Stream Water Column Research Articles

Water Column Nitrogen Removal During Storms in a Low‐Order Watershed

AbstractWater column removal in streams is a nitrogen (N) cycling pathway that has been historically overlooked. Studies filling this knowledge gap have focused on the role of water column N removal in mid‐to‐large‐order rivers with consistently high suspended sediment concentrations. However, smaller streams may provide comparable suspended sediment concentrations during and after storm events, creating favorable conditions for water column N removal. To assess the presence, magnitude, and control of water column N removal during storms in low‐order watersheds, we measured water column denitrification and heterotrophic assimilatory N uptake rates at three locations in a Mid‐Atlantic watershed during five storm events of different magnitude, sediment loads, and nutrient availability. We found large variations in water column denitrification (0–5.56 mg N g−1 d−1) and assimilatory uptake (0.003–1.67 mg N g−1 d−1). Higher rates of N removal occurred during flow recession, with a correlation between suspended sediment organic matter content and denitrification. On average, denitrification rates in the water column were higher when flashy responses to storm events occurred. In contrast to denitrification, water column N removal rates (as both denitrification and heterotrophic assimilation) during storm events were comparable to those measured at baseflow in larger rivers. However, water column denitrification could only account for less than 10% of potential reach‐scale N removal during most of the storm events. Our findings provide insight into the ecological relevance of small stream water columns and suggest that more research is needed to understand the magnitude of stream water column processing on watershed‐scale N removal.

Thermal Threats to Freshwater Mussels: An Empirical Stream Assessment

Freshwater mussels fulfill an important ecological role in aquatic ecosystems, but they currently face many threats, including thermal regime alteration. Thermal transformation of the aquatic environment is associated with climate change, land use alteration, and other pervasive anthropogenic global changes. To enhance our understanding of ecological thermal impacts, we combined extensive field measurements of temperature in the stream water column and substrate depths (5 and 15 cm) at sites where mussels occur, measures of abundance and species richness for mussels and fish, and thermal tolerance knowledge for mussels and fish to generate a comprehensive assessment of the potential threats mussels face as temperatures continue to rise as a result of global change. Mean summer (June–August 2010–2012) temperatures at mussel-occupied sites in the upper Tar River basin of North Carolina, USA, ranged from 16.2 to 34.7 °C. The mean temperature from the hottest 96 h at each site ranged from 23.5 to 31.5 °C. At 80% of sites, a period of moderate drought coincided with the hottest 96 h period. Temperature threshold exceedance durations indicated that chronic, combined chronic/acute, and acute freshwater mussel thermal tolerance thresholds (i.e., 28 °C, 30 °C, and 33 °C, respectively) based on laboratory exposures of glochidia (larvae) and juveniles were commonly exceeded. Water temperatures exceeded 28 °C for at least 24 h at 55% of sites and for at least 96 h at 35% of sites, and they exceeded 30 °C for at least 24 h at 15% of sites. We quantified a thermal buffering effect of the substrate that may be protective of mussels. There was a mean difference of 0.5 °C between the water column and the upper substrate (5 cm) and a mean difference of 0.9 °C between the water column and the lower substrate (15 cm). Maximum differences of up to 5.5 °C between the water column and the upper substrate and 11.5 °C between the water column and the lower substrate were observed. Our models estimating the relation between the water column and substrate temperatures more realistically characterize ambient temperature exposures and have widespread implications for mussel conservation and climate change risk assessment in similar streams. Freshwater mussels currently exist on the edge of their thermal limits, but their abundance and species richness cannot be explained by temperature patterns alone. Fish species richness was related to the thermal regime, indicating that species interactions may be an important driver of freshwater mussel responses to global change.

