IMIS - Marine Onderzoeksgroepen

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 theoptode signal correlated better with the concentration of CO₂,c(CO₂), than with its partial pressure, p(CO ₂). Using the calibrated c(CO₂) and a regional parameterisation of total alkalinity (A_T) as a function of temperature and salinity, we calculated total dissolved inorganic carbon content, c(DIC), which had a standard deviation of 11 µmol kg⁻¹ compared with in situ 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. The calibrated data enabled the calculation of DIC- and O₂-based net community production, N(DIC) and N(O₂). To derive N, DIC and O₂ 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, wefound N(DIC) = (21±5) mmol m⁻² d⁻¹, N(O₂) = (94±16) mmol m⁻² d⁻¹ andan (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. Thehigh-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 materialproduced during the period of increased Chl a inventory decreased N(DIC) to (−3±5) mmol m⁻² d⁻¹ and N (O₂) to (0±2) 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 influenced by the Norwegian Atlantic Current (NwAC) and the Norwegian Coastal Current(NCC). The NCC was characterised by lower c(O₂) and c(DIC) than the NwAC, as well as lower N(O₂) and c_raw(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 O₂ concentrations.