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 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, we found N(DIC) = (21±5) mmol m−2 d−1, N(O2) = (94±16) 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, associated with N(DIC) = (85±5) mmol m−2 d−1 and N(O2) = (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(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 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 O2 concentrations.

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 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.

On the Variability in the Onset of the Norwegian Coastal Current

A high-resolution reanalysis of the circulation in the Kattegat and Skagerrak is used to investigate the mechanisms that control the variability in the onset of the Norwegian Coastal Current. In the reanalysis, the authors have used all available in situ and remote sensing observations of salinity and temperature and use surface current observations from two coastal high-frequency radars that were ideally placed to monitor the exchange between the two basins. This study finds a strong correlation between the variability in the wind forcing in the Skagerrak and the transport in the Norwegian Coastal Current through the Torungen–Hirtshals section. Two cases with winds into and out of the Skagerrak are studied in more detail, and the results suggest asymmetries in the forcing mechanisms. For winds out of the Skagerrak, strong outflows of Baltic Sea Water associated with a deflection of the Kattegat–Skagerrak Front may disrupt local processes in the Skagerrak, which is not accounted for in previously published conceptual models for the variability of the coastal currents in this region.

Modelling dispersal of eggs and quantifying connectivity among Norwegian coastal cod subpopulations

The Norwegian coast is populated by two cod populations: Northeast Arctic cod and Norwegian Coastal cod. In this paper, we use a further division based on life history: oceanic cod, coastal cod, and fjord cod. A numerical ocean model was implemented for the northern Norwegian coast where all these populations have spawning areas. The model results were used to simulate connectivity and retention of cod eggs from the different subpopulations. The model reproduced the observed variability and mesoscale activity in the Norwegian Coastal Current. Eggs released at an oceanic spawning area were transported northwards along the coastline. Coastal cod eggs had intermediate connectivity with each other and fjord cod eggs had high local retention. Although the high retention of eggs in fjord areas is mainly caused by a subsurface distribution of eggs, the intermediate retention of eggs from coastal spawning areas is caused by small-scale eddies in-between many small islands. The high-resolution ocean model made it possible to reveal these specific dispersal patterns. The high retention of early life stages in fjords combined with strong homing to spawning areas indicates that fjord subpopulations may be described as a metapopulation.