Clara Mendoza-Lera

Stream regulation by small dams affects benthic macroinvertebrate communities: from structural changes to functional implications

We studied benthic macroinvertebrate communities upstream and downstream of five small reservoirs (surface release in autumn–winters) (north Spain) to assess the effect of flow regulation on structural and functional characteristics of stream ecosystems. We based our approach on the use of structural metrics (density, biomass, richness and diversity) in combination with two functional diversity indices based on biological and ecological traits: FDPG index, related to species richness, and FDQ, which incorporates evenness across taxa. Although water physicochemical parameters were unaffected by the reservoirs during the study period (autumn–winter), macroinvertebrate metrics were lower below the dams, with detritivores (shredders and collector-gatherers) being the most affected. The alder leaf breakdown rate estimated by the litter-bag technique was related to the density, biomass, richness, diversity and FDPG index of shredders, compromising the ecosystem functioning. The most plausible origin for the observed differences in macroinvertebrate metrics between upstream and downstream reaches was the change of the flow regime caused by the impoundments at downstream sites, leading to droughts in summer in those naturally permanently flowing streams. The observed functional diversity loss might reduce the chances of the community to override natural or man-induced fluctuations in their environment with possible repercussions on important ecosystem functions and services.

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.

Relating hydraulic conductivity and hyporheic zone biogeochemical processing to conserve and restore river ecosystem services

River management practices commonly attempt to improve habitat and ecological functioning (e.g. biogeochemical processing or retention of pollutants) by restoring hydrological exchange with the hyporheic zone (i.e. hyporheic flow) in an effort to increase mass transfer of solutes (nutrients, carbon and electron acceptors such as oxygen or nitrate). However, even when hyporheic flow is increased, often no significant changes in biogeochemical processing are detected. Some of these apparent paradox result from the simplistic assumption that there is a direct relationship between hyporheic flow and biogeochemical processing. We propose an alternative conceptual model that hyporheic flow is non-linearly related with biogeochemical processing. Based on the different solute mass transfer and area available for colonization among hydraulic conductivities, we hypothesize that biogeochemical processing in the hyporheic zone follows a Gaussian function depending on hyporheic hydraulic conductivity. After presenting the conceptual model and its domain of application, we discuss the potential implications, notably for river restoration and further hyporheic research.

The algal lift: Buoyancy‐mediated sediment transport

The role of benthic algae as biostabilizers of sediments is well-known, however, their potential to lift and transport sediments remains unclear. Under low-flow conditions, matured algal mats may detach from the bed and may lift up sediment, thereby causing disturbance to the uppermost streambed sediment. We tested the potential of algal mats to lift sediments in 12 indoor flumes filled with sand (0.2 – 0.8 mm), gravel (2 – 8 mm) or a sand-gravel mixture (25/75% mass). After four weeks, the algal mats covered about 50% of the flumes area. Due to the accumulation of oxygen gas bubbles in the mats, that developed from high primary production at 4.5 weeks, about half of the algal mats detached from the bed carrying entangled sediments. Both the area covered by algal mats and detached area were similar among sediment types, but the amount of sediment transported tended to be higher for sand and sand-gravel mixture compared to gravel. Our results reveal that biologically mediated sediment transport mainly depends on the development of a dense filamentous algal matrix, that traps gas bubbles, increasing the mats buoyancy. This novel mechanism of sediment transport will occur in shallow ecosystems during low-flow periods, with the highest impact for sandy sediments.

Woody debris is related to reach-scale hotspots of lowland stream ecosystem respiration under baseflow conditions

Stream metabolism is a fundamental, integrative indicator of aquatic ecosystem functioning. However, it is not well understood how heterogeneity in physical channel form, particularly in relation to and caused by in‐stream woody debris, regulates stream metabolism in lowland streams. We combined conservative and reactive stream tracers to investigate relationships between patterns in stream channel morphology and hydrological transport (form) and metabolic processes as characterized by ecosystem respiration (function) in a forested lowland stream at baseflow. Stream reach‐scale ecosystem respiration was related to locations (“hotspots”) with a high abundance of woody debris. In contrast, nearly all other measured hydrological and geomorphic variables previously documented or hypothesized to influence stream metabolism did not significantly explain ecosystem respiration. Our results suggest the existence of key differences in physical controls on ecosystem respiration between lowland stream systems (this study) and smaller upland streams (most previous studies) under baseflow conditions. As such, these findings have implications for reactive transport models that predict biogeochemical transformation rates from hydraulic transport parameters, for upscaling frameworks that represent biological stream processes at larger network scales, and for the effective management and restoration of aquatic ecosystems.

Periodic sediment shift in migrating ripples influences benthic microbial activity

Migrating bedforms have high levels of particulate organic matter and high rates of pore water exchange, causing them to be proposed as hot spots of carbon turnover in rivers. However, the shifting of sediments and associated mechanical disturbance within migrating bedforms, such as ripples, may stress and abrade microbial communities, reducing their activity. In a microcosm experiment, we replicated the mechanical disturbances caused by the periodic sediment shift within ripples under oligotrophic conditions. We assessed the effects on fungal and bacterial biomass ratio (F:B), microbial community respiration (CR), and bacterial production (BCP) and compared with stable undisturbed sediments. Interactions between periodic mechanical disturbance and sediment-associated particulate organic matter (POM) were tested by enriching sediments collected from migrating ripples with different qualities of POM (fish feces, leaf litter fragments and no addition treatments). F:B and BCP were affected by an interaction between mechanical disturbance and POM quality. Fish feces enriched sediments showed increased F:B and BCP compared to sediments with lower POM quality and responded with a decrease of F:B and BCP to sediment disturbance. In the other POM treatments F:B and BCP were not affected by disturbance. Microbial respiration was however reduced by mechanical disturbance to similar low activity levels regardless of POM qualities added, whereas fish feces enriched sediment showed short temporary boost of CR. With the worldwide proliferation of migrating sand ripples due to massive catchment erosion, suppressed mineralization of POM will increasingly affect stream metabolism, downstream transport of POM and carbon cycling from reach to catchment scale.

Contrasting habitats but comparable microbial decomposition in the benthic and hyporheic zone

Input of allochthonous leaf litter is the main carbon source for heterotrophic metabolism in low-order forested streams. A major part of this leaf litter is accumulated at benthic retention structures or buried in the hyporheic zone. As a result of hyporheic sediment characteristics, hyporheic physicochemistry differs from that of the benthic zone selecting the microbial community. The present study aimed at understanding the influence of the hydrological and physiochemical differences between the benthic and hyporheic zone on microbial leaf litter decomposition and on the structure and function of the associated microbial community. Leached leaves of Alnus glutinosa were exposed for 62days in 250-μm mesh bags in the benthic zone and buried in the hyporheic zone at a depth of 2-3cm. Decomposition rates were comparable for both zones. In contrast, respiration, bacterial abundance, ergosterol content, fungal spore production and richness of fungal morphotypes were lower associated with hyporheic than with benthic leaves. Microbial community structure displayed zone-dependent temporal dynamics. Thus, the microbial community carried out leaf litter decomposition independently of its structure. These results suggest that carbon processing is not necessarily impaired by environmental constraints because the community structure may compensate those constraints (i.e. functional redundancy).