Phosphorus distribution in sediments of Morales Stream (tributary of the Matanza-Riachuelo River, Argentina). The influence of organic point source contamination

Increased discharges of organic matter from different sources in Morales Stream, one of the main tributaries of the Matanza-Riachuelo River, caused not only an increase in its primary production but also drastic changes in the composition of its sediments, thus favoring eutrophication processes. An in situ study was carried out in order to assess the effects of an organic point source contamination ( from intensive cattle rearing) on the sediments of Morales Stream. Surface water and sediment samples were analysed to determine the chemical characteristics of the water - sediment system. The amounts and forms of sediment phosphorus were determined using the 'EDTA method' (Golterman, 1996) at two sites of the stream having different nutrient loads. The increase in the organic load of Morales Stream waters influences the dynamics of sediment P, producing two main effects: ( 1) an increase in the organic matter amount of the sediment that leads to an increase in the amount of P associated to organic fractions, which may be released by bacterial activity under anoxic conditions; and ( 2) a decrease in the concentration of P in the fraction bound to iron. Morales Stream sediments may act as a potential source of P, which can release this nutrient to water under the reducing conditions originated by uncontrolled discharges of organic residues to this water body.