As atividades antrópicas desenvolvidas nas ultimas décadas têm alterado o ciclo natural dos nutrientes, o que traz efeitos negativos às aos recursos hídricos. Os crescentes problemas com a eutrofização nesses ambientes têm aumentado os casos de florescimento de cianobactérias, que podem trazer riscos aos ecossistemas e comprometer a saúde da população. Em reservatórios, esses efeitos são ainda mais indesejáveis tendo em vista que acumulam água para múltiplos usos. Em vista desse quadro, torna-se importante o estudo da relação dos nutrientes com a comunidade fitoplanctônica. Geralmente os estudos de controle da eutrofização por restrição de nutrientes se atentam apenas à restrição do fósforo. O conhecimento sobre os efeitos das diferentes fontes nitrogenadas em reservatórios tropicais/subtropicais é praticamente inexistente. Para investigar a importância do amônio, nitrato e ureia como fontes de nitrogênio para o fitoplâncton em reservatórios tropicais com diferentes graus de trofia, foram realizadas 4 coletas nos reservatórios de Itupararanga (supereutrófico) e Barra Bonita (hipereutrófico) em dois pontos na coluna de água (profundidades referentes a 50 e 10% da radiação solar fotossinteticamente ativa na subsuperfície). As taxas de assimilação obtidas nessa pesquisa foram relativamente maiores que em trabalhos semelhantes devido à grande biomassa fitoplanctônica em Itupararanga e em Barra Bonita. As taxas de assimilação foram maiores na estação chuvosa, em Barra Bonita que em Itupararanga e, na primeira profundidade. As estratégias de assimilação de nitrogênio foram relacionadas à composição da comunidade fitoplanctônica. Em Itupararanga a comunidade apresentou domínio da cianobactéria Cylindrospemospsis raciborskii e mostrou-se bem adaptada à assimilação de amônio, nitrato e ureia, sendo o amônio a forma nitrogenada preferida, seguido de nitrato e ureia. Em Barra Bonita, a comunidade apresentou domínio da cianobactéria Microcystis aeruginosa durante a estação chuvosa, e ausência de dominância na estação seca. A mudança na composição da comunidade resultou numa maior afinidade pelo nitrato. A intensidade luminosa teve efeito positivo na assimilação da ureia. As cianobactérias parecem bem adaptadas à assimilação de qualquer fonte de nitrogênio disponível, e parte do sucesso desse grupo em reservatórios subtropicais pode ser atribuído a esta característica. / Human activities in the last decades are altering the natural nutrient cycles, with damaging repercussions to the water sources. The growing eutrophication problems are increasing cyanobacterial bloom occurrences and the environmental and health risks associated. In reservoirs, those effects are even more undesirable, due to its waters multipurpose. Therefore, understanding the relations between nutrients and phytoplankton community is needed. Overall, eutrophication control efforts are focus on phosphorus restriction. Little is known on the effects the available nitrogen sources on tropical/subtropical reservoirs primary production. To investigate the importance of ammonium, nitrate and urea to the phytoplankton in subtropical reservoirs with different trophic states, four field campaignes were conducted between October/2013 and June/2014. The water was collected in two points (depths corresponded to 50 e 10% of the surface\'s solar radiation). The uptake rates observed in the work were relatively higher than most similar works, presumably due to the high phytoplanktonic biomass in Itupararanga and Barra Bonita. The uptake rates were higher in the rain season, in the shallower depths and in Barra Bonita than Itupararanga. The uptake also differ among the reservoirs: In Itupararanga, the phytoplankton community was dominated by the cyanobacteria Cylindrospemospsis raciborskii dominance, and was well adapted to the use of ammonium, nitrate and urea (with ammonium been the favorite N source). In Barra Bonita, the phytoplankton community was dominated by the cyanobacteria Microcystis aeruginosa in the rain season and showed no dominance in the dry season. The shift in the phytoplankton community seamed to increase its affinity to nitrate uptake. The light intensity was correlated with urea uptake only. The cyanobacteria ability to use any given nitrogen source can be a key factor to its success.

