Michael Mutz

Effects of hydromorphological integrity on biodiversity and functioning of river ecosystems

River channels tend to a dynamic equilibrium driven by the dynamics of water and sediment discharge. The resulting fluctuating pattern of channel form is affected by the slope, the substrate erodibility, and the vegetation in the river corridor and in the catchment. Geomorphology is basic to river biodiversity and ecosystem functioning since the channel pattern provides habitat for the biota and physical framework for ecosystem processes. Human activities increasingly change the natural drivers of channel morphology on a global scale (e.g. urbanization increases hydrological extremes, and clearing of forests for agriculture increases sediment yield). In addition, human actions common along world rivers impact channel dynamics directly, e.g. river regulation simplifies and fossilizes channel form. River conservation and restoration must incorporate mechanisms of channel formation and ecological consequences of channel form and dynamics. This article (1) summarizes the role of channel form on biodiversity and functioning of river ecosystems, (2) describes spatial complexity, connectivity and dynamism as three key hydromorphological attributes, (3) identifies prevalent human activities that impact these key components and (4) analyzes gaps in current knowledge and identifies future research topics.

Disconnect of microbial structure and function: enzyme activities and bacterial communities in nascent stream corridors.

A fundamental issue in microbial and general ecology is the question to what extent environmental conditions dictate the structure of communities and the linkages with functional properties of ecosystems (that is, ecosystem function). We approached this question by taking advantage of environmental gradients established in soil and sediments of small stream corridors in a recently created, early successional catchment. Specifically, we determined spatial and temporal patterns of bacterial community structure and their linkages with potential microbial enzyme activities along the hydrological flow paths of the catchment. Soil and sediments were sampled in a total of 15 sites on four occasions spread throughout a year. Denaturing gradient gel electrophoresis (DGGE) was used to characterize bacterial communities, and substrate analogs linked to fluorescent molecules served to track 10 different enzymes as specific measures of ecosystem function. Potential enzyme activities varied little among sites, despite contrasting environmental conditions, especially in terms of water availability. Temporal changes, in contrast, were pronounced and remarkably variable among the enzymes tested. This suggests much greater importance of temporal dynamics than spatial heterogeneity in affecting specific ecosystem functions. Most strikingly, bacterial community structure revealed neither temporal nor spatial patterns. The resulting disconnect between bacterial community structure and potential enzyme activities indicates high functional redundancy within microbial communities even in the physically and biologically simplified stream corridors of early successional landscapes.

Influences of Woody Debris on Flow Patterns and Channel Morphology in a Low Energy, Sand-Bed Stream Reach

In lowland areas, such as the glacial landscapes of eastern Germany, sand-bed streams are the most common stream type. They have low gradients and their hydrological regime is often subdued due to the frequent interruption by lakes. Very few is known about the influence of woody debris in these streams, since nearly all previous studies are from high-gradient conditions, where streams have coarse bed sediments and harsh hydrological regimes. The research objectives of this study were first to assess the quasi-natural quantity and quality of wood in a lowland sand-bed stream and second to understand the influence of wood on the channel morphology and the flow patterns at base-flow. The three-dimensional stream bed relief was surveyed by electronic distance measurement. The position and the size of large woody debris was assessed by close-up photography. An acoustic Doppler velocimeter was used to record the patterns of flow velocity and turbulence. Overlay and analysis of the spatial data was done using a Geographic Information System. The standing stock of wood was 1.9 m3 and 39 woody elements per 100 m2 of stream bed. The flow pattern was clearly controlled by the wood. Woody elements elevated above the stream bed deflected flow and locally caused strong secondary current, high turbulence, and scour of the stream bed at baseflow. Wood resting directly on the stream bed, which contributed the majority of the wood inside the bank-full channel, determined the roughness of the stream bed. Near-bed flow patterns observed were isolated roughness flow and wake interference flow, which was registered inside the accumulations of wood. 68% of the stream bed had shear stress above critical. Hence, the secondary morphological structures of the sand-bed were controlled at base-flow by the flow which was determined by the woody debris distribution.

