Geochemical investigation of phosphorus and nitrogen in the hypersaline Dead Sea

Abstract

Dissolved and particulate phosphorus, ammonia, and nitrate concentrations were measured in the Dead Sea water column (330 m depth, salinity ca. 340 g/L), in the Lower Jordan River and in springs along its western coast. Dissolved phosphate in the water column is fairly homogeneous, at about 35 μg P/L. Particulate P shows larger variability, 30–50 μg/L. The main inputs of dissolved phosphate into the lake are diffusion from bottom sediments (58%) and the Jordan River inflow (41%). Input from springs is negligible (1%). Biological uptake is a minor removal pathway because in the present Dead Sea, primary production takes place only when major floods occur and dilute the upper layer, about once in 10 years. When this happens, only about 200 ton P, out of a whole-lake reservoir of ca. 5000 ton P, are removed from the biologically active upper layer of about 7 m. Chemical removal pathways, not yet investigated, such as coprecipitation with authigenic aragonite, could be of importance. The average ammonia concentration in the water column has gradually increased from 5.9 mg/L in 1960 to 8.9 mg/L, in 1991. Diffusion from the bottom sediments is a major source of ammonia to the Dead Sea. The annual input from freshwater inflows has been of lesser importance in the 1960s and 1970s. A pollution-derived increase in the ammonia concentration of the Jordan River in the 1980s may partially explain the concomitant rise in the ammonia load of the lake. It is also suggested that following the 1979 overturn, and the yearly turnovers of the 1980s, ammonia might have been produced within the oxygenated water column by mineralization. Nitrate concentration was very low, 20 μg N/L, in the 1960s and increased to 0.2 to 0.5 mg/L in 1981. This increase is shown to be due to human pollution of the Jordan River. We conclude that nutrient concentration in the Dead Sea water column is controlled by physical and chemical factors, whereas biological effects are minimal.