International audienceProtecting or restoring aquatic ecosystems in the face of growing anthropogenic pressures requires an understanding of hydrological and biogeochemical functioning across multiple spatial and temporal scales. Recent technological and methodological advances have vastly increased the number and diversity of hydrological, bio-geochemical, and ecological tracers available, providing potentially powerful tools to improve understanding of fundamental problems in ecohydrology, notably: 1. Identifying spatially explicit flowpaths, 2. Quantifying water residence time, and 3. Quantifying and localizing biogeochemical transformation. In this review, we synthesize the history of hydrological and biogeochemical theory, summarize modern tracer methods, and discuss how improved understanding of flowpath, residence time, and biogeochemical transformation can help ecohydrology move beyond description of site-specific heterogeneity. We focus on using multiple tracers with contrasting characteristics (crossing proxies) to infer ecosystem functioning across multiple scales. Specifically, we present how crossed proxies could test recent ecohydrological theory, combining the concepts of hotspots and hot moments with the Damköhler number in what we call the HotDam framework
Emergent macrophytes (helophytes) grow in the active channel of fluvial ecosystems. Subsurface flow beneath this area (i.e., hyporheic zone) is considered critical for ecological processes. However, little is known about the influence of helophyte roots on subsurface solute transport. We investigated the effect of three helophyte species with different root architecture (Iris pseudacorus L., Phragmites australis L., and Scirpus lacustris L.) on solute transport along subsurface flow paths. We considered both the physical and the biological roles of the roots, expecting that (1) roots will act as structures that create heterogeneities in the sediment (physical role); thus, root architecture will alter subsurface flow paths; (2) roots will remove water via evapotranspiration (biological role), leading to slower flow velocity; and (3) both scenarios will result in longer water residence times. We performed conservative tracer pulse additions in 12 flow‐through flumes subjected to four treatments: absence of helophytes (Control) and presence of helophytes (Iris, Scirpus, and Phragmites). Tracer breakthrough curves were used to compare solute transport patterns between the treatments by fitting a mobile‐immobile model and by applying temporal moment analysis. Results showed that helophyte roots increase subsurface water residence time by creating heterogeneities in the substrate and by removing water. Furthermore, hydraulic retention increased with the percent volume of fine roots but decreased in the presence of thicker roots. Based on these results we suggest that the root architecture of helophytes and their capacity to remove water via evapotranspiration should be considered when planning stream restoration activities aimed to improve water quality.
In the Mediterranean region, water scarcity compromises stream water quality particularly downstream of wastewater treatment plants (WWTP). We tested the potential of four helophyte species to reduce dissolved inorganic nitrogen (N) and phosphorus (P) from WWTP effluents. We conducted an 11-month mesocosm experiment to assess differences in N and P content among plant compartments and among species. Moreover, we quantified the relative contribution of above and belowground parts of the plants to N and P retention. The experiment was conducted at the Urban River Laboratory (www.urbanriverlab.com) in artificial channels (12 m long x 0.6 m wide x 0.4 m deep) planted with monospecific stands of Iris pseudoacorus, Typha angustifolia, Phragmites australis and Scirpus lacustris. Channels (three replicates per species) received water from the WWTP effluent, which flowed at a constant rate of 5 L min-1 through the sub-surface. The helophytes were planted in November 2014 and biomass standing stocks of carbon (C), N and P were measured in October 2015 at the time of maximum plant biomass. Differences in the concentration of N and P were larger among plant compartments than among species. The highest N concentration was measured in leaves while rhizomes showed the highest P concentration. The total plant biomass varied greatly among species from 11.4 to 4.6 Kg DW m-2 for Iris and Scirpus, respectively. Iris accumulated the highest amount of N (256 g N m-2) and P (27 g P m-2) in biomass. Plants retained from 8% (Scirpus) to 19% (Iris) of total dissolved inorganic N inputs to the channels (10.4 kg N) during the experiment, and from 6% (Phragmites) to 14% (Iris) of total dissolved inorganic P inputs (1.3 Kg P). This study provides quantitative evidence to water managers of the potential role of helophytes to improve water quality in freshwater ecosystems receiving water from WWTP effluents.
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