As florestas subtropicais têm um papel importante nos serviços ecossistêmicos, embora pouco se conheça sobre a sua dinâmica e funcionamento, especialmente na porção sul da Mata Atlântica. A dinâmica florestal é frequentemente avaliada pela variação líquida da biomassa de árvores. Essa dinâmica de biomassa é influenciada por processos demográficos nas comunidades, os quais correspondem ao crescimento (de sobreviventes), recrutamento e mortalidade de indivíduos. Avaliamos a influência de fatores bióticos (atributos funcionais de plantas e a estrutura inicial da floresta) e fatores abióticos (variáveis do solo) sobre a dinâmica de biomassa de uma floresta subtropical, em um intervalo de 18 anos. Também avaliamos o efeito dos processos demográficos sobre a mudança de biomassa líquida da floresta. O estudo foi realizado no sul do Brasil, em uma área de transição entre a Floresta com Araucária e a Floresta Estacional, sem registro de distúrbio há pelo menos 60 anos, mas com histórico anterior de corte seletivo de madeira. Os levantamentos foram realizados em 1999 e em 2017, em 24 parcelas permanentes de 500 m2 cada, mensurando todos os indivíduos com CAP > 30 cm. Atributos funcionais foliares, de densidade de madeira e altura potencial foram obtidos para todas as espécies. Modelos lineares foram utilizados para avaliar o efeito de atributos funcionais (considerando valores médios da comunidade inicial (CWM= ‘community-weighted mean’) e a diversidade funcional (FD)), da estrutura inicial (área basal no tempo inicial) e de variáveis do solo sobre as variáveis resposta (taxa anual de crescimento, recrutamento e mortalidade). Modelos lineares também foram utilizados para avaliar o efeito dos processes demográficos (taxa de crescimento, recrutamento e mortalidade) e incremento em biomassa (biomassa de sobreviventes + recrutas) sobre a variação líquida de biomassa na comunidade. Observamos um efeito positivo do CWM da altura máxima sobre o crescimento de sobreviventes, e um efeito negativo do CWM do conteúdo de fósforo foliar, do FD de conteúdo de matéria seca foliar (LDMC) e da FD da altura máxima. A biomassa de recrutados teve um efeito positivo do pH e negativo do K do solo, enquanto a mortalidade foi positivamente associada à estrutura inicial da comunidade. Observamos ainda que a mudança líquida de biomassa desta floresta teve influência significativa da mortalidade e do incremento total (biomassa de sobreviventes + recrutados). Comunidades com indivíduos mais altos e com atributos relacionados à conservação de recursos em média crescem mais, enquanto que o menor crescimento teve relação com comunidades funcionalmente mais diversas em termos de LDMC e altura, potencialmente associado a espécies de sub-bosque. Solos menos ácidos permitiram o maior recrutamento, enquanto que a alta concentração de K limitou o recrutamento. Comunidades com maior cobertura inicial apresentaram maior mortalidade de espécies no período. Este trabalho traz importantes resultados sobre a dinâmica de florestas subtropicais, demonstrando que as características iniciais da comunidade, tanto bióticas quanto abióticas, tiveram influência no incremento e/ou perda em biomassa deste ecossistema num período de quase duas décadas. / Subtropical forests play an important role on ecosystem services, but the drivers of biomass dynamics and ecosystem functioning specially at the southern portion of Mata Atlântica are poorly understood. The forest dynamics is often evaluated by tree net biomass change, which is underlied by the community demographic processes. These demographic processes correspond to the growth of survival trees, recruitment and mortality. Here we evaluated the influence of biotic factors (plant functional traits and the initial forest structure) and abiotic factors (soil variables) on biomass dynamics of a subtropical forest in an interval of 18 years. We also evaluated the effect of demographic processes on the net biomass change of the forest. This study was carried out in a subtropical forest in southern Brazil and the region is characterized by the Atlantic forest s.l, in a transitional area between Araucaria and Seasonal forest. The study area corresponds to a protected area (Chapecó National Forest) that did not suffer disturbance since at least 60 years but had some selective logging before. The forestry inventories were realized in 1999 and 2017, in 24 permanent plots with 500 m2 each one. All the individuals with circumference at breast height > 30 cm were measured, identified and tagged. Leaf functional traits, wood density and maximum height were obtained for all species. We performed linear models to evaluate the effect of functional traits (following the mean values of initial community, community-weighted mean (CWM) and functional trait diversity (FD)), initial forest structure (community basal area in initial time) and soil variables, on the response variables (annual rate of growth of survival, recruits and mortality). Linear models were also used to evaluate the effect of tree demographic processes (biomass growth of survival, recruits, mortality and biomass increment (growth of survival + recruits)) on the net biomass change. We observed a positive effect of CWM and FD of maximum height, FD of leaf dry matter content (LDMC) and a negative of CWM of leaf nitrogen content on the growth of survivors. The biomass of recruits had a negative effect of soil K content, and a positive effect of soil pH. Biomass mortality was positive related to the initial forest structure. The net biomass change was negatively influenced by the biomass mortality and positively related to the biomass increment. Communities with taller individuals in mean and traits related to resources conserving grew more. The lower growth was related to more diverse communities in terms of LDMC and maximum height, probably associated to understory species. Less acid soils allowed more recruitment, and high availability of soil K limited the recruitment. Communities with more initial tree cover had more mortality along the time. This work provided important results about the biomass dynamics in subtropical forests, showing that biotic and abiotic community initial characteristics influenced the gain or loss of biomass of this ecosystem in a period of almost two decades.

The investigation of the role of basin-scale forcing on the circulation of the Faroe-Shetland Channel (FSC) is important to further understanding of the inter-annual variability of the Atlantic water (AW) fluxes in this region. The FSC plays a key role in the transfer of warm and saline AW towards the Nordic Seas that is an integral part of the Atlantic Meridional Overturning Circulation which is projected to decline over the twenty-first century and might reduce the oceanic heat and salt transports towards the Arctic. So far little attention has been paid to the mechanisms driving the AW fluxes in the FSC, reliable estimates of AW temperature and salt transports time series are lacking. This study presents a new time series of the AW fluxes based on the combination of hydrography and altimetry data. The mechanisms involved in driving the variability of AW fluxes are considered based on observational data and the output from a high-resolution ocean model (VIKING20). The hydrographic observations from 1993 to 2015 show an increase in temperature and salinity of AW. However, there is no evidence of trends in AW volume, temperature or salt transports during the observed period. This analysis confirms that the amount of heat and salt transported through the FSC is dominated by the volume transport. Moreover, this study identifies a bias in the standard deviation of the geostrophic velocity at a depth associated with referencing the geostrophic calculations to the sea surface geostrophic velocity from satellite altimetry. This finding does not strongly influence the AW volume transports in the AW layer, however, it has important implications for estimates of the geostrophic volume transport at depth. This study shows that the Ekman driven up/downwelling and the differential Ekman pumping mechanisms driven by the local wind forcing may influence sea surface height (SSH) and the displacement of isopycnals in the channel, leading to AW volume transport variabilit However, due to the large associated error bars on the surface and subsurface parameters, there is no clear evidence that these mechanisms are significantly responsible for the AW volume transport variability in the FSC. Lagrangian trajectories show evidence of two pathways from the North Atlantic to the FSC that may explain AW variability in the FSC: one pathway involves the flow of warm and saline waters from the Rockall Trough that corresponds to high temperatures and low AW volume transport in the channel, and the other pathway involves the flow of relatively cooler and less saline waters from the Iceland Basin that is linked to low temperatures and stronger volume transport in the FSC. Moreover, we show that the first (second) pathway is associated with the negative (positive) phases of the North Atlantic Oscillation (NAO) and the ocean gyre contraction (expansion). The changes of the NAO index phases explain 26 % of the AW volume transport variance in the FSC. Another important mechanism that leads to stronger (weaker) AW volume transport is stronger (weaker) pressure gradient across the Greenland-Scotland Ridge, reflected by the SSH changes. This mechanism explains 29 % of AW volume transport variance in the FSC.

