Norwegian Sea net community production estimated from O2 and prototype CO2 optode measurements on a Seaglider

We report on a pilot study using a CO2 optode deployed on a Seaglider in the Norwegian Sea from 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 that 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 content, c(DIC), which had a standard deviation of 11 mu mol kg(-1) compared with in situ measurements. The glider was also equipped with an oxygen (O-2) optode. The O-2 optode was drift corrected and calibrated using a c(O-2) climatology for deep samples. The calibrated data enabled the calculation of DIC- and O-2-based net community production, N (DIC) and N (O-2). To derive N, DIC and O-2 inventory changes over time were combined with estimates of air-sea gas exchange, diapycnal mixing 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 (DIC) = (21 +/- 5) mmol m(-2) d(-1), N (O-2) = (94 +/- 16) mmol m(-2) d(-1) and an (uncalibrated) Chl a peak concentration of c(raw)(Chl a) = 3 mg m(-3). During the June period, c(raw)(Chl a) increased to a summer maximum of 4 mg m(-3), associated with N (DIC) = (85 +/- 5) mmol m(-2) d(-1) and N (O-2) = (126 +/- 25) 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 (DIC) to (-3 +/- 5) mmol m(-2) d(-1) and N (O-2) to (0 +/- 2) mmol m(-2) d(-1). The survey area was a source of O-2 and a sink of CO2 for most of the summer. The deployment captured two different surface waters influenced by the Norwegian Atlantic Current (NwAC) and the Norwegian Coastal Current (NCC). The NCC was characterised by lower c (O-2) and c(DIC) than the NwAC, as well as lower N (O-2) and craw(Chl a) but higher N (DIC). Our results show the potential of glider data to simultaneously capture time- and depth-resolved variability in DIC and O-2 concentrations.

Automated summary

The study utilized a CO2 and O-2 optode on a Seaglider in the Norwegian Sea to study the spatial and temporal variability in DIC and O-2 concentrations. Results showed significant influence of different seasons and currents on Chl a inventory and production.