Encerrada Bay (EB) is located in the far south of Argentina, on the north coast of the Beagle Channel and is artificially connected with Ushuaia Bay (UB). This study was carried out between 2004 and 2005; and assesses the impact of Ushuaia city to the nutrient dynamics in EB. It was focused on physical and chemical characterization of discharges, water and sediment quality, nutrient benthic fluxes, and water exchange with UB. The average ammonium, nitrate, phosphate and silicate concentrations in the water bay were 99.5 ± 30.7; 10.0 ± 4.2; 2.0 ± 0.7; 23.5 ± 2.9 µM, respectively. Benthic fluxes showed a consumption of oxygen (50-450 mg m⁻² h⁻¹) and nitrate (20-416 µmol m⁻² h⁻¹) by sediment and release of ammonium (79−4,772 µmol m⁻² h⁻¹) and phosphate (27-36 µmol m⁻² h⁻¹) into the water column. The daily contributions of nitrogen and phosphate from the effluents to EB were between 102 and 517 kg day⁻¹ and between 4 and 22 kg day⁻¹ respectively, while the net average export fluxes to UB were 41.7 kg day⁻¹ of nitrogen and 15.7 kg day⁻¹ of phosphate. The difference between received and exported nutrients is consumed in EB by primary producers, partially buffering the impact of wastewater in UB at its own eutrophication risk.

The Laucala Bay coastal system was investigated from 2003 to 2005 to evaluate the changes in water quality (nutrients) after improvements in sewage effluent quality from Kinoya sewage treatment plant (KSTP), whose output is discharged into the bay. The oxidized nitrogen (NO x -N) values averaged 1.31 µM, but varied from 0.76 to 3.77 µM, and the filterable reactive phosphate or orthophosphate (FRP) averaged 0.77 µM, ranging from 0.24 to 2.37 µM. The mean concentrations of NO x -N and FRP, respectively, were two and ten times higher than concentrations found in unpolluted sites in Fiji. Based on these values and NH₃ concentrations from parallel studies, Redfield ratio estimates suggest that primary productivity of the bay is N limited. No significant decrease in concentration of both NO x -N and FRP was found in Laucala Bay waters during the present investigation compared to concentrations found in two previous studies. This indicates that KSTP effluent may be only a minor source of nutrients into the bay, and to protect the Laucala Bay ecosystem from increasing nutrient pollution, there is a need to control nutrient input from other major point and nonpoint sources.