Overlooked Implication of Sediment Transport at Low Flow: Migrating Ripples Modulate Streambed Phototrophic and Heterotrophic Microbial Activity

David Scheidweiler,Michael Mutz,Clara Mendoza‐Lera,Ute Risse‐Buhl

doi:10.1029/2020wr027988

Abstract

AbstractSandy streambeds are mobile even at low flow velocities at which sediments can be transported as bedload, more specifically as migrating ripples. Small variations in discharge can result in transitions between sediment transport and no‐transport. Despite being inherent processes of streams and rivers, the effect of sediment transport and transport regime transition on the phototrophic and heterotrophic activity of streambed microbial communities remains unclear. We performed a microcosm experiment mimicking sediment transport as migrating ripples (i.e., migrating) and no sediment transport (i.e., stationary), and their transition to observe the response of the phototrophic and heterotrophic microbial community. Both net community production and community respiration were respectively 77% and 40% suppressed in migrating sediments compared to stationary sediments. In migrating sediments, a combination of mechanical stress, light limitation, and limited habitable area likely hampered microbial metabolism. Stationary conditions facilitated an active community of phototrophs, mainly diatoms, as indicated by high net community production, high rates of dissolved organic carbon release and silicon retention. After transitioning migrating to stationary and vice versa, differences were maintained regardless of the change in mechanical stress and associated stressors, most likely as a result of the interaction between their antecedent transport conditions and developmental stage that shaped the microbial community. Our results indicate that sediment transported as migrating ripples at low flow velocity can strongly modulate streambed metabolism, and discharge oscillations resulting in sediment transport transitions will result in a mosaic of microbial activity and biomass that will emerge at larger scales determining reach‐scale metabolism.

Highlights

In streams and rivers, ecosystem metabolism that is carried out by multitrophic microbial communities residing mainly in the streambed is shaped by the master variable flow (Battin et al, 2009, 2016; Risse-Buhl et al, 2020)
Our results indicate that sediment transported as migrating ripples at low flow velocity can strongly modulate streambed metabolism, and discharge oscillations resulting in sediment transport transitions will result in a mosaic of microbial activity and biomass that will emerge at larger scales determining reach-scale metabolism
The experiment consisted of two phases simulating the dynamics of the streambed in terms of sediment transport (Figure 1d): 1. In phase 1 (P1), we studied the effect of migrating ripples on the phototrophic and heterotrophic microbial activity (n = 8)

Summary

Introduction

Ecosystem metabolism (primary production and respiration) that is carried out by multitrophic microbial communities residing mainly in the streambed is shaped by the master variable flow (Battin et al, 2009, 2016; Risse-Buhl et al, 2020). Sediment transport at low flow (i.e., flow of water in a stream during prolonged dry weather conditions [Smakhtin, 2001]) is of much less intensity, affecting mainly the finer sediment grain sizes over longer periods (more than half of the year) (Biggs et al, 2005; Bridge, 2003; Singh et al, 2019) While the former one is a strong shaping force acting episodically on a large temporal scale (once per year [Biggs et al, 2005]), the latter one could potentially shape ecosystem metabolism continuously during much longer periods. The implications for ecosystem metabolism of low flow sediment transport are poorly understood

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Discussion

Conclusion