Cyanobacteria are an important part of phytoplankton communities, however, they are also known for forming massive blooms with potentially deleterious effects on recreational use, human and animal health, and ecosystem functioning. Emerging high-frequency imaging flow cytometry applications, such as Imaging FlowCytobot (IFCB), are crucial in furthering our understanding of the factors driving bloom dynamics, since these applications provide community composition information at frequencies impossible to attain using conventional monitoring methods. However, the proof of applicability of automated imaging applications for studying dynamics of filamentous cyanobacteria is still scarce. In this study we present the first results of IFCB applied to a Baltic Sea cyanobacterial bloom community using a continuous flow-through setup. Our main aim was to demonstrate the pros and cons of the IFCB in identifying filamentous cyanobacterial taxa and in estimating their biomass. Selected environmental parameters (water temperature, wind speed and salinity) were included, in order to demonstrate the dynamics of the system the cyanobacteria occur in and the possibilities for analyzing high-frequency phytoplankton observations against changes in the environment. In order to compare the IFCB results with conventional monitoring methods, filamentous cyanobacteria were enumerated from water samples using light microscopical analysis. Two common bloom forming filamentous cyanobacteria in the Baltic Sea, Aphanizomenon flosaquae and Dolichospermum spp. dominated the bloom, followed by an increase in Oscillatoriales abundance. The IFCB results compared well with the results of the light microscopical analysis, especially in the case of Dolichospermum. Aphanizomenon biomass varied slightly between the methods and the Oscillatoriales results deviated the most. Bloom formation was initiated as water temperature increased to over 15°C and terminated as the wind speed increased, dispersing the bloom. Community shifts were closely related to movements of the water mass. We demonstrate how using a high-frequency imaging flow cytometry application can help understand the development of cyanobacteria summer blooms.

Abstract. In this study, we introduce new observations of sea–air fluxes of carbon dioxide using the eddy covariance method. The measurements took place at the Utö Atmospheric and Marine Research Station on the island of Utö in the Baltic Sea in July–October 2017. The flux measurement system is based on a closed-path infrared gas analyzer (LI-7000, LI-COR) requiring only occasional maintenance, making the station capable of continuous monitoring. However, such infrared gas analyzers are prone to significant water vapor interference in a marine environment, where CO2 fluxes are small. Two LI-7000 analyzers were run in parallel to test the effect of a sample air drier which dampens water vapor fluctuations and a virtual impactor, included to remove liquid sea spray, both of which were attached to the sample air tubing of one of the analyzers. The systems showed closely similar (R2=0.99) sea–air CO2 fluxes when the latent heat flux was low, which proved that neither the drier nor the virtual impactor perturbed the CO2 flux measurement. However, the undried measurement had a positive bias that increased with increasing latent heat flux, suggesting water vapor interference. For both systems, cospectral densities between vertical wind speed and CO2 molar fraction were distributed within the expected frequency range, with a moderate attenuation of high-frequency fluctuations. While the setup equipped with a drier and a virtual impactor generated a slightly higher flux loss, we opt for this alternative for its reduced water vapor cross-sensitivity and better protection against sea spray. The integral turbulence characteristics were found to agree with the universal stability dependence observed over land. Nonstationary conditions caused unphysical results, which resulted in a high percentage (65 %) of discarded measurements. After removing the nonstationary cases, the direction of the sea–air CO2 fluxes was in good accordance with independently measured CO2 partial pressure difference between the sea and the atmosphere. Atmospheric CO2 concentration changes larger than 2 ppm during a 30 min averaging period were found to be associated with the nonstationarity of CO2 fluxes.

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.

Abstract. The direction and magnitude of carbon dioxide exchange between the atmosphere and the sea is regulated by their difference in partial pressure of carbon dioxide (pCO2). Typically, observations of pCO2 are carried out by using research vessels and voluntary observing ships which cannot easily detect 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 during the different seasons.We present pCO2 data from July 2018–June 2019 carried out in the vicinity of the island of Utö in the Archipelago Sea and quantify the relevant physical, biological and chemical processes affecting pCO2. The highest monthly median diurnal pCO2 peak-to-peak amplitude (31 μatm) was observed in August. This high diurnal variation was found to be related predominantly to biological processes. The biological transformations of carbon generated a sinusoidal diurnal pCO2 variation, with a maximum in the morning and a minimum in the afternoon. Compared to the biological carbon transformations, the effect of air sea exchange of carbon dioxide and the effect of temperature changes on pCO2 are smaller, with their monthly median peak-to-peak amplitudes were up to 12 and 5 μatm, respectively. Single diurnal peak-to-peak amplitudes can be significantly larger (up to 500 μatm), during upwelling. If the net exchange of carbon dioxide between the sea and atmosphere on our study site and sampling period is calculated based on a data set that consists of only one measurement per day, the error in the budget depends on the sampling time and can be up to ±12 %.

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