Ecological status of Urdaibai Biosphere reserve based on bacterial communities in a small-drainage estuary

Context The influence of anthropogenic pressures on plankton communities in well-flushed estuaries has been overlooked. Owing to this ecosystem’s short water-residence time, they are foreseen to be mainly composed of microorganisms from neritic–oceanic sources, more resilient to anthropogenic impacts. Aims This study characterises the bacterioplankton of Urdaibai, a small-drainage estuary located at a UNESCO biosphere reserve, to unravel its annual dynamic and ecological status. Methods Amplicon sequencing was used to assess the euhaline bacterioplankton shifts and microgAMBI index depicted the ecological status of the estuary. Key results A differentiated community was identified in upstream waters throughout the year. In addition, organisms commonly associated with faecal waste were detected. The ecological status of the estuary was poor or moderate along the annual cycle, but surprisingly worsened during winter. Conclusions The above suggests that the bacterial community dynamic in this well-fluxed small-drainage estuary is more complex than anticipated. Importantly, the analysis of pollution-indicative bacteria proved that the water policy of this estuary needs to be further evaluated. Implications The completion of an efficient sewage treatment system for this protected and touristic estuary is strongly recommended. Incorporating bacterial information to its current monitoring system will constitute a useful and valuable tool.