Comparing spiraling‐ and transport‐based approaches to estimate in‐stream nutrient uptake length from pulse additions

AbstractThe distance that a nutrient travels as a solute before its removal from the stream water column is known as the uptake length (SW), which is a functional indicator of environmental quality and integrity. Among nutrient enrichment methods, instantaneous nutrient addition (e.g., slug or pulse) have been proposed as an alternative to plateau and labeled nutrient approaches. Two approaches have been commonly used to estimate SW and its associated metrics (i.e., areal uptake rate, U; and uptake velocity, Vf) from pulse additions: the spiraling approach, based on the longitudinal variation in nutrient concentrations, and the transport modeling approach, based on the advective and dispersive transport of solutes. However, little is known in how the choice of such analytical methods impacts the estimation of stream uptake parameters and the conclusions we draw from them. Here, we estimated the SW and Vf of ammonium‐nitrogen (NH4‐N) and soluble reactive phosphorus (SRP) from 16 pulsed additions conducted in four low‐order streams in southeastern Brazil. We compared metrics estimated by the Tracer Additions for Spiraling Curve Characterization (TASCC) and the One‐Dimensional Transport with Inflow and Storage (OTIS) methods, based on the spiraling‐ and transport‐based approaches, respectively. The TASCC:OTIS SW ratio averaged 0.71 for NH4‐N and 1.01 for SRP, whereas the mean of TASCC:OTIS Vf ratio was 2.04 for NH4‐N and 1.03 for SRP. The results showed that both SW and Vf estimates differed significantly between methods for NH4‐N, but no statistical differences were observed in SRP estimates. In our study, we highlighted the significant effects of transient storage and variable nutrient concentration on pulsed enrichments. Such information should be considered when choosing which method is appropriate to use for a particular site. Differences between modeling approaches must be addressed when comparing methods to expand our knowledge on broad temporal and spatial patterns of in‐stream nutrient uptake.

Heavy metal toxicity in the water column and benthic sediments of a degraded tropical stream.

Agriculture and other anthropogenic activities on riparian corridors pose a great ecotoxicological risk to freshwater ecosystem and human health. Using the atomic absorption spectroscopy, concentrations of heavy metals (Cd, Cu, Fe, Zn, Pb and As) in the water column and benthic sediments of a degraded tropical stream (Opa Stream, Ile-Ife, Nigeria), were assessed with a view to determining the stream's health status. Three metals (Cu, Pb, and As) showed significant positive correlations between concentrations in the water column and the benthic sediments. All but one heavy metal (i.e. Zn) had reached disturbing concentrations in the stream's water column and exceeded their recommended limits in surface freshwaters. The geo-accumulation index (Igeo) and contamination factor (CF) indicated that the stream was moderately contaminated with Cd (0 <Igeo< 1; CF much closer to 1). This study concludes that the stream was impacted with heavy metals in its water column and slightly impacted with Cd on its bed, thus raising health concerns for plankton, benthic organisms and all users of the surface water. More conservation attention by relevant stakeholders through monitoring and regulation of human activities in river basins, is recommended for the sustenance of tropical freshwater ecosystem and human health.

Biofilm-specific uptake does not explain differences in whole-stream DOC tracer uptake between a forest and an agricultural stream

Benthic biofilms are often assumed to control terrestrially-derived dissolved organic carbon (tDOC) uptake in streams. We tested this by comparing 13C-enriched ryegrass leachate uptake in an agricultural and a forest stream, hypothesizing that a greater abundance of autotrophic biofilms in the agricultural stream would cause its whole-stream tDOC uptake to be comparatively low. We measured whole-stream and biofilm tDOC tracer uptake, metabolism, bacterial and algal diversity, and nutrient status of benthic epilithic biofilms, and assessed whole-stream hydromorphology. Whole-stream uptake of tDOC was six times lower in the agricultural (3.0 mg m−2 day−1) than in the forest (19.0 mg m−2 day−1) stream, and tDOC uptake velocity indicated lower tDOC demand in the agricultural (1.2 mm min−1) than in the forest (1.9 mm min−1) stream. The agricultural stream differed from the forest stream by slightly lower transient storage capacity and higher benthic biofilm bacterial abundance and production, lower biofilm biomass and lower biofilm molar C:N, C:P, and N:P ratios. Changes in epilithic biofilms contributed little to the differences in whole-stream tDOC tracer uptake between streams, as biofilm tDOC uptake only amounted to 4% and 13% of whole-stream uptake in the forest and agricultural stream, respectively. This comparison of a forest and an agricultural stream suggests that agricultural stressors have the potential to diminish both whole-stream tDOC uptake and uptake efficiency. Furthermore, the weak link between biofilm and whole-stream tDOC uptake implies that benthic biofilms characteristics are poor predictors for human impacts on tDOC uptake in agricultural streams and that hot spots of tDOC uptake are likely situated in the hyporheic zone or in the stream water column.

