Responses of hyporheic invertebrate assemblages to large-scale variation in flow permanence and surface–subsurface exchange

1. Flow permanence (the proportion of time that flowing water is present) strongly influences benthic invertebrate assemblages in ephemeral and intermittent river reaches. Effects of varying flow permanence on hyporheic invertebrate assemblages are not well understood, and have not previously been studied at large spatial scales. 2. We used a 52‐km long flow‐permanence gradient in the alluvial Selwyn River, New Zealand to assess hyporheic assemblage responses to variation in flow permanence and surface–subsurface exchange. The Selwyn mainstem consists of perennial and temporary reaches embedded in longer downwelling (losing) and upwelling (gaining) sections. 3. We predicted that hyporheic invertebrate diversity, density and assemblage stability would increase with increasing flow permanence. We further predicted that assemblage structure would be influenced by the relative contribution of downwelling and upwelling water at the reach‐scale. 4. Hyporheic invertebrates were collected at 15 river cross‐sections over a 13‐month period. As predicted, hyporheic taxon richness, density and assemblage stability varied directly with flow permanence. The distribution of taxa along the flow permanence gradient appeared to be related to desiccation resistance. However, it is possible that proximity to colonist sources also contributed to distribution patterns. 5. Taxon richness was significantly higher at sites in the gaining section compared with the losing section. Sites with high flow permanence in the gaining and losing sections supported distinct hyporheic assemblages, characterised by amphipods and isopods in the gaining section, and ostracods, Hydra sp. and the mayfly Deleatidium spp. in the losing section. 6. Results of the study suggest an expansion of the scope of the Hyporheic Corridor Concept to include large hyporheic flowpaths associated with unbounded alluvial plains rivers. Hyporheic assemblages in alluvial rivers are strongly influenced by large‐scale flow permanence gradients, large‐scale surface water–groundwater exchange, and their interactions.     

Surface–subsurface water exchange rates along alluvial river reaches control the thermal patterns in an Alpine river network

1. Water temperature is a key characteristic of stream ecosystems that is gaining scientific and managerial relevance as maximum temperatures in aquatic ecosystems increase worldwide.
2. To assess the effect of surface–subsurface water exchange on stream water temperature patterns, four alluvial reaches in the Tagliamento River basin (NE Italy), constrained by geomorphic knickpoints at the upper and lower end, and two to four hyporheic flowpaths within each reach, were continuously studied during summer 2007 and winter 2007–08. Water temperature was continuously monitored at the upstream and downstream knickpoints of the floodplains, as well as at discrete upwelling areas within each reach. Discharge and vertical hydraulic gradient were measured along the alluvial reaches, and the residence time and chemistry of upwelling water were assessed four times during the study.
3. Discharge variation along the study reaches revealed that massive hyporheic exchange occurred in all sites, ranging from 21% in reach 2–52% in reach 1. End member mixing analysis showed little influence of ground water, as almost all upwelling water was freshly infiltrated hyporheic water. Importantly, hyporheic exchange flows shaped surface temperature at the upwelling locations in all study reaches, providing potential thermal refugia for aquatic biota. At sites with highest hyporheic flow rates, net temperature change was also reflected at the floodplain scale.
4. The magnitude of the thermal change along a hyporheic flowpath was not related to the flowpath length but to the estimated ²²²Rn water age. Reduction in the diel thermal amplitude by hyporheic flows rather than net temperature change, reduced temperature extremes. Therefore, restoration activities to create thermal refugia should consider the role of hyporheic flows and enhance the exchange between surface and hyporheic waters.     

