Exacerbation of the problem in the future, however, is expected mostly in the developing nations of the tropics, where population growth and attendant urbanization are fastest. Eutrophication—an increased rate of supply of nutrients leading to enhanced primary production—is caused mainly by loading of nitrates and phosphates derived from fertilizers, livestock waste, and fossil fuel emissions. The most heavily affected areas are the Baltic and North Seas, the Mediterranean, and the Northwestern Atlantic. The rates of autotrophic production then exceed the rate of consumption, resulting in the settling of the excess organic material, the decomposition of which leads to extreme effects such as oxygen depletion, buildup of toxins, smothering, and mortality of benthic and some pelagic species. Ecosystem models offer some insight into the contribution of anthropogenic inputs and their fate in watersheds and associated rivers and estuaries.