Temporal and spatial dynamics in microbial community composition within a temperate stream network.

The water column of streams hosts a unique microbial community that is distinct from the microbial communities of the stream benthos and surrounding soil. This community is shaped by complex interacting forces, including microbial dispersal from surrounding environments and in-stream selection. However, how the processes structuring stream communities change over space and time remains poorly understood. In this study, we characterize spatial and temporal trends in microbial community composition throughout a stream network spanning first through fifth order streams. We found that the microbial communities of headwater streams are compositionally diverse, with low representation of freshwater microbial taxa and high representation of soil and sediment-associated taxa. In three out of five seasonal samplings, a successional pattern was identified in which phylotype richness and compositional heterogeneity decreased while the proportion of known freshwater taxa increased with increasing cumulative upstream dendritic distance. However, in two samplings, streams instead exhibited uniformly high microbial diversity across the watershed, and the fraction of freshwater taxa showed no relationship with dendritic distance. Overall, our data suggest that the successional processes that drive microbial diversity in streams are highly dynamic and can be disrupted at landscape scales, potentially in response to variation in temperature and precipitation.

Water and Sediment Microbial Quality of Mountain and Agricultural Streams.

Increased public health risk caused by pathogen contamination in streams is a serious issue, and mitigating the risk requires improvement in existing microbial monitoring of streams. To improve understanding of microbial contamination in streams, we monitored in stream water columns and streambed sediment. Two distinct streams and their subwatersheds were studied: (i) a mountain stream (Merced River, California), which represents pristine and wild conditions, and (ii) an agricultural stream (Squaw Creek, Iowa), which represents an agricultural setting (i.e., crop, manure application, cattle access). Stream water column and sediment samples were collected in multiple locations in the Merced River and Squaw Creek watersheds. Compared with the mountain stream, water column concentrations in the agricultural stream were considerably higher. In both mountain and agricultural streams, concentrations in bed sediment were higher than the water column, and principal component analysis indicates that land use affected water column levels significantly ( < 0.05). The cluster analysis showed grouping of subwatersheds for each basin, indicating unique land use features of each watershed. In general, water column levels in the mountain stream were lower than the USEPA's existing water quality criteria for bacteria. However, the levels in the agricultural stream exceeded the USEPA's microbial water quality criteria by several fold, which substantiated that increased agricultural activities, use of animal waste as fertilizers, and combined effect of rainfall and temperature may act as potential determining factors behind the elevated levels in agriculture streams.

A Metagenome-Based Investigation of Gene Relationships for Non-Substrate-Associated Microbial Phosphorus Cycling in the Water Column of Streams and Rivers.

Phosphorus (P) is a nutrient of primary importance in all living systems, and it is especially important in streams and rivers which are sensitive to anthropogenic P inputs and eutrophication. Microbes are accepted as the primary mineralizers and solubilizers of P improving bioavailability for organisms at all trophic levels. Here, we use a genomics approach with metagenome sequencing of 24 temperate streams and rivers representing a total P (TP) gradient to identify relationships between functional genes, functional gene groupings, P, and organisms within the P biogeochemical cycle. Combining information from network analyses, functional groupings, and system P levels, we have constructed a System Relational Overview of Gene Groupings (SROGG) which is a cohesive system level representation of P cycle gene and nutrient relationships. Using SROGG analysis in concert with other statistical approaches, we found that the compositional makeup of P cycle genes is strongly correlated to environmental P whereas absolute abundance of P genes shows no significant correlation to environmental P. We also found orthophosphate (PO43-) to be the dominant factor correlating with system P cycle gene composition with little evidence of a strong organic phosphorous correlation present even in more oligotrophic streams.

Temperature effects on pathogen inactivation in stream water and sediment

Pathogen contamination in stream water is a serious concern, which can pose risks to public and animal health, and reducing the risks of pathogens requires enhanced knowledge of pathogen sources and pathogen survival under various environmental and controlled conditions. One of the major factors, which affect pathogen survival, is temperature. To improve existing understanding of pathogen inactivation in stream water and sediment at various temperature conditions, we executed a series of microcosm studies in batch mode for assessing pathogen survival. Water and sediment samples from stream water column and bed sediment of Merced River Watershed, California were collected, and stored at 4°C prior to experiments. Subsequently, heat shock experiments were conducted under temperature controlled conditions. Initial pathogen levels in water and sediment samples were controlled by mixing the inoculum of E. coli, Salmonella, and Listeria, which are water and foodborne pathogens, known for causing illnesses in humans. We studied pathogen survival at 30°C, and 50°C. Results showed that the temperature of heat shock and incubation time under heat shock played crucial role in reducing pathogen levels. Low temperature (30°C) heat shocks and limited time heat shocks with relatively higher temperature (50°C) may not have considerable impacts in reducing pathogen levels.

