Long-term (1995-2022) assessment of soft-bottom non-indigenous marine benthic species in the Nerbioi estuary (southeastern Bay of Biscay)

Abstract

In coastal ecosystems, monitoring non-indigenous species (NIS) is essential to prevent new invasions and to develop effective mitigation measures. This study presents the spatial distribution and temporal trends of softbottom marine benthic NIS in the Nerbioi estuary (southeastern Bay of Biscay) using a 28-year dataset. It also evaluates the influence of environmental variables and maritime activities on the distribution and abundance of NIS. From a total of 3195 benthic species records collected between 1995 and 2022 at five sampling stations along the estuary, 459 taxa were identified, of which 27 were classified as NIS. The proportion of NIS relative to native species increased over time. Although maritime traffic is typically considered one of the main introduction pathways, other factors-such as climate change or secondary spread-cannot be ruled out, since the number of vessels in the Nerbioi estuary has decreased. The highest proportion of NIS was observed in the innermost part of the estuary, where salinity and oxygen levels were low. However, the greatest NIS abundance was found in the intermediate part, where contaminant levels were highest and where industrial activity and a sewage treatment plant were located. This suggests that the opportunistic nature of NIS enables them to colonize and thrive in areas subject to anthropogenic stress. Among the most frequently recorded NIS, the mussel Xenostrobus securis was the only species listed as invasive in the Spanish catalogue. These findings underscore the need for targeted monitoring to manage the spread of NIS in the region.