Relevance of large litter bag burial for the study of leaf breakdown in the hyporheic zone

Particulate organic matter is the major source of energy for most low-order streams, but a large part of this litter is buried within bed sediment during floods and thus become poorly available for benthic food webs. The fate of this buried litter is little studied. In most cases, measures of breakdown rates consist of burying a known mass of litter within the stream sediment and following its breakdown over time. We tested this method using large litter bags (15 × 15 cm) and two field experiments. First, we used litter large bags filled with Alnus glutinosa leaves (buried at 20 cm depth with a shovel) in six stations within different land-use contexts and with different sediment grain sizes. Breakdown rates were surprisingly high (0.0011–0.0188 day⁻¹) and neither correlate with most of the physico-chemical characteristics measured in the interstitial habitats nor with the land-use around the stream. In contrast, the rates were negatively correlated with a decrease in oxygen concentrations between surface and buried bags and positively correlated with both the percentage of coarse particles (20–40 mm) in the sediment and benthic macro-invertebrate richness. These results suggest that the vertical exchanges with surface water in the hyporheic zone play a crucial role in litter breakdown. Second, an experimental modification of local sediment (removing fine particles with a shovel to increase vertical exchanges) highlighted the influence of grain size on water and oxygen exchanges, but had no effect on hyporheic breakdown rates. Burying large litter bags within sediments may thus not be a relevant method, especially in clogged conditions, due to changes induced through the burial process in the vertical connectivity between surface and interstitial habitats that modify organic matter processing.     

Early stages of leaf decomposition are mediated by aquatic fungi in the hyporheic zone of woodland streams

1. Leaf litter constitutes the major source of organic matter and energy in woodland stream ecosystems. A substantial part of leaf litter entering running waters may be buried in the streambed as a consequence of flooding and sediment movement. While decomposition of leaf litter in surface waters is relatively well understood, its fate when incorporated into river sediments, as well as the involvement of invertebrate and fungal decomposers in such conditions, remain poorly documented. 2. We tested experimentally the hypotheses that the small interstices of the sediment restrict the access of the largest shredders to buried organic matter without compromising that of aquatic hyphomycetes and that fungal decomposers in the hyporheic zone, at least partly, compensate for the role of invertebrate detritivores in the benthic zone. 3. Alder leaves were introduced in a stream either buried in the sediment (hyporheic), buried after 2 weeks of exposure at the sediment surface (benthic‐hyporheic), or exposed at the sediment surface for the entire experiment (benthic). Leaf decomposition was markedly faster on the streambed surface than in the two other treatments (2.1‐ and 2.8‐fold faster than in the benthic‐hyporheic and hyporheic treatments, respectively). 4. Fungal assemblages were generally less diverse in the hyporheic habitat with a few species tending to be relatively favoured by such conditions. Both fungal biomass and sporulation rates were reduced in the hyporheic treatment, with the leaves subject to the benthic‐hyporheic treatment exhibiting an intermediate pattern. The initial 2‐week stage in the benthic habitat shaped the fungal assemblages, even for leaves later subjected to the hyporheic conditions. 5. The abundance and biomass of shredders drastically decreased with burial, except for Leuctra spp., which increased and was by far the most common leaf‐associated taxon in the hyporheic zone. Leuctra spp. was one of the rare shredder taxa displaying morphological characteristics that increased performance within the limited space of sediment interstices. 6. The carbon budgets indicated that the relative contributions of the two main decomposers, shredders and fungi, varied considerably depending on the location within the streambed. While the shredder biomass represented almost 50% of the initial carbon transformed after 80 days in the benthic treatment, its contribution was <0.3% in the hyporheic one and 2.0% in the combined benthic‐hyporheic treatment. In contrast, mycelial and conidial production in the permanently hyporheic environment accounted for 12% of leaf mass loss, i.e. 2–3 times more than in the two other conditions. These results suggest that the role of fungi is particularly important in the hyporheic zone. 7. Our findings indicate that burial within the substratum reduces the litter breakdown rate by limiting the access of both invertebrate and fungal decomposers to leaves. As a consequence, the hyporheic zone may be an important region of organic matter storage in woodland streams and serve as a fungal inoculum reservoir contributing to further dispersal. Through the temporary retention of litter by burial, the hyporheic zone must play a significant role in the carbon metabolism and overall functioning of headwater stream ecosystems.     