Watershed Urbanization Linked to Differences in Stream Bacterial Community Composition.

Urbanization strongly influences headwater stream chemistry and hydrology, but little is known about how these conditions impact bacterial community composition. We predicted that urbanization would impact bacterial community composition, but that stream water column bacterial communities would be most strongly linked to urbanization at a watershed-scale, as measured by impervious cover, while sediment bacterial communities would correlate with environmental conditions at the scale of stream reaches. To test this hypothesis, we determined bacterial community composition in the water column and sediment of headwater streams located across a gradient of watershed impervious cover using high-throughput 16S rRNA gene amplicon sequencing. Alpha diversity metrics did not show a strong response to catchment urbanization, but beta diversity was significantly related to watershed impervious cover with significant differences also found between water column and sediment samples. Samples grouped primarily according to habitat—water column vs. sediment—with a significant response to watershed impervious cover nested within each habitat type. Compositional shifts for communities in urbanized streams indicated an increase in taxa associated with human activity including bacteria from the genus Polynucleobacter, which is widespread, but has been associated with eutrophic conditions in larger water bodies. Another indicator of communities in urbanized streams was an OTU from the genus Gallionella, which is linked to corrosion of water distribution systems. To identify changes in bacterial community interactions, bacterial co-occurrence networks were generated from urban and forested samples. The urbanized co-occurrence network was much smaller and had fewer co-occurrence events per taxon than forested equivalents, indicating a loss of keystone taxa with urbanization. Our results suggest that urbanization has significant impacts on the community composition of headwater streams, and suggest that processes driving these changes in urbanized water column vs. sediment environments are distinct.

Enrichment of stream water with fecal indicator organisms during baseflow periods.

Fecal indicator organisms (FIOs) are generally believed to be present in surface waters due solely to direct deposition of feces or through transport in runoff. However, emerging evidence points toward hyporheic exchange between sediment pore water and the overlying water column during baseflow periods as a source of FIOs is surface waters. The objective of this work was to (a) propose a mass balance-based technique for estimating changes of FIO concentrations in the same volume of water (or "slug") from the inlet to outlet of stream reaches in baseflow conditions and (b) to use such enumeration to estimate rate of the FIO release to stream water column. Concentrations of Escherichia coli (E. coli) and enterococci were measured in the slug while simultaneously monitoring the movement of a conservative tracer, Br that labeled the slug. Concentrations of E. coli in the slug were significantly larger (P=0.035, P=0.001, and P=0.001, respectively) at the outlet reach in all three replications, while enterococci concentrations were significantly larger in two of three replications (P=0.001, P<0.001, and P=0.602). When estimated without accounting for die-off in water column, FIO net release rates across replications ranged from 36 to 57 cells m-2s-1 and 43 to 87 cells m-2s-1 for E. coli and enterococci, respectively. These release rates were 5 to 20% higher when the die-off in water column was taken into account. No diurnal trends were observed in indicator concentrations. No FIO sources other than bottom sediment have been observed during the baseflow period. FIOs are released into stream water column through hyporheic exchange during baseflow periods.

Aggregated filter‐feeding consumers alter nutrient limitation: consequences for ecosystem and community dynamics