Geomorphic influence on intraspecific genetic differentiation and diversity along hyporheic corridors

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Distribution of solutes, microbes and invertebrates in river sediments along a riffle-pool-riffle sequence

• 1 Unlike riffles, research has focused rarely on the hyporheic zone of pools. To highlight the functioning of a pool, field investigations were performed in a riffle‐pool‐riffle sequence by integrating simultaneously physico‐chemistry, microbes and invertebrates. The study was conducted in a channel characterised by strong downwelling of surface water.
• 2 To include the downstream flux of water within the sediment, a longitudinal profile was studied along six stations situated: at the centre (Station 1) and at the lower end (Station 2) of the first riffle, at the upstream part (Station 3), at the centre (Station 4, at the inflection point) and at the lower end (Station 5) of the pool, and at the centre of the second riffle (Station 6). At each station, three replicate samples were taken and three sample depths were investigated (0.2, 0.5, and 1.0 m below the stream bed) on two dates.
• 3 Physico‐chemical parameters (vertical hydraulic gradient, oxygen concentration and specific conductance) differed between stations depending on infiltration rates. In contrast, organic matter and microbial parameters presented patchy distributions linked with factors other than the geomorphological pattern. Despite not very pronounced geomorphologic features, the slope variation at the centre of the pool (at the inflection point) affects the distribution of epigean and hypogean invertebrates.
• 4 Based upon faunal parameters, the pool could be divided into an upstream and a downstream part, the latter being more strongly influenced by surface water.
• 5 The pool should be considered as a heterogeneous area. In that respect, the inflection point of a pool may be as important as the top of a riffle in the functioning of river sediments.

     

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.     

The three dimensional distribution of the fauna in a single riffle in a stream in Ontario

The surface and hyporheic fauna of a riffle area in Salem Creek near Elmira, Ontario, was investigated with a Surber and a corer sampler. A stratified random sampling design estimated the number of invertebrates at 154 561 (109 805–224 155) m^–2 in June and at 64 407 (40 905–97 996) m^–2 in October with more than 70% of the animals occurring below 5 cm. There was a great spatial variation of the hyporheos density in the three dimensions of the riffle, which seemed to be related mostly to hydrogeological characteristics (superficial groundwater flow and grain size). Upward water movement in the centre of the stream occurred in the upper two transects, especially from the upper right bank, while the lower transect showed little water transmission. Hyporheos composition is discussed in regard to life history and the input of organic materials. Only two of nine pairs of Surber samples showed a high degree of similarity (0.68, 0.81). The surficial spatial density and diversity were examined in relation to the available surface and to substrate heterogeneity. It is concluded that the great variations between samples caused by many factors present severe problems in the collection of reliable quantitative samples.

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Résumé La faune de surface et celle de l'hyporhélique d'un radier de Salem Creek près d'Elmira, Ontario, ont été étudiées à l'aide de l'échantillonneur de Surber et d'un carottier. Un échantillonnage aléatoire stratifié a permis d'estimer le nombre d'invertébrés a 154 561 (109 805–224 155) m^–2 en juin et à 64 407 (40 905–97 996) m^–2 en octobre. Environ 70% de la faune benthique a été retrouvée entre 5 et 65 cm. La densité de l'hyporhéos présente de grandes variations spatiales, et celles-ci semblent etre reliées aux caractéristiques hydrogéologiques (mouvement des eaux souterraines superficielles et taille des particules). Un net mouvement ascendant dirigé vers le centre du radier a été détecté dans les deux transects situés en amont; aucun mouvement n'a été mesuré dans un troisième transect situé an aval. La composition de l'hyporhéos est mise en relation avec les cycles vitaux et avec l'apport des matières organiques. Des 9 paires d'échantillons recueillis au Surber, deux seulement présentent un certain degré de similarité (0.68, 0.81). Les variations spatiales de la densité et de la diversité de la faune de surface furent mises en corrélation avec la surface disponible et l'hétérogénéité du substrat. On en conclu que la grande variabilité entre les échantillons dans les deux habitats est due à plusieurs facteurs et qu'elle pose de sérieux problèms pour l'estimation de paramètres quantitatifs.

