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Abstract. The western Arctic Ocean, including its shelves and coastal habitats, has become a focus in ocean acidification research over the past decade as thecolder waters of the region and the reduction of sea ice appear to promote the uptake of excess atmospheric CO2. Due to seasonal sea icecoverage, high-frequency monitoring of pH or other carbonate chemistry parameters is typically limited to infrequent ship-based transects duringice-free summers. This approach has failed to capture year-round nearshore carbonate chemistry dynamics which is modulated by biological metabolismin response to abundant allochthonous organic matter to the narrow shelf of the Beaufort Sea and adjacent regions. The coastline of the Beaufort Seacomprises a series of lagoons that account for > 50 % of the land–sea interface. The lagoon ecosystems are novel features that cycle between“open” and “closed” phases (i.e., ice-free and ice-covered, respectively). In this study, we collected high-frequency pH, salinity,temperature, and photosynthetically active radiation (PAR) measurements in association with the Beaufort Lagoon Ecosystems – Long Term Ecological Research program – for an entire calendar yearin Kaktovik Lagoon, Alaska, USA, capturing two open-water phases and one closed phase. Hourly pH variability during the open-water phases are someof the fastest rates reported, exceeding 0.4 units. Baseline pH varied substantially between the open phase in 2018 and open phase in 2019 from ∼ 7.85to 8.05, respectively, despite similar hourly rates of change. Salinity–pH relationships were mixed during all three phases, displaying nocorrelation in the 2018 open phase, a negative correlation in the 2018/19 closed phase, and a positive correlation during the 2019 open phase. The high frequency of pH variabilitycould partially be explained by photosynthesis–respiration cycles as correlation coefficients between daily average pH and PAR were 0.46 and 0.64for 2018 and 2019 open phases, respectively. The estimated annual daily average CO2 efflux (from sea to atmosphere) was5.9 ± 19.3 mmolm-2d-1, which is converse to the negative influx of CO2 estimated for the coastal Beaufort Seadespite exhibiting extreme variability. Considering the geomorphic differences such as depth and enclosure in Beaufort Sea lagoons, furtherinvestigation is needed to assess whether there are periods of the open phase in which lagoons are sources of carbon to the atmosphere, potentiallyoffsetting the predicted sink capacity of the greater Beaufort Sea.
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