Tide and wind as forcing factors controlling mesozooplankton community in a shallow temperate estuary (Bahia Blanca, Argentina).

MARÍA C. MENÉNDEZ; MARÍA C. PICCOLO; MÓNICA S. HOFFMEYER

Foz do Iguaçu, Brasil

Otro; 2010 The Meeting of the Americas.; 2010

Zooplankton communities in coastal zones are strongly influenced by physical processes; therefore, the study of the interaction between biotic and abiotic factors is essential to understand the ecosystem dynamics. In estuaries, these interactions are mainly controlled by tides and atmospheric forcing. In the case of the Bahía Blanca estuary, it also occurs but they have not been studied extensively. The main objective of this study is to determine the short-term changes in the zooplankton community in relation to tidal cycles and predominant local winds. Hydrological data and zooplankton samples were obtained bimonthly from December 2004 to April 2006, during 14-h tidal cycles. Zooplankton samples were taken by pumps at 3-h intervals from surface and bottom water. Vertical temperature and salinity profiles as well as water samples for the determination of suspended particulate matter were acquired in each sampling occasion. All data concerning winds were obtained from a meteorological station situated in the study area. Water level was recorded with a local tide gauge. The mesozooplankton community was represented by 33 taxa. Holoplankton (45-97% of the total abundance) dominated numerically on meroplankton (2-27%) and adventitious plankton (1-39%). Concerning holoplanktonic abundance, the highest values were attained by Acartia tonsa. Meroplankton occurred mainly as polychaetes, barnacle and decapods larvae while benthic harpacticoids and Corophium sp. dominated the adventitious component. Semidiurnal tide was the main influence on the holoplankton variability. However, noticeable differences in the abundance pattern as function of wind intensity were detected. Meroplankton did not show a clear variation pattern along the tidal cycle. The occurrence of benthic forms in the water column in relation to tidal cycles differed among the main taxa, denoting differences between passive and actively swimming organisms. Distributional patterns of harpacticoids seem to be modulated mainly by velocity asymmetries in the tidal currents, in the same way as suspended particulate matter. However, the abundance pattern observed to Corophium sp. indicated behavioural responses associated with tides. Active emergence could be the mechanism by which this species reaches the water column. Distributional patterns varied among the main taxa and the differences were probably related to taxon specific responses to changing physical conditions. Results showed that zooplankton was strongly dependent on allochthonous events and highlighted the role of tides as forcing agents on the community structure. Also, meteorological events such as winds had a strong influence on this variability. The results emphasize the importance of the biological-physical interactions at a short-term scale in coastal systems. Much longer data series considering this fine temporal scale are necessary to determine the relationship between climate variability and zooplankton populations. All of this information may help to a better understanding of the zooplankton variability in the Bahía Blanca estuary and other temperate systems.