To a first order, the oxygen content of the ocean interior is determined by the influx of the gas across the air-sea surface (i.e., ventilation) and consumption due primarily to microbial respiration. As these two competing processes vary in space and time, so does the concentration of oxygen in the ocean interior. Although oxygen concentrations on continental margins are declining in many regions due to increased anthropogenic nutrient loadings [e.g., Rabalais et al., 2002], oxygen also appears to be declining in both the central North Pacific Ocean and the tropical oceans worldwide [Emerson et al., 2004; Whitney et al., 2007; Keeling et al., 2010] (see Figure 1). It is unclear whether the loss throughout the basins in the open ocean is a long-term, nonperiodic (secular) trend related to climate change, the result of natural cyclical processes, or a combination of both (Figure 2). If related to climate change, a number of important factors may be involved, including decreased solubility of oxygen as waters warm, decreased ventilation at high latitudes associated with increased ocean stratification, and changes in respiration in the ocean interior.