The fate of carbon in the coastal sea is emerging as a vital component in the global carbon cycle. In a healthy state, the coastal sea acts a carbon sink, but a degraded state could potentially turn the coastal sea to a source of carbon in the form of emitted carbon dioxide (CO₂) and methane (CH₄). However, the carbon budget in the coastal sea is often poorly constrained due to an inadequate amount of available data. A new eddy covariance (EC) flux tower for measuring sea–atmosphere gas fluxes was installed in a semi-enclosed bay in the coastal Baltic Sea in early 2022. The quasi-continuous EC measurements enable for a better-restrained carbon budget. Although the amount of data is limited due to the small magnitude of fluxes and especially the interference of fluxes from land, we are able to obtain flux estimates in three unobstructed sectors around the tower. We report here the preliminary results for CO₂ fluxes between April 2022 and January 2023. The measured mean monthly CO₂ fluxes varied between −0.3 ± 0.4 µmol m⁻² s⁻¹ (± STD, net sink) and 0.4 ± 0.4 µmol m⁻² s⁻¹ (net source). The measured CO₂ fluxes were relatively small, but all monthly mean fluxes differed significantly (p < 0.05) from 0 except in September. There was a significant (p < 0.05) difference in the day- and night-time CO₂ fluxes from April to October, likely reflecting the diurnal changes in the environmental drivers. More analysis is needed in order to distinguish the fluxes from the land interference and to determine which environmental parameters are driving the fluxes.

Primary Presenter: Aki Vähä, University of Helsinki (aki.vaha@helsinki.fi)

Authors:

Aki Vähä, Institute of Atmospheric and Earth System Research / Physics, University of Helsinki, Helsinki, Finland  (aki.vaha@helsinki.fi)

Christoph Humborg, Baltic Sea Centre, Stockholm University, Stockholm, Sweden  ()

Alf Norkko, Tvärminne Zoological Station, University of Helsinki, Hanko, Finland  ()

Joanna Norkko, Tvärminne Zoological Station, University of Helsinki, Hanko, Finland  ()

Kurt Spence, Tvärminne Zoological Station, University of Helsinki, Hanko, Finland  ()

Markku Kulmala, Institute of Atmospheric and Earth System Research / Physics, University of Helsinki, Helsinki, Finland  ()

Ivan Mammarella, Institute of Atmospheric and Earth System Research / Physics, University of Helsinki, Helsinki, Finland  ()