In many aquatic ecosystems, most microbes live in matrix-enclosed biofilms and contribute substantially to energy flow and nutrient cycling. Little is known, however, about the coupling of structure ...and dynamics of these biofilms to ecosystem function. Here we show that microbial biofilms changed the physical and chemical microhabitat and contributed to ecosystem processes in 30-m-long stream mesocosms. Biofilm growth increased hydrodynamic transient storage-streamwater detained in quiescent zones, which is a major physical template for ecological processes in streams-by 300% and the retention of suspended particles by 120%. In addition, by enhancing the relative uptake of organic molecules of lower bioavailability, the interplay of biofilm microarchitecture and mass transfer changed their downstream linkage. As living zones of transient storage, biofilms bring hydrodynamic retention and biochemical processing into close spatial proximity and influence biogeochemical processes and patterns in streams. Thus, biofilms are highly efficient and successful ecological communities that may also contribute to the influence that headwater streams have on rivers, estuaries and even oceans through longitudinal linkages of local biogeochemical and hydrodynamic processes.
Benthic (streambed) biofilms metabolize a substantial fraction of particulate organic matter and nutrient inputs to streams. These microbial communities comprise a significant proportion of overall ...biomass in headwater streams, and they present a primary control on the transformation and export of labile organic carbon. Biofilm growth has been linked to enhanced fine particle deposition and retention, a feedback that confers a distinct advantage for the acquisition and utilization of energy sources. We quantified the influence of biofilm structure on fine particle deposition and resuspension in experimental stream mesocosms. Biofilms were grown in identical 3 m recirculating flumes over periods of 18–47 days to obtain a range of biofilm characteristics. Fluorescent, 8 µm particles were introduced to each flume, and their concentrations in the water column were monitored over a 30 min period. We measured particle concentrations using a flow cytometer and mesoscale (10 µm to 1 cm) biofilm structure using optical coherence tomography. Particle deposition‐resuspension dynamics were determined by fitting results to a stochastic mobile‐immobile model, which showed that retention timescales for particles within the biofilm‐covered streambeds followed a power‐law residence time distribution. Particle retention times increased with biofilm areal coverage, biofilm roughness, and mean biofilm height. Our findings suggest that biofilm structural parameters are key predictors of particle retention in streams and rivers.
Key Points
We quantified biofilm‐particle interactions by fitting a stochastic mobile‐immobile model to observations from laboratory flume experiments
Fine particle retention was positively correlated with biofilm height, roughness, and streambed coverage
Results can be integrated within the stochastic mobile‐immobile framework to assess the effects of biofilms on fine particle dynamics
Sorption of organic molecules to mineral surfaces is an important control upon the aquatic carbon (C) cycle. Organo-mineral interactions are known to regulate the transport and burial of C within ...inland waters, yet the mechanisms that underlie these processes are poorly constrained. Streamwater contains a complex and dynamic mix of dissolved organic compounds that coexists with a range of organic and inorganic particles and microorganisms. To test how microbial metabolism and organo-mineral complexation alter amino acid and organic carbon fluxes we experimented with (13)C-labelled amino acids and two common clay minerals (kaolinite and montmorillonite). The addition of (13)C-labelled amino acids stimulated increased microbial activity. Amino acids were preferentially mineralized by the microbial community, concomitant with the leaching of other (non-labelled) dissolved organic molecules that were removed from solution by clay-mediated processes. We propose that microbial processes mediate the formation of organo-mineral particles in streamwater, with potential implications for the biochemical composition of organic matter transported through and buried within fluvial environments.
The effects of hydrodynamics and organic matter on sediment biofilm esterase activity and 3Hthymidine incorporation were investigated at several depths in the streambed and riparian zone of the ...alpine stream Oberer Seebach (Austria). On the sediment scale, microbial activity increased with both upwelling and downwelling Darcian velocities of interstitial water and was minimal when the surface/subsurface water exchange decreased. As revealed by stepwise multiple regression analyses, sediment carbohydrates and chlorophyll a explained a considerable percentage (up to 97% in summer) of the variance of biofilm esterase activity and 3Hthymidine incorporation, whereas pore-water temperature and concentrations of dissolved organic carbon (DOC) and dissolved oxygen (DO) were poor predictors of biofilm activity. DOC and DO fluxes, however, accounted for a considerable percentage of the variance in 3Hthymidine incorporation, which points toward the importance of flow in biofilm functioning. On the reach scale, the ratio of surface water travel length to the subsurface travel length, which is an index of the surface/ subsurface water exchange frequency, determined average reach biofilm activity. Downstream routing along predominantly subsurface flow paths increased streambed esterase activity, whereas elevated surface travel lengths reduced esterase activity. Esterase activity and 3Hthymidine incorporation were also positively related to the streambed DOC retention efficiency, which underscores the role of biofilms in solute retention. These results high-light the coupling between streambed hydrodynamics and microbial activity and link sediment-scale and reach-scale processes.
