Magalhães, A., Nobre, D.S.B., Bessa, R.S.C., Pereira, L.C.C. and Costa, R.M. da, 2013. Diel variation in the biomass and productivity of Acartia tonsa (Copepoda: Calanoida) in a tropical estuary (Taperaçu, northern Brazil). In: Conley, D.C., Masselink, G., Russell, P.E. and O'Hare, T.J. (eds.)Diel variation in the productivity of the copepods Acartia lilljeborgii and Acartia tonsa was investigated in the Taperaçu estuary in July (rainy season) and October (dry season), 2004. The estimated productivity of A. lilljeborgii (min-max: 3.932,385.41 mg C m−3 d−1; mean: 246.60 mg C m−3 d−1) and A. tonsa (min-max: 0.32–160.21 mg C m−3 d−1; mean: 16.99 mg C m−3 d−1) highlight the importance of their contribution to zooplankton production in the Taperaçu estuary. The absence of significant short-term variation in the productivity of A. lilljeborgii and A. tonsa in relation to the period of the day may be related to the combined effect of the shallow depth of the waters of this estuary and its strong tidal currents. These factors rendered the water column homogeneous, reducing the effect of the vertical migration of the zooplankton as a potential factor responsible for oscillations in the density of these organisms during the course of the day. The influence of tidal variation (flood/ebb) on the dynamics of these two species was also unclear in this study due to a number of morphological and morphodynamic characteristics of the estuary, such as the absence of any measurable fluvial discharge and the small catchment area, which resulted in an minor and infrequent inflow of freshwater into the estuary.