Figure shows that measurements from the 5 m discrete samples and the underway inlet compared well within measurement noise, which confirms that the ship's underway inlet did not contaminate our measurements. Mean underway surface seawater benzene and toluene concentrations were very similar (cruise mean ± standard deviation, benzene 17 ± 11 pmol dm -3 , toluene 17 ± 9 pmol dm -3 ) and both displayed a large range: for benzene from the limit of detection (LOD) to 73 pmol dm -3 , and for toluene from 5 pmol dm -3 to 72 pmol dm -3 . Previous mesocosm experiments have suggested a diurnally varying source of benzene and toluene in the ocean . However, diurnal variability is not obvious in our data (Figure ). Instead, we observed sporadic episodes of very high seawater benzene and toluene concentrations on top of relatively constant background levels. This is a very characteristic distribution for compounds produced by biological processes, as is the case for example for DMS (2) and isoprene . The median seawater concentration of benzene (13 pmol dm -3 ) and toluene (15 pmol dm -3 ) were lower than the mean which illustrates a greater skewness for benzene than for toluene. Despite similar mean concentrations, benzene and toluene usually did not peak at the same time. For example, benzene concentrations in seawater were very high in an area of high Chl a concentration sampled around 13/3/19, while toluene concentrations in this area were near the cruise average. Consequently, benzene and toluene underway seawater concentrations correlated significantly, but weakly during this cruise track (R = 0.07; P<0.001; N=885), suggesting different source and sink strengths for the two compounds. Benzene correlated significantly but weakly with Chl a (R=0.34; P<0.001; N=859) and strongly with DMS (R=0.70; P<0.001; N=868), while toluene did not correlate with either. The correlation of benzene with Chl a is consistent with the findings from the depth profiles. Toluene did correlate weakly with isoprene (R=0.08; P<0.001; N=825). Overall, significant correlations with a phytoplankton proxy (Chl a) and/or other biogenic trace gases (DMS, isoprene) support a biological source for benzene and toluene in seawater.