Differences in regulation of planktonic and epilithic biofilm bacterial production in the middle reaches of a temperate river

To clarify the governing factors of planktonic and epilithic bacterial production (BP) and to quantify their relative contributions to the carbon cycle, we investigated the seasonal variation and regulatory factors of planktonic and epilithic BP in the middle reaches of the Shinano River, Japan, ecosystem from February 2019 to May 2020. Sampling was conducted at 3 stations: upper stream riffle, upper stream pool, and lower stream riffle, where current velocity, water depth, and bed shear stress were distinct. Planktonic and biofilm BP ranged from 5.5 to 466 mgC m(-3) d(-1) and 2.9 to 132 mgC m(-2) d(-1), respectively, showing clear seasonal variation. Biofilm BP was higher in the upper stream riffle than at the other stations, where no spatial variation in planktonic BP was observed. Generalized linear models suggest that BP was primarily regulated by water temperature. Additionally, planktonic BP was significantly correlated with dissolved organic carbon, suggesting carbon limitation. Biofilm BP showed no evidence of resource limitation (nutrients and organic matter), but was significantly explained by current velocity and station. The results suggest that although seasonality is dominant in biofilm BP variation, spatial differences are significant within the seasonal variability. Moreover, current velocity and bottom shear stress related to local geomorphologies such as riffles and pools affect substrate supply rate and biofilm formation processes, regulating biofilm BP variation. This study demonstrated different regulatory factors of planktonic and biofilm BP in the middle reaches of a temperate river.

Automated summary

This study investigated the seasonal variation and regulatory factors of planktonic and epilithic bacterial production in the middle reaches of the Shinano River, Japan. Water temperature was found to primarily regulate bacterial production, with planktonic BP correlated to dissolved organic carbon. Biofilm BP was influenced by current velocity and station, showing spatial differences within seasonal variability. The results highlight the importance of local geomorphological factors in regulating biofilm production in a temperate river ecosystem.