Nutrient cycling is a key process linking organisms in ecosystems. This is especially apparent in stream environments in which nutrients are taken up readily and cycled through the system in a downstream trajectory. Ecological stoichiometry predicts that biogeochemical cycles of different elements are interdependent because the organisms that drive these cycles require fixed ratios of nutrients. There is growing recognition that animals play an important role in biogeochemical cycling across ecosystems. In particular, dense aggregations of consumers can create biogeochemical hotspots in aquatic ecosystems via nutrient translocation. We predicted that filter-feeding freshwater mussels, which occur as speciose, high-biomass aggregates, would create biogeochemical hotspots in streams by altering nutrient limitation and algal dynamics. In a field study, we manipulated nitrogen and phosphorus using nutrient-diffusing substrates in areas with high and low mussel abundance, recorded algal growth and community composition, and determined in situ mussel excretion stoichiometry at 18 sites in three rivers (Kiamichi, Little, and Mountain Fork Rivers, south-central United States). Our results indicate that mussels greatly influence ecosystem processes by modifying the nutrients that limit primary productivity. Sites without mussels were N-limited with -26% higher relative abundances of N-fixing blue-green algae, while sites with high mussel densities were co-limited (N and P) and dominated by diatoms. These results corroborated the results of our excretion experiments; our path analysis indicated that mussel excretion has a strong influence on stream water column N:P. Due to the high N:P of mussel excretion, strict N-limitation was alleviated, and the system switched to being co-limited by both N and P. This shows that translocation of nutrients by mussel aggregations is important to nutrient dynamics and algal species composition in these rivers. Our study highlights the importance of consumers and this imperiled faunal group on nutrient cycling and community dynamics in aquatic ecosystems.

Physicochemical Characterization of Sediment in Northwest Arkansas Streams

Eutrophication of surface waters is a critical concern in regions around the world facing nutrient surpluses as a result of confined animal feeding operations (CAFOs) and subsequent land application of manures. While large amounts of research exist on the transport of nutrient enriched runoff from fields to surface waters less information is available on in-stream processes controlling the transport of P in-stream. Thus, information is needed on the role of stream sediments in regulating transient phosphorus (P) to better understand the relationship between nutrient inputs and water quality. Fine-sized sediments ( 2-mm) consume and store P. From fine-sized sediment a modified P saturation ratio (PSRmod), related to the sediment’s ability to bind P and determined from Mehlich-3 extracted nutrients, has been correlated to in-stream dissolved reactive P (DRP) concentrations. The objectives of this study were to determine the relative size distribution of total- and fine-sized sediment (sand, silt clay) fractions among streams, determine the optimum sample number needed to characterize Mehlich-3 P (M3P) and PSRmod, and finally determine the applicability of PSRmod, as an indicator of stream water column DRP concentrations. Stream sediments were sampled from the 0- to 3-cm depth from stream reaches ranging from (25 – 75 m) in August, 2008 for characterization along with water samples collected from the thalweg for DRP concentration determination. Additional water column samples were collected along with fine-sized sedi- ment chemical properties in February, May, and June 2009. The distribution of sediment size classes was statistically similar, with 2- to 20- and 20- to 75-mm sized sediment dominating. Fine-sized sediment (mod, were determined to typically be sufficiently characterized by a sample scheme utilizing three samples points. Modified P saturation ratio of mod, has the potential to be used as an indicator of the ability of stream sediments to enrich stream water with P. Thus, fine-sized sediment nutrient concentrations appear to be key regulators of water column P concentrations.

Do benthic biologists pay enough attention to aggregates formed in the water column of streams and rivers?

AbstractAggregation of organic matter occurs commonly in the water column of streams and rivers. Aggregates are formed by physicochemical or biological mechanisms, and these aggregations result in the transformation of suspended matter into much larger units. Microaggregates are formed by spontaneous assembly of organic molecules and resemble single particles. In contrast, macroaggregates (flocs and snow) are formed by binding of components with exopolymers (EPS), and they vary in size, shape, and porosity. Fecal pellets are a 3rd type of aggregate and usually are bound by EPS but are more compact than flocs or snows. Transport of aggregates to the substratum plays a significant role in the use of organic matter by microorganisms and other benthic biota, and we need to integrate these processes into our studies of lotic ecosystems. Understanding aggregation is essential to basic science, and it plays a role in planning for conservation and management at a time when flowing waters and their catchments are ...

