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The variability of near surface properties of the Baltic Sea and the Kattegat was investigated using data from research vessel sampling and data from a FerryBox system during the years 2010–2013. The FerryBox system was mounted on a cargo vessel with a route covering the Gulf of Bothnia, the Baltic proper and the Kattegat twice a week. Water samples from the FerryBox system were analysed from 2011 to 2013 to investigate the quality of the FerryBox-underway data. Salinity from water samples and underway measurements showed a strong correlation. Chlorophyll a samples collected in the Belt Sea and the Kattegat and corresponding in vivo fluorescence underway data showed a correlation (R 2 = 0.75, p < 0.001). Temperature and salinity showed a strong correlation between the FerryBox-underway measurements and the research vessel based data. Oxygen was measured using an optode sensor in the FerryBox system. In general the results were well correlated with oxygen data from research vessels based on the Winkler method. Chlorophyll a data from water samples collected using research vessels in general had a weak consistency with the chlorophyll fluorescence data from the FerryBox system. The use of FerryBox systems for observations related to descriptors in the EU Marine Strategy Framework Directive is discussed. [ABSTRACT FROM AUTHOR]