Nutrients in the oceans are moved about by biological and physical process. In the euphotic zone in the surface open oceans, nutrients are utilized mainly by phytoplankton and cyanobacteria. Respiration and the decomposition of marine organisms cause nutrients to be released back into the seawater, mostly in the deep waters. As a result, nutrient concentrations are generally low near surface but high in deeper waters, except in polar regions where intensive upwelling brings cold, nutrient-rich seawater back to the surface. Aside from upwelling, physical processes play an important role in moving seawater horizontally and in, therefore, transporting nutrients as well. For instance. Ocean currents transport the nutrient-rich Mississippi River water to form the death zone off the delta.