This research studied the seasonal and spatial dynamics for abundance of picophytoplanktons (including Prochlorococcus spp., Synechococcus spp. and picoeukaryotes) in the South China Sea (SCS) and the Kuroshio. Waters were collected during five cruises between August 2009 and December 2010. Growth rates were determined in two size fractioned waters, <2 um and <10 um, after incubation. The differences of growth rates between the two size fractions were defined as the grazing rates. Before the incubation, waters were enriched with FeCl3, EDTA, or NH4Cl to examine the possible shortage of Fe or nitrogen. Abundances of picophytoplanktons and nanoflagellates were examined using a flow-cytometry and a microscope, respectively. Prochlorococcus was more abundant in the warm than the cold seasons and in the Kuroshio and the basin of the SCS than in the shelf and slope of the SCS. In the high abundance seasons/regions, low irradiance enhanced the growth rates of Prochlorococcus. Although both of the growth rates and grazing rates were high during then, the growth rates were found higher than the grazing rates. Addition of EDTA enhanced the growth rates that was likely attributed to its chelating with toxic trace metals (such as Cd2+, Cd2+) and/or with growth necessity trace metals (such as Co2+). The seasonal/spatial distributions for Synechococcus were in contrast to that of Prochlorococcus. High growth rates of Synechococcus were related to high nitrate concentrations and the low irradiance. The growth rates were higher than the grazing rates in the high nitrogen seasons/regions when/where irradiance was also relatively low. EDTA also enhanced the growth of Synechococcus, and was likely due to its chelating to remove Cd2+ and/or to retain Co2+. Distributions of picoeukaryotes were similar to that of Synechococcus. Factors affected its dynamics were not clear because of its complicated compositions.

Population dynamics of picophytoplanktons, including Prochlorococcus, Synechococcus, and picoeukaryotes, were investigated in the upstream Kuroshio. Data were collected during eight cruises between July 2007 and May 2009. Sampling stations were located along 21¢X55¡¦N and between 121¢X00¡¦E and 122¢X10¡¦E in the Kuroshio off the Southeast Taiwan. Monitoring experiments including light shadding experiment, nutrient enrichment, temperature control, and grazing experiments were conducted to better understand the mechanisms that affect the growths of the picophytoplanktons. The abundances of the picophytoplanktons were measured using a flow cytometry.Water column integrated (0~200 m) abundance of Prochlorococcus was higher (26.63 ¡Ó 3.87 ¡Ñ 1012 cells m-2) in spring than either summer (19.07 ¡Ó 4.08 ¡Ñ 1012 cells m-2), autumn (16.05 ¡Ó 2.80 ¡Ñ 1012 cells m-2), or winter (17.89 ¡Ó 5.41 ¡Ñ 1012 cells m-2). During winter, the abundance was significantly (p<0.05) higher at the offshore station (17.89 ¡Ó 5.41 ¡Ñ 1012 cells m-2) than the inshore station (3.19 ¡Ó 2.07 ¡Ñ 1012 cells m-2). The abundance of Prochlorococcus was positively related to water temperature, nitracline depth (Dni), and euphotic depth (Deu), and negatively to surface concentration of N+N or SRP. Prochlorococcus was abundant (>100 ¡Ñ 103 cells ml-1) in the upper 100-m water column. Its maximum (200~300 ¡Ñ 103 cells ml-1) often occurred at the depth shallower than 75 m. The cell density sustained at >25 ¡Ñ 103 cells ml-1 between 100~150 m and was almost nil at the depth deeper than 150 m. There was no significant seasonal differences for either the abundances of Synechococcus (0.32~1.07 ¡Ñ 1012 cells m-2) or picoeukaryotes (0.16~0.24 ¡Ñ 1012 cells m-2). During winter, the abundances of Synechococcus was significantly (p<0.05) higher in the offshore Kuroshio water (2.94 ¡Ó 0.32 ¡Ñ 1012 cells m-2) than that of the inshore Kuroshio water. Similar trend of offshore (0.52 ¡Ó 0.05 ¡Ñ 1012 cells m-2) higher than the inshore was observed for picoeukaryotes in winter. The dynamics of Synechococcus abundance was positively related to surface SRP concentration and negatively to Dni. The picoeukaryotes abundance was positively related to surface N+N concentration, and SRP and negatively to Temp, Dni, and Deu. Vertical distribution of Synechococcus showed that the maximum abundance often occurred above 75 m, but was almost nil below 100 m. By contrast, the maximum abundance for picoeukaryotes often occurred between 50~125 m. The abundance of Synechococcus was positively related to the abundance of picoeukaryotes. And their abundance were negatively related to that of Prochlorococcus. Many environmental factors fluctualed parallelly. Dynamics of surface Temp, Dni and Deu were positively correlated to each other and either of them was negatively correlated to the dynamics of surface concentration of N+N or SRP. Surface N+N was positively correlated with surface SRP. The result of light shadding experiment showed that Prochlorococcus and picoeukaryotes, compared to Synechococcus, were much sensitive to high intensity of light. This suggest that Synechococcus was more tolerant to high light intensity or required more light energy than Prochlorococcus or picoeukaryotes. The results of nutrient enrichment experiments showed that addition of EDTA significantly enhanced the growth of three groups of picophytoplanktons. However, there was no significant difference after addition of either nitrate, Fe, or Cu. Prochlorococcus grew better at 27 ¢XC than 30 ¢XC in the temperature experiment. But there was no difference in the growth rate between 27 ¢XC and 30 ¢XC for Synechococcus or picoeukaryotes The result of grazing experiment showed that there was no difference between the growth rate with and without grazers in the incubation for any of the three groups of picophytoplanktons.

