Alba Argerich

Resazurin as a “smart” tracer for quantifying metabolically active transient storage in stream ecosystems

[1] We propose the experimental use of resazurin (Raz) and develop a metabolically active transient storage (MATS) model to include processes that may provide additional information on transient storage from a biogeochemical perspective in stream ecosystems. Raz is a phenoxazine compound that reduces irreversibly to resorufin (Rru) in the presence of aerobic bacteria. Raz was added as a stream tracer to a 128-m reach of the forested second-order Riera de Santa Fe del Montseny (Catalonia, NE Spain), along with a conservative tracer, NaCl. Raz was transformed to Rru at a rate of 0.81 h � 1 in the hyporheic zone and only at a rate of 9.9 � 10 � 4 h � 1 in the stream surface channel. Raz transformation and decay and Rru production and decay were both correlated with O2 consumption measured at wells. The ratio of Raz to Rru concentration at the bottom of the reach was moderately correlated with instantaneous rates of net ecosystem production (NEP) measured over the whole reach. Data for Raz, Rru, and chloride were well fitted with the MATS model. The results from this study suggest that Raz transformation to Rru can be used as a ‘‘smart’’ tracer to detect metabolic activity, specifically aerobic respiration, associated with transient storage zones in stream ecosystems. Therefore, the Raz-Rru system can provide an assessment of the amount of transient storage that is metabolically active, an assessment that complements the physical characterization of transient storage obtained from conventional hydrologic tracers. The use of both physical and metabolic parameters of transient storage obtained with these tracers may increase our understanding of the relevance of transient storage on stream biogeochemical processes at whole reach scale, as well as the contribution of the different transient storage compartments to these processes.

Development of a “smart” tracer for the assessment of microbiological activity and sediment‐water interaction in natural waters: The resazurin‐resorufin system

Received 16 November 2007; revised 25 February 2008; accepted 14 March 2008; published 23 July 2008. [1] A ‘‘smart’’ tracer is a tracer that provides, directly or through measurement of its concentration or in combination with another compound, at least one ‘‘bit’’ more of information about the environment through which it travels than a conservative tracer. In this study we propose and present the chemical compound resazurin as a smart tracer to assess the coupling between solute transport and microbiological activity in sediment-water interfaces in freshwaters. Resazurin is a weakly fluorescent redox-sensitive dye that undergoes an irreversible reduction to strongly fluorescent resorufin under mildly reducing conditions, most commonly in the presence of living microorganisms. To investigate the suitability of resazurin as a smart tracer, we characterized the decay, sorption, reaction, and transport behavior of resazurin and resorufin in various waters and sediments using laboratory experiments. Results show that resazurin irreversibly and rapidly reacts to resorufin in colonized sediment with pseudo-first-order behavior and a rate coefficient of 1.41 h � 1 . This reaction is 3 orders of magnitude faster than that in stream water alone, indicating the tracer is sensitive to microbiological activity and associated sediment-water interactions. The compounds are affected by significant sorption, with an approximately linear isotherm and a Kd of 6.63 mL/g for resorufin in sediment with 2.19% organic carbon. The compounds are stable over weeks in natural water, except in the presence of strong light where significant photochemical decay may occur more rapidly.

Quantification of metabolically active transient storage (MATS) in two reaches with contrasting transient storage and ecosystem respiration

a deep alluvial deposit. The MATS zones measured 0.002 m 2 in the bedrock reach (37% of transient storage) and 0.291 m 2 in the alluvial reach (100% of transient storage). The effective rate coefficient of Raz transformation in the MATS of the bedrock reach was approximately 16 times that of the alluvial reach. However, when we take into account the contribution of the MATS zone to overall metabolic activity, Raz transformation in the MATS zone was 2.2 times slower in the bedrock reach than in the alluvial reach. The difference was similar to the difference in ecosystem respiration, which was 1.8 times lower in the bedrock reach than in the alluvial reach, suggesting that the MATS zones were important contributors to ecosystem respiration. Results indicate that the quantification of MATS can improve our understanding of the role that transient storage zones play on stream metabolic processes and demonstrate the utility of Raz as a “smart” tracer that provides new information on metabolic activity at a whole‐reach and at smaller scale. Citation: Argerich, A., R. Haggerty, E. Marti, F. Sabater, and J. Zarnetske (2011), Quantification of metabolically active transient storage (MATS) in two reaches with contrasting transient storage and ecosystem respiration, J. Geophys. Res., 116, G03034, doi:10.1029/2010JG001379.

