That's in the open ocean  in enclosed basins, for example the Black Sea, many fjords and sea lochs, the deep waters are not renewed by water masses moving in from other areas in the way that they are in the open ocean  and there anoxia can occur in the deep waters, that is the oxygen can be completely removed by biological activity  particularly in degradation processes of organic matter, bacterial respiration  so anoxic conditions can occur in isolated deep basins  but low oxygen concentrations are actually very rare in the open ocean.