The copepods Acartia clausi and Paracalanus parvus are the most abundant species of mesozooplankton in most areas. The macrozooplankton species Calanus helgolandicus is also important in terms of biomass. The distribution of warm‑water copepod species like Temora stylifera and Calanoides carinatus has moved northwards across the ecoregion. The seasonal pattern of the mesozooplankton in off‑shelf areas sees an annual maximum in spring and a secondary peak in late summer–early autumn. This pattern is modified in many coastal areas by the summer upwelling (western Iberian shelf, western Cantabrian sea), and river plumes, which have strong influence in shelf areas across the ecoregion as well as to a minor extent in the Cantabrian sea. Both the upwelling and the plumes enhance zooplankton growth in summer.
Decreasing trends have been observed in copepod abundance in off‑shelf areas since the late 1950s. Conversely, increasing trends in coastal zooplankton off Galicia have been observed since the late 1980s, while no recent trends are evident in the eastern Cantabrian region.