This study aims to identify the controlling processes of phytoplankton distributions in the Alboran Sea (western Mediterranean Sea). A three-dimensional primitive equation model, the Navy Layered Ocean Model, forced by satellite-measured winds from the European Remote Sensing satellites (ERS-1 and 2) and the NASA scatterometer (NSCAT), is used to examine circulation from January 1996 to December 1998 and to simulate the distribution of passive tracers within the basin. We discuss the impact of the basin circulation on the observed patterns of chlorophyll observed by the Sea-viewing Wide Field-of-view Sensor (Sea WiFS) and the Ocean Color and Temperature Scanner (OCTS). The upper layer is fertilized with nutrients which are transported by the western anticyclonic gyre from the coastal upwelling sites in the northwest. Nutrients are supplied also by eddy-induced upwelling in the periphery of the gyre path (found to be 20 to 60% of the supply due to advection in the present simulations). Circulation controls the supply of nutrients and the observed phytoplankton biomass in the upper layer during the nonbloom months (late spring, summer, and early fall). During the bloom regime, seasonal destratification is the dominant fertilizing process for the entire basin. ¿ 2001 American Geophysical Union