The diurnal cycle of <i>p</i>CO<sub>2</sub> in the coastal region of the Baltic Sea

Abstract

Abstract. The direction and magnitude of carbon dioxide fluxes between the atmosphere and the sea are regulated by the gradient in the partial pressure of carbon dioxide (pCO2) across the air–sea interface. Typically, observations of pCO2 at the sea surface are carried out by using research vessels and ships of opportunity, which usually do not resolve the diurnal cycle of pCO2 at a given location. This study evaluates the magnitude and driving processes of the diurnal cycle of pCO2 in a coastal region of the Baltic Sea. We present pCO2 data from July 2018 to June 2019 measured in the vicinity of the island of Utö at the outer edge of the Archipelago Sea, and quantify the relevant physical, biological, and chemical processes controlling pCO2. The highest monthly median of diurnal pCO2 variability (31 µatm) was observed in August and predominantly driven by biological processes. Biological fixation and mineralization of carbon led to sinusoidal diurnal pCO2 variations, with a maximum in the morning and a minimum in the afternoon. Compared with the biological carbon transformations, the impacts of air–sea fluxes and temperature changes on pCO2 were small, with their contributions to the monthly medians of diurnal pCO2 variability being up to 12 and 5 µatm, respectively. During upwelling events, short-term pCO2 variability (up to 500 µatm within a day) largely exceeded the usual diurnal cycle. If the net annual air–sea flux of carbon dioxide at our study site and for the sampled period is calculated based on a data subset that consists of only one regular measurement per day, the bias in the net exchange depends on the sampling time and can amount up to ±12 %. This finding highlights the importance of continuous surface pCO2 measurements at fixed locations for the assessment of the short-term variability of the carbonate system and the correct determination of air–sea CO2 fluxes.

Persistent hot spots of CO2 and CH4 in coastal nearshore environments

“…Most likely driven by inorganic carbon uptake by primary producers, phytoplankton biomass was observed to have a negative effect on pCO 2 at chl a concentrations higher than 8 μg L À1 . Although the pCO 2 followed a typical diurnally oscillating pattern (Honkanen et al 2021), this effect was small compared to other drivers. We also observed strong seasonality in pCO 2 partial pressures, suggesting high CO 2 uptake during spring bloom and high mineralization rates in autumn, resulting from high primary productivity in summer.…”

“…The pronounced seasonality of high-latitude ecosystems adds further complexity to quantification of biogeochemical processes, leading to high variability of concentration and flux estimates of CO 2 and CH 4 from coastal environments. Accordingly, the partial pressures of CO 2 and CH 4 as well the associated fluxes at the water-atmosphere interface display pronounced diurnal (Honkanen et al 2021;Roth et al 2022) and seasonal variation (Borges et al 2018). For example, methane emissions ranging between 1 and 5000 μmol m À2 d À1 were observed in a coastal bay adjacent to the Atlantic Ocean (Burgos et al 2018).…”

A carbonate system time series in the Eastern Mediterranean Sea. Two years of high-frequency in-situ observations and remote sensing

“…Observations at high frequency are important as they provide more information on changes in ocean chemistry essential for the assessment of the marine ecosystem's responses to warming, eutrophication, and the oceanic CO 2 uptake (Flecha et al, 2015), on how diel variations in plankton production and respiration contribute to the variability of the carbonate system (Honkanen et al, 2021), as well as on developing/validating algorithms and exploring drivers connected with sub-mesoscale phenomena. Measuring from moving platforms may compromise the results as biological diurnal oscillations may not be appropriately accounted for (Honkanen et al, 2021). The present study provides the first marine pCO 2 time series in the Eastern Mediterranean measuring pCO 2 directly and at high frequency (hourly scale) and the second pH time-series cycle also at high frequency (a previous annual cycle done in Saronikos Gulf in 2013; Gonzaĺez-Davila et al, 2016).…”

Shipping and algae emissions have a major impact on ambient air mixing ratios of NMHCs and methanethiol on Utö island in the Baltic Sea

“…The station produces real-time, highfrequency measurements of the physical, chemical and biological features of the water column and atmospheric concentrations of trace gases and aerosols (Fig. 1a; Laakso et al, 2018;Kraft et al, 2021;Honkanen et al, 2018Honkanen et al, , 2021Honkanen et al, , 2023Rautiainen et al, 2023).…”