Seasonal and interannual variability in pH and dissolved oxygen concentration in the Atlantic Water inflow north of Svalbard

The slope area north of Svalbard is a key gateway between the North Atlantic and the Arctic Ocean. The inflow of Atlantic Water is a major source of heat and nutrients and strongly influences biological productivity and the marine ecosystem. At the same time, Atlantic Water is rich in anthropogenic CO₂ and characterised by low pH, potentially affecting acidification and thus marine organisms in the Arctic. While recent efforts have increased our understanding, there remains large uncertainty around seasonal processes and the interplay between physics, biogeochemistry and biology especially in winter due to a lack of observations.

Here, we present time series observations of pH and dissolved oxygen concentrations near the surface and at the bottom from moored sensors placed in the Atlantic Water inflow on the continental slope north of Svalbard in 2019-2020 and 2021-2022.

We find high seasonality in the near-surface layer both in dissolved oxygen and pH whereas the bottom layer is more stable. Variability in pH is mostly driven by the strength of the Atlantic Water inflow and water mass present both near the surface and at depth. In contrast dissolved oxygen near the surface is strongly influenced by both physical processes including Atlantic Water inflow, sea ice presence and air-sea interaction, but also biological activity such as the spring bloom and respiration and degradation of organic matter. In the deep layer gradual respiration is counteracted by periodic ventilation events linked to the inflow of more oxygen-rich Atlantic origin waters.

The strong influence of Atlantic Water on pH and dissolved oxygen at the Svalbard slope highlights how changes in North Atlantic circulation and water mass properties have cascading effects on the biogeochemistry of this Arctic sentinel region. Concurrently, the pronounced difference between the two years of pH and dissolved oxygen, and the interplay between various advective and local biogeochemical processes, highlight that no single driver fully explains the observed variability. This complexity underlines the urgent need for longer time series and sustained year round observations to robustly identify the processes shaping the biogeochemistry in the Arctic-Atlantic transition zone, and how these may evolve under future climatic change.