The Realized Niche Of Phytoplankton Assemblages In The Bohai Sea:An Analysis Based On Pigments

The Bohai Sea coastal zone is a key area for economic growth.The Bohai Sea region receives a lot of nutrients from the high intensity human activity in the surrounding area.Additionally,because of the limited water exchange,the region has experienced significant eutrophication,the ecological disasters occur frequently and the fishery resources decline.Previous researches have demonstrated that the fundamental niche for phytoplankton development has altered as a result of both the eutrophic environment and climate change.The scale and frequency of red tide outbreaks increase as a result of the phytoplankton community structure transitioning from the diatom dominance to non-diatom dominance.The habitat in which a species can continue to increase is its fundamental niche.The rise of non-diatom is intimately correlated with the fundamental niche’s change.However,the fundamental niche does not take into account the interactions between organisms.Therefore,it is necessary to further study the realized niche of phytoplankton assemblages,that is,to understand the future and evolution trend by assessing the degree of niche difference and population competitiveness of different assemblages.The Bohai Sea’s center regions were chosen as the study’s research focus.The CHEMTAX software was used to analyze the seasonal variation characteristics of phytoplankton pigment assemblages,based on survey data from 8 voyages between 2013 and 2021 and the analysis method of photosynthetic phytoplankton pigments.The realized niche of the main phytoplankton assemblages in the Bohai Sea were then calculated using Maximum Entropy Models（MaxEnt）and Generalized Additive Models（GAMs）.The results of this study have scientific reference value and practical significance for understanding the evolution mechanism of phytoplankton community structure and the early warning and prediction of red tide outbreak in the Bohai Sea.The main results are as follows:1.The seasonal variation and spatial distribution characteristics of the phytoplankton pigment assemblagesThis study analyzed the photosynthetic pigment data from 222 sampling sites on 8 voyages between 2013 and 2021.The results showed that the Chlorophyll-a concentration in the Bohai Sea was significantly higher during the spring and summer seasons（1.34～2.70μg/L）compared to the autumn and winter seasons（0.55～0.68μg/L）.In spring,Bacillariophyceae were dominant in all the layers,accounting for60.26～70.77%of total Chlorophyll-a（Chl-a）concentration.In summer,Synechococcus（21.13～30.36%）,Prasinophyceae（4.76～27.27%）,Chlorophyceae（6.86～23.6%）and Cryptophyceae（9.78～21.28%）were dominant in the surface layer.Haptophyceae（8.14～28.74%）,Chlorophyceae（21.04～24.49%）and Bacillariophyceae（10.01～24.43%）were dominant in the middle layer.Bacillariophyceae（14.89～35.28%）,Haptophyceae（11.61～26.20%）and Chlorophyceae（3.27～23.86%）were dominant in the bottom layer.In autumn,Bacillariophyceae（26.89～62.57%）,Chlorophyceae（5.37～58.14%）and Prasinophyceae（2.24～30.87%）.In winter,Bacillariophyceae and Haptophyceae became dominant assemblages in the surface layer,accounting for33.42～55.59%and 6.38～20.09%of total Chl-a concentration,respectively.Bacillariophyceae and Cryptophyceae were dominant in the middle layer,accounting for 57.76～59.67%and 10.75～17.78%of total Chl-a concentration,respectively.Bacillariophyceae and Chlorophyceae were dominant in the bottom layer,accounting for 54.22～60.90%and 15.97～19.33%of total Chl-a concentration,respectively.2.Correlation analysis of phytoplankton pigment assemblages and environmental factors in the Bohai SeaThis study utilized Canonical Correspondence Analysis（CCA）to analyze the correlation between the phytoplankton pigments assemblages and environmental factors in different seasons.The results indicated that The proportion of phytoplankton pigment assemblages in different seasons was restricted by different environmental factors.Bacillariophyceae were negatively correlated with temperature（Temperature,T）in all seasons and positively correlated with