Dead zones are caused by an increase in chemical nutrients in the water, known as eutrophication. Eutrophication can lead to an increase in the density of certain types of phytoplankton, known as an algae bloom. These algae blooms die off and as they do, oxygen is used up and due to the oxygen being used up it causes hypoxia conditions.
Dead zones are caused naturally but also by human activities. Fertilizers is the major human related cause. The natural causes are coastal upwelling, changes in the wind and water circulation patterns, runoff from sewage, and urban land use.
Due to low oxygen levels, there have been problems with the reproduction of fish, low egg counts, and lack of spawning. It's been found in the study of the Gulf killifish by the Southeastern Louisiana University that the never of eggs in females living in hypoxia waters were only 1/7 the number of eggs in fish living in normal oxygen level waters.