Studies On Phytoplankton Community Under Integrated Multi-trophic Aquaculture Modes In Sanggou Bay

Six surveys were carried out between 2015 and 2016 in Sanggou Bay to investigate the phytoplankton community structure and environmental factors in different mariculture regions.In addition,mesocosm experiments were conducted in October 2015.The purpose of this study is to analyze the spatio-temporal distribution and size composition of phytoplankton community and photosynthetic pigments using microscopic and HPLC analyses.The relationships among phytoplankton community structure,aquaculture modes and environmental factors were discussed.The results of this study will provide a scientific basis for optimizing mariculture structure.Some important results are drawn as followings:1)Nutrients(DIN,DIP,DSi)fluctuated significantly between seasons.DIN and DIP peaked in November(autumn)and DSi peaked in August(summer).Concentrations of DIN and DSi decreased from inner to outer bay.Concentration of DIP peaked in shellfish+fish culture area,with an average of 3.29 ± 2.00 ?mol/L.2)A total of 97 taxa belonging to 52 genera of 4 phyla,were identified in Sanggou Bay.There were 72 species of diatoms in 36 genera,22 species of dinoflagellates in 13 genera,2 species of chrysophytes in 2 genera and 1 species of cyanobacteria in 1 genus.The average of cell number was 59.51 cells/m L.Cell number peaked in summer,and was lowest in autumn,with an average of 96.50 cells/m L and 16.63 cells/m L,respectively.Diatom was the predominant group of phytoplankton with average cell number of 56.10 cells/m L.The common predominant species included Skeletonema costatum,Paralia sulcata,Leptocylindrus danicus,Chaetoceros lorenzianus,Pseudo-nitzschia delicatissima and so on.Phytoplankton species diversity and evenness indexes showed similar patterns,and reached maximum values in November 2015.3)Cell numbers were higher in aquaculture areas than the open sea.Within the culture areas,cell number was low in shellfish culture area with the averages of 30.11 cells/m L.Dominant species differed in different sea areas,e.g.Skeletonema costatum,Leptocylindrus danicus,Chaetoceros lorenzianus and Pseudo-nitzschia delicatissima in shellfish culture area,Skeletonema costatum and Leptocylindrus danicus in shellfish+fish culture area,Paralia sulcata,Pseudo-nitzschia delicatissima and Leptocylindrus danicus in macroalga culture area,and Paralia sulcata and Chaetoceros lorenzianus in the open sea.Phytoplankton species diversity and evenness indexes were lower in shellfish+fish culture area with the average of 2.37 and 0.53,respectively.4)Twenty-two pigments were detected in Sanggou Bay based on HPLC analysis.Fucoxanthin was the major pigment,with an average of 0.31 ?g/L.The result indicated that diatom dominated the phytoplankton community in Sanggou Bay as the same results with microscopic analysis.Concentrations of chlorophyll a peaked in summer,and was low in autumn,with an average of 0.40 ?g/L and 0.14 ?g/L,respectively.Concentrations of chlorophyll a were higher in the cultural areas,and low in the open sea.5)Microplankton and nanoplankton dominated in phytoplankton community of Sanggou Bay.While picoplankton were in low levels in most seasons except for August 2016.Nanoplankton dominated in shellfish+fish culture area,with the average proportion of 62.81%.Other sea areas inluding the open sea,shellfish culture area,and macroalga culture area were dominated by microplankton with the average proportions of 71.55%,62.48%,60.06%,respectively.Meanwhile,picoplankton were higher in shellfish+fish culture area.6)Patinopecten yessoensis and Gracilaria lemaneiformis were selected as experiment materials in the mesocosm experiment.Four groups were set in the experiment including the control group,shellfish group,macroalga group,and shellfish+macroalga group.A total of 32 taxa which belong to 28 genera of 4 phyla were identified in the mesocosm experiment.There were 25 species of diatoms in 21 genera,4 species of dinoflagellates in 4 genera,2 species of chrysophytes in 2 genera and 1 species of cyanobacteria in 1 genus.Diatoms was the predominant group of phytoplankton,with the average cell number and proportion of 0.89 cells/m L and 78.13%,respectively.Followed by dinoflagellates with the average cell number and proportion of 0.02 cells/m L and 12.50%,respectively.Cell number was higher in the control group,with the average of 1.29 cells/m L.Cell numbers in the shellfish,macroalga and shellfish+macroalga groups were 0.88,0.76,0.73 cells/m L,respectively.The results indicated shellfish,macroalga cultures and their co-culture reduced phytoplankton biomass.Nineteen pigments were detected in mesocosm experiment based on the results of HPLC analysis.Fucoxanthin was the major pigment.Chl a contents in shellfish,macroalga groups and shellfish+macroalga groups were significantly lower than those in control.