One of the most documented dead zones on Earth is in the northern Gulf of Mexico in the summer when solar heating increases the buoyancy of surface waters thereby reducing mixing between the surface and the bottom. Phytoplankton -- powered by the same sunlight and fed by a rich nutrient broth flowing out of local rivers -- bloom, die, sink, and get remineralized by bacteria which use up all the available oxygen in the isolated bottom waters which then become dead zones.
The above view looking eastwards from Louisiana on the left towards northern Florida in the distance on the right was collected in winter when oxygenated water is more easily mixed down to the sea floor. The tell-tale tan and greenish-brown plumes from the Mississippi and Atchafalaya rivers show that even though dead zones may not form in winter, transport of the suspended sediments that are usually accompanied by nutrients continues all year long.
You may click on the above Aqua MODIS image for a larger version. A still larger 7.3 megabyte version is also available.