Differences in the colonization success and impact of non-indigenous and other expanding copepod species on the zooplankton of two contrasting estuaries of the Bay of Biscay

The colonization success of non-indigenous species (NIS) of copepods that arrived in the estuaries of Bilbao and Urdaibai (Bay of Biscay) and other expanding species (OES) of copepods that arrived in the former estuary during the 1998–2015 period, as well as the impact of such species on the density and seasonal patterns of congeneric species, total copepods and total zooplankton have been analysed. The estuary of Bilbao was successfully colonized by the NIS Acartia tonsa and Oithona davisae (since 2003) and by the NIS Pseudodiaptomus marinus and the OES Calanipeda aquaedulcis (since 2010), coinciding with the progressive improvement of environmental conditions in the rehabilitation process of this estuary. The three NIS species also arrived in the estuary of Urdaibai during the same time period, but only A. tonsa became abundant, likely due to inherent natural constraints for the other two species. A. tonsa in both estuaries, and O. davisae and C. aquaedulcis in the estuary of Bilbao became influential in the density, composition and dynamics of total copepods and zooplankton in water-masses of 33 or lower salinity. The highest impact was found in the inner estuary of Bilbao, where total copepods and zooplankton reached densities comparable to those of the outer estuary and copepod maxima moved from spring to summer due to the contribution of the colonizing copepod species. The congeneric species most affected by the NIS were the neritic species Acartia clausi and Oithona nana in the estuary of Bilbao, and the brackish species Acartia bifilosa in the estuary of Urdaibai.