salinity（Salinity,S）,suggesting that Bacillariophyceae are adapted to low-temperature,high-salinity environments.In spring,Bacillariophyceae were positively correlated with dissolved silicate（DSi）,indicating that DSi concentration was conducive to the growth of Bacillariophyceae.In summer,Bacillariophyceae were negtively correlated with temperature（Temperature,T）and were positively correlated with salinity（Salinity,S）and dissolved inorganic nitrogen（DIN）,suggesting that relatively low temperature,and relatively high salinity and DIN concentration promoted their growth.Bacillariophyceae have a stronger positive correlation with ammonium（NH₄⁺）than nitrate（NO₃^-）,suggesting that NO₃^-romoted diatoms more significantly than NH₄⁺.The correlation between Synechococcus and temperature was positive,while the correlation with dissolved inorganic phosphorus（DIP）was significantly negative,indicating that relatively high temperature and phosphorus（P）limitation promote the growth of Synechococcus.Cryptophyceae were positively correlated with NH₄⁺,indicating that they are suitable for living in environments with high NH₄⁺concentrations.In autumn,Bacillariophyceae were positively correlated with DSi,and NO₃^-/NH₄⁺,indicating that high DSi concentrations and high NO₃^-/NH₄⁺ratios in the water are conducive to the growth of Bacillariophyceae.Prasinophyceae were positively correlated with salinity,indicating that high salinity in the water is conducive to the growth of Prasinophyceae.In winter,Bacillariophyceae were positively correlated with salinity and NH₄⁺,suggesting that relatively high salinity and NH₄⁺concentration promoted their growth.3.Assessment of the realized niche of phytoplankton pigment assemblages in the Bohai SeaThe MaxEnt model and GAMs were used to evaluate the actual niche of phytoplankton groups in the Bohai Sea.The average niche（μ）and niche width（σ）of different pigment groups of phytoplankton were analyzed and obtained by considering temperature,salinity and,nutrient concentration and structure.The results show that the assemblages can be separated from each other through the following suitable niche characteristics:The suitable niches for Chlorophyceae wereμT=15.2℃,μS=30.3 andμDIN=6.92μM.The suitable niches for Bacillariophyceae wereμT=8.1℃,μS=31.1,μDSi=7.47μM andμNO₃^-/NH₄⁺=5.54.The suitable niches for Dinophyceae wereμT=9.8℃,μS=30.9 andμDIP=0.15μM.The suitable niches for Prasinophyceae wereμT=15.9℃,μS=31.2,μDIP=0.37μM andμNO₃^-/NH₄⁺=5.68.The suitable niches for Chrysophyceae wereμT=17.1℃,μS=31.2 andσNH₄⁺=1.56μM.The suitable niches for Cryptophyceae wereμT=20.2℃andμS=31.0.The suitable niches for Haptophyceae wereμT=19.1℃,μS=31.6 andσNO₃^-/NH₄⁺=3.16.The suitable niches for Synechococcus wereμT=22.5℃,μS=31.0 andσNO₃^-=3.53μM.The competitiveness of Bacillariophyceae and non-Bacillariophyceae can be reduced by relatively high temperatures and relatively low DIP concentrations,which explains the realized niche changes in favor of Synechococcus and Chlorophyceae becoming dominant groups.This is discovered by comparing the niche of environmental factors of different phytoplankton assemblages.In summary,through the study on the realized niche of phytoplankton photosynthetic pigment assemblages,it is found that the phytoplankton community structure in the Bohai Sea is dominated by Bacillariophyceae in spring,autumn and winter,and non-Bacillariophyceae become the dominant assemblages in summer.Further analysis of the realized niche of phytoplankton photosynthetic pigment assemblages showed that the dominant pattern of Bacillariophyceae in summer was changed through the competition of the realized niche of different assemblages,which explained the separation of the reasons for the decline of Bacillariophyceae under the condition of global warming.However,the realized niche should take into account not only environmental factors and interspecific competition,but also the feeding relationship.Due to the influence of time and funds,the feeding pressure of zooplankton was not considered in this study,so the accuracy of the realized niche can be further improved in future studies.