Calcareous nannofossil assemblages have been investigated in three sections from around the margins of the Early Cretaceous North Sea Basin. Both absolute (number of nannofossils per gram) and relative (percent of the assemblage of different taxa) abundance data have been collected. Absolute abundances (up to 10 billion nannofossils per gram) are somewhat higher than those reported previously from other stratigraphic intervals. Stratigraphic correlation of trends in assemblage data in different sections points toward the existence of large, uniform water masses, stable over long (0.5 to 1 m.y.) time periods. These water masses contained a characteristic nannoplankton, which appears to have been driven largely by variations in fertility possibly tied to changes in sources of surface waters, which in turn may have been controlled by relative sea level. Fine fraction isotopic data show fluctuations on shorter time periods (0.1 to 0.5 m.y.). We postulate that some of these isotopic fluctuations were controlled by relative changes in sea level which affected the balance of surface waters derived from runoff and those imported from the Boreal Sea. These oceanographic changes appear to have affected nannofossil assemblages only in discrete intervals. ¿ American Geophysical Union 1995