where currents flowing from the open ocean come up against narrow continental shelves nutrientrich oxygenpoor water can be brought up into coastal waters and produce hypoxic zones zones with low levels of dissolved oxygen or even dead zones zones with insufficient oxygen to support life also called anoxic zones examples of this effect are found on the western coasts of america north and south of the equator the western coast of subsaharan africa and the western coast of the indian subcontinent increases in the flow of some ocean currents may intensify this effect chaps some greenhouse gases especially chlorofluorocarbons have an impact on the ozone layer in the stratosphere