How does the volume of Oxygen Minimum Zones change in time? How does this affect the cycling of nitrate?

Concentrations of dissolved oxygen are decreasing in many areas of the ocean interior ([Stramma et al., 2008]; [Keeling et al., 2010]) and oxygen minimum zones (OMZs) may be expanding. Evidence now suggests that the area and volume of OMZ regions may oscillate on a decadal to centennial time scale, linked to variability in trade winds ([Deutsch et al., 2014; Deutsch et al., 2011]). As oxygen is depleted in OMZs, nitrate is consumed by processes such as classical denitrification and anammox. In addition, respiration of sedimenting particulate material in an OMZ may slow down, resulting in an enhanced flux of carbon to greater depth [Roullier et al., 2014]. The available data sets suggest that nitrate stocks may show multi-decadal scale oscillations, as well [Deutsch et al., 2011]. Long-term decreases in oxygen may reduce ocean productivity as a result. However, oxygen measurements at the very low concentrations found in OMZs are difficult and the historical data are often suspect [Bianchi et al., 2012]. Oxygen sensors on profiling floats are now capable of producing high quality data that rival the consistency of the best shipboard data when sensors are calibrated in the atmosphere on each profile ([Bittig and Körtzinger, 2015] [Johnson et al., 2015]). Global arrays of floats with oxygen and nitrate sensors could produce unparalleled records of variability in OMZs ([Stanev et al., 2013]; [Whitmire et al., 2009]; [Prakash et al., 2012]). They would help elucidate the mechanism of deoxygenation within the intermediate waters of the ocean, such as that occurring in the North Pacific [Emerson et al., 2004; Whitney et al., 2007; Sasano et al., 2015], and its linkage to climate

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