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

Kenji Tsuchiya*, Ayato Kohzu, Victor S. Kuwahara, Shin-ichiro S. Matsuzaki, Masatoshi Denda, Kimio Hirabayashi

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ABSTRACT: To clarify the governing factors of planktonic and epilithic bacterial production (BP) and to quantify their relative contributions to the carbon cycle, the seasonal variations and regulatory factors of planktonic and epilithic bacterial production (BP) in the middle reaches of the Shinano River, Japan ecosystem were investigated from February 2019 to May 2020. Sampling was conducted at three stations; upper stream riffle and 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 variations. Biofilm BP in the upper stream riffle was higher than those at the other stations, where no spatial variation of planktonic BP was observed. Generalized linear models suggest that BP was primarily regulated by water temperature. Additionally, planktonic BP was significantly correlated to dissolved organic carbon, suggesting carbon limitation. Biofilm BP showed no evidence of resource limitation (nutrients and organic matters), but was significantly explained by current velocity and station. The results suggest that although seasonality is dominant in biofilm BP variations, spatial differences are significant within the seasonal variability. Besides, current velocity and bottom shear stress related to local geomorphologies such as riffles and pools affects substrate supply rate and biofilm formation process, regulating biofilm BP variations. The present study demonstrated different regulation factors of planktonic and biofilm BP in the middle reaches of a temperate river.