton communities above 170 um particle size（p<0.05）.These environmental factors were consistent with the measure values of sea water at the sewage when red tide burst at stations A1 and C1 in section 1 near the sewage outlet,and indicating that the use of planktonic bacterial communities was more sensitive for assessing ecological the risk of red tide outbreak in nearshore waters.Three planktonic bacteria（Planktomarina,Acinetobacter and Verrucomicrobiaceae）and one planktonic eukaryotic microorganism（Haptopphyceae）were related to OPI index（p<0.05）.The correlation between their abundance changes in different seasons and the occurrence pattern of red tides was analyzed in the sewage outlet.The results showed that Planktomarina was significantly（p<0.05）associated with the red tide of C.tripos blooms at A1 station.At the same time,it had great asssociation with TN,DIN and NO₃-N（p<0.05）,which showed the red tide outbreaks of Ceratium tripos were caused by the high concentrations of TN,DIN and NO₃-N in A1 station.Acinetobacter and Verrucomicrobiaceae were significantly associated with the red tide of S.costatum blooms at C1 station（p<0.05）.At the same time,there was great correlation with PO₄-P（p<0.05）.The results showed that Planktomarina was significantly（p<0.05）associated with the red tide of C.tripos blooms at A1 station,while Acinetobacter and Verrucomicrobiaceae were significantly（p<0.05）associated with the red tide of S.costatum blooms at C1 station（p<0.05）.Nutrient pollution had important effects on planktonic and microbial communities,and their distributions showed obvious spatial and seasonal heterogeneity.During the red tide season,real-time monitoring of TN,DIN and NH₄-N in seawater environment and the dynamic changes of these three sensitive organisms could help early warning of red tide,and the method provided important support for red tide risk assessment.