The Distribution Of Ultraplankton In Different Marine Environments

In this dissertation, temporal and spatial variations of ultraplankton (Synechococcus,Prochlorococcus, picoeukaryotes, nanoeukaryotes and heterotrophic bacteria) abundanceswere studied in different marine environments (in the East China Sea, Bay of Marseille inthe NW Mediterranean Sea and Jiaozhou Bay in the Yellow Sea) based on flow cytometryanalysis. The relationships between environmental conditions and ultraplankton abundancewere also studied in order to search for the possible controlling factors on ultraplanktondistribution in different marine environments.Picoplankton distribution in different water masses of the East China Sea (ECS) wasinvestigated in November2006and February2007. The cruises tracked crossed three majorwater masses: Coastal Water Mass (CWM), Shelf Mixing Water Mass (MWM) andKuroshio Water Mass (KWM). Picoplankton composition was resolved in four main groupsby flow cytometry, namely Synechococcus, Prochlorococcus, picoeukaryotes, andheterotrophic bacteria. The average Synechococcus, picoeukaryotes and heterotrophicbacteria abundances were (10.63±10.88)×10~3,(1.61±1.16)×10~3,(3.39±1.27)×10~5cells/mLin autumn and (6.45±8.60)×10~3,(3.23±2.63)×10~3,(3.76±1.37)×10~5cells/mL in winter,respectively. Prochlorococcus was seldom observed in surface samples and not found inCWM, neither in autumn nor in winter. In contrast, Prochlorococcus was present in MWMand KWM with abundance at around10~3cells/mL in autumn and winter. Synechococcusdistribution showed marked variations with respect to water masses with the maximumabundance in KWM and minimum abundance in CWM. The depth-averaged integratedabundance of Synechococcus was about5-fold higher in KWM than in CWM, which mightbe mainly influenced by temperature. In autumn particularly, at several stations in MWM,Synechococcus was resolved as two subgroups. Picoeukaryotes were less affected bywater-mass differences than Synechococcus. Heterotrophic bacteria depth-averaged integrated abundance exhibited the smallest seasonal variations with respect to watermasses. Correlation analysis showed obvious differences between picoplankton abundancesand environmental parameters (temperature, nutrients and chlorophyll a) in three watermasses, indicated the three water masses are likely to have different influences onpicoplankton (particularly Synechococcus) distribution.In the Bay of Marseille (NW Mediterranean), the composition of ultraplankton (<10μm) sampled in surface (1m depth) was investigated from May2010to April2012.Ultraplankton (<10μm) composition was resolved in four main groups by flow cytometry,namely Synechococcus, picoeukaryotes, nanoeukaryotes and heterotrophic bacteria.Prochlorococcus was seldom observed in our study, even though in the previous studies,Prochlorococcus has been widely reported. In the Bay of Marseille, Synechococcusdominated the ultraphytoplankton community in terms of cell abundance. Synechococcusshowed a clear seasonal pattern with higher abundance in spring (April and May,>90×10~3cells/mL) and lower abundance in autumn and winter (<15×10~3cells/mL). Synechococcusabundance varied more than40-fold between a spring maximum (May2010,129.59×10~3cells/mL) and winter minimum (March2012,2.29×10~3cells/mL) values. Picoeukaryoteshad a similar distribution pattern as Synechococcus and their high abundance occurred inwinter (January-March,>10.72×10~3cells/mL), slightly earlier than Synechococcus.Nanoeukaryotes exhibited much lower abundance than Synechocccus and picoeukaryotes.The distribution pattern of nanoeukaryotes was similar to that of Synechococcus. FromMay to September in2010, the data of heterotrophic bacteria abundance were missingbecause of technical reasons. Heterotrophic bacteria exhibited pronounced fluctuationsduring the study period and reached their maximum abundance (9.74×10~5cells/mL) inMarch2012. On12July2010, in addition to groups of Synechococcus, Prochlorococcus,picoeukaryotes, and nanoeukaryotes, an unidentified additional cluster was observed.These unknown particles were sorted out by flow cytometry and then observed byepifluorescence microscopy: they appeared to be chain-forming microorganisms, just liketrichomes of some diazotroph cyanobacteria with one heterocyst, but not unambiguouslydistal. The chains observed after cell sorting were not straight, but rather folded. The shortoccurrence of free trichomes in the Bay of Marseille with such a high abundance remains to be explained. High temperature in summer, induced stratification of the water column,and nitrate depletion, may be related to this event. High frequency survey ofultraphytoplankton assemblages at the single cell level appears necessary to observe moreefficiently such phenomenon and document more extensively its full dynamics.The seasonal variability of picoplankton abundances in the semi-enclosed Jiaozhou Bay(Yellow Sea) were investigated monthly from January2007to December2010by flowcytometic analysis. Picoplankton community in the studied area was comprised ofSynechococcus, picoeukaryotes and heterotrophic bacteria. Prochlorococcus was notdetected at any station during the whole year. A pronounced seasonality was observed inpicoplankton community structure and abundances inside the bay, in the Bay mouth andoutside the bay. Synechococcus exhibited a clear seasonal pattern, with maximumabundance occurring in August when temperature was highest and minimum abundance inwinter and spring. The maximum abundance of picoeukaryotes occurred slightly earlier(April-July) than Synechococcus. Picoeukaryotes abundance outnumbered Synechococcusin most of the year except in August. Heterotrophic bacteria exhibited two peaks ofabundances in August (major peak) and April (minor peak), which were quite similar asChlorophyll a distribution. Principal Component Analysis (PCA) suggested that seasonalvariations of picoplankton were mainly driven by temperature and nutrient concentrations(NO3-, NO2-, NH4-and SiO32-). The significantly positive correlations betweentemperature/nutrients and picoplankton abundances also implied the importance oftemperature and nutrients in picoplankton distribution in Jiaozhou Bay.The ultraphytoplankton composition, abundance and fluorescence in the Bay ofMarseille and Jiaozhou were compared in2011and2012. In the Bay of Marseille andJiaozhou, Synechococcus, picoeukaryotes and nanoeukaryotes were observed in all thesamples. Prochlorococcus was seldom observed in the Bay of Marseille and neverobserved in the Jiaozhou Bay. Besides, in the Bay of Marseille, a possible bloom of freetrichomes was observed in a short period during summer2010. While in the Jiaozhou Bay,an unknown and abundant cluster was obsvered in all the samples. With flow cytometryand scanning electron microscopy, it is difficult to identify this species. In the next step,molecular technique should be introduced for further identification. In the seasonal scale, ultraphytoplankton in the Bay of Marseille and Jiaohzou exhibited quite differentdistribution patterns. In the Bay of Marseille, Synechococcus and nanoeukaryotes presentedthe maximum abundance in spring and minimum abundances in autumn and winter.Picoeukaryotes showed the maximum abundance in winter, slightly earlier thanSynechococcus and nanoeukaryotes. While in the Jiaozhou Bay, Synechococcus abundancewas highest in August (T>24°C) and lowest in winter and spring (T<15°C). Picoeukaryotesand nanoeukaryotes exhibited highest abundances in July, one month earlier thanSynechococcus, and lower abundances in winter and spring, similar as Synechococcus. Itseemed in the oligotrophic Bay of Marseille, nutrient supply is more important forultraphytoplantkon distribution, while in the Jiaozhou Bay, temperature is the keycontrolling factor for ultraphytoplantkon distribution. In both bays, cell abundanceexhibited a clear opposite trend compared to cellular fluorescence.