Response of native and non-indigenous zooplankton to inherent system features and management in two Basque estuaries: A niche decomposition approach

The realized ecological niches of native zooplankton and non-indigenous species (NIS) were analysed in the contrasting estuaries of Bilbao and Urdaibai from 1998 to 2015 in order to study their responses to the particular features of each estuary and the biotic and abiotic changes along the study period. The marginality and tolerance of zooplankton taxa was estimated for the two estuaries together using the OMI analysis, whilst, by means of the WitOMI analysis, niches were differentiated into spatial subniches to assess the differences between estuaries and into temporal subniches to determine the variations in time within each estuary. The changes in the niche overlap of the main native and NIS species were calculated with the D-metric. Results showed that the combined effect of salinity gradients and seasonal variations, linked to temperature, defined the main spatio-temporal niches of zooplankton taxa in the overall environmental scenario of both estuaries. Thus, those factors separated winter-spring neritic, summer-autumn neritic, warm water affinity brackish and limnetic species. Secondarily, river discharge and hydrological stability accounted for the higher presence of freshwater and some brackish species in the estuary of Bilbao and tychoplanktonic organisms in the estuary of Urdaibai. At the regional scale the marginality and tolerance of zooplankton taxa reflected a combination of spatial and seasonal niche breadth and difference of abundance between estuaries. The number of taxa with niches not clearly explained by the environmental variables studied was higher in the estuary of Urdaibai, whilst the new arriving species occupied realized niches well defined by the environmental variables and overall zooplankton taxa showed a better discrimination from generalist to specialist behaviours in the estuary of Bilbao. This was related to inherent abiotic and biotic features of each estuary, which make the estuary of Urdaibai more refractory to the settlement of NIS species. Moreover, in the estuary of Bilbao, several neritic and brackish species experienced temporal changes in niche breadth and overlap that was attributed both to the environmental improvement related to rehabilitation plans in the system and the impact of the species that arrived during the study period. In the estuary of Urdaibai only small changes attributable to the limited impact of NIS were inferred, since no meaningful changes in environmental conditions were perceived.