The name of the thesis: Practical exercises using the theme of tropical and subtropical fruits in teaching natural history and biology describe the basic issue that the following text is devoted to. This thesis is connected to my bachelor thesis, "Tropical Fruit in Natural History and Biology Textbooks," defended in 2010. The main objectives of the thesis was to complete the contentual analysis of natural history and biology textbooks for the 2nd degree of primary schools, lower grades of grammar schools, and secondary schools. I focused on the issue of the incorrect inclusion of certain plants in chapters for tropical and subtropical fruits. Based on the total contentual analysis of textbooks, I have chosen the processing of practical exercises with the theme of tropical and subtropical fruits as the main objective. Based on the evaluation of the questionnaire, which served as a basis for practical exercises, in which teachers of natural history and biology were addressed, I was focused primarily on lab exercises, worsheets, proposals for project days, didactic games, and excursion worksheets, when creating the practical exercises. The subsequent objective was to verify one laboratory exercise in teaching. The result of this thesis is not only a brief review of natural history and biology...

The Marion Plateau is a carbonate-platform complex on the passive northeast Australian margin. During ODP Leg 194, a series of eight sites, on two transects, were drilled through Oligocene to Holocene mixed carbonate and siliciclastic sediments that record the depositional history of the Marion Plateau. Major sediment constituents, including benthic foraminifers, were counted from thin sections made from samples taken from three drilling sites: bryozoan-dominated northern highstand platform site 1193 and northern lowstand ramp site 1194, and coralline red algae-dominated southern platform site 1196. Quantitative multivariate analysis of benthic foraminiferal assemblages revealed three paleoenvironmentally relevant groups of taxa. The first group, dominated by Amphistegina and Lepidocyclina, was found at all three sites and is interpreted to be representative of euphotic open-shelf deposition in less than 50 m water depth. The second group was dominated by Cycloclypeus, Operculina and planktonic foraminifers. This group was found at all three sites, but was more common from sites on the northern platform and is interpreted to represent deposition in deep, oligophotic water, in greater than 50 m water depth. The third group, consisting of soritids, alveolinids, Austrotrillina and Flosculinella, was found only in sediments from the southern platform and is interpreted to represent deposition in a restricted, shallow-marine environment, in less than 20 m water depth, consistent with a sea-grass meadow. These three groups of taxa record changing paleoenvironments on the Marion Plateau, and reveal the depositional histories of the Marion Plateau's carbonate platforms.

As global temperatures rise due to anthropogenic climate change, water resources, thus economies, are threatened. A geologically recent period of increased temperatures is the mid-Holocene and an investigation of its climate may allow for a better understanding of future precipitation and changes to regional water resources. The regions of interest are tropical Northern Central America and subtropical North America with Belize and Florida representing each climate zone. By reconstructing mid-Holocene climate in Florida and Belize, I hope to provide a better understanding of how increased temperatures and a reduced latitudinal temperature gradient impacts both precipitation patterns and variability. Today, drivers of changes in precipitation include climate systems such as the Atlantic Multidecadal Variability (AMV), North Atlantic Oscillation (NAO), and the Intertropical Convergence Zone (ITCZ). Therefore, it is imperative to determine their latitudinal influences during the mid-Holocene and consequently their potential impact on water resources in the near future. Speleothems from Chen Ha Cave, Vaca Plateau, Belize and Brown's Cave, West Central Florida, provided high-resolution (sub-annual to decadal) oxygen and carbon stable isotope data that allowed for a detailed investigation of mid-Holocene climate. The speleothems were sampled along the growth axis of a cross-section for oxygen and carbon isotopic analysis. 234U-230Th dating was used to create a chronology for each record and determine the time step between each isotope sample. Time series analysis with variations of Fourier transforms, including Lomb-Scargle, wavelet analysis, and multi-taper method, was used to extract periodicities for each oxygen isotope record. To determine which atmospheric-oceanic modes influenced mid-Holocene precipitation, the speleothem periodicities were compared to those of known periodicities of atmospheric-oceanic modes, such as the AMV and NAO. Finally, the Florida and Belize records were assessed for coherency using cross wavelet analysis. The Floridian speleothem recorded less precipitation compared to present levels due to a westward expansion and intensification of the North Atlantic Subtropical High (NASH) with a quasi-persistent but less influential AMV. Relative to today, the mid-Holocene in Belize was slightly wetter which I suggest is a result of a more northerly ITCZ and an intensification of the NASH that increased the strength of the Caribbean Lower Level Jet (CLLJ). The Seuss solar cycle was also significant in Belize, contributing 7.2% of the precipitation variability. Wavelet coherency assessment reveal very little connectivity between the Florida and Belize speleothem reconstructions, potentially due to the blocking influence of the ITCZ. Comparison to other records from the mid-Holocene supports the hypothesis of an intensified NASH and more northerly ITCZ. A future increase in precipitation in Belize may lead to increased soil erosion, the need for crop adaptation, and risk to the population of low lying areas, such as Belize City. In Florida, reduced precipitation may result in a decrease in agricultural output and threats to the state's freshwater supply.

