The Transmission Effectiveness of the Dissolved Nutrients (N, P, Si) Through a South Mediterranean Estuary (Seybouse, Algeria) Under Large Anthropogenic Forcing

Abstract

The Seybouse Estuary (SE) is the downstream of the second great river in Algeria draining one of the main country’s watersheds (6471 km2, 1.5 million inhabitants). This study highlights the effect of the estuarine part on the distribution and the flux of dissolved nitrogen (N), phosphorus (P) and silicon (Si) through the SE. Surface water sampling was taken and analysed in the dry (September 2016) and wet (April 2017) seasons. Physical parameters and water discharge data were jointly recorded in parallel to sampling dates. Very low flows varying between 0.3 and 4.7 m3 s-1 were recorded at the outlet in dry and wet seasons, respectively. The SE outlet waters were 2.6 and 10 folds higher than upstream in dissolved inorganic nitrogen (DIN) and phosphates (PO4) levels, respectively. However, the silicates (SiO4) levels decreased by about 1/2 in the outlet waters. In terms of flux, the SE delivered to the coast 84 t·yr-1, 9 t·yr-1 and 56 t·yr-1 of DIN, PO4 and SiO4, respectively. The high DIN:PO4 = 137 and low SiO4:DIN = 0.52 molar ratios classify SE as one of the strong anthropised estuaries. The estuarine part amply modifies cycles and budgets of riverine nutrients introduced to the coast.