CONICET | Buscador de Institutos y Recursos Humanos

The objectives of the present study were to describe the influence of tidal cycle on mesozooplankton variability and to compare the mesozooplankton distribution between two zones in the main channel of the estuary: its northern margin and the deeper central zone. Sampling was carried out on 12, May 2006, during a transition from spring to neap tides. Zooplankton samples were collected during the daytime every 3-h (from a boat in the central main channel), beginning at high tide. Two submersible pumps were used to obtain simultaneous surface and bottom samples and a PVC hose linked the pumps to 200 µm pore size plankton nets. Water was filtered through the nets for 10-20 min, for a total sample volume between 1.5 and 2.9 m3. The collected material was preserved in a buffered solution of 4% formaldehyde. Vertical profiles of temperature and salinity were obtained with a digital multisensor Horiba U-10 and additional water samples were taken to determine suspended particulate matter (SPM). Measurements of current velocity were made using a 1.5 MHz Acoustic Doppler Profiler (ADP). A. tonsa abundances registered in the channel margin were higher near the surface throughout the tidal. The abundance of this species was markedly higher during ebb tide than during flood, low or high tide events. Contrary to what occurs in the margin area, the abundances in the central main channel were higher near the bottom during the entire tidal cycle and no marked increase of the abundance was detected during ebb tide. These results showed a daytime scenario in which the short-term temporal variability of the dominant species A. tonsa was strongly affected by tidal dynamics. This study presents a contribution to the knowledge of the mesozooplankton dynamics over tidal time scales. The results highlight the role of tides as forcing agents on the mesozooplankton community structure and emphasize the importance of the biological-physical interactions at a short-term time scale in coastal systems. In addition, and although this study is restricted to samples collected during a short period of time, the results show how variable the mesozooplankton community structure can be over short time scales in mesotidal temperate estuaries. This variability should be taken into account for any mesozooplankton monitoring program conducted in a temperate system with a high-tidal regime.