     

Denitrification in a nitrogen-limited stream ecosystem

Denitrification was measured in hyporheic, parafluvial, and bank sediments of Sycamore Creek, Arizona, a nitrogen-limited Sonoran Desert stream. We used three variations of the acetylene block technique to estimate denitrification rates, and compared these estimates to rates of nitrate production through nitrification. Subsurface sediments of Sycamore Creek are typically well-oxygenated, relatively low in nitrate, and low in organic carbon, and therefore are seemingly unlikely sites of denitrification. However, we found that denitrification potential (C & N amended, anaerobic incubations) was substantial, and even by our conservative estimates (unamended, oxic incubations and field chamber nitrous oxide accumulation), denitrification consumed 5–40% of nitrate produced by nitrification. We expected that denitrification would increase along hyporheic and parafluvial flowpaths as dissolved oxygen declined and nitrate increased. To the contrary, we found that denitrification was generally highest at the upstream ends of subsurface flowpaths where surface water had just entered the subsurface zone. This suggests that denitrifiers may be dependent on the import of surface-derived organic matter, resulting in highest denitrification rate at locations of surface-subsurface hydrologic exchange. Laboratory experiments showed that denitrification in Sycamore Creek sediments was primarily nitrogen limited and secondarily carbon limited, and was temperature dependent. Overall, the quantity of nitrate removed from the Sycamore Creek ecosystem via denitrification is significant given the nitrogen-limited status of this stream.     

Oligochaete communities in the hyporheic zone of a glacial river,the Roseg River,Switzerland

We examined the composition and distribution of oligochaete communities in the hyporheic zone of the Roseg River, a glacial river in southeastern Switzerland. Ten oligochaete species were collected from 11 sites distributed along an 11-km reach of the river, downstream of the Tschierva Glacier. The most frequently encountered species were Dorydrilus michaelseni, Propappus volki, Cernosvitoviella atrata and Cernosvitoviella carpatica. Six other species were relatively scarce and exhibited a discontinuous distribution pattern only in the downstream sites. At the most downstream site, a population of eyeless Nais communis was discovered with anatomical traits apparently related to a subterranean life-adaptation of this surface-living species. The species richness and abundance of oligochaete communities were relatively constant over time, but increased at sites where groundwater entered the stream. The spatial distribution of several oligochaete species was linked to the longitudinal arrangement of groundwater upwelling areas. This observation suggested that groundwater was an important upstream migration pathway for oligochaetes during the glacial retreat.     

Key role of streambed moisture and flash storms for microbial resistance and resilience to long‐term drought

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Alpha and beta diversity of connected benthic–subsurface invertebrate communities respond to drying in dynamic river ecosystems

Drying disturbances are the primary determinant of aquatic community biodiversity in dynamic river ecosystems. Research exploring how communities respond to disturbance has focused on benthic invertebrates in surface sediments, inadequately representing a connected community that extends into the subsurface. We compared subsurface and benthic invertebrate responses to drying, to identify common and context‐dependent spatial patterns. We characterized community composition, alpha diversity and beta diversity across a gradient of drying duration. Subsurface communities responded to drying, but these responses were typically less pronounced than those of benthic communities. Despite compositional changes and in contrast to reductions in benthic alpha diversity, the alpha diversity of subsurface communities remained stable except at long drying durations. Some primarily benthic taxa were among those whose subsurface frequency and abundance responded positively to drying. Collectively, changing composition, stable richness and taxon‐specific increases in occurrence provide evidence that subsurface sediments can support persistence of invertebrate communities during drying disturbances. Beta‐diversity patterns varied and no consistent patterns distinguished the total diversity, turnover or nestedness of subsurface compared to benthic communities. In response to increasing drying duration, beta diversity increased or remained stable for benthic communities, but remained stable or decreased for subsurface communities, likely reflecting contrasts in the influence of mass effects, priority effects and environmental filtering. Dissimilarity between subsurface and benthic communities remained stable or increased with drying duration, suggesting that subsurface communities maintain distinct biodiversity value while also supporting temporary influxes of benthic taxa during drying events. As temporary rivers increase in extent due to global change, we highlight that recognizing the connected communities that extend into the subsurface sediments can enable holistic understanding of ecological responses to drying, the key determinant of biodiversity in these dynamic ecosystems.     