Trends in stream nitrogen concentrations for forested reference catchments across the USA

To examine whether stream nitrogen concentrations in forested reference catchments have changed over time and if patterns were consistent across the USA, we synthesized up to 44 yr of data collected from 22 catchments at seven USDA Forest Service Experimental Forests. Trends in stream nitrogen presented high spatial variability both among catchments at a site and among sites across the USA. We found both increasing and decreasing trends in monthly flow-weighted stream nitrate and ammonium concentrations. At a subset of the catchments, we found that the length and period of analysis influenced whether trends were positive, negative or non-significant. Trends also differed among neighboring catchments within several Experimental Forests, suggesting the importance of catchment-specific factors in determining nutrient exports. Over the longest time periods, trends were more consistent among catchments within sites, although there are fewer long-term records for analysis. These findings highlight the critical value of long-term, uninterrupted stream chemistry monitoring at a network of sites across the USA to elucidate patterns of change in nutrient concentrations at minimally disturbed forested sites.

Quantifying spatial differences in metabolism in headwater streams

Abstract: Stream functioning includes simultaneous interaction among solute transport, nutrient processing, and metabolism. Metabolism is measured with methods that have limited spatial representativeness and are highly uncertain. These problems restrict development of methods for up-scaling biological processes that mediate nutrient processing. We used the resazurin—resorufin (Raz-Rru) tracer system to estimate metabolism at different spatial scales (habitat, subreach, and reach) in 2 headwater streams of the H. J. Andrews Experimental Forest (Oregon, USA), and present a mathematical framework for its application. We investigated the relationship between metabolism and hydrodynamics, i.e., geomorphic units (e.g., pool—riffle, pool—cascade), bed materials (i.e., alluvium vs bedrock channels), and type of transient storage (i.e., pure hyporheic exchange, pure surface transient storage, and a combination of both). The metabolic hotspots detected by the Raz-Rru system in both watersheds were related to hydrodynamic conditions known to increase biological processing. Higher respiration rate coefficients were found in subreaches with extensive hyporheic flow and flow through large woody-debris complexes, and higher reaeration rate coefficients were found in subreaches with intensive respiration activity and higher flow velocities. Because such hydrodynamic conditions and their effects on stream processing are difficult to quantify in headwater streams without the use of tracer techniques, the Raz-Rru system proved to be a good integrator of solute transport and stream metabolism processes.

Influence of transient storage on stream nutrient uptake based on substrata manipulation

Quantification of the transient storage zone (As) has become critical in stream biogeochemical studies addressed to examine factors controlling nutrient uptake. It is expected that higher As may enhance the interaction between nutrients and biota and thus, increase nutrient uptake. However, results from the literature are controversial. We hypothesized that besides of the size of As, the intrinsic physical and biological characteristics of stream structures that generate As are also relevant for nutrient uptake. We performed 24 additions of phosphate, ammonium, and chloride in four reaches of a man-made channel where we introduced three types of naturally colonized substrata packs (mud, sand and cobbles) to modify As. We estimated ammonium and phosphate uptake coefficients in both the main channel and As using a solute transport model (OTIS-P) and compared the results among reaches with different substrata types. The introduction of substrata packs decreased water velocity and increased As similarly among treatments. Nutrient uptake coefficients in the main channel were similar among reaches with different type substrata packs; however, nutrient uptake coefficients measured in As differed among them as well as the ratio between ammonium and phosphorus uptake coefficients in As, which were 1.6 in reaches with mud packs and 0.02 in reaches with sand or cobble packs. Results obtained in this study suggest that the contribution of As in nutrient uptake not only depends on the size of As but on the type of materials used to increase As, and thus, have biogeochemical implications on restoration projects aimed to modify channel morphology.

Ecohydrological interfaces as hot spots of ecosystem processes

The movement of water, matter, organisms, and energy can be altered substantially at ecohydrological interfaces, the dynamic transition zones that often develop within ecotones or boundaries between adjacent ecosystems. Interdisciplinary research over the last two decades has indicated that ecohydrological interfaces are often “hot spots” of ecological, biogeochemical, and hydrological processes and may provide refuge for biota during extreme events. Ecohydrological interfaces can have significant impact on global hydrological and biogeochemical cycles, biodiversity, pollutant removal, and ecosystem resilience to disturbance. The organizational principles (i.e., the drivers and controls) of spatially and temporally variable processes at ecohydrological interfaces are poorly understood and require the integrated analysis of hydrological, biogeochemical, and ecological processes. Our rudimentary understanding of the interactions between different drivers and controls critically limits our ability to predict complex system responses to change. In this paper, we explore similarities and contrasts in the functioning of diverse freshwater ecohydrological interfaces across spatial and temporal scales. We use this comparison to develop an integrated, interdisciplinary framework, including a roadmap for analyzing ecohydrological processes and their interactions in ecosystems. We argue that, in order to fully account for their nonlinear process dynamics, ecohydrological interfaces need to be conceptualized as unique, spatially and temporally dynamic entities, which represents a step change from their current representation as boundary conditions at investigated ecosystems.

