Small headwater streams interlink catchment soils with the river network. As water makes its way from the hillslopes to the stream, it may transport dissolved organic carbon (DOC) and potentially soil microbes into stream water. In this study, we aimed at quantifying the dynamic transfer of DOC and microbial life, namely bacteria from catchment soils into streams. We hypothesized that increased soil saturation enhances the lateral inflow of bacteria and DOC into streams. To address this hypothesis, we sampled six first order streams and three soil transects at two different depths located within the pre-alpine Oberer Seebach (OSB) catchment in Austria over a duration of 2 years. We found a strong variation in DOC concentrations (range 0.4-5.6 mg L −1 ) and bacterial abundances (range < 500,000-3,863,000 cells mL −1 ) measured by flow-cytometry. The highest values of DOC and bacterial cells occurred during high flow events. DOC concentration and bacterial abundance were correlated across all streams and seasons. In soils, DOC ranges were higher and were also correlated with bacterial abundance, while DOC concentrations were ∼ 10 times higher per bacterial cell than in streams. Overall we show that soils provide a dynamic inflow of bacteria and DOC to first order streams. Most probably, this results in a dynamic and reoccurring inoculation of small streams from catchment soils during runoff events. We propose that this dynamic microbial inoculation of small streams is potentially relevant for microbial community dynamics of downstream receiving waters.Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Trait‐based approaches are increasingly popular in ecology to describe communities and their responses to natural or anthropogenic changes. Morphology is an integrative trait that combines functional and evolutionary information. However, the objective and quantitative description of the morphological diversity is quite challenging. Modern morphometrics encompass an array of mathematical approaches that turn shapes into quantitative variables. For models with no or only a few homologous points, outline analysis (e.g., elliptical Fourier transform) converts the outline geometry into quantitative variables, which can be used in the multivariate framework. The elliptical Fourier transform here describes the shape of 85 fish species found in the North Sea. This study shows that the main component of morphological diversity in the North sea is the elongation and development of dorsal, pelvic, and caudal fins. The spatial distribution of morphological diversity decreases along a latitudinal gradient, with higher diversity in the southern part of the study area. Compared to species diversity, our results indicate that environmental conditions filter morphological traits in the northern North Sea. Outline analysis is a powerful approach to provide an objective description of fish morphology and to improve our understanding of the diversity of large marine ecosystems.

Flood events are now recognized as potentially important occasions for the transfer of soil microbes to stream ecosystems. Yet, little is known about these “dynamic pulses of microbial life” for stream bacterial community composition (BCC) and diversity. In this study, we explored the potential alteration of stream BCC by soil inoculation during high flow events in six pre-alpine first order streams and the larger Oberer Seebach. During 1 year, we compared variations of BCC in soil water, stream water and in benthic biofilms at different flow conditions (low to intermediate flows versus high flow). Bacterial diversity was lowest in biofilms, followed by soils and highest in headwater streams and the Oberer Seebach. In headwater streams, bacterial diversity was significantly higher during high flow, as compared to low flow (Shannon diversity: 7.6 versus 7.9 at low versus high flow, respectively, p < 0.001). Approximately 70% of the bacterial operational taxonomic units (OTUs) from streams and stream biofilms were the same as in soil water, while in the latter one third of the OTUs were specific to high flow conditions. These soil high-flow OTUs were also found in streams and biofilms at other times of the year. These results demonstrate the relevance of floods in generating short and reoccurring inoculation events for flowing waters. Moreover, they show that soil microbial inoculation during high flow enhances microbial diversity and shapes fluvial BCC even during low flow. Hence, soil microbial inoculation during floods could act as a previously overlooked driver of microbial diversity in headwater streams.