The Guapimirim estuary is the main tributary of Guanabara bay and is located in the northeast portion. Although it is protected, this estuary has been experiencing strong anthropogenic pressure, which has led to changes in the natural characteristics. Large amounts of sewage are dumped into the bay through tributaries, thereby changing the water and bottom sediment quality. One of the main elements of sewage is phosphorus. Despite its importance to life, a high concentration of this nutrient in the environment can result in eutrophication. This work describes the phosphorus distribution in its different fractions in the bottom sediment at 16 stations located in the main channel of the Guapimirim estuary. These results are correlated with data on grain size, organic matter and calcium carbonate content in the bottom sediment and with physicochemical parameters of the bottom water. The grain size decreases toward the mouth of the estuary, whereas the organic matter and carbonate content increase. The salinity increases significantly at 3.5 km upstream from the mouth, where there is also a notable increase in fine sediments; the same site is the mean position of the salinity front. The temperature and pH increase in the same direction. The Pinorg-total ranges between 3.18 and 7.13 µmol g−1, increasing toward the mouth. The same trend is observed for the other phosphorus fractions P-Ca, P-Fe and P-f.a., which range from 0.68 to 1.91, 0.79 to 1.71 and 0.03 to 0.93 µmol g−1, respectively. The P-Ca and P-Fe fractions are the most representative in the Pinorg-total, occurring at 26.3 and 26.0 %, respectively.

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ABSTRACT. This work was conducted in the Guapimirim estuary, the main tributary of the Guanabara Bay. This system is located in a preserved area in the catchment basin of the bay, inside the Guapimirim Environmental Protection Area. The estuary presents a meandering channel with 3 meters mean depth. The aim of this work is to evaluate the salinity intrusion dynamics in response to the tide and freshwater discharge, as well as its effects on the water column structure and residence time. Surveys were performed to assess the salinity structure along the estuary main channel under different freshwater discharges (dry and rainy period) and tide (neap and spring). The results showed significant differences in the water column structure, presenting high stratification in neap tide and conditions homogeneous in spring tide. The salinity front (SF) showed a bigger displacement in the spring tide. Under highly stratified conditions, in neap tide, its displacement was bigger with smaller tidal amplitude. The location of SF inside of the estuary, in the high water, was influenced manly by freshwater discharge. The residence time in the Guapimirim estuary was longer in neap tide and dry period, presenting a good relationship with freshwater discharge.Keywords: Guapimirim estuary, salinity intrusion, residence time.RESUMO. Este trabalho foi desenvolvido no estuário de Guapimirim, principal tributário da Baía de Guanabara. Este sistema se localiza em umaárea ainda preservada na bacia de drenagem da baía, dentro daÁrea de Proteção Ambiental de Guapimirim. O estuário apresenta um canal meandrante com uma profundidade média de 3 metros. O objetivo deste trabalho consiste em avaliar a dinâmica da intrusão salina em resposta a atuação dos processos físicos como a vazão fluvial e a maré, assim como, sua consequência na estrutura da coluna d'água e tempo de residência. Foram realizados levantamentos sobre a estrutura salina ao longo do canal principal do estuário em diferentes condições de vazão fluvial (período seco e chuvoso) e maré (quadratura e sizígia). Os resultados mostraram diferenças marcantes na estrutura da coluna d'água sendo estratificado em maré de quadratura e mais homogêneo em maré de sizígia. A frente salina apresentou um maior deslocamento em maré de sizígia.Em condições de alta estratificação, em maré de quadratura, o maior deslocamento da frente salina esteve associado a uma menor amplitude da maré. A vazão fluvial foi o principal processo físico que atuou no alcance máximo da frente salina. O tempo de residência de uma parcela deágua dentro do estuário de Guapimirim foi maior em condição de maré de quadratura e em período seco, apresentando boa relação com a vazão fluvial.Palavras-chave: estuário de Guapimirim, intrusão salina, tempo de residência.