patiotemporal variations in the abundance, biomass, fecundity, and production of Oithona brevicornis (Copepoda: Cyclopoida) in a West African tropical coastal lagoon (Grand-Lahou, Côte d'Ivoire). Zoological Studies 51(5): 627-643. Oithona brevicornis is a very important zooplankton species in coastal and estuarine aquatic ecosystems of West Africa and was recorded in very high abundances in coastal lagoons of Côte d'Ivoire. To better understand the functional role of this key species, spatio-seasonal variations in O. brevicornis abundance, biomass, fecundity, and production were investigated at 21 stations in Grand-Lahou Lagoon during an annual cycle from Jan. to Dec. 2004. The abundance and biomass showed more or less important spatiotemporal variations. Two peaks were recorded for the mean spatial values in Feb.-Mar. and Sept. During the long (LDS) and short dry seasons (SDS), all developmental stages showed their weakest density and biomass in the channel zone and near the mouth of the Boubo River. During the long (LRS) and short rainy seasons (SRS), O. brevicornis almost disappeared or showed very low abundances (< 1 individual/L) in the northern part and near the channel. Spatiotemporal variations in O. brevicornis fecundity and production showed no particular pattern during the dry seasons (LDS and SDS), while during the rainy seasons (LRS and SRS), very low fecundity and production values were recorded at stations influenced by river inputs. Correlation analyses showed that temperature and salinity were the main variables explaining these spatio-seasonal variations.