Saco do Justino (RS-Brazil): water column and pore water ammonium and phosphate in an uncontaminated shallow inlet

Abstract

The “Saco do Justino” is an important inlet of the Patos Lagoon estuary and is used as a nursery for many valuable commercial specie of fish and crustacean. This ecosystem bears an abundant seagrass beds, indicating high productivity. Since there is no direct input of any anthropogenic nutrients into this inlet, this research investigated the source of the nutrients for this vegetation. Monthly, from 1994 to 1995, the concentrations of ammonium, phosphate, Eh and the salinity of water samples and pore water were determined. The comparison of these data with those of the euthrophic inlet “Saco da Mangueira” showed that the Saco do Justino is not euthrophicated. Moreover, the concentrations of nutrients increased towards the bottom, with a gradient about 10 between the ammonium of the water column and the interstitial water. The gradient of phosphate was about 2, showing that the natural and the main source of ammonium and phosphate for the water column is the interstitial water, through advection nutrients for the water column. The sea water disturbed the stability of the sedimentary column and breached the stratification formed between the oxidant condition of the superficial layers and the reducing condition of the deepest layers, where highly contaminated interstitial water accumulated.