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The study of near‐pristine estuaries can be used as a tool for mitigation projects of harmful effects in anthropogenic eutrophic systems, since one can analyze the effect of temporal and spatial variations generated mainly by natural forces. Phytoplankton taxonomy has been used as a classical indicator to assess changes in transitional water communities, however alternative methods based on morphological, behavioral and physiological traits offer the opportunity to compare sites or moments with different taxonomic compositions. Our goal, in this context, is to evaluate phytoplankton community short‐term and seasonal variability in a near‐pristine estuary, Barra Grande estuary (Ilha Grande, RJ, Brazil), through species functional traits and thus community functional diversity. Samplings were carried out in a mooring in complete tidal cycles, seasonally during 2012. Our results showed a diverse phytoplankton community with 38 frequent and abundant taxa, marked by density variation (1.2 × 10.4–2 × 10.7 cell L−1) in depth, with abundance inversely related to tidal currents. The functional structure of the phytoplankton community measured by functional diversity (FD), varied seasonally in and across a gradient of tidal energy. A core community, mainly represented by flagellates and dinoflagellates, occurred in all observations and was highly functional (high FD), exploiting a variety of habitats. The chain‐forming diatoms were associated with high tidal energy, and occurred in higher densities during summer. Phytoplankton cell size, cell shape, and the ability to form colonies are extremely plastic traits that can be regulated by the environment, however, isolated tychopelagic diatoms were present in the study area across all seasons, with higher contributions in autumn and winter, reflecting the shallow characteristic of this system. During the winter, an exposed sandbar was formed, and the lack of connection with the ocean resulted in an abundance of riverine and brackish water taxa. In this near‐pristine estuary the densities and occurrences of HAB phytoplankton are low. Trait‐based analyses add information about community structure, which can be impacted by anthropogenic actions in urbanized coastal systems. Thus the information provided by this study regarding phytoplankton functional diversity and its relation to nutrients and hydrography in Barra Grande Estuary can be applied as a baseline model for the development of public policies.

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Recebido em 15/9/08; aceito em 11/3/09; publicado na web em 4/8/09 HYDROCARBONS AND STEROLS AS INDICATORS OF SOURCE AND FATE OF ORGANIC MATTER IN SEDIMENTSFROM SEPETIBA BAY, RIO DE JANEIRO. The inputs of organic matter derived from natural and anthropogenic sources to Sepetiba Bay were investigated by using aliphatic hydrocarbons and sterols in superficial sediments. Concentrations ranged from 0.26 to 2.65 μg g -1 , <0.01 to 17.41 μg g -1 and 0.77 and 9.24 μg g -1 for n-alkanes, UCM (unresolved complex mixture) and total sterols, respectively. The selected markers and ratios among individual compounds showed the major contribution of terrestrial sources to the total pool of sedimentary organic matter in the bay, but the accumulation of autochthonous organic matter increased with distance from the shoreline. The input of petroleum hydrocarbons and sewage (coprostanol in the range 0.01 to 0.43 μg g -1 ) were also detected, especially in the more urbanized regions of the bay, but at lower levels of contamination when compared to estuaries in other Brazilian coastal regions.Keywords: Sepetiba Bay; sediments; organic matter. INTRODUÇÃOA matéria orgânica em sedimentos costeiros tem origem em diversas fontes -terrestres, aquáticas, atmosféricas e antropogênicas, no caso de áreas urbanizadas -que apresentam grande variabilidade espacial e temporal nas suas contribuições relativas.1-3 De acordo com sua origem, a matéria orgânica apresenta reatividades bastante distintas frente aos gradientes significativos nas propriedades físico-químicas, químicas e bióticas dos sistemas aquáticos, particularmente em regiões estuarinas.4,5 A elucidação dos processos que determinam a produção, transporte e acúmulo de matéria orgânica em sistemas aquáticos é fundamental para a compreensão dos ciclos biogeoquí-micos globais, mas ao mesmo tempo representa um dos grandes desafios da geoquímica orgânica em decorrência da multiplicidade dos fatores envolvidos. 4,6 Diversas classes de compostos na fração de lipídios (n-alcanos, ácidos graxos, alcoóis lineares e esteróis, entre outras) são utilizadas na caracterização da matéria orgânica sedimentar devido à boa especificidade em relação à fonte e à maior resistência aos processos de degradação bacteriana, quando comparados com outros grupos de compostos orgânicos, como carboidratos e proteínas. 3,7,8 Os n-alcanos, por exemplo, apresentam diferenças em relação ao tamanho da cadeia de carbono que permitem a identificação de fontes autóctonas e alóc-tonas de matéria orgânica, incluindo a contaminação petrogênica. [9][10][11]