This thesis investigates possible dynamical pathways through which variability in the extra-tropical Pacific Ocean influences decadal fluctuations of tropical Pacific sea surface temperatures (SST). Specifically, we examine the hypothesis that low-frequency changes in the Pacific‟s meridional subtropical cells (STCs), which transport subsurface water masses equatorward from the extra-tropical into the tropical Pacific upwelling system, modulate decadal variations of the equatorial SST. The relationship between the STCs and equatorial Pacific SST anomalies is explored statistically using the monthly hindcast output from the Ocean General Circulation Model (OGCM) for the Earth Simulator (OFES). We find that decadal variability of the subsurface heat transport of the southern branch of the STC is more closely correlated (R = -0.74) with eastern equatorial SST anomalies on timescales longer than 8 years. The northern branch of the STC is overall not well correlated with equatorial SSTa; however, we find that in the period before the 1976 climate shift, the northern cell is more strongly and significantly correlated with equatorial SSTa (R = -0.89, >99%), while the southern cell is not (R = -0.32). The physical significance of these findings remain unclear and requires isolating mechanisms that could lead to an asymmetry in the role of the northern and southern STC in modulating eastern equatorial SSTa during different states of the Pacific climate. This will be a critical step to attribute physical significance to the statistical changes observed before and after the 1976 climate shift.

In the North Pacific Subtropical Gyre, a nearly-annual phytoplankton bloom forms near the subtropical front at ~30° N. Mixed communities of nitrogen-fixing diatom symbioses (diatom-diazotroph associations) increase 10²-10³ fold in these blooms. In July 2008 (31.46˚N 140.49˚W) and August 2009 (25.18 °N 154 °W), two blooms were sampled to determine diatom-diazotroph association species composition, physical, and chemical characteristics of the water column. In both 2008 and 2009, the dominant diatom-diazotroph association was the Hemiaulus hauckii-Richelia intracellularis symbiosis. The 2009 subtropical front bloom was missed; however, another bloom closer to Hawaii was sampled where diatom-diazotroph association abundance was 10-fold lower (10² cells Lˉ¹) than 2008 despite surface chlorophyll a values that were 3 times greater. Both blooms showed substantial changes in phytoplankton size structure with the >10 μm size chlorophyll a fraction increasing from 10 to 40 % in 2008. In the 2009 bloom, the non-symbiotic pennate diatom Mastogloia woodiana numerically dominated (>150,000 cells Lˉ¹) and formed aggregates that resulted in substantially higher % of netplankton chlorophyll a fractions. Summer open ocean blooms from the two years share a common trend of Hemiaulus dominance of the diatom-diazotroph association population and size structure changes. However, non-symbiotic species can dominate the overall bloom, and diatom-diazotroph association species may not be responsible for the chlorophyll a increase. These two years may represent different types of blooms or temporal changes within summer diatom blooms. The increased biomass in the larger-size fraction suggests these blooms are potential sites for carbon export from the surface layer. / text

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Lizards are considered model organism in ecology, allowing the understanding of patterns and relations between organisms and environment. Tropidurus torquatus is a species with a broad distribution, with populations in different habitats and under diverse environmental pressures. However, the knowledge of diverse aspects of this species are unknown, specially in the subtropical region of the continent. These characteristics make the species in this study excellent to understand questions about geographic variation and environmental variable relations. Our purpose with this study is try to solve some questions that comprehend reproduction and growth from individuals of this specie from two populations, in the Pampa biome, in the south of Brazil. To elucidate the reproductive ecology questions, we collected 315 animals from a population localized in Alegrete/RS, between September of 2007 and August of 2008. The collected animals were dissected and their gonads were analyzed in