Hourly oxygen and total gas tension measurements at the Southern Ocean Time Series site reveal winter ventilation and spring net community production

Abstract

Using a moored instrument package at 35 m depth at the Southern Ocean Time Series (SOTS) site near 46°56′S 142°15′E from September 2010 to April 2011 (219 days), we obtained the first Southern Ocean Time Series of dissolved oxygen (from an optode sensor) and nitrogen (from a total gas tension sensor). Nitrogen was consistently supersaturated (100.8%–102.9%), while oxygen was highly subsaturated in early spring (as low as 93.5%) and reached supersaturation (maximum 104.9%) during only 37 days in early summer. The low oxygen levels in spring illustrate the importance of deep mixing in the Subantarctic Zone in ventilating the upper limb of the global overturning circulation. Using nitrogen as a proxy for physical processes, we isolated biological contributions to the oxygen time series to obtain net community production (NCP). Almost all NCP occurred in spring in the presence of deep mixed layers, with only small additional contributions in summer after water column stratification. The temperature and salinity time series also revealed distinct parcels of water. Rapid changes at their interfaces generated unrealistic NCP events in the standard calculation model, which were removed, while still retaining NCP contributions from each parcel. NCP totaled 2.2 ± 1.2 mol O2 m−2 over the deployment, within the range of previous estimates from low temporal resolution techniques. Examination of errors revealed particular sensitivity to entrainment, suggesting more rigorous understanding of this process is required, e.g., via profiling instruments.