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The present study aimed to analyze the structure and the temporal variation of the phytoplankton of Ajuruteua beach (Bragança, Pará) and to investigate the influence of environmental variables on the dynamics of this community to provide a basis about the trophic state of this environment. Biological, hydrological and hydrodynamic samplings were performed during a nyctemeral cycle in the months of November/08, March/09, June/09 and September/09. We identified 110 taxa, which were distributed among the diatoms (87.3%), dinoflagellates (11.8%) and cyanobacteria (0.9%), with the predominance of neritic species, followed by the tychoplankton species. Chlorophyll-a concentrations were the highest during the rainy period (24.5 mg m -3 ), whereas total phytoplankton density was higher in the dry period(1,255 x 10 3 cell L -1 ). However, phytoflagellates density was significantly higher during the rainy period.Cluster Analysis revealed the formation of four groups, which were influenced by the monthly differences in the environmental variables. The Principal Component Analysis indicated salinity and chlorophyll-a as the main variables that explained the components. Spearman correlation analysis supported the influence of these variables on the local phytoplankton community. Overall, the results obtained suggest that rainfall and strong local hydrodynamics play an important role in the dynamic of the phytoplankton of Ajuruteua beach, by influencing both environmental and biological variables.

Quaternary and Environmental Geosciences (2014) 05(2):125-136Papel da praia na proteção da costa e as alterações oceanográficas em diferentes escalas temporais Processos morfodinâmicos em uma praia de macromaré no litoral amazônico . Equipamentos oceanográficos foram fundeados por 25h em cada mês de coleta. As amostras superficiais de sedimentos foram coletadas ao longo do perfil de praia, entre a zona de dunas e o início da antepraia. Em laboratório, a granulometria foi realizada utilizando um agitador automático com peneira variando de -1.0 to 4.0 φ. As marés são semi-diurnas e assimétricas, com períodos de vazantes mais longos (6,5 e 7,5 h). A oscilação da maré durante as sizígias varia entre 4,0 e 5,5 m. Na área predomina a corrente de maré com direção SE-NW durante a maré vazante e de NW-SE durante as marés enchentes. As velocidades das correntes são maiores durante os períodos de enchente (até 0,7 m/s). Durante a maré baixa, o estirânico possui cerca de 250-300 m. Esta praia é composta por areia fina (2,6-2,8φ). As ondas são modulados sobre os bancos de areia, de acordo com a fase da maré, com alturas mínimas de ondas H s durante a maré vazante, mas superior a um metro durante a preamar. Os valores de RTR e Ω são claramente modulados de acordo com a fase da maré. Durante condições de alta energia, a praia apresenta característica de planície de maré, enquanto que durante condições de baixa energia apresenta características ultradissipativa. Morphodynamic processes in a macrotidal beach in the Amazon littoral Palavras-chave:Morfodinâmica; praia arenosa; macromarés; ondas moduladas; litoral amazônico. AbstractOceanographic processes operating in the Amazon coastal zone result from the superposition of several dominant forces (e.g., macrotides, moderate wave energy, strong currents, elevated fluvial discharge and intense winds) that are responsible for continuous changes in the profile of the beach. The aim of this study was to understand the changes in beach morphology related to parameters such as tidal range (TR), breaker height (H b ), wave period (T), and sediment fall velocity (w s ). The study took place on a sandy Amazonian beach (Ajuruteua beach) during equinoctial (March, 2009, rainy season and September, 2009, dry season) and non-equinoctial (December, 2008, dry season and June, 2009, rainy season) periods. Waves and currents were measured for 25h using a bottom-mounted mooring to which a mini-current meter and wave and tide data loggers were attached. Surface sediment samples were collected along the beach profile. Sediment samples were then separated by size in the laboratory using a sieve shaker with nests ranging from -1.0 to 4.0 φ. Local tides are semidiurnal and asymmetric, with longer ebb tides (6.5 to 7.5 h) and spring tides normally reaching heights of between 4.0 and 5.5 m. Circulation patterns are dominated by tidal currents, which flow to the northwest during the ebb tide and to the southeast during the flood, with speeds of up to 0.7 m/s. During low tide, the intertidal zone is wide (...