The response of invertebrates to a gradient of flow reduction – an instream channel study in a New Zealand lowland river

1. The impacts of low flows on invertebrates were assessed in six experimental channels placed in a lowland stream. Each channel consisted of replicate stacked gravel‐filled baskets (buried 0–7, 8–14 and 15–21 cm deep) from which invertebrates were sampled. Samples were collected before, and 1 and 2 months after flows were reduced by varying amounts. We predicted that invertebrate communities would change most with the lowest flows that persisted longest. The effects of reduced flow on channel and intragravel‐velocities, temperature, dissolved oxygen and algal biomass were also monitored. 2. Mean velocity (17 cm s⁻¹) and depth (20 cm) were similar in all channels prior to flow reduction, but were reduced in five channels by 25% to 98%, a sixth channel serving as a control. Mean intragavel velocity prior to flow reduction (1.7 mm s⁻¹) was also reduced from 17% to 51%. Flow reduction had no effect on temperature, which ranged from 11.2 °C (night) to 19.6 °C (day). Dissolved oxygen was generally high (mean = 94% saturation) but variable (range from 50% to 145%), and did not differ between channels after flow reduction. Periphyton biomass (chlorophyll a and AFDM_algae) increased over time and was positively related to velocity after 1 month of flow reduction. After 2 months, however, this relationship reversed, with higher chlorophyll and AFDM_algae in lower velocity channels. Organic matter biomass (AFDM_total) was significantly higher in channels with lower velocities, and did not change with time. There was no relationship between AFDM_total and velocity in baskets from different depths. 3. Greater than 85% of animals were found in the upper baskets (0–7 cm deep), and flow reduction had no influence on the vertical distribution of invertebrates. After 1 month, invertebrate density had declined roughly in proportion to the magnitude of flow reduction in all channels where flows were reduced more than 25%. Densities recovered to pre‐reduction levels within 2 months. The amphipod, Paracalliope fluviatilis dominated all channels numerically prior to flow reduction, but its density declined markedly during the study. Densities of Ostracoda, Oxyethira albiceps and Cladocera increased dramatically after 2 months of flow reduction, especially in the lowest flow channels. Of the physical variables measured, chlorophyll a biomass and discharge best explained the temporal changes in the invertebrate community. 4. The effects of increased duration and magnitude of flow reduction on invertebrate communities were restricted to changes in the relative abundances of just a few taxa. Our results suggest that invertebrates in this lowland stream were resistant to the experimental flow reduction, presumably because of their broad ecological tolerances. We also found that more prolonged low flows did not result in predictable changes. This finding may have implications in terms of using hydraulic‐habitat models to set minimum flows in lowland streams if invertebrates can persist at flows much lower than their supposed optima.     

溪流潜流层大型无脊椎动物生态学研究进展

溪流潜流层是溪流表层水和地下水相互作用的群落交错区,生物多样性丰富,是溪流生态系统的重要组成部分.大型无脊椎动物位于潜流层食物网的顶层,直接影响着潜流层物质和能量动态,是河流健康潜在的指示生物,调节着潜流层的环境净化和生态缓冲功能,对溪流生态系统发挥着至关重要的作用.潜流层大型无脊椎动物类群按生活史划分为偶入动物、非典型潜流层动物和典型潜流层动物.潜流层孔隙大小、孔隙水流速、溶解氧、温度、可利用的食物源、渗透系数和水力停留时间是影响大型无脊椎动物在潜流层分布的主要因素.对于潜流层这样一个特殊的生态界面,针对不同的研究目的应该选择合适的取样和调查方法.潜流层大型无脊椎动物的生活史和生活史对策,在溪流生态系统物质循环和能量流动中作用的定量化分析,基于潜流层大型无脊椎动物的河流健康评价体系,以及潜流层作为“庇护地”对于大型无脊椎动物分布和进化的生态学意义,都值得进一步关注和深入研究.     

