The northern part of the Caspian Sea is plagued by a process called eutrophication, in which agricultural run-ff rich in fertilizers stimulates rampant growth of algae in the water. The death and decay of these algae robs the water of oxygen, with obvious negative consequences for aquatic life. This image of the Caspian Sea shows swirls of green and blue near the mouth of the Volga River (top center), which indicate the presence of algae. The bright blue color of the northeastern part of the sea may be due to a mixture of plant life and sediment, for this is where the sea is most shallow. This image is from the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra satellite on June 11, 2003.