Water that contains 2 mg oxygen per liter or less is termed hypoxic, since at that concentration many aquatic aerobic organisms are unable to survive. how does the depth at which hypoxia is observed change over time

Global and local climate change also makes marine ecosystems more vulnerable to lower oxygen levels (hypoxia). Aquatic organisms cannot survive without the presence of dissolved oxygen in the water.
When the climate is warmer, the surface water warms up more quickly and its density becomes lower than that of the underlying waters. However, it is the mixture of waters of different depths that helps to oxygenate the deeper waters. If the surface water is warmer and less dense, it mixes less well with the more dense bottom waters.  As a result, the oxygen content will be lower in some places (deeper places for example).
At oxygen levels below 30%, species that do not tolerate hypoxic conditions must migrate to other geographic areas. If they can not migrate, their survival may be threatened.