Norwegian Sea Net Community Production Calculated Using O2 And CO2 Optodes on a Seaglider

The ocean carbon cycle is influenced by processes such as respiration, photosynthesis, air-sea gas exchange, vertical and horizontal mixing and CaCO₃ dissolution. To resolve these processes on all relevant temporal and spatial scales, higher resolution-sampling than daily CTD casts on research cruises is needed. In this study, for the first time a CO₂ optode was deployed on a Seaglider for eight months in the Norwegian Sea, together with an O₂ optode and a chlorophyll fluorometer, repeating the same transect eight times. The raw CO₂ optode measurements were not accurate and needed correction for drift, thermal lag and recalibration using discrete samples of total alkalinity (A_T) and dissolved inorganic carbon (C_T). Carbon (NCP_C) and oxygen (NCP_O)-based net community production were derived from the dataset.

In particular, using a salinity of 35 the data were divided between the fresh Norwegian Coastal Current and the saline Norwegian Atlantic Current. Then, the data were binned into 0.1° latitude bins to calculate the NCP through the repeat glider transects. The inventory changes were corrected for air-sea flux and entrainment to calculate NCP. NCP_O and NCP_C were found to agree with each other and in line with corresponding changes in the chlorophyll concentration. It was possible to identify two phytoplankton blooms at the beginning of May and June. During the first bloom, NCP_O increased to 74 mmol m^-2 d^-1 and NCP_C to 17 mmol m^-2 d^-1. During the second bloom, NCP_O increased to 178 mmol m^-2 d^-1 and NCP_C to 90 mmol m^-2 d^-1. Moreover, during all of summer the surface water was oversaturated with O₂ creating a continuous mean flux to the atmosphere of 19 mmol m^-2 d^-1 and making it the main driver for NCP_O changes. In contrast, in the case of carbon the Norwegian Sea took up 3 mmol m^-2 d^-1 of CO₂ during all of summer and the NCP_C changes were mainly driven by the inventory.