Interactions between fauna and sediment control the breakdown of plant matter in river sediments

1. A substantial portion of particulate organic matter (POM) is stored in the sediment of rivers and streams. Leaf litter breakdown as an ecosystem process mediated by microorganisms and invertebrates is well documented in surface waters. In contrast, this process and especially the implication for invertebrates in subsurface environments remain poorly studied. 2. In the hyporheic zone, sediment grain size distribution exerts a strong influence on hydrodynamics and habitability for invertebrates. We expected that the influence of shredders on organic matter breakdown in river sediments would be influenced strongly by the physical structure of the interstitial habitat. 3. To test this hypothesis, the influence of gammarids (shredders commonly encountered in the hyporheos) on degradation of buried leaf litter was measured in experimental systems (slow filtration columns). We manipulated the structure of the sedimentary habitat by addition of sand to a gravel‐based sediment column to reproduce three conditions of accessible pore volume. Ten gammarids were introduced in columns together with litter bags containing alder leaves at a depth of 8 cm in sediment. Leaves were collected after 28 days to determine leaf mass loss and associated microbial activity (fungal biomass, bacterial abundance and glucosidase, xylosidase and aminopeptidase activities). 4. As predicted, the consumption of buried leaf litter by shredders was strongly influenced by the sediment structure. Effective porosity of 35% and 25% allowed the access to buried leaf litter for gammarids, whereas a lower porosity (12%) did not. As a consequence, leaf litter breakdown rates in columns with 35% and 25% effective porosity were twice as high as in the 12% condition. Microbial activity was poorly stimulated by gammarids, suggesting a low microbial contribution to leaf mass loss and a direct effect of gammarids through feeding activity. 5. Our results show that breakdown of POM in subsurface waters depends on the accessibility of food patches to shredders.     

Mesocosm experiments reveal the direction of groundwater–surface water exchange alters the hyporheic refuge capacity under warming scenarios

相似文献   

Water-sediment exchanges control microbial processes associated with leaf litter degradation in the hyporheic zone: a microcosm study

The present study aimed to experimentally quantify the influence of a reduction of surface sediment permeability on microbial characteristics and ecological processes (respiration and leaf litter decomposition) occurring in the hyporheic zone (i.e. the sedimentary interface between surface water and groundwater). The physical structure of the water–sediment interface was manipulated by adding a 2-cm layer of coarse sand (unclogged systems) or fine sand (clogged systems) at the sediment surface of slow filtration columns filled with a heterogeneous gravel/sand sedimentary matrix. The influence of clogging was quantified through measurements of hydraulic conductivity, water chemistry, microbial abundances and activities and associated processes (decomposition of alder leaf litter inserted at a depth of 9 cm in sediments, oxygen and nitrate consumption by microorganisms). Fine sand deposits drastically reduced hydraulic conductivity (by around 8-fold in comparison with unclogged systems topped by coarse sand) and associated water flow, leading to a sharp decrease in oxygen (reaching less than 1 mg L⁻¹ at 3 cm depth) and nitrate concentrations with depth in sediments. The shift from aerobic to anaerobic conditions in clogged systems favoured the establishment of denitrifying bacteria living on sediments. Analyses performed on buried leaf litter showed a reduction by 30% of organic matter decomposition in clogged systems in comparison with unclogged systems. This reduction was linked to a negative influence of clogging on the activities and abundances of leaf-associated microorganisms. Finally, our study clearly demonstrated that microbial processes involved in organic matter decomposition were dependent on hydraulic conductivity and oxygen availability in the hyporheic zone.     