We investigated possible effects of the hydrodynamics at the water/sediment interface on river bed biofilms within the reservoir Freudenau (Vienna, Austria) of the Danube River during the period ...1996/97. Two study sites (OBB and SSF) that differed in the magnitude of surface/subsurface water exchange were selected and intersite comparisons revealed higher organic matter, bacterial cell numbers, and esterase activity in SSF with lower horizontal outflow. Concentrations of colloidal carbohydrates and uronic acids were unaffected by hydrodynamics. The relative contribution of uronic acids to bulk colloidal carbohydrates was higher in the low-flow site SSF. The distribution patterns of this relative contribution generally matched the subsurface flow pattern. Shortly after impoundment in March 1996 and along with decreased surface flow velocity, maximal biofilm carbohydrate exopolymers concurred with minimal esterase activity in OBB. We hypothesize that this inverse relationship is due to increased diffusional resistance within the exopolymer biofilm matrix that reduces mass transfer and hydrolytic activity. These results, to our knowledge, are the first evidence for microbial participation in the clogging of a large river bed. Biofilm-associated organic carbon increased significantly by a factor of ∼3.3 to 4.4 with progressive clogging as determined by the sediment leakage coefficient, which increased ∼3.8 times. Concomitantly, with ongoing clogging, esterase activity exhibited increasingly higher values at the interface relatively to deeper sediment layers, which translates into steeper depth gradients. Furthermore, minimal inflow from the surface water into the river bed along with steepest esterase gradients concurred with a senescent benthic algal bloom. This suggests an important role for algae in clogging. Either algae obstruct voids mechanically, or their exudates fuel heterotrophic bacteria that in turn are involved in clogging processes. However, our data do not allow unequivocal differentiation between biogenic and physical clogging mechanisms.
SUMMARY
1. We monitored streamwater and streambed sediment porewaters from White Clay Creek (WCC), SE Pennsylvania, for dissolved organic carbon (DOC), dissolved oxygen (DO) and conductivity to ...investigate organic matter processing within the hyporheic zone. Dissolved organic carbon and DO concentrations were higher in the streamwater than in the porewaters and, in many cases, concentrations continued to diminish with increasing depth into the streambed.
2. Hydrological exchange data demonstrated that the permeability of the stream bed declines with depth and constrains downwelling, effectively isolating porewaters >30 cm from streamwater.
3. End‐member mixing analysis (EMMA) based on conductivity documented a DOC source and DO sink in the hyporheic zone. We calculated hyporheic streambed DOC fluxes and respiration from the EMMA results and estimates of water flux. Based upon our calculations of biodegradable DOC entering the hyporheic zone, we estimate that DOC supports 39% of the hyporheic zone respiration, with the remaining 61% presumably being supported by entrained particulate organic carbon. Hyporheic respiration averaged 0.38 g C m−2 d−1, accounted for 41% of whole ecosystem respiration, and increased baseflow ecosystem efficiency from 46 to 59%.
4. Advective transport of labile organic molecules into the streambed concentrates microbial activity in near‐surface regions of the hyporheic zone. Steep gradients in biogeochemical activity could explain how a shallow and hydrologically constrained hyporheic zone can dramatically influence organic matter processing at the ecosystem scale.
Colored dissolved organic matter (CDOM) and dissolved organic carbon (DOC) concentrations were investigated in a blackwater tributary (river Surumoni) of the upper river Orinoco (South Venezuela) ...during November and December 1996. DOC concentrations were high (∼10
mg C
L
−1), relatively invariant in time and largely decoupled from discharge. The Surumoni has the highest CDOM levels reported hitherto with slopes of ln-linearized absorption spectra ranging from 0.0085–0.0152
nm
−1, a mass specific absorption coefficient at
λ=300
nm of 6.45
L
mg
−1
m
−1. Optical signatures indicate that the CDOM is highly aromatic in nature and of terrestrial origin in the Surumoni, whereas autochthonous sources are also likely to contribute significantly to the Orinoco CDOM pool. Hydrologic connectivity of the active channel with fringing floodplains largely determined the spatiotemporal variation of CDOM in the Surumoni. The upriver channel is straight, braided in some reaches, and CDOM optical properties remained largely invariant along its course. Downriver, the meandering channel receives substantial amounts of highly degraded, aromatic material from extensive floodplains.
•Glacier retreat decreases diurnal power and frequency of discharge and conductivity.•Diurnal variations of turbidity are less affected.•Water temperature is impacted but the main driver is air ...temperature.•Impact on hydrological parameters is greater in Ecuador than in the Swiss Alps.
The impacts of glacier mass and area loss are assumed to affect stream hydrology and related ecosystems. We applied wavelet analysis to high frequency time series of discharge, conductivity, turbidity, air and water temperature in mountain streams influenced by a gradient of diverse glacier catchment coverages from 0 to 28%. All parameters were recorded during 200–250 day period in 24 mountain streams in Switzerland and Ecuador. We interpreted the range of glacier coverage as a proxy of glacier melt effect linked to modification in glacier cover based on a space-for-time substitution approach. We used the diurnal variation power and frequency as hydrological parameters to quantify the intensity and occurrence of significant diurnal variations in the different mountain catchments. Our results show that the reduction in glacier cover would reduce the intensity and occurrence of diurnal variations in discharge, turbidity and conductivity, and the intensity of diurnal variations in water temperature. Discharge and conductivity were affected in a similar way, while the impact on turbidity was less significant. Furthermore, due to their different climatic regimes, the diurnal discharge in the tropical Andes was more impacted than in the Alps, indicating that the impacts of global change on glacier streams would be stronger in the tropical Andes than in the Alps.
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