Abstract. The ocean plays a crucial role in the global carbon cycle by absorbing and storing substantial amounts of atmospheric carbon dioxide (CO₂). It is estimated that the ocean has sequestered approximately 26 % of CO₂ emissions over the last decade, resulting in significant changes in the marine carbon system and impacting the marine environment. The sea-surface microlayer (SML) plays a crucial role in these processes, facilitating the transfer of materials and energy between the ocean and the atmosphere. However, most studies on the carbon cycle in the SML have primarily addressed daily variability and overlooked nocturnal processes, which may lead to inaccurate global carbon estimates. We analysed temperature, salinity, pH_T25, and pCO₂ using data collected over three complete diel cycles during an oceanographic campaign along the Croatian coast near Šibenik in the Middle Adriatic. Our analysis revealed statistically significant differences (p < 0.05) between daytime and nighttime measurements of temperature, salinity, and pH_T25. These differences may be related to the occurrence of buoyancy fluxes, which are typically more pronounced during the day and could enhance CO₂ fluxes, as observed with values of 1.98 ± 2.52 mmol cm⁻² h⁻¹ during the day, while at night, they dropped to 0.01 ± 0.02 mmol cm⁻² h⁻¹. These findings emphasise the importance of considering complete diurnal cycles to accurately capture the variability in thermohaline features and carbon exchange processes, thereby improving our understanding of the role of the ocean in climate change.