Importance of transient storage zones for ammonium and phosphate retention in a sandy-bottom Mediterranean stream

1. The ability of hyporheic sediments to exchange water and retain ammonium and phosphate in the Riera Major stream ,North-East Spain, under different discharge conditions was measured by conducting short-term nutrient and chloride additions. 2. The mean exchange coefficients from free-flowing water to the storage zone (k₁) and vice versa (k₂) were 0.82 × 10–4 s^??1 and 7 × 10^??3 s^??1, respectively. The ratio of storage zone cross-sectional area to stream cross-sectional area (A_S/A) averaged 2.8 × 10–2 and was negatively correlated with discharge (r = –0.85, d.f. = 13, P < 0.001). 3. The percentage of hyporheic zone water which came from surface water varied as a function of discharge and hyporheic depth, ranging between 33% and 95% at 25 cm depth, and between 78% and 100% at 10 cm depth. 4. The nutrient retention efficiency in the hyporheic zone at 10 cm depth measured as uptake length (S_wh) was less than 3.3 cm for ammonium and 37 cm for phosphate. Higher nutrient retentions were measured in the sediments at 10 cm depth than at 25 cm, indicating that near-surface sediments were involved more actively in phosphate retention than the deeper hyporheic sediments. The lack of ammonium at any depth of the hyporheic zone showed that ammonium was very rapidly taken up in the surfacial sediments.     

Vertical distribution of ammonia-oxidizing archaea (AOA) in the hyporheic zone of a eutrophic river in North China

Nitrification plays a significant role in the global nitrogen cycle, and this concept has been challenged with the discovery of ammonia-oxidizing archaea (AOA) in the environment. In this paper, the vertical variations of the diversity and abundance of AOA in the hyporheic zone of the Fuyang River in North China were investigated by molecular techniques, including clone libraries, phylogenetic analysis and real-time polymerase chain reaction. The archaeal amoA gene was detected in all sediments along the profile, and all AOA fell within marine group 1.1a and soil group1.1b of the Thaumarchaeota phylum, with the latter being the dominant type. The diversity of AOA decreased with the sediment depth, and there was a shift in AOA community between top-sediments (0–5 cm) and sub-sediments (5–70 cm). The abundance of the archaeal amoA gene (1.48 × 10⁷ to 5.50 × 10⁷ copies g^?1 dry sediment) was higher than that of the bacterial amoA gene (4.01 × 10⁴ to 1.75 × 10⁵ copies g^?1 dry sediment) in sub-sediments, resulting in a log₁₀ ratio of AOA to ammonia-oxidizing bacteria (AOB) from 2.27 to 2.69, whereas AOB outnumbered AOA in top-sediments with a low log₁₀ ratio of (?0.24). The variations in the AOA community were primarily attributed to the combined effect of the nutrients (ammonium-N, nitrate-N and total organic carbon) and oxygen in sediments. Ammonium-N was the major factor influencing the relative abundance of AOA and AOB, although other factors, such as total organic carbon, were involved. This study helps elucidate the roles of AOA and AOB in the nitrogen cycling of hyporheic zone.     

Flood effects on invertebrates, sediments and particulate organic matter in the hyporheic zone of a gravel-bed stream

1. We investigated the effects of a flood on the fauna and physical habitat of the hyporheic zone of the Kye Burn, a fourth order gravel‐bed stream in New Zealand. 2. Freeze core hyporheic samples (to 50 cm depth) and benthic samples (to 10 cm) were taken, along with measurements of vertical hydrological gradient, before, 2 days after and 1 month after the flood (estimated return period: 1.5 years, estimated Q_max = 10.4 m³ s^?1). 3. The composition of the hyporheos differed over the three sampling occasions with fewer taxa collected immediately postflood than preflood. The equitability of the community was higher on both postflood occasions, consistent with the reduced densities of two abundant taxa (Leptophlebiidae and Copepoda). 4. Total invertebrate abundance was lower on the postflood occasions than preflood in both benthic (0–10 cm) and hyporheic (10–50 cm) sediments. Several taxa, including asellotan isopods and amphipods, recovered within 1 month of the event. Hyporheic densities of larval Hydora and nematodes did not differ among the three sampling occasions, but the water mite Pseudotryssaturus was more abundant 1 month after the flood than preflood. There was no evidence of vertical movements (to 50 cm) by any taxa in response to the flood. 5. The proportion of fine sediments (<1 mm) in the subsurface sediments (10–50 cm) increased over the three sampling occasions and median particle size declined, but sediment porosity did not change. More particulate organic matter was found in the sediments after the flood. 6. Our study provides little evidence that the hyporheic zone (to 50 cm) acted as a significant refuge during the flood event, although movements to or recolonisation from sediments deeper than 50 cm could explain the recovery of many crustacean and mite taxa within 1 month.     

