The impact of the changes in seasonal grazing of the copepod Eucalanus subtenuis on chlorophyll a concentrations in the water column along the coastal zone of the south-central Caribbean Sea, Colombia, were studied during eleven oceanographic cruises carried out between September 1999 and May 2000. The cruises covered two coastal stations in Salamanca Gulf and two off the Tayrona Natural National Park. Ingestion rates on phytoplankton were determined for two copepod size classes (prosome length 250–600 µm and >600 µm). During each cruise, chlorophyll a was determined in the upper 35 m of the water column for three phytoplankton size classes: pico- (0.2–2.0 µm), nano- (2.0–20.0 µm), and microphytoplankton (20.0–200.0 µm). Our results showed no differences in the ingestion by E. subtenuis between the copepod size classes, nor strong variations in ingestion between areas. However, differences in hydrography (lower salinity and higher temperature during the rainy season), phytoplankton biomass (higher in the rainy season), and copepod ingestion (higher gut contents and ingestion rates during the rainy season) occurred between climatic periods. Within both areas and seasons, the grazing impact of E. subtenuis was low (<1% of the phytoplankton standing stock) and its variability was mainly determined by fluctuations in copepod density and phytoplankton supply, rather than by temperature changes or copepod size classes. Thus, in the southcentral Caribbean shelf, E. subtenuis grazing seems to play a minor role in determining the phytoplanktonic seasonal biomass fluctuations, especially when compared with the effects of the important input of water over the shelf from the continent and coastal lagoons.