Temporal variation of hydrological exchange and hyporheic biogeochemistry in a headwater stream during autumn

Abstract The hyporheic zone is of great interest for stream ecologists because of its role in stream biogeochemical processing. Our study addresses the effects of leaf-litter inputs and varying discharge on surface–hyporheic water exchange and their possible consequences for the hyporheic zone biogeochemistry. Our study was conducted during autumn in Riera de Santa Fe (northeastern Iberian Peninsula), a stream with a well developed deciduous riparian canopy. We placed 15 wells spaced at 5-m intervals longitudinally down the study reach and measured surface and hyporheic nutrient and dissolved O2 (DO) concentrations on 23 sampling dates (15 during the leaffall period and 8 after a flood that washed out 65% of the accumulated leaf biomass). We assessed changes in surface-water exchange and in hyporheic NH4-N and soluble reactive P (SRP) uptake via coinjection of a conservative tracer and nutrients. Compared to surface water, hyporheic water had lower DO, higher SRP and NO3-N concentrations, and similar NH4-N concentration. Hyporheic water had higher DO saturation (p  =  0.00) and higher NH4-N concentration (p  =  0.00) in downwelling than in upwelling wells, whereas SRP and NO3-N concentrations did not differ significantly between well types (p > 0.05). Hydrologic connectivity was higher in downwelling than in upwelling wells and decreased with leaf-litter accumulation in the stream channel and increased with stream discharge. Increased connectivity after a flood reduced the difference in DO between surface and hyporheic compartments in upwelling and downwelling wells and in NO3-N in upwelling wells. NH4-N and SRP uptake responded differently to these changes. Hyporheic SRP uptake rate was controlled by hyporheic SRP concentration, which did not vary with changes in connectivity, whereas NH4-N uptake rate was indirectly affected by changes in connectivity through the influence of connectivity on DO availability. Last, although no NO3-N was added during the solute injections, we observed an increase in hyporheic NO3-N that probably was caused by nitrification. Together these results illustrate how the combination of stream hydrology and organic matter accumulation can dictate seasonal changes in hyporheic biogeochemistry.

Ecosystem respiration increases with biofilm growth and bed forms: Flume measurements with resazurin

In a set of streamside mesocosms, stream ecosystem respiration (ER) increased with biofilm biomass and flow heterogeneity (turbulence) generated by impermeable bed forms, even though those bed forms had no hyporheic exchange. Two streamside flumes with gravel beds (single layer of gravel) were operated in parallel. The first flume had no bed forms, and the second flume had 10 cm high dune-shaped bed forms with a wavelength of 1.0 m. Ecosystem respiration was measured via resazurin reduction to resorufin in each flume at three different biomass stages during biofilm growth. Results support the hypothesis that ER increases with flow heterogeneity generated by bed forms across all biofilm biomass stages. For the same biofilm biomass, ER was up to 1.9 times larger for a flume with 10 cm high impermeable bed forms than for a flume without the bed forms. Further, the amount of increase in ER associated with impermeable bed forms was itself increased as biofilms grew. Regardless of bed forms, biofilms increased transient storage by a factor of approximately 4.

Comprehensive multiyear carbon budget of a temperate headwater stream

Headwater streams comprise nearly 90% of the total length of perennial channels in global catchments. They mineralize organic carbon entering from terrestrial systems, evade terrestrial carbon dioxide (CO2), and generate and remove carbon through in-stream primary production and respiration. Despite their importance, headwater streams are often neglected in global carbon budgets primarily because of a lack of available data. We measured these processes, in detail, over a 10 year period in a stream draining a 96 ha forested watershed in western Oregon, USA. This stream, which represents only 0.4% of the watershed area, exported 159 kg C ha−1 yr−1, similar to the global exports for large rivers. Stream export was dominated by downstream transport of dissolved inorganic carbon (63 kg C ha−1 yr−1) and by evasion of CO2 to the atmosphere (42 kg C ha−1 yr−1), leaving the remainder of 51 kg C ha−1 yr−1 for downstream transport of organic carbon (17 kg C ha−1 yr−1 and 34 kg C ha−1 yr−1 in dissolved and particulate form, respectively).