This research investigated the seasonal and spatial distributions of picophytoplankton, including Prochlorococcus, Synechococcus and picoeukaryotes, in the northern South China Sea. Monitoring experiments including light intensity control and nutrient enrichment were conducted concurrently with on board sampling to examine factors affecting their cell densities dynamics. Quantification of cell numbers was carried out by flow cytometry. Averaged Synechococcus abundance in the South China Sea was 1¡Ñ104 cells ml-1, high in winter (1.37¡Ó0.30¡Ñ104 cells ml-1) and low in summer or fall (0.51¡Ó0.13¡Ñ104 cells ml-1 and 0.53¡Ó0.22¡Ñ104 cells ml-1, respectively). During a same season of the year, there was more Synechococcus in the shelf-slope region than in the basin. The cell density in summer, but not in winter, was significantly positively related to surface water nutrient concentration. Nutrient enrichment experiment carried out in winter also indicated that the growth of Synechococcus did not respond to addition of nitrate. On the other hand, Synechococcus seemed to prefer high illumination. In the light intensity experiment, Synechococcus collected from surface water grew better at 100% surface illumination than <100% light intensities. Synechococcus collected from deep water grew best at 30% and 18% of surface illuminations. Vertically, Synechococcus concentrated mostly in surface water with maximum cell number occurring at the surface or a few meters deep. Nutrient enrichment experiment in winter also showed that Synechococcus responded significantly to iron addition. Average cell density of picoeukaryotes was always less than 0.5¡Ñ104 cells ml-1, being high in winter (0.46¡Ó0.10¡Ñ104 cells ml-1) and low in summer or fall (0.15¡Ó0.02¡Ñ104 cells ml-1 and 0.19¡Ó0.03¡Ñ104 cells ml-1, respectively). Picoeukaryotes was always more concentrated in the shelf-slope region than in the basin, especially in winter when cell density in the shelf-slope region was 0.70¡Ó0.11¡Ñ104 cells ml-1. Although in winter picoeukaryotes was significantly positively related to surface water nutrient concentration, enrichments of nitrate or iron did not enhance their growth. Prochlorococcus had a cell density > 5.5¡Ñ104 cells ml-1 in the euphotic zone, and distributed as deep as 200 m. Light intensity monitoring experiment showed that Prochlorococcus from surface water grew better under high illumination than those from deep water and vice versa. Under 9% of surface illumination, deepwater Prochlorococcus population showed a positive growth, corresponding well with its deep distribution. Nutrient enrichment experiment conducted in winter showed that Prochlorococcus did not respond to enrichment of nitrate or iron.

