Norwegian Sea net community production estimated from O ₂ and prototype CO ₂ optode measurements on a Seaglider

Abstract. We report on a pilot study using a CO2 optode deployed on a Seaglider in the Norwegian Sea for 8 months (March to October 2014). The optode measurements required drift- and lag-correction, and in situ calibration using discrete water samples collected in the vicinity. We found the optode signal correlated better with the concentration of CO2, c(CO2), than with its partial pressure, p(CO2). Using the calibrated c(CO2) and a regional parameterisation of total alkalinity (AT) as a function of temperature and salinity, we calculated total dissolved inorganic carbon concentrations, CT, which had a standard deviation of 10 µmol kg−1 compared with direct CT measurements. The glider was also equipped with an oxygen (O2) optode. The O2 optode was drift-corrected and calibrated using a c(O2) climatology for deep samples (R2 = 0.89; RMSE = 0.009 µmol kg−1). The calibrated data enabled the calculation of CT – and oxygen-based net community production, N(CT) and N(O2). To derive N, CT and O2 inventory changes over time were combined with estimates of air-sea gas exchange and entrainment of deeper waters. Glider-based observations captured two periods of increased Chl a inventory in late spring (May) and a second one in summer (June). For the May period, we found N(CT) = (24±5) mmol m−2 d−1, N(O2) = (61±14) mmol m−2 d−1 and an (uncalibrated) Chl a peak concentration of craw(Chl a) = 3 mg m−3. During the June period, craw(Chl a) increased to a summer maximum of 4 mg m−3, which drove N(CT) to (64±67) mmol m−2 d−1 and N(O2) to (166±75) mmol m−2 d−1. The high-resolution dataset allowed for quantification of the changes in N before, during and after the periods of increased Chl a inventory. After the May period, the remineralisation of the material produced during the period of increased Chl a inventory decreased N(CT) to (−80±107) mmol m−2 d−1 and N(O2) to (−15±27) mmol m−2 d−1. The survey area was a source of O2 and a sink of CO2 for most of the summer. The deployment captured two different surface waters: the Norwegian Atlantic Current (NwAC) and the Norwegian Coastal Current (NCC). The NCC was characterised by lower c(O2) and CT than the NwAC, as well as lower N(O2), N(CT) and craw(Chl a). Our results show the potential of glider data to simultaneously capture time and depth-resolved variability in CT and O2.

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