Treating causes not symptoms: restoration of surface-groundwater interactions in rivers

Many river restoration projects seek to address issues associated with impaired hydrological and ecological connectivity in longitudinal (e.g. effects of dams, weirs) or lateral (e.g. alienated floodplain) dimensions. Efforts to restore the vertical dimension of impaired stream–groundwater exchange are rare, hampered by limited understanding of the factors controlling this linkage in natural alluvial rivers. We propose a simplified two-axis model of the ‘primary drivers’ (sediment structure and vertical hydraulic gradient) of stream–groundwater exchange that acknowledges their interaction and provides a practical template to help researchers and river managers pose hypothesis-driven solutions to restoration of damaged or lost vertical connectivity. Many human activities impact on one or both of these drivers, and we review some of the tools available for treating the causes (rather than symptoms) in impacted stream reaches. For example, creating riffle-pool sequences along stream reaches will enhance vertical hydraulic gradient, whereas flushing flows can remove clogging layers and sustain sediment permeability. Our model is a first step to specifying mechanisms for recovery of lost vertical connectivity. Assessing results of river restoration using this approach at reach to catchment scales will provide scientific insights into the interplay of hydrology, fluvial geomorphology and river ecosystem function at appropriately broad scales.

Microbial activity and sediment disturbance modulate the vertical water flux in sandy sediments

Abstract. Little research has been conducted on the influence of microbial communities, sediment disturbances, and their interaction on the Vertical Water Flux (VWF) across a hydraulically complex streambed. Our study was aimed at the effects of microbial activity and shallow-sediment disturbance on VWF in sandbed flumes. We assessed the dynamics of VWF and the development of a microbial community during 30 d (June–July 2010) in 16 outdoor flumes with 2 types of bedform shape (level and ripple). We operated 8 flumes in constant darkness (no-light), and 8 in daylight to gain information on the relative significance of algae and heterotrophic microorganisms. We simulated a shallow-sediment disturbance after 21 d and compared microbial activity and VWF before and after the disturbance. We measured organic matter content, abundance of organisms, microbial activity, precipitation of CaCO3, and O2 bubbles resulting from primary production before and after the disturbance. VWF differed among treatments after 13 d. Algal and bacterial cells embedded in an extracellular polymer network, algal and microbial precipitation of CaCO3, and production of O2 bubbles in the uppermost sediment blocked the sediment pore space and disrupted VWF under daylight conditions, whereas bacterial cells and microbial precipitation of CaCO3 reduced VWF in no-light flumes. Microbial activity and organic matter were not affected by the shallow-sediment disturbance, but VWF was restored. VWF can be controlled by microbial activity and shallow-sediment disturbances and should be seen as spatially heterogeneous and fluctuating in time on small scales.

Long-term leaf litter decomposition and associated microbial processes in extremely acidic (pH <3) mining waters

Leaf packs were exposed in extremely acidic mining waters, removed at intervals throughout a two-year period and analysed for mass loss, microbial respiration and fungal biomass (ergosterol content). The mass loss followed an exponential trend with asymptotic values of 45-53 % of initial mass reached within the first 5 months. After this initial stage, leaves were completely encrusted by iron oxyhydroxide precipitates. The highest respiration occurred during the initial period and subsequently decreased to low levels comparable to those on an inert substrate (plastic strips) that was covered by iron precipitates. These precipitates create a barrier against microbial attack and mechanical abrasion of leaf litter and trap considerable amounts of organic matter. Ergosterol contents were highest after 3 months and varied throughout the subsequent exposure period with higher contents in autumn/winter. Unidentified HPLC-peaks may indicate that dead fungal hyphae are enclosed by iron plaques instead of being degraded. The formation and accumulation of refractory leaf litter/iron plaque layers has implications for the future development of the mining waters.

Litter Supply as a Driver of Microbial Activity and Community Structure on Decomposing Leaves: a Test in Experimental Streams

ABSTRACT Succession of newly created landscapes induces profound changes in plant litter supplied to streams. Grasses dominate inputs into open-land streams, whereas tree litter is predominant in forested streams. We set out to elucidate whether the activity and structure of microbial communities on decomposing leaves are determined by litter quality (i.e., grass or tree leaves colonized) or whether changes during riparian succession affecting litter standing stocks on the stream bed play an overriding role. We used 15 outdoor experimental streams to simulate changes in litter supplies reflecting five stages of riparian succession: (i) a biofilm stage with no litter, (ii) an open-land stage characterized by grass litter inputs, (iii) a transitional stage with a mix of grass and tree litter, (iv) an early forested stage with tree litter, and (v) an advanced forested stage with 2.5 times the amount of tree litter. Microbial activities on tree (Betula pendula) and grass (Calamagrostis epigejos) litter were unaffected by either the quantity or type of litter supplied to the experimental streams (i.e., litter standing stock) but differed between the two litter types. This was in stark contrast with bacterial and fungal community structure, which markedly differed on grass and tree litter and, to a lesser extent, also among streams receiving different litter inputs. These patterns reveal distinct responses of microbial community structure and activity to the bulk litter available in streams but consistent responses to the litter type colonized.