Cold event in the South Atlantic Bight during summer of 2003: Anomalous hydrographic and atmospheric conditions

Unusually cold seawater temperatures were observed along much of the U.S. eastern seaboard during the summer of 2003. In this study, hydrographic and atmospheric observations from spring through summer were analyzed to track the evolution of the cold water event in the South Atlantic Bight (SAB) and investigate links to various forcing mechanisms. The hydrographic observations included 13 cross‐shelf transects over the central region of the SAB, surface temperature time series from several NDBC stations, and bottom temperatures from a mid shelf mooring. Atmospheric data were obtained from NDBC stations. Additional data included water level from NOS stations and river discharge from USGS stations. The conditions observed during spring and summer of 2003 were compared with climatological values. Record precipitation and increased river discharge during spring produced strong salinity stratification over the inner and mid shelf. Anomalously intense and persistent upwelling‐favorable winds were present from May until August. On the mid and outer shelf the resulting upwelling and subsurface shoreward penetration of cold water acted as a feedback mechanism to preserve the stratified conditions through the summer. The characteristics of the upwelled water corresponded to water from the lower part of the Gulf Stream water column. On the shelf the resulting temperature values under the thermocline were significantly lower than climatological temperatures by 5°–7°C.

Corrigendum

Freshwater BiologyVolume 51, Issue 5 p. 986-987 Free Access Corrigendum First published: 13 April 2006 https://doi.org/10.1111/j.1365-2427.2006.01548.xCitations: 4AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat There is an error in eqn 3, Fig. 3 and associated text in Cross et al. (2005). The corrected equation, figure and text are printed below. The corrected text begins with the last paragraph of page 1905 in Cross et al. (2005). Figure 3Open in figure viewerPowerPoint Hypothetical relationship between stream water column N and P concentrations, uptake rates and uptake lengths (as determined by solute addition experiments) for various scenarios. In scenario I, the water column is the sole source of nutrients for benthic biomass production, assimilation by benthic organisms represents the only nutrient demand, and nutrients are not differentially recycled in the benthic compartment. In scenarios II–IV, these assumptions are altered to assess how each affects nutrient spiralling ratios as determined by solute addition techniques. Solid vertical arrows represent flows of dissolved inorganic N and P. Dotted vertical arrows represent flows of dissolved and/or particulate organic N and P. Correction (corrected text in italics): The utility of beginning with this simplified case is that it helps identify several characteristics of stream ecosystems that can cause UN : UP (and SW–N : SW–P) to vary from the value predicted by BN : BP. For example, the supply of N and P is rarely, if ever, fully provided by dissolved inorganic elements extracted from the water column, although uptake length calculations are almost exclusively determined from these forms. Alternate avenues of nutrient delivery to benthic organisms include local mineralisation of organic matter, perfusion of ground water through stream sediments, and direct use of nutrients in organic matter (Sheibley et al., 2003; Brookshire et al., 2005). Regardless of the mechanism, differential subsidies of N and P will cause UN : UPto vary from BN : BP(Fig. 3). Variable fractions of N and P demand met by flowing water supplies (FNandFP, respectively) will change relative uptake rates such that:(3) For example, if 10% of N demand and 40% of P demand is supplied by non-water column sources (F N and F P equal 0.9 and 0.6, respectively), U N : UPandSW−N : SW−Pwould increase to about 22.5 and 0.5, respectively (Fig. 3, scenario II). In this scenario, gross UN : UP(assimilation of N and P by organisms) has not changed and remains equivalent to BN : BP(=15). However, differential use of N and P from non-water column sources has exaggerated the degree of N limitation. Discrepancies between observed UN : UP and BN : BP could also arise from differential internal recycling of N and P (Fig. 3, scenario III) and non-assimilatory removal of nutrients from the water column (Fig. 3, scenario IV). Denitrification, nitrification, and sorption are a few processes that result in the loss of particular forms of nutrients from solution and are included in tracer-derived estimates of nutrient uptake in streams. Reference Cross W.F., Benstead J.P., Frost P.C. & Thomas S.A. (2005) Ecological stoichiometry in freshwater benthic systems: recent progress and perspectives. Freshwater Biology, 50, 1895– 1912. Wiley Online LibraryCASWeb of Science®Google Scholar Citing Literature Volume51, Issue5May 2006Pages 986-987 FiguresReferencesRelatedInformation

Bacterial communities associated with benthic organic matter in headwater stream microhabitats

