AbstractThe SAR11 clade and the Roseobacter clade affiliated (RCA) cluster belong to the most prominent bacterioplankton groups in temperate to polar seas. Despite some insights into biological controls of both lineages, little is known about environmental, hydrographic and biogeochemical controls. Therefore, we assessed the abundance of both lineages using quantitative PCR in the southern North Sea, subjected them to a multiple linear regression analysis and related their occurrence to current patterns by backtracking the water masses found at individual stations for the preceding 24 to 27 d. SAR11 constituted <1 to 47% of total bacterial 16S rRNA genes. The abundance of this clade was inversely correlated to the salinity change of the water masses at the stations, indicating a preference for stable and presumably nutrient depleted waters. The RCA cluster constituted <1 to 5% of total bacterial 16S rRNA genes but did not exhibit any correlation to hydrographic properties. However, a multiple linear regression analysis showed that the RCA cluster was significantly correlated to a suite of biogeochemical parameters, bacterial abundance, concentrations of chlorophyll a, particulate organic carbon and suspended particulate matter and salinity changes, explaining 94.3% of the variability of the RCA data. These results show that backtracking water masses and relating them to bacterioplankton populations aids in the understanding of the growth dynamics of specific bacterioplankton populations and sheds new light on why high abundances of the SAR11 clade are usually found in stratified water masses.