ABSTRACT Studies involving coastal sediment resuspension have shown that trace metals could be released to the water column due to changes in physical-chemical conditions. Therefore, if environmental agencies adopt screening protocols with insufficient evaluations of contaminant mobilization risks for a given area, the prediction of environmental impacts caused by dredging may be limited. This work evaluates the influence of spatial variation on the geochemical mobility of trace metals (Cd, Cu, Ni, Pb, Zn) after artificial sediment resuspension, using as study case the Iguaçu River estuary (Guanabara Bay, Brazil). The Iguaçu River drains the industrial complex of the metropolitan area of Rio de Janeiro State, besides the presence of agriculture and the input of untreated domestic wastes. Surface estuarine sediments were submitted to resuspension experiments in an open system, during 1h and 24h of agitation on local water. A clear tendency of metals’ solubilization was observed after resuspension, especially considering the dissolved concentrations of Cu (average: 8.0 µg L-1) and Zn (average: 0.9 mg L-1), especially for the samples from the transects 2 and 3. However, evaluations of water quality changes due to sediment resuspension are not requested by the legislation regulating the dredging activities. In the sediments, the results suggested a higher geochemical mobility of Cu, indicated by the massive increase on the bioavailable fraction after resuspension, mainly on the transect number 3, on the river’s mouth. The effects of resuspension were distinct between samples, suggesting that even in a small scale, important differences on metals’ mobility are found. Thus, the combined assessment of changes in metal concentrations in water and in the metal partitioning linked to the solid-phase was demonstrated to be a promising tool for predicting the environmental risks of dredging due to changes in the bioavailability of metals.

Resumo Objetivo: Existem poucos estudos que tratam dos processos que ocorrem em pequenos estuários que recebem uma elevada carga de matéria orgânica em regiões tropicais. O objetivo do presente trabalho foi caracterizar o comportamento biogeoquímico de nutrientes presentes em água superficial da foz do rio Iguaçu, durante condições específicas (maré de quadratura, vazante), localizado no setor interno de uma baía altamente eutrofizada (baía de Guanabara, SE Brasil). Métodos Variáveis físicas e químicas foram medidas in situ (pH, temperatura, condutividade, salinidade, total de sólidos dissolvidos, transparência, oxigênio dissolvido), já o material particulado em suspensão, clorofila a e os nutrientes (carbono e as formas de nitrogênio e fósforo) foram medidos em laboratório, ao longo de um gradiente estuarino mesohalino. Resultados A foz do rio Iguaçu está em alto estágio de eutrofização, considerando-se as concentrações de nutrientes, clorofila a e de transparência da coluna d' água encontrados. Os resultados indicam uma transição de condições heterotróficas para autotróficas, visto que as concentrações dos nutrientes diminuíram ao longo do gradiente salino, enquanto a clorofila a aumentou ao longo dos transecções. O pH e a razão clorofila : feopigmentos estiveram relacionados com a quantidade e qualidade da matéria orgânica em decomposição, sobretudo nos transecções sob maior influência marinha. Mais de 95% do nitrogênio dissolvido estava na forma de amônio (NH4+), proveniente do processo de amonificação da matéria orgânica em excesso na região, indicando presença de esgoto sem tratamento. Conclusão Neste estudo, o rio Iguaçu contribuiu com altas cargas de nutrientes que suportam uma importante produção primária na região interna da baía, estando relacionada à absorção de CO2 e metabolismo autotrófico, mostrando a importância de se identificar os comportamentos biogeoquímicos de nutrientes em áreas estuarinas, mesmo em pequena escala.

Abstract Aim: There are few studies dealing with the biogeochemical processes occurring in small estuaries receiving high sewage loading in tropical regions. The aim of this investigation was to characterize the biogeochemical behavior of nutrients in superficial waters collected at the Iguaçu estuarine system, during specific conditions (neap tide), located at the inner sector of a heavily eutrophicated embayment (Guanabara Bay, SE Brazil). Methods Physical and chemical variables were measured in situ (pH, temperature, conductivity, salinity, total dissolved solids, transparency, dissolved oxygen), whereas suspended particulate matter, chlorophyll a, phaepigments and nutrients (carbon, nitrogen and phosphorus forms) were measured in laboratory across the mesohaline estuarine gradient. Results The Iguaçu River mouth is in a high stage of eutrophication, considering nutrient concentrations, chlorophyll a and transparency of water column. Results indicate a transition from heterotrophic conditions to autotrophic conditions, since the nutrients concentrations showed a decreasing pattern along the saline gradient, while the chlorophyll an increased over the transects. The pH values and chlorophyll : phaeopigments ratios are significantly related to the amount and quality of organic matter contents, especially at transects under strong marine influence. More than 95% of the dissolved and total nitrogen concentrations are represented by NH4+ contributions, which are related to the ammonification of organic matter contents in this region, indicating the existence of untreated sewage loads in this area. Conclusion In this study, the Iguaçu River seemed to contribute with high inputs of nutrients that support important phytoplankton production at the inner regions of the bay related to the CO2 sink and autotrophic metabolism, showing the importance of verifying the biogeochemical behaviors of nutrients in estuarine areas, even in small scale.