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.     

The impact of flash floods on microbial distribution and biogeochemistry in the parafluvial zone of a desert stream

1. The impact of flash flooding on microbial distribution and biogeochemistry was investigated in the parafluvial zone (the part of the active channel lateral to the surface stream) of Sycamore Creek, a Sonoran Desert stream in central Arizona. 2. It was hypothesized that subsurface bacteria were dependent on the import of algal-derived organic matter from the surface stream, and it was therefore predicted that microbial numbers and rates of microbially mediated processes would be highest at locations of surface to subsurface hydrologic exchange and at times when algal biomass was high. 3. Prior to a flash flood on 19 July 1994, chlorophyll a was high (≈ 400 mg m^–2) in the surface stream and microbial numbers were highest at the stream–parafluvial interface and declined along parafluvial flowpaths, supporting the hypothesized algal–bacterial linkage. Immediately following the flash flood, chlorophyll a was low (≈ 7 mg m^–2), and microbial numbers were reduced at the stream–parafluvial interface. 4. Counter to expectations, parafluvial functioning (in terms of nitrate production and dissolved oxygen decline along flowpaths) re-established immediately after the flood receded. Therefore, material other than algal exudates supported parafluvial metabolism immediately postflood, and terrestrially derived dissolved organic matter is the likely source. 5. Algae in the surface stream recovered quickly following flooding, but recovery of parafluvial bacteria lagged somewhat behind. These results highlight the importance of surface–subsurface interaction to stream ecosystem functioning and show that the nature of these interactions changes substantially in successional time.     

The metabolism of organic matter in the hyporheic zone of a mountain stream, and its spatial distribution

Community respiration in hyporheic sediments (HCR) was studied in a characteristic riffle-pool-sequence of a mountain stream. HCR activity at the riffle site strongly exceeded that at the corresponding pool site with a mean ratio of 5.3. The vertical distribution of HCR activity was homogeneous in the pool, while there was a distinct maximum in the uppermost layer in the riffle. Similarly, the spatial distribution of certain fractions of particulate organic matter (POM), and their turnover, was largely determined by stream morphology. Mean annual HCR per unit area of stream bed was estimated as 1.71 g O₂ m⁻² d⁻¹. Hence, HCR contributes significantly to total heterotrophic activity in streams, thus enhancing the relative importance of heterotrophic processes in running waters containing hyporheic zones.     

Detecting dispersal of Nuphar lutea in river corridors using microsatellite markers

1. River corridors are well-known for their role in plant dispersal. The buoyancy of seeds, the possibility of dispersal by vegetative fragments, and the frequency and efficiency of dispersal among different river catchments determine linear distribution patterns. Little is known about the relative importance of these factors to observed patterns of genetic variation.
2. One hundred and fifty-six Nuphar lutea individuals from forty-four sampling sites in the river catchments of the Cidlina River, the Mrlina River and the Labe River (Czech Republic) were studied using ten microsatellite markers. Interpretation of patterns in genetic variation allowed several conclusions about dispersal mechanisms.
3. Vegetative long-distance dispersal is probably very limited in this species. Only one multilocus genotype was found in more than one sampling site. The distance between the sites was about 75 km.
4. To explain the distribution of Bayesian based clusters of related multilocus genotypes, both along-river and inter-river long-distance dispersals have to be invoked.
5. A marginally significant tendency for higher genetic diversity in the lower part of the river Cidlina was detected. Continuous downstream dispersal of seeds by water currents could be a valid explanation.
6. Significant positive autocorrelation was found among individuals at within-river distances of up to 25 km. Repeated dispersal of seeds over distances in the range of tens of kilometers is common.     

