A River's Liver – Microbial Processes within the Hyporheic Zone of a Large Lowland River

Abstract

Little is known on microbial activities in the sediments of large lowland rivers despite of their potentially high influence on biogeochemical budgets. Based on field measurements in a variety of sedimentary habitats typical for a large lowland river (Elbe, Germany), we present results on the abundance and production of sedimentary bacteria, the potential activity of a set of extracellular enzymes, and potential nitrification and denitrification rates. A diving bell was used to access the sediments in the central river channel, enabling us to sample down to 1 m sediment depth. Depth gradients of all measures of microbial activity were controlled by sediment structure, hydraulic conditions, as well as by the supply with organic carbon and nitrogen. Microbial heterotrophic activity was tightly coupled with the availability of carbon and nitrogen, whereas chemolithotrophic activity (nitrification rate) was related to the available surface area of particles. In the central bed of the river, bacterial production and extracellular enzyme activity remained high down to the deepest sediment layers investigated. Due to the large inner surface area and their connectivity with the surface water, the shifting sediments in the central channel of the river were microbially highly active There, vertically integrated bacterial production amounted to 0.95 g C m À3 h À1 , which was 2.9 to 5.5 times higher than in the nearshore habitats. We conclude that carbon and nitrogen cycling in the river is controlled by the live sediments of the central river channel, which thus represent a ''liver function'' in the river's metabolism.

Riverbed Clogging and Sustainability of Riverbank Filtration

“…In 2015, a median leakage coefficient of L = 4 × 10 −6 s −1 was determined during a summer long low-flow period with moderate abstraction. Even though one may expect stronger clogging under these drought conditions, the high water temperatures, higher biological activity [20][21][22] and the ship traffic during lower river stage with stronger impact of propulsion systems on the riverbed [23] are assumed to have a positive effect on riverbed leakage.…”

Methanogenic System of a Small Lowland Stream Sitka, Czech Republic

“…The river hyporheic zone, volume of saturated sediment beneath and beside streams containing some proportion of water from surface channel, plays a very important role in the processes of self-purification because the river bed sediments are metabolically active and are responsible for retention, storage and mineralization of organic matter transported by the surface water (Hendricks 1993;Jones & Holmes 1996, Baker et al 1999, Storey et al 1999, Fischer et al 2005. The seemingly well-oxygenated hyporheic zone contains anoxic and hypoxic pockets ("anaerobic microzones") associated with irregularities in sediment surfaces, small pore spaces or local deposits of organic matter, creating a 'mosaic' structure of various environments, where different microbial populations can live and different microbially mediated processes can occur simultaneously (Baker et al 1999, Morrice et al 2000, Fischer et al 2005.…”

“…The seemingly well-oxygenated hyporheic zone contains anoxic and hypoxic pockets ("anaerobic microzones") associated with irregularities in sediment surfaces, small pore spaces or local deposits of organic matter, creating a 'mosaic' structure of various environments, where different microbial populations can live and different microbially mediated processes can occur simultaneously (Baker et al 1999, Morrice et al 2000, Fischer et al 2005. Moreover, hyporheic-surface exchange and subsurface hydrologic flow patterns result in solute gradients that are important in microbial metabolism.…”

The role of organisms in hyporheic processes: gaps in current knowledge, needs for future research and applications

“…Recent works have highlighted the diversity of microbial communities, both for bacteria (Iribar et al, 2008;Lowell et al, 2009) and fungi (Ba¨rlocher et al, 2008;Cornut et al, 2010). Similarly, the activities of these microbes have been studied in a wide range of rivers, from small streams (Schmid and Schmid-Araya, 2010) to large rivers (Claret et al, 1997(Claret et al, , 1998Fischer et al, 2005). All these studies have converged toward the idea that the hyporheic zone plays an essential role in the global metabolism of the rivers (Boulton et al, 1998Dahm et al, 1998;Robertson and Wood, 2010;Krause et al, 2011).…”