Fungal–bacterial dynamics and their contribution to terrigenous carbon turnover in relation to organic matter quality

Ecological functions of fungal and bacterial decomposers vary with environmental conditions. However, the response of these decomposers to particulate organic matter (POM) quality, which varies widely in aquatic ecosystems, remains poorly understood. Here we investigated how POM pools of substrates of different qualities determine the relative contributions of aquatic fungi and bacteria to terrigenous carbon (C) turnover. To this end, surface sediments were incubated with different POM pools of algae and/or leaf litter. 13C stable-isotope measurements of C mineralization were combined with phospholipid analysis to link the metabolic activities and substrate preferences of fungal and bacterial heterotrophs to dynamics in their abundance. We found that the presence of labile POM greatly affected the dominance of bacteria over fungi within the degrader communities and stimulated the decomposition of beech litter primarily through an increase in metabolic activity. Our data indicated that fungi primarily contribute to terrigenous C turnover by providing litter C for the microbial loop, whereas bacteria determine whether the supplied C substrate is assimilated into biomass or recycled back into the atmosphere in relation to phosphate availability. Thus, this study provides a better understanding of the role of fungi and bacteria in terrestrial–aquatic C cycling in relation to environmental conditions.

Palatability of Leaves Conditioned in Streams Affected by Mine Drainage: A Feeding Experiment with Gammarus Pulex (L.)

Both the absence of leaf shredding macroinvertebrates and low microbial activity are of major importance in determining slow and incomplete leaf decay in extremely acidic (pH<3.5) mining streams. These streams are affected by a heavy ochre deposition causing the formation of massive iron plaques on leaf surfaces that hinder microbial exploitation. An investigation was carried out to determine whether iron plaques and leaf conditioning status (acid conditioned with and without iron plaques, neutral conditioned, unconditioned) affect the feeding preference of the shredder Gammarus pulex (L.). Leaf respiration rates and fungal biomass (ergosterol contents) were measured to determine microbial colonization. Neutral conditioned leaves had significantly higher microbial colonization than acid conditioned leaves with iron plaques. Notwithstanding, leaves of both conditioning types were consumed at high rates by G. pulex. The microbial colonization had no influence on feeding preference in the experiment. It is presumed that iron adsorbed organic material caused the high palatability of leaves with iron plaques. The results indicate that the large deposits of leaves coated with iron plaques will be available to the stream food web when water quality will be restored to neutral as planed in scenarios for the future development of mining streams.

Fungal importance extends beyond litter decomposition in experimental early-successional streams

Fungi are important decomposers of leaf litter in streams and may have knock-on effects on other microbes and carbon cycling. To elucidate such potential effects, we designed an experiment in outdoor experimental channels simulating sand-bottom streams in an early-successional state. We hypothesized that the presence of fungi would enhance overall microbial activity, accompanied by shifts in the microbial communities associated not only with leaf litter but also with sediments. Fifteen experimental channels received sterile sandy sediment, minimal amounts of leaf litter, and one of four inocula containing either (i) fungi and bacteria, or (ii) bacteria only, or (iii) no microorganisms, or (iv) killed microorganisms. Subsequently, we let water from an early-successional catchment circulate through the channels for 5 weeks. Whole-stream metabolism and microbial respiration associated with leaf litter were higher in the channels inoculated with fungi, reflecting higher fungal activity on leaves. Bacterial communities on leaves were also significantly affected. Similarly, increases in net primary production, sediment microbial respiration and chlorophyll a content on the sediment surface were greatest in the channels receiving a fungal inoculum. These results point to a major role of fungal communities in stream ecosystems beyond the well-established direct involvement in leaf litter decomposition.