Vertical migration in streams: seasonal use of the hyporheic zone by the spinous loach <Emphasis Type="Italic">Cobitis shikokuensis</Emphasis>

Vertical hydrological connectivity between the surface stream and benthic and hyporheic zones plays a key ecological role in the biodiversity of lotic ecosystems because it allows surface and benthic organisms to use the hyporheic zone as a seasonal habitat and refuge. Use of the hyporheic zone by surface/benthic organisms has been well studied in invertebrates, but little is known about the importance of this connectivity for fishes. We investigated streambed surface and hyporheic densities (5–10, 15–20 and 20–25 cm below the streambed surface) of a stream fish, Cobitis shikokuensis, over a 20-month period in the Shigenobu River, southwestern Japan, to test the hypothesis that it uses the hyporheic zone for spawning and overwintering. In total, 1,804 individuals (13–58 mm total length) were captured from 33 streambed surface samplings and 102 individuals (10–46 mm total length) were present in 1,147 samples of 57 hyporheic samplings. Population densities in both zones peaked in late summer–early autumn due to the recruitment of age 0+ fish and a female with eggs was found in the hyporheic zone during the reproductive season. Both 0+ and older fish were absent from the streambed surface during winter, and fish densities were also lower in the hyporheic zone at this time. However, the vertical distribution of the fish tended to be skewed towards the deeper hyporheic layers from autumn to spring. These findings indicate that C. shikokuensis vertically migrates between the streambed surface and the hyporheic zone for spawning, rearing and overwintering, suggesting that the integrity of vertical hydrological connectivity in lotic systems is crucial for certain fish species.     

Sources and seasonal patterns of dissolved organic matter (DOM) in the hyporheic zone

The hyporheic zone is a region underneath streambeds that integrates surface and groundwater. Although its location is central to biogeochemical linkages between the riparian zone, dissolved nutrients, and benthic biota, the seasonal quality and likely sources of dissolved organic matter (DOM) in the hyporheic zone are not well understood. To investigate DOM characteristics in the hyporheic zone, water from the surface and subsurface (at depths 20, 60, and 100 cm below the streambed) was sampled every 4 weeks from 2007 to 2008 in a third-order stream in southern Ontario. Using UV spectroscopy, measures of spectral slopes, aromaticity, and A ₂₅₄/A ₃₆₅ ratios (molecular weight) were obtained. Temporal changes in these measures were consistent with watershed processes such as shedding of leaf litter in the fall, and photochemical and biofilm influence in the spring and summer. The fluorescence index (a measure of relative DOM source) suggested that at the surface and in the downwelling zone, DOM microbial sources increased with depth in the sediment, regardless of the season. Excitation–emission matrices (EEMs) showed seasonally distinct, protein-like DOM components of bacterial origin that were stronger in the fall. Leachates from specific allochthonous DOM sources—leaf litter from Betula papyrifera (white birch) and Thuja occidentalis (white cedar)—and an autochthonous source, biofilm, were isolated and incubated with unfiltered surface water. EEMs from these leachates indicated that these sources could indeed help explain observed patterns of DOM in surface and subsurface waters. These results suggest that although DOM sources were relatively constant, biogeochemical processing within the hyporheic zone resulted a DOM pool that was temporally dynamic and altered the nature of organic matter transported downstream into lakes and rivers.     

Synergistic impacts of sediment contamination and dam presence on river functioning

1. Dam presence is commonly associated with strong accumulation of polluted sediments. In spite of this context of multiple stressors, physical effects are often solely considered in the ecological assessment of the dam impacts. 2. We studied four ‘reservoir/downstream reach’ systems differing in levels of sediment contamination in reservoirs. Using assemblages and biotrait (i.e. ecological or biological attribute) responses of macroinvertebrate communities and leaf litter breakdown, we examined the individual effects and potential interactions between sediment contamination and dam presence along the gradient of ecotoxic pressure. 3. Leaf breakdown rates ranged from 0.0044° per day in the most contaminated reservoir to 0.0120° per day in the reference reservoir. Comparisons of community trait profiles among reservoirs highlighted a gradient of trait responses to sediment contamination. 4. In the absence of toxic contamination, the dam‐induced modifications in biotraits of invertebrate assemblages were not related to a reduction of leaf litter breakdown. Conversely, contaminated sediment in reservoir induced strong functional disturbances (i.e. bioecological shifts and reduction of leaf litter breakdown) downstream of dams. 5. Key biotrait categories positively related to leaf litter breakdown rate have been identified. They corresponded mainly to shredders and/or small‐sized (<0.5 cm) insects, using aquatic (e.g. crawlers) or aerial (e.g. fliers) active dispersal strategies. In addition, trait categories positively correlated to contamination level have been considered as ‘response’ traits. They corresponded to large‐sized (>4 cm) species, having several generations per year (polyvoltin), using asexual reproduction and/or disseminating by drift (aquatic, passive). 6. In the current context of ecological continuity restoration, this study has identified the risks associated with the presence of historical contamination in the run‐of‐river reservoirs for downstream ecosystem health.     

