An anomalous winter phytoplankton bloom was observed across the central Gulf of Maine during a winter cruise in late February 1999, but not during a similar mission to the same region in late February 2000. During the cruise in 1999, 0 to 40 m mean chl a concentrations were >2 μg 1-1, while in 2000 total chl a concentrations were <1.0, and >5 pm size fractions were <0.5 μg 1-1. The presence of the bloom in 1999 could not be explained in terms of the critical depth hypothesis given conditions present at the time of the bloom. The large amount of colder, lower-salinity Scotian Shelf water present at the surface of the central Gulf of Maine in 1999 may have provided conditions suitable for the initiation of the bloom earlier during the winter. The presence of the winter bloom had a major effect on higher trophic levels. Zooplankton were >10x more abundant in 1999 and included species typically not present in the central Gulf during winter. All developmental stages of planktonic copepods were present, including large numbers of older copepodid stages of species such as Calanus finmarchicus, indicating that the bloom had been proceeding for some time. RNA:DNA ratios of surface C. finmarchicus stage C5 and adult females were at or near maximum temperature-dependent values in 1999, but were considerably reduced in 2000. Egg production rates of C. finmarchicus were high in 1999 but reduced in 2000. Both showed a curvilinear relationship with chl a. Analysis of gonads of females of other copepod species indicate similar between-year differences in reproductive rates. We suggest that the presence of the winter bloom in the central Gulf of Maine allows an extra generation of C. finmarchicus and other copepods to develop, leading to a buildup of the population during winter. This will have significant effects on downstream regions such as Georges Bank, which depend on re-supply from the Gulf of Maine.