Aquatic and marine dead zones can be caused by increase in chemical nutrients in the water, know as eutrophication.these chemicals are the fundamental building block of single-celled, plants-like organisms that live in the water column, and whose growth is limited in part by the availabality of these materials.Eutrophication can lead to rapid increase in the density of certian types of phytoplankton, a phenomennon know as an algal bloom.Although these algae produce oxygen in the daytime via photosynthesis,during the nigth hours they continue to undergo cellular respiration and can therefore deplete the water column to of available oxygen.In addiition,when algal blooms die off,oxygen is used up further during bacterial decomposition of the dead algal cells.Both of these processes can result in a significant depletion of dissolved oxygen in the water, creating hypoxic comditions. Dead zones can be caused by natural and anthropogenic factors. Use of chemical fertilizers is causes include coastal upwelling and changes in wind and water circulation patterns. Runoff from sewage, urban land use, and fertilizers can also contribute to eutrophication.