t EDP Sciences, Les Ulis DOI : 10. 1051/jp4 : 20030284 Manganese and iron as oxygen carriers to anoxie estuarine sediment F. M. M. Brayner and B. Matvienko' Instituto Tecnologico do Estado de Pernambuco-Brazil, Av. Prof. Luiz Freire 700, Ciudade Universitaria, Recife/PE, Brazil 1 University of Sao Paulo, Sao Carlos Engineering School, Hydraulics Department CRHEA, Caixa Postal 717, Sao Carlos SP 13560-970, Brazil Abstract : We studied the concentration of a series of transition metals ineluding lvln and Fe in an estuarine fishpond. Tlie pond is situated at latitude 8OlO'S and longitude 34°55'W, in the Capibaribe River estuary, within the Recife city boundaries, which is located in Pernambuco, a state of the Brazilian Northeast Pond area is 1. 5 ha and it bas a 0. 5 m depth. It is separated from the river by dikes. Water temperature at 28 C is stable throughout the year. Light breezes keep the water aerated, but intense ongoing decomposition makes the sediment anoxie. The area, originally of mangrove type, has been changed by antropic action on its fauna and vegetation. The study focuses on changes in behaviour of heavy metals. Samples of bottom sediments wore collected by Eckman dredge sediment sampler and total metal concentration was determined by the lithium borate fusion method. Water, recent sediment, and consolidated sediment were examined in this fishpond where Mn and Fe are brought in periodically by water and then gradually go into the sediment at respective rates of 10. 52 and 1332 mg m-2a-1. Strong bioturbation re-suspends sediment while simultaneously re-dissolution of these ions is going on fhrough reduction in thé anoxie sédiment. As soluble species these ions migrate from sediment to water and are there continually oxidized by dissolved oxygen, becoming insoluble. With their precipitation, chemically bound oxygen is carried down to the sediment, constituting a parallel channel of transport in addition to migration into the sediment bydiffusion of the oxygen dissolved in the water. The estimated flow rates are 3. 25 and 76 mg 02 ni-2a-' due to Mn and Fe respectively. The rates were established using natural silicon as a tracer. 1. INTRODUCTION In the studied fishpond fish are raised exclusively on the biomass originating from photosynthesis within the pond. From the 1. 5 ha area anmually about 700 kg offish are harvested. Based on other studies [1] we estirnate that fish, the final member of the food chain in the pond, incorporate carbon at a rate of 19 mg C rif2d-'. The fishpond is part of an estuarine system and is kept isolated by dikes. Once per year it is ahnost completely drained and part of its sediment is used to repair the dykes. The ongoing reworking of the sediment precludes dating by 21OPb. For this reason an altemate tracer, silicon, was used to establish sedimentation rates, which is possible due to favorable pH conditions prevailing in the tropics. Silicon, in the form of soluble silicate is brought in by the water during replenishments which are donc at high tide, about once every fortnight. Between replenishments the silicon concentration in water declines due to its use by algae which transform soluble silicates into insoluble silica. However this removal of silicic acid does not rise the pH because the intense