Flow cytometry (FCM) is a well established technique used for enumeration and characterization of marine biological particles, which fulfills the scientific demands of rapid cell counting automation. FCM allows the discrimination of pico- and nanoplankton populations regarding its abundance, cell size, and pigment content both by natural or induced fluorescence. The cyanobacterium Prochlorococcus is widespread in the euphotic zone of the tropical and subtropical oceans, and is considered the smallest and most abundant photosynthetic organism in the planet. Synechococcus, other important cyanobacterium genus present in the picoplankton, is highly diverse and is widely distributed in marine ecosystems from cold and mesotrophic, to warm, open ocean oligotrophic waters. Photosynthetic pico- and nanoeukaryotes display a range of physiologies and life strategies. Although its abundance is generally lower, the larger cell size leads to a significant contribution to the epipelagic community biomass. In this thesis I aimed to investigate the abundance and carbon biomass distribution of heterotrophic bacteria, Prochlorococcus, Synechococcus, autotrophic pico- and nanoeukaryotes in the Southwest Atlantic Ocean off Brazil, their relation to the different water masses and the influence of hydrodynamic (South Atlantic Central Water intrusion) and biological processes (Trichodesmium spp. and Mesodinium rubrum blooms) on such distributions. / A citometria de fluxo (FCM, sigla em inglês) é uma técnica bem estabelecida, usada para enumeração e caracterização de partículas biológicas marinhas, a qual supre a demanda científica por automação rápida de contagem de células. A FCM permite a discriminação de populações pico- e nanoplanctônicas no que concerne à sua abundância, tamanho de célula e pigmentação, tanto por fluorescência natural ou induzida. A cianobactéria Prochlorococcus é amplamente disseminada pela zona eufótica dos oceanos tropicais e subtropicais, sendo considerada o menor e mais abundante organismo fotossintético do planeta. Synechococcus, outro importante gênero de cianobactérias presente no picoplâncton, é diverso e amplamente distribuído em ecossistemas marinhos, de águas frias e mesotróficas à regiões oceânicas quentes e de águas oligotróficas. Pico- e nanoeucariotos fotossintéticos apresentam uma grande variedade de fisiologias e estratégias de vida. Embora a abundância destes grupos seja geralmente menor, o maior tamanho de suas células resulta numa contribuição significante dos mesmos para a biomassa da comunidade epipelágica. Na presente tese eu objetivei investigar a abundância e a distribuição da biomassa de carbono de bactérias heterotróficas, Prochlorococcus, Synechococcus, pico- e nanoeucariotos autotróficos no sudoeste do Oceano Atlântico ao largo do Brasil, sua relação com as diferentes massas d\'água e a influência de processos hidrodinâmicos (como a intrusão da Água Central do Atlântico Sul) e biológicos (florações de Trichodesmium spp. e Mesodinium rubrum) em tais distribuições.

Flow cytometry (FCM) is a well established technique used for enumeration and characterization of marine biological particles, which fulfills the scientific demands of rapid cell counting automation. FCM allows the discrimination of pico- and nanoplankton populations regarding its abundance, cell size, and pigment content both by natural or induced fluorescence. The cyanobacterium Prochlorococcus is widespread in the euphotic zone of the tropical and subtropical oceans, and is considered the smallest and most abundant photosynthetic organism in the planet. Synechococcus, other important cyanobacterium genus present in the picoplankton, is highly diverse and is widely distributed in marine ecosystems from cold and mesotrophic, to warm, open ocean oligotrophic waters. Photosynthetic pico- and nanoeukaryotes display a range of physiologies and life strategies. Although its abundance is generally lower, the larger cell size leads to a significant contribution to the epipelagic community biomass. In this thesis I aimed to investigate the abundance and carbon biomass distribution of heterotrophic bacteria, Prochlorococcus, Synechococcus, autotrophic pico- and nanoeukaryotes in the Southwest Atlantic Ocean off Brazil, their relation to the different water masses and the influence of hydrodynamic (South Atlantic Central Water intrusion) and biological processes (Trichodesmium spp. and Mesodinium rubrum blooms) on such distributions. / A citometria de fluxo (FCM, sigla em inglês) é uma técnica bem estabelecida, usada para enumeração e caracterização de partículas biológicas marinhas, a qual supre a demanda científica por automação rápida de contagem de células. A FCM permite a discriminação de populações pico- e nanoplanctônicas no que concerne à sua abundância, tamanho de célula e pigmentação, tanto por fluorescência natural ou induzida. A cianobactéria Prochlorococcus é amplamente disseminada pela zona eufótica dos oceanos tropicais e subtropicais, sendo considerada o menor e mais abundante organismo fotossintético do planeta. Synechococcus, outro importante gênero de cianobactérias presente no picoplâncton, é diverso e amplamente distribuído em ecossistemas marinhos, de águas frias e mesotróficas à regiões oceânicas quentes e de águas oligotróficas. Pico- e nanoeucariotos fotossintéticos apresentam uma grande variedade de fisiologias e estratégias de vida. Embora a abundância destes grupos seja geralmente menor, o maior tamanho de suas células resulta numa contribuição significante dos mesmos para a biomassa da comunidade epipelágica. Na presente tese eu objetivei investigar a abundância e a distribuição da biomassa de carbono de bactérias heterotróficas, Prochlorococcus, Synechococcus, pico- e nanoeucariotos autotróficos no sudoeste do Oceano Atlântico ao largo do Brasil, sua relação com as diferentes massas d\'água e a influência de processos hidrodinâmicos (como a intrusão da Água Central do Atlântico Sul) e biológicos (florações de Trichodesmium spp. e Mesodinium rubrum) em tais distribuições.