INTRODUCTION
Estuaries are transitional environments that link continental, limnic environments to marine systems, and are characterized primarily by the constant dynamic of the tidal cycle (Schaeffer-Novelli, 1995), which provokes daily variation in abiotic variables such as salinity, temperature, dissolved oxygen and nutrient concentrations, and tidal currents (Park and Marshall, 2000). This variability is reflected in the structure of the resident and/or transitional populations of different organisms, and determines fluctuations in the composition of ecological communities, and the spatial-temporal variation in the distribution of species (Little, 2000).
These environments are also considered to be natural nurseries, providing important spawning grounds for an ample variety of aquatic vertebrates and invertebrates, and constitute one of the most productive biological systems in the world. The productivity of phyto and zooplankton, and zoobenthic organisms is especially marked.
The zooplankton has a special role in the estuarine biota, occupying a key role in the trophic web of these systems, including nutrient recycling and the regulation of phytoplankton populations