Replacing the Nile: Are Anthropogenic Nutrients Providing the Fertility Once Brought to the Mediterranean by a Great River?

摘要

Prior to construction of the Aswan High Dam, the annual Nile flood delivered about 7-11 * 10~3 t of biologically available phosphorus (P), at least 7 * 10~3 t of inorganic nitrogen (N), and 110 * 10~3 t of silica (Si) to the Mediterranean coastal waters off Egypt. These nutrients stimulated a dramatic "Nile bloom" of diatoms which supported a productive fishery. After closure of the dam in 1965, flow from the Nile was reduced by over 90%, and the fishery collapsed. It remained unproductive for about 15 years. The fishery began a dramatic recovery during the 1980s, coincident with increasing fertilizer use, expanded agricultural drainage, increasing human population, and dramatic extensions of urban water supplies and sewage collection systems. Calculations of the potential anthropogenic contribution of nutrients (P and N, but not Si) are consistent with the hypothesis that human sewage and agricultural drainage now support the fertility once provided by the Nile, though the nature of the productive ecosystem now supporting the fishery appears to be quite different from the historical one.