Continuous long-term observations of the carbonate system dynamics in the water column of a temperate fjord

A cabled underwater observatory with more than 30 sensors delivering data in real-time was used to study the dynamics of the upper pelagic carbonate system of the Koljo Fjord, western Sweden, from September to April during two consecutive years (2011-2012 and 2012-2013). In the dynamic upper ca 15 m of the water column, salinity and temperature varied by up to 10 and 20 °C throughout the recorded periods, respectively. Partial pressure of CO2 (pCO2), measured with newly developed optical sensors (optodes) at three water depths (5, 9.6 and 12.6 m), varied between 210-940 μatm, while O2 varied between 80-470 μmol/L. Redfield scaled graphs (ΔO2:ΔDIC = − 1.30), in which DIC was derived from pH or pCO2 and salinity-derived alkalinity (ATsal), and oxygen was measured by the sensors, were used as a tool to assess timing and occurrence of different processes influencing the dynamics of these parameters. Distinctive short-term variations of pCO2 and O2 were induced by either tidal oscillations, wind-driven water mass transport in the mixed layer or occasional transport of deep-basin water from below the thermo/halocline to the surface layer. Intensified air-sea gas exchange during short storm events was usually followed by stabilization of gas-related parameters in the water column, such as O2 concentration and pCO2, on longer time-scales characteristic for each parameter. Biological processes including organic matter degradation in late summer/autumn and primary production in early spring were responsible for slower and gradual seasonal changes of pCO2 and O2. Net primary production (NPP) rates in the Koljo Fjord were quantified to be 1.79 and 2.10 g C m− 2 during the spring bloom periods in 2012 and 2013, respectively, and ratios of O2 production:DIC consumption during the same periods were estimated to be − 1.21 ± 0.02 (at 5 m depth in 2013), − 1.51 ± 0.02 (at 12.6 m in 2012) and − 1.95 ± 0.05 (at 9.6 m in 2013). These ratios are discussed and compared to previously reported O2:C ratios during primary production.