The study states that 10-30% of the oxygen loss between 1960 and 2010 is attributed to warming-induced decline in solubility, and that globally "the majority of oxygen loss has been caused by changes in ocean circulation and associated ventilation with oxygen from the ocean surface" (i.e. probably also climate change).

Further, from the study it's clear that "deoxygenation recorded in tropical regions often reflects mostly untreated sewage inputs and agricultural runoff (areas identified in Diaz & Rosenberg, 2008)," with many other citations provided as well.