Interactive influences of the marine yabby (<Emphasis Type="Italic">Trypaea australiensis</Emphasis>) and mangrove (<Emphasis Type="Italic">Avicennia marina</Emphasis>) leaf litter on benthic metabolism and nitrogen cycling in sandy estuarine sediment

A previous study has demonstrated that in sandy sediment the marine yabby (Trypaea australiensis) stimulated benthic metabolism, nitrogen regeneration and nitrification, but did not stimulate denitrification, as the intense bioturbation of the yabbies eliminated anoxic microzones amenable to denitrification. It was hypothesised that organic matter additions would alleviate this effect as the buried particles would provide anoxic microniches for denitrifiers. To test this hypothesis a 55-day microcosm (75 cm × 36 cm diameter) experiment, comprising four treatments: sandy sediment (S), sediment + yabbies (S + Y), sediment + A. marina litter (S + OM) and sediment + yabbies + A. marina litter (S + Y + OM), was conducted. Trypaea australiensis significantly stimulated benthic metabolism, nitrogen regeneration, nitrification and nitrate reduction in the presence and the absence of litter additions. In contrast, the effects of litter additions alone were more subtle, developed gradually and were only significant for sediment oxygen demand. However, there was a significant interaction between yabbies and litter with rates of total nitrate reduction and denitrification being significantly greater in the S + Y + OM than all other treatments, presumably due to the decaying buried litter providing anoxic micro-niches suitable to nitrate reduction. In addition, both T. australiensis and litter significantly decreased rates of DNRA and its contribution to nitrate reduction.     

Biomass and Decay Rates of Roots and Detritus in Sediments of Intermittent Coastal Plain Streams

Biomass and breakdown of tree roots within streambed sediments were compared with leaf and wood detritus in three Coastal Plain headwater intermittent streams. Three separate riparian forest treatments were applied: thinned, clearcut, and reference. Biomass of roots (live and dead) and leaf/wood was significantly higher in stream banks than in the channel and declined with depth strata (0–10 > 10–20 > 20–30 cm). Riparian roots (live and dead combined) contributed on average 24 and 42% of coarse particulate organic matter (CPOM) biomass within the top 30 cm of channel and streambank sediments, respectively. Estimated mean surface area of live riparian roots within sediments was 1084 cm² m⁻³. Streambed temperatures showed greater fluctuation at the clearcut site compared to thinned and reference treatments. However, breakdown rates among buried substrate types or riparian treatments did not differ after 1 y. Slow decay rates were associated initially with anaerobic conditions within sandy sediments and later with dry sediment conditions. Riparian roots represent a direct conduit between streamside vegetation and the hyporheic zone. In addition to contributing to organic matter storage, the abundance of riparian roots within streambed sediments suggests that roots play an important role in biogeochemical cycling within intermittent headwater streams of the Coastal Plain.     

Aquatic Hyphomycete Species Are Screened by the Hyporheic Zone of Woodland Streams

