Abstract
The Elbe Estuary (Germany) stretches 142 km from the weir in Geesthacht to the North Sea. It is classified as mesotidal, partially mixed and heavily impacted by anthropogenic activities and modifications. Despite well-documented changes in ecosystem status, little is known about the microbial community in its surface water. In this study, we used 16S rDNA sequencing to characterise bacterial communities in surface water of the Elbe Estuary. Samples were collected across three seasons (winter, spring, and summer) in 2021 and 2022, to assess the relationship between environmental factors and bacterial community structure. Our analyses revealed that bacterial community diversity and composition varied seasonally and along the estuary stretch and were closely linked to physicochemical properties. Alpha diversity was highest in winter and in oligohaline samples. Distance-based redundancy analysis showed that salinity, discharge, temperature, inorganic nitrogen (NO(2)), and silicate are key factors in shaping the bacterial community compositions. Although spatial differences were observed, seasonal variation was the main determinant of bacterial diversity and community structure. Overall, our results show that anthropogenic pressures and seasonal changes are reflected in a dynamic microbial community with metabolic functions strongly shaped by human activity.