Seasonal measurements of photosynthetic pigments were performed in the Caribbean Sea to investigate the potential effects of Orinoco River discharge on the vertical distributions of the major phytoplankton groups. Samples were collected during periods of low (April 1988) and high (September 1988) riverine input. The chlorophyl a concentration of the subsurface chlorophyll maximum (SCM) sampled at inshore and offshore stations displays only minor seasonal variations. However, the depth and pigment composition of the SCM during periods of low and high outflow are markedly different. A ''two-layer'' phytoplankton distribution pattern is observed for all stations occupied during the spring and those stations not influenced by Orinoco River discharge in the fall: an upper ''light-adapted'' population dominated by prokaryotic phytoplankton (cyanobacteria and possibly prochlorophyte-like phytoplankton) and a deeper ''shade-adapted'' population comprised of chromophytes and ''green algae.'' The depth of the SCM at these stations averaged 77¿12 m. By contrast, stations influenced by Orinoco River discharge during the fall where characterized by near-surface diatom-dominated communities and shallower SCM depths (39¿16 m). Nutrient loading, elevation of absorption and attenuation coefficients, and transport of coastal ''seed'' phytoplankton populations are possible mechanisms by which the Orinoco River modifies the composition and distribution of phytoplankton in the Caribbean Sea. ¿ American Geophysical Union 1993