Estimates of benthic fluxes of nutrients across the sediment-water interface (Guarapiranga reservoir, Sao Paulo, Brazil)

Concentration profiles of nutrients (dissolved organic carbon, nitrate, nitrite, ammonium and soluble reactive phosphorus) were determined in pore waters from sediment from the Guarapiranga reservoir (Sao Paulo, Brazil). Redox potential and acid volatile sulfide measurements on bulk sediment samples were determined in the field and laboratory, respectively. The sediment redox potential ranged from - 170 to - 220 mV at 0-1 cm and increased to somewhat higher values at 20 cm. The acid volatile sulfide (AVS) profile had a bimodal pattern with concentration peaks at 3 cm (27-55 mg kg(-1)) and 14 cm (70-110 mg kg(-1)). Dissolved organic carbon (DOC) concentrations increased from the surface (4.7-5.6 mg l(-1)) to 20 cm (values up to 12 mg l(-1)). The concentration of ammonium increased significantly with depth, with maximum concentrations occurring at 15 cm; nitrate-nitrite concentrations only increased appreciably at 10 cm. The SRP profiles increased in concentration from the surface to approximately 10-cm depth, with a maximum value of 1200 mug H2PO4- l(-1). Benthic fluxes from the sediment into the pore water ranged from 278 to 339 mg cm(-2) year(-1) for ammonium ions and from 8 to 18 mg cm(-2) year(-1) for SRP. These upward diffusive fluxes correspond to 47-70% and to 10-24% of the total deposition of N and P measured in the reservoir, respectively. The burial rates for N and P in these sediments are 30-54% and 76-89%, respectively. (C) 2001 Elsevier Science B.V. All rights reserved.