污染物在农田溪流生态系统中的动态变化

氮是地表水体发生富营养化的主要因子,河流系统是氮输出的主要运移通道,养分在河流生态系统中的持留和趋向控制着污染物的输出。以巢湖流域一个受人为活动严重影响的农田源头溪流——六岔河为研究对象(包括4个渠道型、1个水塘型和3个河口型断面,对应长度分别为1．3km、0．15km和0．36km),设置9个监测点研究总氮(TN)、硝酸盐(NO3-N)、氨态氮(NH4^ -N)和总悬浮物(TSS)在溪流生态系统中的持留,评价人为严重干扰下的农田溪流生态系统在非点源污染物运移中的生态功能。结果表明：TN、NO3-N、NH4^ -N和TSS在溪流中的持留、释放受溪流的河流形态影响,水塘型和河口型断面是污染物持留的主要区域;TN、NO3-N、NH4^ -N和TSS在水塘型和河口型断面内的持留量分别占溪流持留量(基流、径流持留量的和)的61％、47％、75％和56％。降雨-径流过程中发生的持留是污染物持留的主要部分,TN、NO3-N、NH4^ -N和TSS的持留量分别占溪流持留量的93％、97％、89％和96％;渠道型断面是溪流最主要的内在污染源,污染物释放量占溪流释放量的90％以上;受水塘型断面出口处的水坝影响,位于水塘前的渠道型断面在基流和降雨-径流过程中均有效地持留污染物,而其余渠道型断面在不同水文条件下呈现出不同的持留特性。非点源污染物在溪流中的持留和释放的空间动态变化是溪流生态系统对自然和人为干扰的一种综合响应,有必要恢复溪流生态系统功能,控制农业非点源污染。     

Subsurface hydrology and degree of burial affect mass loss and invertebrate colonisation of leaves in a woodland stream

SUMMARY 1. In deciduous forest streams, fallen leaves form a large component of the total organic matter budget, and many leaves become buried within stream sediments. We examined the processing of buried leaves as compared with those at the surface, and the influence of subsurface hydrology on processing rates.
2. Leaf packs were secured on the streambed surface or buried 10 cm deep in upwelling and downwelling reaches of a second-order stream in Michigan, U.S.A. Mass loss and invertebrate colonisation were measured from October to February.
3. Leaves buried in upwelling reaches lost mass more slowly (exponential decay coefficient, k =−0.0097) than did leaves from the other treatments (buried downwelling: −0.017; surface upwelling: −0.022; surface downwelling: −0.021).
4. Initially, more invertebrates colonised surface leaf packs than buried packs. During the remainder of the study, however, hydrology had a greater effect on invertebrate abundance than did burial, as more invertebrates were found in packs in downwelling reaches than in upwelling reaches.
5. Local subsurface hydrology and degree of burial, factors rarely considered in studies of detritus processing, can significantly influence mass loss and invertebrate colonisation of fallen leaves in streams. Furthermore, because of slower processing, subsurface zones may function as organic matter reservoirs that gradually 'spiral' carbon to downstream subsurface and surface habitats.     

Nitrate depletion in the riparian zone of a small woodland stream

Field enrichments with nitrate in two spring-fed drainage lines within the riparian zone of a small woodland stream near Toronto, Ontario showed an absence of nitrate depletion. Laboratory experiments with riparian substrates overlain with nitrate enriched solutions revealed a loss of only 5–8% of the nitrate during 48 h incubation at 12°C. However, 22–24% of the initial nitrate was depleted between 24 and 48 h when a second set of substrate cores was incubated at 20°C. Short-term (3 h) incubations of fresh substrates amended with acetylene were used to estimate in situ denitrification potentials which varied from 0.05–3.19 g N g^–1 d^–1 for organic and sandy sediments. Denitrification potentials were highly correlated with initial nitrate content of substrate samples implying that low nitrate levels in ground water and riparian substrates may be an important factor in controlling denitrification rates. The efficiency of nitrate removal in spring-fed drainage lines is also limited by short water residence times of < 1 h within the riparian zone. These data suggest that routes of ground water movement and substrate characteristics are important in determining nitrate depletion within stream riparian areas.