Bacterial communities associated with a variety of benthic detritus types were studied in three streams in the context of the chemical characteristics of the sediment material and the stream water. A cell purification assay was developed for a quantitative microscopic evaluation of bacterial community structure in detritus samples by fluorescence in situ hybridization (FISH). The efficiency of FISH with fluorescently monolabelled probes was compared with FISH with signal amplification by catalysed reporter deposition (CARD-FISH). In detritus types poor in organic carbon and nitrogen, the numbers of prokaryotes were related to the chemical characteristics of the stream water column, whereas no such relationship was found for detritus types rich in organic carbon and nitrogen. These results might help to provide criteria for the selection of detritus types for river ecosystem assessment and monitoring. The percentage of bacteria detected by FISH with monolabelled probes was correlated with the detritus total organic matter (OM). This is likely attributed to a higher ribosome content of microbial cells on substrates rich in OM. Cell detection by CARD-FISH did not show any correlation with OM content, indicating that this technique renders the results more independent from the activity state of cells. Fluorescence in situ hybridization with four group-specific probes suggested a relationship between substrate quality and the composition of the microbial assemblages on the various types of detritus. The improved protocol for cell purification and CARD-FISH may facilitate future investigations on the relationship between the riverine benthic detritus quality and microbial community composition.

Spatial and temporal water column and streambed temperature dynamics within an alpine catchment: implications for benthic communities

Despite the significant influence of temperature upon alpine stream benthic communities, thermal regimes of the water column and hyporheic zone of these mountain streams have received limited attention. This paper reports upon a detailed spatio-temporal study of water column and streambed temperatures undertaken within the Taillon–Gabiétous catchment, French Pyrénées, that aims: (1) to characterize the nature and dynamics of alpine stream water column and streambed thermal patterns; (2) to investigate stream thermal variability under a range of hydroclimatological conditions; and (3) to consider the implications of (1) and (2) for alpine stream benthic communities. The catchment contains four highly dynamic hydrological sources and pathways: (1) two cirque glaciers (Taillon and Gabiétous); (2) seasonal snowpacks; (3) a karst groundwater system; and (4) hillslope aquifers. Water column temperatures were monitored continuously at four sites located along the Taillon glacial stream and at three groundwater springs (two karstic and one hillslope) over the 2002 summer melt season. An eighth site (Tourettes) was established on a predominantly groundwater-fed stream with limited meltwater input. Bed temperatures (0·05, 0·20 and 0·40 m depth) and river discharge were measured at three sites: (1) the Taillon stream; (2) the Tourettes stream; and (3) below the confluence of (1) and (2). Air temperatures, incoming short-wave radiation and precipitation were recorded to characterize atmospheric conditions. Glacial stream water column temperatures increased downstream, although groundwater tributaries punctuated longitudinal patterns. Karstic groundwater streams were cooler and more thermally stable than the glacial stream (except at the glacier snout). Hillslope groundwater stream temperatures were most variable and, on average, the warmest of all sites. Streambed temperatures in the glacial stream were coldest and most variable whilst the warmest and least variable streambed temperatures were recorded in an adjacent groundwater tributary. Temperature variability was strongly related to: (1) dynamic water source and pathway contributions; (2) proximity to source; and (3) prevailing hydroclimatological conditions. The high thermal heterogeneity within this catchment may sustain relatively diverse benthic communities, including some endemic Pyrénéan macroinvertebrate taxa. Copyright © 2004 John Wiley & Sons, Ltd.

Introduced bivalves in freshwater ecosystems: the impact of Corbicula on organic matter dynamics in a sandy stream.

Previous research on Corbicula fluminea (a well-established, non-native bivalve) has clearly shown that this single species impacts ecosystem processes such as nutrient and dissolved organic carbon cycling in the water column of streams. Surprisingly, little was known about how Corbicula might influence similar processes in streambed sediments. Here, we used both laboratory and field experiments to determine how filter- and pedal-feeding by Corbicula impact organic matter dynamics in the sandy streambed (Goose Creek, Virginia). Corbicula consumed significant quantities of organic material in the streambed when conditions favored pedal-feeding but increased buried organic matter stores when filter-feeding promoted deposition of organic matter (by production of feces and pseudofeces). Corbicula contributed significantly to total benthic community respiration (and thus carbon dioxide production), and used pedal-feeding on benthic organic material to grow at a faster rate than that possible by filter-feeding alone. Corbicula should be an important coupler between benthic and pelagic processes because this bivalve uses organic matter from both the water column and the stream sediments. Given the widespread occurrence of this species, we speculate that the introduction of Corbicula may have had major implications for organic matter dynamics in this and many other streams in the United States.