There is a widespread view that physical forcing and biological factors interact to produce a relatively constant background of small phytoplankton cells in the ocean, with variability superimposed only when large cells respond to fluctuations such as pulsed nutrient inputs. The view has not been adequately evaluated in coastal ecosystems. We used automated submersible flow cytometry to monitor seasonal changes in phytoplankton abundance, cell size, and pigmentation at the Martha's Vineyard Coastal Observatory. The study site is a cabled facility 2-km offshore in 15-m deep water of the open northeastern US continental shelf. Our observations reveal that picophytoplankton (< 2 micrometers in diameter) abundance and biomass are highly variable ( similar to 10-fold for biomass) and exhibit striking and reproducible seasonal patterns. Biomass of picoeukaryotes tends to increase in late spring and remain relatively high all summer, while Synechococcus exhibit a sustained period of net population increase in spring that culminates with peak abundances sometime between mid- June and early July, when they comprise about half of the picophytoplankton biomass. Both groups, but especially Synechococcus, decline again in autumn, in conjunction with decreasing solar insolation and decreasing water temperature. These findings emphasize that picophytoplankton communities are not always regulated to constant biomass levels by tight coupling with fast-growing microheterophs; at this temperate coastal location, the "lawn" analogy holds only considering a relatively dormant wintertime phase with low or absent grazing. Temperature limitation of picoplankton growth rates likely plays a critical role in regions with cold winters, but the combination of factors that interact during seasonal transitions remains poorly understood.