the laboratory. Growth data were obtained from a study between December 2006 and December 2008, in Santo Antão locality, Santa Maria, Rio Grande do Sul. Every month the individuals were captured, measured, and the difference between the captures were used to calculate the growth rate. The reproductive period was between September and January, with eggs produce from October to January. Males reach the sexual maturity earlier and with smaller size than females. Females produces in average 6,21 eggs per clutch and not produces multiple clutch in the same reproductive period. Temperature, photoperiod, insolation, precipitation and humidity do not affect the reproductive period, but can cause more effect in growth rate. Adult males grow up more than females in summer. Both males and females decrease growth rate when there are less photoperiod, insolation and temperature at the winter. The decrease in growth rate during the winter, has as main consequence there are less animals reproducing in the following season after their birth. The smaller growth rate, late maturation, small reproductive period and eggs incubation are the main characteristics that make the south population different from the others in the tropical climate. Others characteristics like clutch size and eggs size, time of reproduction and offspring size seem to have less variation between populations in this species, and even between species of the genera Tropidurus. The data this study suggest that historic factors and phenotypic plasticity, in response to environment variables, act in this population, in the south of Brazil. / Lagartos são considerados organismos modelos em ecologia, possibilitando o entendimento de padrões e relações entre organismos e ambiente. Tropidurus torquatus é uma espécie amplamente distribuída, com populações vivendo em diversos habitats e sob diferentes pressões ambientais. Apesar disso, diversos aspectos da ecologia da espécie são desconhecidos, principalmente na região subtropical do continente. Essas características tornam a população estudada excelente para o entendimento de questões ligadas a variação geográfica e a relação com as variáveis ambientais. Devido a isso, com esse estudo buscamos resolver algumas questões envolvendo a reprodução e o crescimento dos indivíduos de duas populações, localizadas no Pampa, no sul do Brasil. Para elucidar as questões referentes à ecologia reprodutiva, 315 animais foram coletados, de uma população em Alegrete/RS, mensalmente entre setembro de 2007 e agosto de 2008. Os animais coletados foram dissecados e tiveram suas gônadas analisadas. Dados referentes ao crescimento dos indivíduos foram obtidos de um estudo de captura-recaptura realizado de dezembro de 2006 a dezembro de 2008, na localidade de Santo Antão, Santa Maria, Rio Grande do Sul. Mensalmente, indivíduos eram capturados e a diferença entre sucessivas medidas do comprimento rostro-cloacal utilizadas para calcular a taxa de crescimento. O período reprodutivo estendeu-se de setembro a janeiro, com produção de ovos entre outubro e janeiro. Machos atingiram a maturidade mais precocemente e com menor tamanho em relação às fêmeas. Fêmeas depositaram 6,21 ovos, em média, por ninhada e não há evidencias de produção de ninhadas múltiplas. Temperatura, comprimento do dia, insolação, pluviosidade e umidade não estiveram relacionadas com o período reprodutivo, mas causam grande efeito sobre as taxas de crescimento. Machos adultos cresceram mais do que fêmeas somente no verão. Machos e fêmeas tem o crescimento fortemente restrito pela diminuição do fotoperíodo, insolação e temperatura no inverno. A redução no crescimento durante o inverno tem como principal conseqüência um menor número de animais reproduzindo na estação subseqüente ao seu nascimento. A menor taxa de crescimento, maturação tardia, menor período reprodutivo e de incubação dos ovos são as principais características que diferenciam a população do sul do Brasil de outras de clima tropical em relação aos parâmetros estudados. Outras características, como tamanho da ninhada e dos ovos, época que ocorre a reprodução e tamanho dos filhotes parecem ter menor variação entre populações da espécie e até entre espécies do gênero Tropidurus. Os dados apresentados sugerem a atuação de fatores históricos e da plasticidade fenotípica, frente às variáveis ambientais, nas populações de Tropidurus torquatus do sul do Brasil.

