See complete methodology in Haskell et al. (2016). In summary:
This study is part of an effort aimed at characterizing the biological response to upwelling at SPOT on 21 cruises between January 2013 and June 2014; the Upwelling Regime In-Situ Ecosystem Efficiency (Up.R.I.S.E.E.) study.

Dissolved oxygen/argon ratios: Samples for O2/Ar analysis were collected from Niskin bottles in 300 and 500 mL glass flasks equipped with airtight Louwers-Hapert valves (with 3 high-vacuum greased Viton O-rings in valve stem) and side arms. Prior to sampling, each bottle was poisoned with 150 uL saturated HgCl2, dried in an oven at 50 C, then evacuated to <1 mtorr and weighed. Samples were later sent to the laboratory of Rachel Stanley (WHOI) where they were analyzed for O2 (m/z = 32) and Ar (m/z = 40) by peak jumping using a Finnigan MAT 253 IRMS.

CTD oxygen sensor calibration: The CTD oxygen sensor (Seabird SBE 43) was calibrated using Winkler titrations on samples collected in duplicate at 5 depths on every CTD cast. Winkler titrations were performed the day after each cruise for most of the one day cruises, and typically two days following each two-day cruise, with equipment and procedure outlined by Langdon (2010). In one case (Up-1), Winkler analyses were performed 4 days after the cruise.

Oxygen mixed layer depths were determined as the depth at which the oxygen concentration was 0.5% different from the surface concentration (Castro-Morales and Kaiser, 2012) and ranged between 10 m and 35 m. Euphotic depths were typically between 35 m and 65 m, with the exception of Up-8 and Up-15 (30 m and 70 m). For 9 cruises (Up-13 to Up-21), the base of the euphotic zone was determined with the CTD as the depth at which 1% of the surface photosynthetically available radiation (PAR) is measured. Prior to Up-13, there was no PAR sensor on the CTD unit. Since the deep chlorophyll-a maximum (DCM; determined from CTD fluorescence profiles) is almost always above the euphotic depth in Southern California Bight (Cullen and Eppley, 1981), for the remainder of the cruises, the euphotic depth was defined as 10 m below the DCM. However, during Up-2, -3, and -4, the DCM was within the mixed layer. For these three cruises, the euphotic depth was defined as 15 m below the MLD.