Carbon Sink Of Phytoplankton And Fecal Pellets In Bohai And Yellow Sea

Phytoplankton sinking and zooplankton fecal pellets package effects are two important methods of carbon-sink-related processes in marine biological pump.Based on the National Natural Science Foundation of China,the phytoplankton community and its sinking rates and carbon flux were studied,at the same time,the carbon biomass,production rates and export flux of copepods fecal pellets were clarified in Bohai and Yellow sea during summer in 2018.What is worth mentioning is that our study about carbon export of fecal pellets is the first of its kind,and these results will provide meritorious understanding in the regional carbon sinking related study in this area.The phytoplankton community collected from 42 stations in Bohai and Yellow sea were studied by Utermohl method.A total of 105 taxa which belong to 40 genera of 4 phyla were identified.The phytoplankton community was mainly composed of diatoms and dinoflagellate.The dominant species were Melosira sulcata,Skeletonema dohrnii,Ceratium fusus,Thalassionema nitzschioides,Coscinodiscus subtilis and Dictyocha fibula,etc.The high abundance of phytoplankton was mainly concentrated in the waters adjacent to Laizhou bay and Yangtze estuary,while in the cold water mass area(bottom temperature<10?)the abundance of phytoplankton was low.In the vertical distribution,the phytoplankton abundance in the Bohai sea is highest in surface layer and the phytoplankton in the Yellow sea is mainly concentrated in middle layer.Using cell biovolume to convert carbon biomass,we found the depth-weighted average carbon biomass of phytoplankton in the surveyed area ranged from 0.15 to 44.26 mg C m-3,with an average of 6.00±7.45 mg C m-3.The carbon biomass of dinoflagellates in the investigated waters was significantly higher than that of diatoms(t-test,P<0.05).The high value area of carbon biomass is located on the south coast of Liaodong peninsula.To estimate their carbon flux,the SETCOL method was used to study the phytoplankton sinking rates at 31 stations in the Bohai and Yellow sea area.The sinking rates ranged from-0.20 to 0.41 m d-1(mean=0.43±0.38 m d-1).Correlation analysis showed that phytoplankton were negatively correlated with salinity and positively correlated with other environmental factors(silicate,ammonium salt,nitrate,etc.).In the coastal estuary and Laizhou bay estuary,the sinking rates of phytoplankton were high and the sinking rates of phytoplankton in the Yellow sea is relatively low.The sinking rate in the middle layer was lower than that in the bottom layer in the surveyed area(t-test,p<0.05).The phytoplankton carbon flux at three layer ranged-2.21?44.45 mg C m-2 d-1(mean=2.64±6.22 mg C m-2 d-1),which was significantly different from each other in different sea areas.The high value of phytoplankton carbon flux was mainly located in the northeast sea off the Yangtze estuary and the south coast of the liaodong peninsula,and the carbon flux was relatively low at the Yellow sea cold water mass area in three layers.In terms of vertical distribution,the variation trend of phytoplankton carbon flux in the Yellow sea and Bohai sea is different with the increase of depth,which is roughly the same as the trend of phytoplankton sinking rates in the two sea areas.The concentration and production rates of copepods fecal particles in 10 stations in the Bohai and Yellow sea were studied by means of vertical trawl and field culture of zooplankton,and the carbon flux of copepods fecal pellets was finally estimated.It was found that the carbon biomass concentration of fecal pellets was 0.21±0.19 mg C m-3 in the surveyed area,of which the carbon biomass of fecal pellets in the Bohai sea was the highest,followed that in the north Yellow sea,and the cold water group station in the Yellow sea was the lowest.The production rates of copepods ranged from 0.33 to 1.80 pellets ind-1 h-1.And the fecal pellets production rates of copepods in the 500-1000 ?m size fraction were greater than those of copepods in the size fraction>1000 ?m(t-test,p<0.05),which was mainly affected by the abundance of phytoplankton cells in water.The carbon flux of fecal pellets in the surveyed station was between 2.79 and 22.39 mg C m-2 d-1,with an average of 10.72±2.79 mg C m-2 d-1.The carbon flux of fecal pellets of cold water group station is low,and the highest value is in the Bohai sea low dissolved oxygen(DO)content area station.The contribution of large fecal pellets(L>300?m)to fecal carbon flux can reach more than 70%at the cold water mass station,and in the non-cold water mass area and low DO content area stations,the carbon flux proportion of small fecal pellets(L<200 ?m)and medium-sized(200 ?m<L<300 ?m)is significantly higher than that of cold water mass stations.Although the carbon biomass concentration of fecal pellets in the surveyed station is far less than that of phytoplankton(2.10±2.52 mg C m-3),the carbon flux of fecal pellets is much higher than that of phytoplankton due to the fact that the sinking rates of fecal pellets is 2?3 orders of magnitude higher than that of phytoplankton.