As atividades antrópicas desenvolvidas nas ultimas décadas têm alterado o ciclo natural dos nutrientes, o que traz efeitos negativos às aos recursos hídricos. Os crescentes problemas com a eutrofização nesses ambientes têm aumentado os casos de florescimento de cianobactérias, que podem trazer riscos aos ecossistemas e comprometer a saúde da população. Em reservatórios, esses efeitos são ainda mais indesejáveis tendo em vista que acumulam água para múltiplos usos. Em vista desse quadro, torna-se importante o estudo da relação dos nutrientes com a comunidade fitoplanctônica. Geralmente os estudos de controle da eutrofização por restrição de nutrientes se atentam apenas à restrição do fósforo. O conhecimento sobre os efeitos das diferentes fontes nitrogenadas em reservatórios tropicais/subtropicais é praticamente inexistente. Para investigar a importância do amônio, nitrato e ureia como fontes de nitrogênio para o fitoplâncton em reservatórios tropicais com diferentes graus de trofia, foram realizadas 4 coletas nos reservatórios de Itupararanga (supereutrófico) e Barra Bonita (hipereutrófico) em dois pontos na coluna de água (profundidades referentes a 50 e 10% da radiação solar fotossinteticamente ativa na subsuperfície). As taxas de assimilação obtidas nessa pesquisa foram relativamente maiores que em trabalhos semelhantes devido à grande biomassa fitoplanctônica em Itupararanga e em Barra Bonita. As taxas de assimilação foram maiores na estação chuvosa, em Barra Bonita que em Itupararanga e, na primeira profundidade. As estratégias de assimilação de nitrogênio foram relacionadas à composição da comunidade fitoplanctônica. Em Itupararanga a comunidade apresentou domínio da cianobactéria Cylindrospemospsis raciborskii e mostrou-se bem adaptada à assimilação de amônio, nitrato e ureia, sendo o amônio a forma nitrogenada preferida, seguido de nitrato e ureia. Em Barra Bonita, a comunidade apresentou domínio da cianobactéria Microcystis aeruginosa durante a estação chuvosa, e ausência de dominância na estação seca. A mudança na composição da comunidade resultou numa maior afinidade pelo nitrato. A intensidade luminosa teve efeito positivo na assimilação da ureia. As cianobactérias parecem bem adaptadas à assimilação de qualquer fonte de nitrogênio disponível, e parte do sucesso desse grupo em reservatórios subtropicais pode ser atribuído a esta característica. / Human activities in the last decades are altering the natural nutrient cycles, with damaging repercussions to the water sources. The growing eutrophication problems are increasing cyanobacterial bloom occurrences and the environmental and health risks associated. In reservoirs, those effects are even more undesirable, due to its waters multipurpose. Therefore, understanding the relations between nutrients and phytoplankton community is needed. Overall, eutrophication control efforts are focus on phosphorus restriction. Little is known on the effects the available nitrogen sources on tropical/subtropical reservoirs primary production. To investigate the importance of ammonium, nitrate and urea to the phytoplankton in subtropical reservoirs with different trophic states, four field campaignes were conducted between October/2013 and June/2014. The water was collected in two points (depths corresponded to 50 e 10% of the surface\'s solar radiation). The uptake rates observed in the work were relatively higher than most similar works, presumably due to the high phytoplanktonic biomass in Itupararanga and Barra Bonita. The uptake rates were higher in the rain season, in the shallower depths and in Barra Bonita than Itupararanga. The uptake also differ among the reservoirs: In Itupararanga, the phytoplankton community was dominated by the cyanobacteria Cylindrospemospsis raciborskii dominance, and was well adapted to the use of ammonium, nitrate and urea (with ammonium been the favorite N source). In Barra Bonita, the phytoplankton community was dominated by the cyanobacteria Microcystis aeruginosa in the rain season and showed no dominance in the dry season. The shift in the phytoplankton community seamed to increase its affinity to nitrate uptake. The light intensity was correlated with urea uptake only. The cyanobacteria ability to use any given nitrogen source can be a key factor to its success.