Lack of migration of Acartia tonsa in an homogeneous temperate estuary (Bahía Blanca, Argentina)

EMILIO J. GARIBOTTI, M.CINTIA PICCOLO, PATRICIA M. CERVELLINI

Montreal, Canadá

Otro; Joint Assembly MOCA-09; 2009

IAMAS, IAPSO y IACS

The extent to which zooplankton responds to different environmental processes is not completely understood in coastal waters since parameters like temperature, salinity and currents may affect migratory behavior. Estuarine organisms show activity patterns that are synchronized with day-night, tidal cycles, currents and environmental conditions. Acartia tonsa is the dominant zooplankton species during summer and autumn in the Bahía Blanca Estuary. The objective of the present study was to test that A. tonsa does not experiment vertical migrations in a water column with no stratified environmental conditions and food availability. To analyze the vertical distribution of A. tonsa and its relations to environmental parameters, measurements were performed every 3 h during two tidal cycles (25 h) on November 2007. Abundance of A. tonsa and oceanographic parameters (salinity, temperature, nutrients and tidal currents) were measured at three water levels (surface, mid-water and bottom). Data from a meteorological station located in the study site was also employed. Results of this study show that A. tonsa was more abundant near the bottom that at mid-water and surface. Salinity and temperature gradients did not explain the A. tonsa distribution in the water column because it presented an homogeneous vertical distribution. Tidal currents produced significant water turbulence and, therefore, phytoplankton was also available at the bottom waters. Furthermore, the magnitude of the vertical component of the tidal currents might explain the abundance of A. tonsa near the bottom. Under these conditions the species tends to remain near the bottom avoiding predators and reducing the cost of migration.