Eutrophication of lakes. Some lakes also demonstrate the reverse process (meiotrophication), becoming less nutrient rich with time. The main difference between natural and anthropogenic eutrophication is that the natural process is very slow, occurring on geological time scales. Eutrophication is a common phenomenon in coastal waters. In contrast to freshwater systems, nitrogen is more commonly the key limiting nutrient of marine waters; thus, nitrogen levels have greater importance to understanding eutrophication problems in salt water. Estuaries tend to be naturally eutrophic because land-derived nutrients are concentrated where run-off enters a confined channel. Upwelling in coastal systems also promotes increased productivity by
Baltic Sea hypoxia lack of oxygen (anoxia) kills bottom-living organisms and creates dead zones. The rapid increase in hypoxia in coastal areas around the world is due to the excessive inputs of plant nutrients, such as nitrogen and phosphorus by human activities. The sources of these nutrients include agriculture, sewage, and atmospheric deposition of nitrogen containing compounds from the burning of fossil fuels. The nutrients stimulate the growth of algae causing problems with eutrophication. The algae sink to the bottom and use the oxygen when they decompose. If mixing of the bottom waters is slow, such that oxygen stocks are not renewed, hypoxia