Aquatic hyphomycetes strongly contribute to organic matter dynamics in streams, but their abilities to colonize leaf litter buried in streambed sediments remain unexplored. Here, we conducted field and laboratory experiments (slow-filtration columns and stream-simulating microcosms) to test the following hypotheses: (i) that the hyporheic habitat acting as a physical sieve for spores filters out unsuccessful strategists from a potential species pool, (ii) that decreased pore size in sediments reduces species dispersal efficiency in the interstitial water, and (iii) that the physicochemical conditions prevailing in the hyporheic habitat will influence fungal community structure. Our field study showed that spore abundance and species diversity were consistently reduced in the interstitial water compared with surface water within three differing streams. Significant differences occurred among aquatic hyphomycetes, with dispersal efficiency of filiform-spore species being much higher than those with compact or branched/tetraradiate spores. This pattern was remarkably consistent with those found in laboratory experiments that tested the influence of sediment pore size on spore dispersal in microcosms. Furthermore, leaves inoculated in a stream and incubated in slow-filtration columns exhibited a fungal assemblage dominated by only two species, while five species were codominant on leaves from the stream-simulating microcosms. Results of this study highlight that the hyporheic zone exerts two types of selection pressure on the aquatic hyphomycete community, a physiological stress and a physical screening of the benthic spore pool, both leading to drastic changes in the structure of fungal community.     

Effects of burial on leaf litter quality,microbial conditioning and palatability to three shredder taxa

Summary 1. Heterotrophic microorganisms are crucial for mineralising leaf litter and rendering it more palatable to leaf‐shredding invertebrates. A substantial part of leaf litter entering running waters may be buried in the streambed and thus be exposed to the constraining conditions prevailing in the hyporheic zone. The fate of this buried organic matter and particularly the role of microbial conditioning in this habitat remain largely unexplored. 2. The aim of this study was to determine how the location of leaf litter within the streambed (i.e. at the surface or buried), as well as the leaf litter burial history, may affect the leaf‐associated aquatic hyphomycete communities and therefore leaf consumption by invertebrate detritivores. We tested the hypotheses that (i) burial of leaf litter would result in lower decomposition rates associated with changes in microbial assemblages compared with leaf litter at the surface and (ii) altered microbial conditioning of buried leaf litter would lead to decreased quality and palatability to their consumers, translating into lower growth rates of detritivores. 3. These hypotheses were tested experimentally in a second‐order stream where leaf‐associated microbial communities, as well as leaf litter decomposition rates, elemental composition and toughness, were compared across controlled treatments differing by their location within the streambed. We examined the effects of the diverse conditioning treatments on decaying leaf palatability to consumers through feeding trials on three shredder taxa including a freshwater amphipod, of which we also determined the growth rate. 4. Microbial leaf litter decomposition, fungal biomass and sporulation rates were reduced when leaf litter was buried in the hyporheic zone. While the total species richness of fungal assemblages was similar among treatments, the composition of fungal assemblages was affected by leaf litter burial in sediment. 5. Leaf litter burial markedly affected the food quality (especially P content) of leaf material, probably due to the changes in microbial conditioning. Leaf litter palatability to shredders was highest for leaves exposed at the sediment surface and tended to be negatively related to leaf litter toughness and C/P ratio. In addition, burial of leaf litter led to lower amphipod growth rates, which were positively correlated with leaf litter P content. 6. These results emphasise the importance of leaf colonisation by aquatic fungi in the hyporheic zone of headwater streams, where fungal conditioning of leaf litter appears particularly critical for nutrient and energy transfer to higher trophic levels.     

Faunal response to benthic and hyporheic sedimentation varies with direction of vertical hydrological exchange

相似文献   

Benthic and hyporheic macroinvertebrate distribution within the heads and tails of riffles during baseflow conditions

The distribution of lotic fauna is widely acknowledged to be patchy reflecting the interaction between biotic and abiotic factors. In an in situ field study, the distribution of benthic and hyporheic invertebrates in the heads (downwelling) and tails (upwelling) of riffles were examined during stable baseflow conditions. Riffle heads were found to contain a greater proportion of interstitial fine sediment than riffle tails. Significant differences in the composition of benthic communities were associated with the amount of fine sediment. Riffle tail habitats supported a greater abundance and diversity of invertebrates sensitive to fine sediment such as EPT taxa. Shredder feeding taxa were more abundant in riffle heads suggesting greater availability of organic matter. In contrast, no significant differences in the hyporheic community were recorded between riffle heads and tails. We hypothesise that clogging of hyporheic interstices with fine sediments may have resulted in the homogenisation of the invertebrate community by limiting faunal movement into the hyporheic zone at both the riffle heads and tails. The results suggest that vertical hydrological exchange significantly influences the distribution of fine sediment and macroinvertebrate communities at the riffle scale.     

