The Study Of Phytoplankton Community And Carbon Sink In The Eastern Indian Ocean

Phytoplankton community in the East Indian Ocean was studied during seven cruises carried out in 2014-2015 and 2017-2021,and sinking rate of phytoplankton,zooplankton fecal pellets were studied in 2019.These studies will provide useful insights into the interannual variability of phytoplankton in the Eastern Indian Ocean and will also contribute to carbon sink-related studies in the region.The interannual and seasonal variability of phytoplankton communities in the equatorial eastern Indian Ocean showed that the phytoplankton in the surveyed area fluctuated over the years.Among the seven cruises,the higher phytoplankton cell abundance in 2017 and 2021 was mainly due to the significantly higher abundance of cyanobacteria in the Bay of Bengal.The vertical distribution of phytoplankton showed that cyanobacteria and dinoflagellates were distributed in the upper layers of the water,and diatoms were mainly distributed in the lower and middle layers.A comparison of the 2020survey with other years showed that dinoflagellates and cyanobacteria were predominant in the spring,and diatoms in the autumn.The results of the generalised additive models showed that an increase in temperature promoted the growth of cyanobacteria and dinoflagellates,while diatoms showed a shift from growth to inhibited at 26°C;The N:P fit showed that cyanobacteria and dinoflagellates were better adapted when phosphorus was limited.The phytoplankton diversity index in the surveyed area showed a patchy distribution and was mainly influenced by cyanobacteria.The SETCOL method was applied to study the sinking rates of phytoplankton in spring2019 and autumn 2020 and to estimate the carbon sink fluxes of phytoplankton.The sinking rates in 2019 and 2020 were-0.095～1.277 m/d and-0.051～1.823 m/d,respectively,indicating higher sinking rates in autumn 2020 than in spring 2019.All stations in both cruises had lower sinking rates at the and higher sinking rates at the bottom.Sub-regional comparisons indicated that sinking rates were greater in the Bay of Bengal and Equator in 2020 than in 2019,indicating that phytoplankton communities sink more rapidly when diatoms were dominant.Comparing carbon fluxes in the Bay of Bengal and Equatorial separately from spring 2019 to 2020,we found that carbon fluxes in Equatorial were greater in 2020 than in 2019,mainly duo to the survey area was dominated by diatoms in 2020,whereas carbon fluxes in the Bay of Bengal were significantly higher in Bengal in2019 than in 2020,mainly due to the large amounts of Trichodesmium spp.in 2019 at Station E87-26.Concentrations of zooplankton fecal pellets in the Eastern Indian Ocean were investigated in 2019 and a zooplankton fecal productivity experiment was conducted.fecal pellets abundance varied little between water layers,but there were significant differences between stations.The results of the correlation analysis showed that the distribution of zooplankton fecal pellets was not significantly related to environmental factors.The production rate of zooplankton fecal pellets in the survey area ranged from 0.015～0.521pellets ind^-1h^-1and was strongly influenced by the phytoplankton biomass in the water column,the station with high phytoplankton biomass having higher fecal pellet production rates.the zooplankton fecal pellet production rates were difference at different stations,Fecal particle production rate showing 500μm greater than 200～500μm.Fecal pellet production rates for zooplankton in the 200～500μm particle size range from 0.015～0.169 pellets ind^-1h^-1with a mean value of 0.067 pellets ind^-1h^-1.Fecal production for zooplankton in the 500μm ranges from 0.079～0.521 pellets ind^-1h^-1with a mean value of0.226 pellets ind^-1h^-1.Fecal pellet production ranged from 0.398 to 18.191 mg C m^-2d^-1with a mean value of 7.452 mg C m^-2d^-1;The production of fecal pellets was greatest at EQ-11 and the zooplankton>500μm produced more fecal pellets and could account for more than 70%of the total production at all stations and were the main contributors.This study shows that temperature is the main factor controlling phytoplankton community structure in the oligotrophic waters of the eastern Indian Ocean,with a shift in community structure to a dominance of dinoflagellates and cyanobacteria as temperatures increase.Both phytoplankton deposition and zooplankton fecal pellet deposition are important routes of carbon export from the upper water column to the bottom water column.