The horizontal distributions of particle size spectrum and chlorophyll a concentration across the northern edge of the North Atlantic subtropical gyre along 21¿W were studied as part of the Joint Global Ocean Flux Study North Atlantic Bloom Experiment in March--April, 1989. An abrupt change in the particle size spectrum at 32¿40'N was located within the broad thermohaline front between the central gyre water in the south and the Azores Current in the north. Whereas the small particulate fraction (1--10 μm) and chlorophyll a concentration were similar (within a factor of 2), the abundance of the ''diatom'' size fraction (28--72 μm) showed a drastic increase by more than an order of magnitude in the Azores Current. The diatom fraction seemed to contribute little to the chlorophyll pool, indicating a postbloom condition. Different trophic pathways, i.e., grazing of the picoplankton in the gyre versus the diatom bloom with the following detrital loop in the Azores Current and northward, can be conjectured. The increased scattering versus absorption due to the different particle size spectrum and composition could bias the algorithms of pigment retrieval from remote sensing data. ¿1991 American Geophysical Union