Influence of streambed sediment clogging on microbial processes in the hyporheic zone

1. The hyporheic zone plays a key role in hydrological exchange and biogeochemical processes in streambed sediments. The clogging of sediments caused by the deposition of particles in the bed of streams and rivers can decrease sediment permeability and hence greatly affect hyporheic microbial processes. 2. The main objective of this study was to determine the influence of sediment clogging on hyporheic microbial processes in three French rivers (the Usses, Drôme and Isère). In each river, microbial abundance and activity were studied at three depths (10, 30 and 50 cm) in the sediment at one unclogged (high porosity) and one clogged site (low porosity). 3. The results showed that the sediment clogging had inconsistent effects on microbial processes in the three rivers. Increases (Usses) or decreases (Drôme and Isère) in both aerobic and anaerobic processes were detected at the clogged sites compared to unclogged sites. These results suggest that microbial changes because of the sediment clogging are mainly mediated by the residence time of water within the hyporheic sediments. 4. A single model predicting the effect of clogging on hyporheic microbial processes cannot be applied generally to all rivers because the degree of clogging creates heterogeneous effects on flow rates between surface and interstitial waters. As a consequence, the influence of heterogeneous clogging on surface water–hyporheic exchanges needs to be evaluated by water tracing and hydraulic modelling to determine the links between microbial processes and hydraulic heterogeneity induced by clogging in hyporheic sediments.     

Distribution and microhabitats of freshwater mussels in waterbodies in the terrestrialized floodplains of a lowland river

Even when anthropogenically altered, river floodplains continue to contribute to biodiversity. This study examined the distribution of freshwater mussels in relation to environmental factors in waterbodies in the terrestrialized floodplain of a lowland river. Mussels were captured, and environmental measurements were taken in November of 2013 and 2014 in quadrats established in three floodplain waterbodies (FWBs), which were isolated from the main river channel. Among the three FWBs, mussel abundance was highest in a shallow FWB (depth range 18–45 cm) that had intermediate conditions of mud depth and fine sediment rate. Mussel abundance showed a hump-shaped relationship with water depth (the peak 45–50 cm) and mud depth (the peak 8–12 cm). Mussel abundance was also negatively related to the abundance of benthic litter. Litter abundance was positively related to branch abundance and the presence of tree cover, and negatively related to the distance to tree cover, indicating that benthic litter was derived from riparian trees. Our results indicate that relatively shallow (≤ 50 cm) FWBs with moderately accumulated mud, which are not scoured even during flooding, appear to be suitable habitats for mussels. Moreover, it is possible that riparian trees negatively impact mussel distribution in FWBs. Possible short-term measures for improving mussel habitat in FWBs may include the elimination of riparian trees and benthic litter.     

In situ studies on the breakdown ofNymphoides peltata (Gmel.) O. Kuntze (Menyanthaceae); Some methodological aspects of the litter bag technique

Summary Thein situ breakdown ofNymphoides peltata (Gmel.) O. Kuntze has been studied with special attention for methodology by: (1) using fresh and pre-dried material to establish the influence of pre-drying on breakdown and losses of nutrient stocks during decomposition; (2) enclosing different amounts of material in litter bags; (3) using litter bags with different mesh sizes, and (4) placing litter bags in water (floating leaves, petioles), on the sediment (long shoots) and in the hydrosoil (short shoots, roots). Of the material incubated in water, the floating leaves decomposed at a faster rate than the petioles, while the long shoots had the slowest breakdown. In the sediment the short shoots disappeared at a faster rate than the roots. By incubating the same morphological structure, both in the water and the sediment it appeared that the rate of breakdown was faster in the upper layers of the sediment. Pre-dried plant parts showed in water a larger initial weight loss than normal senescent plant parts, while in the sediment dried plant parts had a significantly slower loss of mass than the freshly incubated structures. Losses of nutrient stocks during decomposition were also markedly altered by pre-drying the material. When a larger amount ofNymphoides material was enclosed in the bags a tendency of a faster decay could be demonstrated. Macro-invertebrates colonized the litter bags with the 0.5 mm mesh size but usually could not-enter the 0.25 mm mesh size bags. The browsing of the detritivores did not result in a faster disappearance of organic matter, but organic matter must have been transported into the bags resulting in a larger amount of remaining organic matter when compared with the 0.25 mm mesh size bags.