Long-term and seasonal zooplankton dynamics in the northwest Iberian shelf and its relationship with meteo-climatic and hydrographic variability

Long-term and seasonal dynamics of zooplankton abundance and taxonomic composition and its relationship with meteo-climatic and hydrographic factors were investigated on the northwest Iberian shelf. Zooplankton were collected monthly (1995–2011) at two locations within and off the Rı́a of Vigo (Station 1 and Station 3, respectively). Total abundance of zooplankton (NZT) varied annually following on average a unimodal cycle, with peak values between late spring and early autumn. In the long term, the time series of NZT exhibited three contrasting periods: A) 1995–2001, characterized by low abundance and low amplitude seasonality, with a stepped increase towards 2001; B) 2001–2006, of high abundance and marked seasonality enclosing the maximum values of the time series; and C) 2006–2010, of intermediate abundance and amplitude of the seasonal cycle. The most common taxonomic groups showed similar long-term and seasonal patterns. Principal component analysis revealed a shift in zooplankton dynamics from 2001 onwards, which affected annual averages and seasonality of all taxa. The observed changes in zooplankton dynamics were concomitant with sustained trends for upwelling intensity (increasing), precipitation (decreasing) and Gulf Stream North Wall position (equatorward) between 2000 and 2005. The results stress the importance of hydrodynamics, driven by meteo-climatic conditions, in the control of the abundance levels of zooplankton at seasonal and long-term interannual scales.