SeaWiFS algorithms for estimating chlorophyll-a concentration (chl-a) from space were evaluated at the Rio de la Plata estuary and its influence area. Twenty-eight stations were explored during two surveys that took place in the 2003 austral spring (November and December). Surface water samples were collected for chl-a, plankton taxonomy, plankton flow cytometry, and absorption by CDOM and suspended particles. Radiometry was measured with both above-water SIMBAD and below-water Li-Cor PAR radiometers. SeaWiFS-derived chl-a values were obtained using NASA's OC4-V4 bio-optical algorithm. A systematic overestimation of chl-a by SeaWiFS was observed, especially in the complex estuarine waters, by 110% on average, and was largely attributed to the bio-optical algorithm, not the atmospheric correction scheme. The overestimation was most dramatic in two areas: the mouth of the Rio de la Plata and the external region of the continental shelf. In the Rio de la Plata estuary, large CDOM absorption seemed to explain the discrepancies. Interestingly, in the surface waters of the outer edge of the continental shelf a conspicuous change in the phytoplankton composition was detected at two contiguous stations. A complete lack of diatoms and dinoflagellates was reported at those stations, and cocoid forms (<2μm) dominated phytoplankton composition (99%). The flow cytometry data also exhibited some peculiar features, i.e., pronounced peaks in picoplankton (0.2-2μm) abundance. At one of the two stations, relatively large specific absorption by particles contributed to chl-a overestimation by the SeaWiFS bio-optical algorithm.