| Summary: | We have investigated zooplankton temporal dynamics in the northwest Iberian shelf, a temperate ecosystem subject to coastal upwelling-donnwelling processes. To this aim, we have applied wavelet analysis, a methodology able to cope with non-stationary dynamics, to monthly time series of zooplankton abundance and biomass acquired between 1995 and 2011 at two locations over the shelf and to environmental variables known to affect functioning of this ecosystem (wind regime, Ekam transport and river outflows). The seasonal signal of total zooplankton abundance and of the main taxonomic groups showed an abrupt increase in amplitude around 2001 that persisted until the end of the series in 2011. Concurrent with the change in amplitude, there was a synchronization of the seasonal cycle of abundance among taxonomics groups (e.g. copepods, larvaceans, chaetognats.) and copepod species, which persisted for several years although it decreased at the end of the series. Between 2001 and 2004, significant changes in wind regime patterns, linked to variability of the North Atlantic Oscillation, were observed: westerly winds became predominant, river outflow increased and offshore Ekman transport decreased. This meteo-climatic configuration favors retention mechanisms over across-shelf exchage processes due to the reinforcement of the western Iberian buoyant plume (WIBP) and the prevalence of downwelling. We hypothesized that the observed changes in zooplankton dynamics are governed by the amplification of the seasonal signal of these environmental drivers causing enhancement of the retention phenomena.
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