Valdes Bay constitutes a very interesting land-sea interface (LSI) at Peninsula Valdes. A gravel bank separates the bay from the open sea. The general characteristics (physical, chemical and biological) of this ecosystem and the chemical composition of the seawater coming in and out were analyzed. Nutrient cycling (nitrogen and phosphate) was very intense and involved a high primary productivity. These nutrients were principally produced at the gravel bank, where oxygen was consumed by heterotrophic activity and nitrification processes. The latter process consumed 1 to 7% of the oxygen. Extrapolating to the 35-km bank, a value of 6.8 metric tons of nitrogen would be produced daily.